cadcam cae notes

CAD/CAM/CAE

Asst. Prof. Abhay S. Gore

Dept. of Mechanical Engineering,

Maharashtra Institute of Technology, Aurangabad

MECHANICAL ENGINEERING CAD/CAM/CAE

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CHAPTER 1. INTRODUCTION

Syllabus

• Introduction to CAD, CAM, CIM and CAE,

• Product cycle,

• Mathematical model for product life cycle. Introduction

• In today‟s global competition, industries cannot survive unless they introduce new products or existing ones with:

– Better quality

– Lower cost

– Shorter lead time

• Computer aided design (CAD) can be defined as the use of computer systems to assist in creation, modification, analysis and optimization

• Computer aided machining (CAM) can be defined as the use of computer systems to plan, manage and control a manufacturing plant through either

direct or indirect computer interface with the plant‟s production resources. CAM

• 3D CAD data can be read by CAM software which takes 3D data & CNC

machine parameters as inputs and delivers a tool path that cuts metal as per the part designs.

• The tool path so generated can be simulated on the screen to evaluate tool gauging so that the machining run is perfect.

Computer Integrated manufacturing

• Computer-integrated manufacturing (CIM) is manufacturing supported by computers. It is the total integration of Computer Aided Design /

Manufacturing and also other business operations and databases.

• Definition: CIM is the integration of total manufacturing enterprise by using integrated systems and data communication coupled with new managerial

philosophies that improve organizational and personnel efficiency.

• The CIM concept is that all the operations related to the production function in an integrated computer system to assist, enhance and /or automate the operations.

• The computer system is spread throughout the firm , touching all the activities that support manufacturing.

• In this integrated computer system, the output of one activity serves as the input to the next activity, through the chain of events that starts with the sales

order and finishes with shipment of the product.


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Figure 1 Scope of CAD/CAM and CIM

Figure 2 Computerized elements of CIM system

• Customer orders are initially entered by the company‟s sales force into a computerized order-entry system. The orders contain the specifications describing the product.

• The specifications serves as input to the design

• New products are designed on a CAD system. The components that comprise product are designed, the BOM is complied, and assembly dwgs. are prepared.

• The output of design serves as input to mfg. engg, where process planning, tool design and similar activities are accomplished to prepare for production

• The output of mfg. engg. Provides input to the PPC, where MRP and scheduling is performed

Computer-Aided Engineering


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• Computer-aided Engineering analysis (often referred to as CAE) is the application of computer software in engineering to analyze the robustness and

performance of components and assemblies.

• It encompasses simulation, validation and optimization of products and manufacturing tools.

• Parts and assemblies designed in CAD software can be „analyzed‟ for their field performances right on the computer screen.

• Most of the olden-day destructive testing methods have found mathematical replacements in the modern-day CAE software.

• CAE software delivers results that help analyzing designs.

• Analysis tools are available for

• static stress-strain,

• deflection,

• thermal,

• flow,

• motion,

• vibration

• This allows designers to “design-right-the-first-time”.

• Use of computer systems to analyze CAD geometry

• Allows designer to simulate and study how the product will behave, allowing for optimization

• Finite-element method (FEM)

• Divides model into interconnected elements

• Solves continuous field problems Ranges of CAE

• Kinematics Analysis: to determine motion paths and linkage velocities in mechanism. Pro/E

• Finite Element Analysis (FEA): Solid Mechanics analysis (stress/strain), Heat Transfer, Flow, and other continuous fields.

Pro/Mechanica, ANSYS, CATIA, NASTRAN

• Computational Fluid Dynamics (CFD): Fluid Simulation. Fluent, Phoenix, CFX

Product Cycle (Design and Manufacturing )

Figure 3 Product Cycle (Design and Manufacturing)


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• Driven by customers and markets which demand the product

• In some cases design functions are performed by the customer and the product is manufactured by a different firm.

• In other cases , design and manufacturing is accomplished by the same firm 1. The product cycle begins with a concept, an idea of a product.

2. This concept refined, analyzed, improved and translated into a plan for the product through the design engineering process.

3. The plan is documented by drafting a set of engineering drawings showing how the product is made and providing a set of specifications indicating how the product should perform

4. A process plan is formulated which specifies sequence of production operations required to make the product.

5. New equipment and tools sometimes be acquired to produce the new product. 6. Scheduling provides a plan that commits the company to manufacture of

certain quantities of the product by certain dates.

7. Once all these plans are formulated product goes into production. 8. It is followed by quality testing and delivery to the customer.

Product Cycle revised with CAD/CAM overlaid

Figure 4 Product Cycle revised with CAD/CAM overlaid

Computer aided design and computer automated drafting are utilized in

conceptualization, design and documentation of the product.

Computers are used in process planning and scheduling to perform these functions

more efficiently.

Computers are used in production to monitor and control the manufacturing

operations.

In quality control , computer are used to perform inspections and performance

tests on the product and its components Mathematical Model for product life cycle

• Let T1 be the time required to produce 1 unit of product.

– This includes sum of individual process times plus time to assemble,

inspect and package a single product.


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• T2 be time associated with planning and setting for each batch of production

– This include the ordering of raw materials by purchasing department,

time required in production planning to schedule the batch, setup times for each operation etc.

• T3 be the time required for designing the product and for all other activities that are accomplished once for each different product.

– These include process planning, cost estimating and pricing, building of special tools and fixtures.

• B be the number of batches produced throughout the product life cycle.

• Q be the number of units produced in each batch.

• The aggregate time spent on the product throughout its life cycle can be

The average time spent on each unit of product during its life cycle.

• In mass and batch production the T2 and T3 terms can be spread over a large

number of units.

• Their relative values ,therefore becomes less important as the production quantities increase. The T1 term becomes most important term.

• In job shop manufacturing , the T2 and T3 terms can become significant because the quantities are so low.

Relationship between CAD/CAM & Automation

• The goal of CAD/CAM and automation is to reduce the various time elements in product life cycle.

• With this there is increase in productivity and improve in standard of living.

• The automation technology is concerned with reducing the T1 and T2 elements, with emphasize on the unit production cost (T1).

• The CAD/CAM technology is concerned with all the three terms but is perhaps focused on the T3 and T2 terms in the life cycle model.

• CAD/CAM has made important contribution towards integrating the functions of design and manufacturing.


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CHAPTER 2. FUNDAMENTALS OF CAD Syllabus

The basic design Process and computer

The manufacturing database

Benefits of CAD

Principles of Concurrent Engineering

The Design Process

Recognition of need involves the realization by someone that a problem exists for

which some corrective action should be taken e.g. defect in current machine design

Definition of problem involves a through specifications of the item to be designed.

This specifications includes physical and fundamental characteristics, cost, quality and operating performance.

Synthesis and analysis are closely related and highly iterative in design process

This iterative process is repeated until the design has been optimized within the

constrained imposed on the designer

The philosophy, functionality and uniqueness are all determined during synthesis.

The major financial commitments to turn conceived product into reality are made during synthesis.

Information generated during synthesis phase is qualitative and hard to capture by computer system.

The end goal of synthesis is a conceptual design

Evaluation is concerned with measuring the design against the specifications

established in problem definition phase.

This evaluation often requires fabrication and testing of a prototype model to

assess performance, quality, reliability and other criteria.

Presentation includes documentation of design by means of drawing , material

specifications, assembly list and so on.

The Application of computer for Design


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Geometric modeling corresponds to the synthesis phase in which physical design

project takes form on the ICG system

Engineering analysis corresponds to phase 4 , dealing with analysis and

optimization

Design review and evaluation is the fifth step in the general design procedure

Automated drafting involves a procedure for converting the design image data residing in computer memory into a hard copy document.

Geometric Modeling

It is concerned with the computer compatible mathematical description of the geometry of the object.

To use geometric modeling, the designer constructs the graphical image of the object on the CRT screen of the ICG system by inputting three types of commands

1. The first type of command generates basic geometric elements such as points, lines and circles.

2. The second command type is used to accomplish scaling, rotation and other transformations of these elements.

3. The third type of demand causes the various elements to be joined into the

desired shape of the object being created on the ICG system. During this process , the computer converts the commands into a mathematical

model, stores it in the computer data files, and displays it as an image on CRT screen.

The model can be called from the data files for review, analysis, or alteration.

There are several different methods of representing the object in geometric modeling.

The basic form uses wire frames to represent the object. In this form the object is displayed by interconnecting lines. Wire frame modeling is classified into three types

1. 2D. Two dimensional representation is used for a flat object


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2. 2 ½ D. In this a three dimensional object to be represented as long as it has no

side wall details. 3. 3D. This allows for full three dimensional modeling of a more complex

geometry

Figure 5 Wire-frame drawing of a part

Even three dimensional wire frame representations of an object are inadequate for complicated drawings

Wire frame models can be enhanced by several methods 1. First uses dashed lines to show the rear edges of the object, those which would

be invisible from front 2. It removes the hidden lines completely, thus providing a less cluttered picture

of the object for the viewer.

Wire-frame drawing

Figure 6 Dashed lines to show rear edges of part Hidden line removal

3. By providing surface representation which makes the object appear solid to

the viewer. 4. The most advanced method uses solid geometry shapes called primitives to

construct the object. 5. Another feature of some CAD system is color graphics capability

Solid Modeling


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Engineering Analysis

The analysis may involve stress strain calculations, heat transfer computations, or use of dynamic behaviour of the system being designed.

The computer can be used to aid in this analysis work. Two important examples of this type are: 1. Analysis of mass properties

2. Finite element analysis 1. Analysis of mass properties:

For solid object: surface area , weight , volume, centre of gravity, and moment of inertia.

For a plane surface (or c/s of solid object) the corresponding computations

may include the perimeter, area, and inertia properties. 2. Finite element analysis:

With this technique the object is divided into large number of finite elements (usually triangular or rectangular in shapes) which form an interconnecting network of concentrated nodes.

By using a computer with significant computer capabilities, the entire object can be analyzed for stress-strain, heat transfer and other characteristics by

calculating the behaviour of each node. By determining the interrelating behavior of all the nodes in the system, the

behavior of the entire object can be assessed

Design Review and Analysis

Semi automatic dimensioning and tolerancing routines which assign size

specifications to surfaces indicated by the user help to reduce the possibility of the dimensioning errors.

The designer can zoom in on part design details and magnify the image on

graphics screen for close scrutiny A procedure called layering is often helpful in design review. E.g. a good

application of layering involves overlaying the geometric image of the final shape of the machined part on the top of the image if the rough casting.

This ensures that sufficient material is available on the casting to accomplish

the final machined dimensions. Another related procedure for design review is interference checking. This

involves the analysis of an assembled structure in which there is a risk that the components of the assembly may occupy the same space.


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The risk occurs in the design of large chemical plants, cold boxes, and other

complicated piping structures. One of the feature of CAD systems is kinematics. The available kinematics

packages provide the capability to animate the motion of simple designed mechanisms such as hinged components and linkages.

Automated Drafting

It involves the creation of hard copy engineering drawings directly from the CAD database.

CAD systems includes features like automatic dimensioning, generation of crosshatched areas, scaling of the drawing, and the capability to develop sectional views and enlarged views of particular part details

Different views like oblique, isometric and perspective views can be of significant assistance in drafting.

The manufacturing database from CAD/CAM

In the conventional manufacturing cycle practiced for so many years in industry,

• Engineering drawings were Prepared by design draftsman and then used by manufacturing engineer to prepare process plan (“Route Sheet”)

• The activities involved in the designing the product were separated from the

activities associated with process planning.

• Essentially a two step procedure was employed.

• This was both time consuming and involved duplication of efforts by design and manufacturing personnel

• In an integrated CAD/CAM system, a direct link is established between product design and manufacturing.


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• It is goal of CAD/CAM not only to automate certain design phases of design and certain phases of mfg., but also to automate the transition from design to

manufacturing.

• Computer- based systems have been developed which create much of the data and documentation required to plan and manage the mfg. operations for the

product.

• The manufacturing data base is an integrated CAD/CAM data base.

• It includes all the data on the product generated during design (geometry data, BOM , material specifications) as well as additional data required for mfg.,

much of which based on the product design

Figure 7 The manufacturing database from CAD/CAM

Benefits of CAD

• Productivity improvement in Design

• Shorter lead times

• Reduced engineering personnel requirement

• Customer modifications are easier to make

• Rapid response of the design analysis

• Improved accuracy of design

• Avoidance of subcontracting to meet schedules

• Provide better functional analysis to reduce prototype testing

• Avoidance of subcontracting to meet schedules

• Assistance in preparation of documentation

• Minimized transcription errors

• Designs have more standardization

• Fewer errors in NC part programming

• Better communication interfaces and greater understanding among engineers, designers, drafters, management and different project groups.

Principles of Concurrent Engineering

Definition

• It is a methodology of restructuring the product development activity in an organization using a cross functional team and is a technique adopted to

improve efficiency of the product design and reduce product cycle time.

• Concurrent engineering brings together a wide spectrum of people from several functional areas in the design and mfr. of a product.


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• Representatives from R&D, engineering, manufacturing, materials management, quality assurance marketing etc develop the product as a team.

Traditional Process of Serial Engineering

• Functions Separated

• Functions Serially Executed

• No Interaction

• Maintenance Usually an Afterthought

• Time Consuming

• Costly

Figure 8 Serial Engineering

Costs involved in Design changes

Figure 9 Costs involved in Design changes

Concurrent vs. Serial Engineering

• All Viewpoints Solicited

• Interdisciplinary Teams

• Life Cycle Cost Considered

• Attempt to represent Concept Early - Before Committing to Detail Design

• Data/Information/Knowledge Exchange Planned and Encouraged

• Cycle Time and Cost Reduced


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Figure 10 A Concurrent Engineering Model


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CHAPTER 3. HARDWARE REQUIREMENTS OF CAD Syllabus

Hardware configuration of a CAD system

Display devices like raster scan devices, plasma panels, liquid crystal displays. Types of images like halftone, grayscale, colour Memory requirements for raster scan graphics terminal

Concept of pointing and positioning. Design Workstation

CAD system would include the following hardware components One or more workstations. These would consist of A graphics terminal

Operator input devices One or more plotters and other output devices

Central Processing unit (CPU) Secondary storage

Figure 11 Configuration of CAD Workstation

The Design workstation Functions of CAD workstation

1. It must interface with the central processing unit 2. It must generate a steady graphic image for the user 3. It must provide digital descriptions of the graphics image

4. It must translate computer commands into operating functions 5. It must facilitate communication between the user and the system.

The Graphics Terminal

• Image generation in computer graphics

• Graphics terminals for computer aided design Image generation in computer graphics

• All computer graphics terminals available today use the cathode ray tube (CRT) as the display device.

The operation of CRT is as follows

• A heated cathode emits a high speed electron beam onto a phosphor-coated glass screen.

• The electron energize the phosphor coating, causing it to glow at points where the beam makes contact

• By focusing the electron beam , changing its intensity , and controlling its

point of contact against the phosphor coating through the use of deflector system , the beam can be made to generate picture on the CRT screen


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Figure 12 Cathode Ray Tube

Image generation in computer graphics

• There are two basic techniques used in computer graphics terminal to generating the image on the CRT screen

1. Stroke writing 2. Raster scan

Other names for stroke writing technique include line drawing, random position, vector writing and directed beam

Other names to raster scan technique include digital TV and scan graphics Stroke Writing

• It uses an electron beam which operates like a pencil to create a line image on the CRT screen.

• The image is constructed out of straight segments

• Each line can be drawn on the screen by directing the beam to move from one point on the screen to the next, where each point is defined by its x and y

coordinates.

• Although the procedure results in images composed of only straight lines, smooth curves can be approximated by making the connecting line segments

short enough.

Raster scan

• In this viewing screen is divided into a large number of discrete phosphor picture elements, called pixels.

• The number of pixels in the raster display might be range from 256 X 256 (a total of 65,000 points) to 1024 X 1024 (a total of 10,00,000 points)

• Each pixel on the screen can be made to glow with a different brightness.

• Color screens provide for the pixels to have different colors as well as brightness


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Graphics Terminal for CAD

1. Directed-beam refresh

2. Direct-view storage tube (DVST) 3. Raster scan (digital TV)

Directed-beam refresh

• Utilizes stroke writing approach to generate the image on the screen

• The image must be regenerated many times per second to avoid flickering

• The phosphor elements on the screen surface are capable of maintaining their brightness only for a short time (sometimes measured in micro seconds)

• On densely filled screens it is difficult to avoid flickering of the image

• As the image is continuously refreshing, selective erasure and alteration of the image is readily accomplished

• It is also possible to provide animation of the image with refresh tube.

• Other names are vector refresh and stroke writing refresh Direct-view storage tube (DVST)

• This also uses stroke writing approach to generate image on the CRT screen

• The term “storage tube” refers to the ability of the screen to retain the image which has been projected against it, thus avoiding the need to rewrite the image constantly

• This is possible with the use of an electron flood gun directed at the phosphor coated screen which keeps the phosphor elements illuminated once they have been energized by the stroke writing electron beam.

• The resulting image will be flicker free.

• In this individual lines cannot be erased selectively

• It lacks color capability

• These are lowest cost terminals

• They are capable of storing large amount of data Raster Scan Terminals

• The operation is similar to commercial Television set

• The difference is that TV uses analogue signals generated by video camera to

construct image on CRT screen, while raster scan ICG terminal uses digital signals generated by computer.

• Lowest cost terminals using two beam intensity levels, on or off . This means that each pixel in the viewing screen is either illuminated or dark

• A picture tube with 256 lines of resolution and 256 addressable points per line would form the image will require 256 X 256 or over 65,000 bits of storage.

• Each bit of memory contains the on/off status of the corresponding pixel on the CRT screen.


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• This memory is called as frame buffer or refresh buffer

• Picture quality can be improved in two ways: by increasing the pixel intensity

and or adding a grey scale (or color).

• Increasing picture intensity means adding more lines of resolution and more addressable points par line.

• A 1024 X 1024 raster screen would require more than 1 million bits of storage

in the frame buffer.

• A grey scale is accomplished by expanding the number of intensity levels which can be displayed on each pixel.

• This requires additional bits for each pixel to store the intensity level.

• Two bits are required for four levels, three bits required for eight levels, and so forth.

• Five or six bits required to achieve approximation for continuous achieve an approximation of continuous grey scale

• For color display , three times as many bits require to get various intensity levels for each of the three primary colors: red, blue and green.

• A raster scan graphics terminal with high resolution and grey scale can require very large capacity refresh buffer.

Colour CRT Monitors

• Two basic techniques used for producing colour display with a CRT are the 1. Beam penetration method 2. Shadow mask method

Beam Penetration Method

• It is used with random scan monitors.

• Two layers of phosphor, usually red and green, are coated onto the inside of the CRT screen, and the displayed colour depends upon how far the electron

beam penetrates into the phosphor layer

• A beam of slow electrons excites only the outer red layer.

• A beam of fast electrons penetrates through the red layer and excites the inner green layer.

• At intermediate speed, combination of red and green light are emitted to show two additional colours, orange and yellow

• The speed of electrons, hence the screen colour at any point is controlled by the beam acceleration voltage

Delta Delta Shadow Mask Method

• These are most widely used because they produce wide variety of range.

• A shadow mask CRT has three phosphor dots at each pixel position

• One phosphor dot emits a red light, another emits a green and the third emits a blue light.

• This type of CRT has three electron guns one for each colour dot, and shadow mask grid just behind the phosphor coated screen

• The electron beams are deflected and focused as a group onto the shadow mask, which has series of holes aligned with phosphor dot patterns.

• When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small colour spot on the screen.

• The phosphor dots are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask.


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Comparison of Graphics terminal features

Direct Beam

storage

DVST Raster scan

Image Generation Stroke writing Stroke writing Raster scan

Picture quality Excellent Excellent Moderate to good

Data content Limited High

Selective erase Yes No Yes

Grey scale Yes No Yes

Color capability Moderate No Yes

Animation capability Yes No Moderate

Flat Panel Displays

• The term flat panel display refers to a class of video devices that have reduced volume, weight and power requirements as compared to CRT.

• Applications of flat panel displays include small TV monitors, calculators, Pocket video games, laptop computers, an advertising boards etc.

Flat panel displays are classified into two types

1. Emissive displays (emitters)

• covert electrical energy into light

• Plasma panels, thin film electroluminescent displays and light emitting diodes etc.

2. Non emissive displays (or non emitters)

• use optical effects to convert sunlight or light from some other source to graphics patterns

• Liquid crystal device

Plasma Panels

• Also called as gas-discharge displays.

• Constructed by filling the region between two glass plates with a mixture of gases that usually includes neon

• A series of vertical ribbons is placed on one glass panel and a set of horizontal ribbons is built into the another glass panel.

• Firing voltage is applied to a pair of horizontal and vertical conductors cause the gas at intersection of the two conductors to break down into a glowing

plasma of electrons and ions.


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• Picture definition is stored into a refresh buffer, and the firing voltage is applied to refresh the pixel positions (at the intersection of the conductors) 60

times per second.

• Alternating current method are used to provide faster application of the firing voltages, and thus brighter images.

• The disadvantage is that they are strictly monochromatic devices. Basic design of plasma panel display device

Liquid Crystal Displays

• LCDs are commonly used in small systems such as calculators and portable laptop computers

• These non-emissive devices produce a picture by passing a polarized light from the surroundings or from an internal light source through a liquid crystal material that can be aligned to block or transmit the light.

• Liquid crystal refers to that these compounds have a crystalline arrangement of molecules, yet they flow like liquid.

• Flat panel display use nematic (threadlike) liquid crystal compounds that tend to keep the long axis of the rod shaped molecules aligned.

• Two glass plates , each containing a light polarizer at right angles to other plate, sandwich the liquid crystal material

• Rows of transparent conductors are built in one glass plate, and columns of vertical conductors are put into another plate.

• The intersection of two conductors defines the pixel position

• In “on state” the molecules are aligned as shown.

• Polarized light passing through the material is twisted so that it will pass though the opposite polarizer.

• The light is then reflected back to the viewer

• To turn off the pixel, a voltage is applied to the intersecting conductors to align the molecules so that the light is not twisted.

• Picture definition is stored into a refresh buffer, and the firing voltage is applied to refresh the pixel positions (at the intersection of the conductors) 60 times per second.

• This type of flat panel device is referred to as a passive matrix LCD

• Another method of constructing LCDs is to place a transistor at each pixel location, using thin film transistor technology.

• The transistors are used to control voltage at pixel locations and to prevent charge from gradually leaking out of the liquid crystal cells.


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• These devices are called as active matrix displays.

Concept of pointing and positioning

• In conventional user interaction, the user enters data and selects or enters commands through keystrokes made on a keyboard.

• In interactive graphics these are supplemented by the activities of: 1. Positioning, or location, generally for the data entry within the program

2. Pointing to, or picking, graphical or other elements on the display screen , for object or command selection.

Positioning

• The CAD user inputs the positional information for such tasks as the location of the geometry within the model, or for the indication of an image to be magnifies in a zoom or other display control application

• Position may be precisely indicated by the entering of the coordinate values.

• But where approximate location is satisfactory the position may be given by a positioning device.

• The cursor is used to indicate the current screen position to the user.

• The movement of the positioning device is normally reflected in the movement of the cursor, and a button on the positioning device or a keyboard

keystroke is used to indicate a specific location

• Devices used : joystick , mouse, thumbwheels, digitizers Pointing

• The most commonly used device is light pen, which could be pointed at illuminated parts of the displays to select lines in the image.

• The light pens are rarely used. Halftone


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• Continuous tone photographs are reproduced for publications in newspaper, magazines, and books with a printing process called halftoning, and the

reproduced pictures are called halftones.

• For a black and white photograph, each intensity area is reproduced as a series of black circles on a white background.

• The diameter is propositional to the darkness required for that intensity level.

• Darker regions are printed with large circles and lighter regions are printed with small circles.

• Book and magazine halftones are printed on high quality paper using approximately 60 to 80 circles of varying diameter per centimeter.

• Newspaper use lower quality papers and lower resolution (about 20 to 30 dots per centimeter.)


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Chapter 4. Software in CAD

Computer Graphics Software

• The graphic software is the collection of programs written to make it convenient for user to operate the computer graphics system.

• It includes programs to generate images on the CRT screen, to manipulate the

images, and to accomplish various types interaction between the user and the system.

• Some other programs for implementing specialized functions of CAD/CAM are

1. Design analysis programs (FEA & kinematic simulation)

2. Manufacturing planning programs (e.g. automated process planning and numerical control part programming )

Ground rules in Designing Software

1. Simplicity : The graphics software should be easy to use 2. Consistency: The package should operate in a consistent and predictable way

to the user 3. Completeness: There should be no inconvenient omissions in the set of

graphics function 4. Robustness : The graphics system should be tolerant of minor instances of

misuse by the operator

5. Performance: Within limitations imposed by the system hardware, the performance should be exploited as much as possible by software. Graphics

program should be efficient and speed of response should be fast and consistent.

6. Economy: Graphics programs should not be so large or expensive as to make

their use prohibitive The software configuration of a graphics system

In the operation of the graphics system by the user , a variety of activities take place, which can be divided into three types

• Interact with the graphics terminal to create and alter images on the screen

• Construct a model of something physical out of the images on the screen. The models are sometimes called as application models

• Enter the model into computer memory and/or secondary storage. Graphics software

The graphics software can be divided into three modules

• The graphics package • The application program

• The application data base

Application Program


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• It controls the storage of data into and retrieves data out of application

database • The application program is driven by the user through the graphics package

• The application program is implemented by the user to construct the model of a physical entity whose image to be viewed on the graphics screen.

• Application programs are written for particular problem areas.

• Problem areas in engineering design would include arch, construction, mech. Components, ext, chem, aerospace.

• Problem areas other than deign include flight simulators, graphical display of data, mathematical analysis, and even artwork

Graphics Package

• The graphics package is the software support between the user and the graphics terminal.

• It manages the graphical interaction between the user and the system • It also serves as a s/w support between the user and the application software • The graphics package consist of input subroutines and output subroutines

• The input subroutine accepts the input commands and data from the user and forward them to the application program

• The output subroutines control the display terminal (or other output device) and converts the application models into 2D or 3D graphics pictures.

Application data base

• The database contains mathematical, numerical and logical definitions of the application models, such as electronic ckts, mechanical components,

automobile bodies, and so forth. • It also contains alphanumeric information associated with the models, such as

BOM, mass properties and other data.

• The contents of the data base can be readily displayed on the CRT or plotted out in hard copy form.

Functions of a Graphics package

• Generation of graphics elements • Transformations

• Display control and windowing function • Segmenting functions

• User input functions Generation of graphics elements

• A graphic element in computer graphics is a basic image entity such as a dot

(or point), line segment, circle, and so forth • The collection of elements in the system could also include alphanumeric

characters and special symbols • There is often a special hardware component in the graphics system associated

with the display of many of the elements.

• This speeds up the process of generating the element • The user can construct the application model out of collection of elements

• The term primitive is used in reference to the graphics element • E.g. sphere, cube, or cylinder • In 3D wire frame models and solid modelling, primitives are used as building

blocks. Transformations

• Transformations are used to change the image on the display screen • Transformations are applied to graphics elements in order to aid the user in

constructing an application model


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• It includes enlargement and reduction of the image by a process called scaling,

repositioning the image or translation, and rotation. Display control and windowing

• This provides the user with the ability to view the image from the desired angle and at the desired magnification

• Another aspect of display control is hidden line removal.

Segmenting function • Segmenting provides user with the capability to selectively replace, delete or

otherwise modify portions of the image. • The term segment refers to a particular portion of the image which has been

identified for the purpose of modifying it.

• Storage type CRT is unsuited to segmenting function.

User input output functions

• User input functions constitute a critical set of functions in the graphics

package because they permit the operator to enter commands or data to the system.

• The entry is accomplished by means of operator input devices. Constructing the geometry

• The use of graphics elements

• Defining the graphics elements • Editing the geometry

The use of graphics elements

The graphics system accomplishes the definition of the model by constructing it

out of graphics elements

These elements are called by the user during the construction process and added one by one, to create the model

There are several aspects about this construction process. 1. Each new element is being called but before it is added to the model, the user

can specify its size, its position and orientation. These specifications are necessary to form the model to proper shape and size

For this purpose various transformations are utilized 2. Graphics element can be subtracted as well as added

Another way of saying this is that the model can be formed out of negative

elements as well as positive elements.

3. A third feature available during model building is the capability to group

several elements together into units which are sometimes called cells A cell in this context , refers to a combination of elements which can be called to use anywhere in the model e.g. a bolt is to be used in several places in

mechanical assembly Define the graphics elements

• The user has variety of ways to call a particular graphic element and position it on the geometric model

• Points, lines, arcs , circles can be defined in various ways through interaction

with the ICG system


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• E.g. point can be defined simply by x, y and z coordinates. A polygon would

be defined as an ordered set of points representing the corners of the polygon Editing the geometry

• A CAD provides system provides editing capabilities to make corrections and adjustments in the geometric model

• When developing the model, the user must be able to delete, move, copy and

rotate the components of the model. • The editing involves selecting the particular portion of the model, and

executing the appropriate command Editing features in CAD

1. Move an item. This involves the translation of item from one place to

another 2. Duplication an item at another location. The copy function is similar to

move function except it preserves a copy of an item at its original location 3. Rotate an object. This is the rotation transformation, in which the item is

rotated through specified angle from its original orientation

4. Mirror an item. This crates a mirror image of an item about a specified plane.

5. Delete an item. This causes selected segment of the model to be removed from the screen and from the database.

6. Trim an line or other component

7. Scale an item

The method of selecting the segment of the model varies from system to system

1. With cursor control common method is for a rectangle to be formed on the CRT screen around the model segment. The rectangle is defines by entering the upper

left and lower right corners of the rectangle 2. It involves light pen to be placed over the component t be selected

The computer must some how show indicate to the user which portion of the

model has been selected

This includes placing mark on the segment, making segment brighter than the rest

of the image, and making the segment blink

Transformations • 2-D Transformations

• 3-D Transformations Two-dimensional Transformations

• To locate a point in a two-axis Cartesian system, the x and y coordinates are

specified. • These coordinate can be treated together as a 1 x 2 matrix : (x, y), e.g. the

matrix (1,4) would be interpreted to be point which is 1 unit from the origin in the x-direction and 4 units from the origin in the y-direction.

• This method of representation can be extended further to define a line as a

2 x 2 matrix by giving x and y coordinates of the two end points of the line. The notation would be,

Translation

• Translation involves moving the element from one location to another x‟= x + m, y‟ = y + n

Where,


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x‟, y‟ = coordinates of the translated point

x, y = coordinates of the original point m, n= movements in the x and y direction

In the matrix notation this can be represented as, (x‟, y‟) = (x, y) + T Where,

T = (m, n), the transformation matrix • Scaling

Scaling of an element is used to enlarge it or reduce its size. The scaling need not necessarily be done equally in x and y directions. e.g. a circle can be transformed into ellipse by using unequal x and y scaling factors.

The points of an element can be scaled by scaling factor as follows: (x‟, y‟) = (x, y )S

Where

• This would produce an alternation in the size of the element by the factor m in the x-direction and by the factor n in the y direction

• It also repositions the element • If the scaling factor is <1 , it is moved closer to origin • If the scaling factor is >1 , it is moved farther from the origin

Rotation In this transformation, the points of an object are rotated about the origin by an

angle θ For a positive angle, this rotation is in the counter clockwise direction This accomplishes rotation of the object by the same angle, but it also moves

the object. In matrix notation the procedure will be as follows: (x‟, y‟) = (x, y) R

Where

Example : Translation Consider the line defined by,

Suppose the line to be translate in space by 2 units in x direction and 3 units in the y direction.


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Example : Scaling

• Apply scaling factor of 2 to the line

The new line will be,

Example : Rotation

Rotate the line about origin by 30°

The new line would be defined as:

• The new line will be,


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3 Dimensional Transformation

• Transformations by matrix methods can be extended to three dimensional space.

Translation • The translation matrix for a point defined in three dimensional matrix would

be, T = (m, n, p)

And would be applied by adding increments m, n and p to the respective coordinates of each of the points defining the three-dimensional geometry

elements Scaling The scaling transformation is given by,

For equal values of m, n and p, the scaling is linear Rotation

• Rotation in three dimensions can be defined for each of the axes

• Rotation about z axis by angle θ is accomplished by the matrix

• Rotation about y axis by angle θ is accomplished by the matrix

Rotation about x axis by angle θ is accomplished by the matrix

Concatenation


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• Single transformations can be combined as a sequence of transformations.

This is called as concatenation, and the combined transformations are called concatenated transformations.

• During editing process when a graphics model is being developed , the use of concatenated transformation is quite common

• More than one transformations are usually requires to accomplish the desired

transformation e.g.

1. Rotation of the element about an arbitrary point the element 2. Magnifying the element but maintaining the location of one of its points in the

same location

• In the first case transformations would be: translation to the origin, then rotation about the origin, then translation back to original location

• In the second case , the element would be scaled (magnified) followed by a translation to locate the desired point as needed

• The objective of the concatenation is to accomplish a series of image

manipulations as a single transformation • The concatenation is the product of the two transformation matrix

• It is important that order of matrix multiplication be the same as the order in which the transformations are to be carried out.

Example : Concatenation

• A point to be scaled by a factor of 2 and rotated by 45°. Suppose point under consideration was (3, 1) . This may be one of the several points defining a

geometric element • First accomplish the transformations sequentially,

First consider the scaling,

(x‟, y ‟) = (x, y)S

Next rotation can be performed (x”, y”) = ( x‟, y‟) R

The same result can be accomplished by concatenating the two transformation matrices, The product of the two matrices would be

Now applying this concatenated transformation to the original matrix


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Examples

• A line is defined in 2 D space by its end points (1,2) and (6,4). Express this in

matrix notation and perform the following transformation in succession on this line

1. Rotate the line by 90° about the origin 2. Scale the line by a factor of 0.5 (Dec/Jan 04/05, 8 Marks)

• A square of side 30 units has its coordinates A(10,10), B(40,10), C(40,40) and

D(10,40), Perform the following transformation in succession and show it on graph

paper 1. Rotate about origin 20° anticlockwise 2. Scale it by factor 1.5

3. Perform the above sequence of transformation by concatenation (May/June 04, 18 Marks)

Homogeneous representation

• In order to concatenate the transformations matrices, all transformation matrices should be multiplicative type.

• However, the translation matrix is vector additive while all other are matrix multiplications.

• It is desirable , to express all geometric transformations in form that ,they can be concatenated by matrix multiplication only.

• In homogeneous representation, an n-dimensional space is

mapped into (n+1) dimensional space. • Thus a 2 dimensional point [x, y] is represented with the homogeneous

coordinate triple (xh,yh,h) Where,

• Thus , a general homogeneous coordinates can also be written as (h.x, h.y, h).

• For two-dimensional geometric transformations, homogeneous parameter h to be any nonzero value.

• Thus , there is an infinite number of equivalent homogeneous representations for each coordinate point (x, y)

• A convenient choice is simply to set h=1.

• Each two-dimensional position is then represented with coordinates (x, y, 1) • This facilitates the computer graphics operations where the concatenation of

multiple transformations can be easily carried out. • The translation matrix in multiplication form can be given as,

The transformation operation can be written as


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• The scaling matrix in multiplication form can be given as,


The rotation matrix in multiplication form can be given as,


Problem (Nov/Dec 2004)

A square ABCD of side 50 units having one of its vertex at A(10,10) is to be scaled by 0.8 along x axis and 1.2 along y-axis. The model is then rotated about origin by 30° in anticlockwise direction. Perform the necessary

transformations using homogeneous transformation matrix and show them on graph paper.


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CHAPTER 6. 3-D Modelling

Syllabus

Preparing an application model by Boolean operations Primitive instancing

Boundary representations CSG (Constructive Solid Geometry )

Wire frame modeling and solid modeling Sweep representations Cell decomposition

Boolean operations

• Boolean operations are used to make more complicated shapes by combining simpler shapes

• 3 types of operations are possible: 1. union („U‟) or “join” 2. intersection („∩‟)

3. difference („-‟) or “subtract”

Boolean operation on solids. (a) Objects A and B, (b) A U B, (c) A ∩ B, (d) A - B, (e) B - A


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Boolean operations applied to a cube and a sphere

Primitive Instancing

• In primitive instancing modeling approach, primitives are simple 3D solid shapes, which form the base for creating a solid model

• These primitives are parameterized by geometric as well as physical properties.

• The solid model of any object can be created with different combinations of these primitives.

• e.g. one primitive object may be a regular pyramid with a user defined number of faces meeting at the apex.

• A parameterized primitive may be thought of as defining family of parts whose members vary in few parameters, an important CAD concept known as

group technology.

• Primitive instancing is often used for relatively complex objects, such as gears , bolts, that are tedious to define in terms of Boolean combinations of simpler

objects.

• e.g. A gear may be parameterized by its diameter or no. of teeths.

• Primitive instancing is based on the concept of families of objects or parts

• All parts having same topology but different dimensions are grouped into family

• Each individual part in the family is called a primitive instance.

• e.g. a cylinder is represented by diameter(D) and height (h)

• Each primitive instancing is defined by specific (D) and (H)

• A number of such cylinder primitive instancing creates a family of cylinders

• A group of such families can define a solid Boundary representations (B-rep)

• Boundary representations or b-reps describe the solid object in terms of its

boundaries, that is the vertices, edges and faces.

• In this model, face is bounded by edges and each edge is bounded by vertices.

• The entities which constitute a B-rep model are:

Geometrical Entities Topological entities

Point Vertex


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Curve, line Edge

Surface Face

B – Rep Model

An Edge-Based Model

v1 v2

v3

v4

e1

e2e3

e4

e6

e5

Faces:

f1 e1 e4 e5

f2 e2 e6 e4

f3 e3 e5 e6

f4 e3 e2 e1

Edges:

e1 v1 v2

e2 v2 v3

e3 v3 v1

e4 v2 v4

e5 v1 v4

e6 v3 v4

Vertices:

v1 x1 y1 z1

v2 x2 y2 z2

v3 x3 y3 z3

v4 x4 y4 z4

v5 x5 y5 z5

v6 x6 y6 z6

v1 v2

v3

v4

e1

e2e3

e4

e6

e5

v1 v2

v3

v4

e1

e2e3

e4

e6

e5

Faces:

f1 e1 e4 e5

f2 e2 e6 e4

f3 e3 e5 e6

f4 e3 e2 e1

Edges:

e1 v1 v2

e2 v2 v3

e3 v3 v1

e4 v2 v4

e5 v1 v4

e6 v3 v4

Vertices:

v1 x1 y1 z1

v2 x2 y2 z2

v3 x3 y3 z3

v4 x4 y4 z4

v5 x5 y5 z5

v6 x6 y6 z6


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• Though B-rep models is constructed of surfaces of solids, computation of

mass and volumetric properties is possible.

• The range of models that can be modeled with B-rep technique is very large.

• The Boundary representation approach requires the user to draw the outline or boundary of the object on CRT screen using an electronic tablet and pen or

analogous procedure.

• The user would sketch various views of the object (front, side and top , or more views if needed), drawing interconnecting lines among the views to

establish their relationship.

• Various transformations and other specialized editing procedure are used to refine the model to the desired shape.

• Unusual shapes can be formed with the help of this representation

• e.g. aircraft fuselage or wing shapes and automobile body styling

• This model is stored in the database as stores precise definition of the model boundaries.

• The model requires more storage space but less computation effort in reconstruction of the model and its image.

• It is relatively simple to convert back and forth between a boundary representation and a corresponding wire frame.

Constructive Solid Geometry (CSG or C-rep)

• A CSG model is based on the concept that a physical object can be divided into a set of primitives (basic elements or shapes) that can be combined in a certain order following a set of rules (Boolean operations) to form the object.

• Each primitive is bounded by a set of surfaces; usually closed and orientable.

• A CSG model is fundamentally and topologically different from a B-rep model in that the former does not store explicitly the faces, edges, and vertices.

• There is a wide variety of primitives available commercially to users.

However, the four most commonly used are the block, cylinder, cone and sphere.

• Initial formulation the object is easier in case of C-rep

• It is relatively easy to construct a precise solid model out of regular solid

primitives by use of Boolean operations.


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• The building block approach is results in more compact file of the model in the data base

• Unusual shapes can not be formed easily with this representation

• This model stores the model by combination of data and logical procedures (the Boolean model)

• This model requires less storage but more computation to reproduce the model

and its image

• Because of relative advantages and disadvantages of the two approaches , hybrid systems have been developed which combine the CSG and B-rep approaches.

Construction of a Wrench


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Wire frame modeling

• In construction of the wire-frame model, the edges of the object are shown as lines

• The image assumes the appearance of a frame structured out of wire- so it is called as Wire frame model

• Very often designers also build physical models to help in the visualization of a design.

• This may require the construction of „skeleton‟ models using wires to represent the edges of an object or component.

• Wire frame modeling, as used currently in computer-aided engineering techniques, is the computer-based analogue of this process.

• A wire-frame model consists of a finite set of points together with the edges connecting various pairs of these points,

• There are limitations to the models which use the wire frame approach.

• These limitations are more prominent in case of three-dimensional models.

• These models are more suitable for two-dimensional representation.

• The more remarkable limitation is that all the lines that define edges of the model are shown in the image.

• Many of 3-D wire frame systems does not have automatic hidden line removal feature.

• The image becomes much complicated to understand and in some cases it might be interpreted in number of ways.

• There is also limitation in defining the model in CAD database


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Solid modeling

• An improvement in the wire frame modeling, both in terms of realism to the user and definition to the computer.

• In this models are displayed with less risk of misinterpretation

• When color are added the picture becomes realistic one.

• The solid modeling has wide range of applications other than CAD and manufacturing.

• These includes color illustrations in magazines and technical publications, animation in movies and training simulators

The solid models are used widely because of following factors:

1. Increasing awareness among users of the limitation of the wire frame systems 2. Continuing development of computer hardware and software which makes

solid modeling possible

• These models require high computational power in terms of both speed and memory, in order to operate

Sweep Representations

• When a curve /shape is moved along a curved path a new object created is called a sweep

• When a 2D object is swept along a linear path , then the resulting object is known as extrusion.

• Rotational sweeps are defined by rotating an area about an axis.

• Sweeps of solids are useful in modelling the regions swept by a machine tool or a tool path.

(a) (b) (c)

Wireframe ambiguity:

Is this object (a), (b) or (c) ?

(a) (b) (c)

Wireframe ambiguity:

Is this object (a), (b) or (c) ?


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• Sweeps whose generating volume or area changes in shape, size or in orientation as they swept, are known as generalized sweeps.

• Sweep representation is useful in generating extruded solids and solids of revolution.

• The sweeping operation is based on sweeping a curve or surface.

• Applications involves simulations of material removal due to machining

operations and interference detection of moving objects in space.

• There are three types of sweep 1. Linear

• Translational

• Rotational

2. Non linear 3. Hybrid


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Cell Decomposition

• An object can be modelled by decomposing its volume into smaller volume of cells which are mutually continuous and do not penetrate into each other.

• The shape in this need not be a cube nor they should be identical.

• It can be seen that some cells are partly outside the boundary, while some of them

are partly inside the boundary

• This is approximate representation of an object.

• In such a case , a smaller hole or cavity gets neglected if the size of cavity is

smaller of the cells or other than squares in this scheme.


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• This difficulty can be overcome by permitting various shapes of cells other than squares and rectangles, such as triangles.


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CHAPTER 7. Graphics Standards

• The need for portability of the geometric model among different hardware platforms has led to the development of device independent graphics

• Simultaneously standards for exchange of drawing data base among software packages have been developed to facilitate integration of design and

manufacturing operations

• The heart of any CAD model is the component database.

• This includes the graphics entities like points, lines, arcs etc. and coordinate points which define the location of these entities

• This geometric data is used in all downstream applications of CAD, which include finite element modeling and analysis, process planning, estimation, CNC programming, Robot programming, programming of CMM, MRP

systems To achieve high level of integration between CAD, analysis and manufacturing operations, the database must contain:

• Shapes of the components ( based on 3-D wire frame or solid model )

• Bill of materials (BOM), of the assembly which the components are used.

• Materials of the components

• The manufacturing, test and assembly procedures to carried out to produce a component so that it is capable of functioning as per requirements of design

In designing data structure for CAD database the following factors to be considered:

• The data must be neutral

• The data structure must be user friendly

• The data base must be portable Features of GKS The feature of Graphics Kernel system include

1. Device independent: The does not assume that any of the input or output devices have particular features or restrictions

2. Text/ annotations : All text or clarification are in a neutral languages like

English 3. Display management: A complete set of display management functions, cursor

control and other features are provided. 4. Graphics Functions: Graphics functions are provided in 2D or 3D

GKS Implementation in a CAD W/S


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• The drivers in GKS is also includes metafile drivers

• Metafiles are devices with no graphics capability

GKS offers two routine to define user created pictures 1. Primitive functions 2. Attribute functions

1. Primitive functions Examples

• POLYLINE to draw set of connected straight lines

• FILL AREA to draw a closed polygon with interior fill

• TEXT to create characters

• GDP (Generalized Drawing Primitive ) to specify the standard drawing entities like circles, ellipse etc.

2. Attribute functions defines appearance of the image e.g. color, line type etc.

Exchange of data between CAD packages

E.g. to transfer CAD model created in Pro/E to I-DEAS or Unigraphics To transfer geometric data from one software to another e.g. to carry out modeling in PRO/E and analysis in ANSYS

One method to meet this need is to use translators. This means developer will have to produce its own translators

• A solution to translators is to use neutral files

• The neutral files have standard formats and software packages can have preprocessors to convert the file data to neutral file and post processors to convert neutral file data to drawing file

DATA EXCHANGE

• The need for exchanging modeling data is directly motivated by the need to integrate and automate the design and mfg. processes to obtain max. benefits

from CAD/CAM.

• The component database can be used in downstream applications like FEA, Process planning, Robot programming, MRP, C. N. C. programming.

• There should be tie between the two or more systems to form an application

that shares common data.

• Data exchange files are neutral files. Following are two types of neutral files:


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1. DXF (Data Exchange File)

2. IGES file (Initial Graphics Exchange Specification ) DXF (Data Exchange File)

• DXF files are std. ASCII text files, which can be easily translated to the formats of other CAD systems

• The DXF facility is available in several drafting packages.

• In case of Auto CAD , the data regarding the created drawing may require to view , modify or plotting.

• Also data can be used for analysis purpose like FEA

• In case of Auto CAD , a drawing interchange file (DXF) can be generated from the existing drawing by means of DXFOUT command.

• On the other hand a drawing interchange file can be converted into Auto CAD drawing by means of DXFIN command

• The general structure of DXF file is as follows: HEADER Section : general information of drawing TABLE Section :

Linetype (LTYPE) table Layer table Text style (STYLE) table

User coordinate system (UCS) table View port configuration file (VPORT) table

BLOCKS Section ENTITIES Section END OF FILE

IGES (Initial Graphics Exchange Specification )

• It is standard exchange format developed for communicating product data among dissimilar CAD/CAM systems.

• It has been used for two purposes:

– Transfer of data within dissimilar systems

– Digital communication between company and its suppliers, customers i.e. IGES enable data transfer from one CAD system to another

• The software which translates data from CAD system to IGES is known as preprocessor

• The software which translates data from IGES data to a CAD system is known as postprocessor

• IGES defines a database, in the form of a file format, which describes an

“IGES Model” of modeling data of a given product

• IGES model can be read and interpreted by dissimilar CAD/CAM systems.

• IGES model based on the concept of entities.

• The fundamental unit of information of model i.e. IGES file, is the entity, all product definition data are expressed as a list of predefined entities.

• Each entity defined by IGES is assigned a specific entity type number to refer

to it in the IGES file.

• Entities are categorized as geometric and non geometric.


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• Geometric entities represent the definition of the product shape and include curves and surfaces.

• Non geometric entities provide views and drawings of the model to enrich its representation and include annotation and structural entities.

• Annotation entities include types of dimensions (linear, angular) symbols, centerlines, cross hatching.

• Structure entities include views, drawings, attributes (i.e. line and text fonts, colors, layers etc.), properties (e.g. mass), symbol (e.g. mechanical and electrical ) and macros (to define parametric parts)

IGES file structure

Flag section

Start section

Global section

Directory entry section

Parameter data section

Terminate section

• Start section contains comments that can be used to describe the drawing, identify its source, comment on its format an so on.

• Global section includes information that describes global characteristics of the IGES file, such as name of the file, the system that created the file, units of measures, and precision

• Directory entry section describes the entities in the drawing. This section

contains attributes such as color, line style. Also provides an index to the entities in the file

• Parameter data section contains data to describe each entity, such as point coordinates, coefficients of curve and surface equations, text characters and

other attributes.

• Terminate section IGES Limitations

• IGES is complex and wordy

• These files are about five times larger than an equivalent product file

• Several entities required for specialized CAD applications are not available in IGES file

• It does not convey the extensive product information needed in the design and mfg. cycle.

STEP


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• Standard for the Exchange of Product Model Data, officially the ISO standard 10303, Product Data Representation and Exchange, is a series of International

Standards with the goal of defining data across the full engineering and mfg. life cycle.

• The ability to share data across applications, across vendor platforms and between contractors, suppliers and customers, is the main goal of this standard.

Scope of STEP

• The standard method of representing the information necessary for completely defining a product throughout its entire life, i.e. from the product conception to the end of useful life.

• Standard method for exchanging the data electronically between two different systems.

• It provides worldwide standard for storing, sharing and exchanging product information among different CAD systems.

• It includes methods of representing all critical product specifications such as shape information, materials, tolerances, finishes and product structure.

• It is data exchange that would apply to a wide range of product areas, including architectural, engineering and construction.

STEP Architecture

• It has main four components 1. EXPRESS modelling language

2. Data schemes including attributes such as geometry, topology, features and tolerances.

3. Application interface called Standard Data Access Interface (SDAI) to enable applications to access and manipulate STEP data

4. STEP database


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CHAPTER 8. ROBOTICS

Definition

A robot is a programmable , multi function manipulator deigned to move

material, parts, tools or special devices through variable programmed motion for the performance of a variety of tasks

An industrial robot is a general purpose , programmable machine processing

certain anthropomorphic characteristics What are the parts of a robot?

• Manipulator • Pedestal • Controller

• End Effectors • Power Source

Manipulator (Mimics the human arm) • Base • Appendages

o Shoulder o Arm

o Grippers Pedestal (Human waist)

• Supports the manipulator.

• Acts as a counterbalance. Controller (The brain)

• Issues instructions to the robot. • Controls peripheral devices. • Interfaces with robot.

• Interfaces with humans. End Effectors (The hand)

• Spray paint attachments • Welding attachments • Vacuum heads

• Hands • Grippers

Power Source (The food) • Electric • Pneumatic

• Hydraulic ROBOT PHYSICAL CONFIGURATION

1. Cartesian configuration 2. Cylindrical configuration 3. Polar configuration 4. Joint-arm configuration Cartesian Configuration

Robots with Cartesian configurations consist of links connected by linear joints (L). Gantry robots are Cartesian robots (LLL).

Cartesian Robots It consists of three orthogonal slides. Three slides are parallel to x, y and z axes of the Cartesian coordinate system

Commonly used for:

pick and place work

assembly operations handling machine tools


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arc welding

Advantages:

ability to do straight line insertions into furnaces.

easy computation and programming. most rigid structure for given length.

Disadvantages:

requires large operating volume. exposed guiding surfaces require covering in corrosive or dusty environments.

can only reach front of itself Cylindrical Configuration

Robots with cylindrical configuration have one rotary ( R) joint at the base and linear (L) joints succeeded to connect the links.

Cylindrical Robots

In this, the robot body is a vertical column that swivels about vertical axis The arm consist of several orthogonal slides which allow the arm to be moved

up and down and in or out w.r.t. to body Commonly used for: handling at die-casting machines

assembly operations handling machine tools

spot welding Advantages: can reach all around itself

rotational axis easy to seal relatively easy programming

rigid enough to handle heavy loads through large working space good access into cavities and machine openings

Disadvantages:

linear axes is hard to seal won‟t reach around obstacles

exposed drives are difficult to cover from dust and liquids Spherical/Polar Robots A robot with 1 prismatic joint and 2 rotary joints – the axes consistent with a polar

coordinate system. Commonly used for:

• handling at die casting or fettling machines

• handling machine tools

• arc/spot welding Advantages:

large working envelope. two rotary drives are easily sealed against liquids/dust.

Disadvantages:

complex coordinates more difficult to visualize, control, and program. exposed linear drive.

low accuracy Joint-arm Configuration


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The jointed-arm is a combination of cylindrical and articulated configurations.

The arm of the robot is connected to the base with a twisting joint. The links in the arm are connected by rotary joints. Many commercially

available robots have this configuration.

Advantages and Disadvantages of 4 Robot Types

Configuration Advantages Disadvantages

Cartesian coordinates

3 linear axes, easy to visualize, rigid structure, easy to program

Can only reach front of itself, requires large floor space, axes

hard to seal

Cylindrical

coordinates

2 linear axes +1 rotating, can

reach all around itself, reach and height axes rigid, rotational axis easy to seal

Can‟t reach above itself, base

rotation axis as less rigid, linear axes is hard to seal, won‟t reach around obstacles

Spherical

coordinates

1 linear + 2 rotating axes, long

horizontal reach

Can‟t reach around obstacles,

short vertical reach

Joined-arm

Revolute coordinates

3 rotating axes can reach above or

below obstacles, largest work area for least floor space

Difficult to program off-line, 2

or 4 ways to reach a point, most complex manipulator

Basic Motion Systems Six degrees of freedom

It provides the robot the capability to move and perform predetermined task These six degrees of freedom are intended to follow the versatility of

movement possessed by the human arm

The six basic motions consists of three arm and body motions and three wrist motions

Arm and body motions

1. Rotational transverse : Rotation about the vertical axis (right or left swivel of the robot arm )

2. Vertical transverse : Up and down motions of the arm , caused by pivoting the entire arm about a horizontal axis or moving the arm about vertical slide.

3. Radial transverse : extension and retraction of the arm (in and out movement) Wrist motions

4. Wrist swivel : Rotation of the wrist

5. Wrist bent : Up and down movement of the wrist , which also involves a rotational movement

6. Wrist yaw : swivel of the wrist


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ROBOT MOTION SYSTEMS

Classification Based on Control Systems: 1. Point-to-point (PTP) control robot 2. Continuous-path (CP) control robot

Point to Point Control Robot (PTP):

The PTP robot is capable of moving from one point to another point.

The locations are recorded in the control memory. PTP robots do not control the path to get from one point to the next point.

Common applications include:

component insertion spot welding

hole drilling machine loading and unloading assembly operations

Continuous-Path Control Robot (CP):

The CP robot is capable of performing movements along the controlled path.

All the points along the path must be stored explicitly in the robot's control memory.

Straight-line motion is the simplest example for this type of robot.

Some continuous-path controlled robots also have the capability to follow a smooth curve path that has been defined by the programmer.

In such cases the programmer manually moves the robot arm through the desired path and the controller unit stores a large number of individual point locations along the path in memory

Typical applications include: spray painting

finishing gluing arc welding operations

Programming with Robot

1. Manual method

2. Walkthrough method 3. Leadthrough method 4. Off- line programming


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Manual method

It is more like setting machine rather than programming It involves setting mechanical stops, cams, switches, or relays in the robot‟s

control unit Used for short work cycles (e.g. pick-and-place operations )

Walkthrough Method

In this programmer manually moves robots arm and hand through the sequence of the work cycles.

Each movement is recorded into memory for subsequent playback during the production

Speed is controlled independently

Appropriate for spray painting, arc welding robots Leadthrough Method

Uses teach pendent to power drive the robot through its motion sequence Teach pendent is device, consists of switches and dials to control the robot‟s

physical movements

Each motion is recorded into memory for future playback during the workcycle

Popular because of its ease and convenience Off-line Programming

It involves the preparation of robot program off-line , in a manner similar to

NC part programming It is accomplished on computer terminal

After preparation it is entered into the memory of the computer for use during the work cycle.

The advantage is that production time of robot is not lost to delays in teaching

the robot a new task Robotic Sensors

Sensors provide feedback to the control systems and give the robots more flexibility.

Sensors such as visual sensors are useful in the building of more accurate and

intelligent robots. The sensors can be classified as follows:

Position sensors • Position sensors are used to monitor the position of joints. • Information about the position is fed back to the control systems that are used

to determine the accuracy of positioning Range sensors

Range sensors measure distances from a reference point to other points of importance.

Range sensing is accomplished by means of television cameras or sonar

transmitters and receivers. Velocity Sensors

They are used to estimate the speed with which a manipulator is moved. The velocity is an important part of the dynamic performance of the

manipulator.

The DC tachometer is one of the most commonly used devices for feedback of velocity information.

Proximity Sensors They are used to sense and indicate the presence of an object within a

specified distance without any physical contact.

This helps prevent accidents and damage to the robot.


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infra red sensors

acoustic sensors touch sensors

force sensors tactile sensors for more accurate data on the position

The Hand of a Robot: End-Effectors

The end- effecter (commonly known as robot hand) mounted on the wrist enables the robot to perform specified tasks.

Various types of end-effectors are designed for the same robot to make it more flexible and versatile.

End-effectors are categorized into two major types: grippers and tools.

Grippers Grippers are generally used to grasp and hold an object and place it at a desired location.

mechanical grippers vacuum or suction cups

magnetic grippers adhesive grippers hooks, scoops, and so forth


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Tools At times, a robot is required to manipulate a tool to perform an operation on a

workpiece. In such applications the end- effecter is a tool itself spot-welding tools arc-welding tools

spray-painting nozzles rotating spindles for drilling

rotating spindles for grinding

Technical features of Robot

1. Work volume 2. Precision of movement

3. Speed movement 4. Weight carrying capacity 5. Types of drive system

Work Volume o It refers to the space within which the robot can operate

o It is the spatial region within which the end of the robot‟s wrist can be manipulated

o The work volume is determined by its physical configuration , size, and limits

of its arms and joint manipulations o E.g. work volume of Cartesian coordinate robot is rectangular, for polar it is

partial sphere etc. Precision of movements 1. Spatial resolution:

The spatial resolution of a robot is the smallest increment of movement into which the robot can divide its work volume.


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It depends on the system‟s control resolution and the robot's mechanical

inaccuracies. 2. Accuracy :

Accuracy can be defined as the ability of a robot to position its wrist end at a desired target point within its reach. Accuracy is closely related to spatial resolution , since the robot‟s ability to

reach a particular point in space depends on its ability to divide its joint movements into small increments

In terms of control resolution, the accuracy can be defined as one-half of the control resolution.

The accuracy of a robot is affected by many factors. For example, when the

arm is fully stretched out, the mechanical inaccuracies tend to be larger because the loads tend to cause deflection.

3. Repeatability:

It is the ability of the robot to position the end effector to the previously positioned location.

It depends on the stability of the control system and is affected by temperature, load etc.

Speed of movement

The speed with which the robot can manipulate the end effector ranges up to a maximum of about 1.5 m/s

The speed is determined depends of factors like weight of the object being

moved, the distance moved, and the precision required. Heavy objects can not be moved as fast as light objects because of inertia

problem Objects must be moved slowly when high positional accuracy is required

Weight carrying capacity

It covers wide range At higher end, there are robots which can lift up to 500 kgs to 1000 kgs.

At lower end, there are robots which can be used for 0.75 to 1.5 kgs. Types of drive system


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1. Hydraulic drive

Provide fast movements Preferred for moving heavy parts

Preferred to be used in explosive environments Occupy large space area There is a danger of oil leak to the shop floor

2. Electric drive

Slower movement compare to the hydraulic robots

Good for small and medium size robots Better positioning accuracy and repeatability stepper motor drive: open loop control

DC motor drive: closed loop control Cleaner environment

The most used type of drive in industry 3. Pneumatic drive

Preferred for smaller robots

Less expensive than electric or hydraulic robots Suitable for relatively less degrees of freedom design

Suitable for simple pick and place application Relatively cheaper


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CHAPTER 9. N. C. MACHINE TOOLS

SYLLABUS

N. C. system

Components and procedure Coordinate systems, axes Part Programming

Manual Part Programming, Formats N. C. Codes

Computer assisted part programming , Programming Languages Part programming for turning, milling and drilling by both methods Part Programming with APT

Basic components of CNC and DNC Introduction

Numerical control is a form of programmable automation in which the processing equipment is controlled by a set of instructions called as program (which contains numbers, letters, and symbols)

The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular work part or job.

When the job changes, the program of instructions changed. The capability of changing programs makes NC suitable for low and medium

volume production

Basic components of NC

1. Program of Instructions

2. Machine Control Unit 3. Processing Equipment

Basic components of NC

1. Program of Instructions

The program of instructions is the detailed step by step commands that direct the processing equipment

Commands refer to position of spindle w. r. t. worktable on which the part is fixtured

More advanced instructions include selection of spindle speeds, cutting tools etc.

The common medium used for coding of program is 1- in. wide punched tape.

Some recently used are punched cards, magnetic tape cassettes and floppy diskettes

2. Machine Control Unit (MOU)

MCU consists of the electronics control and hardware that read and interpret program of instruction and convert it into mechanical actions of the machine

tool or other processing equipment.


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The elements of machine tool are consists of a tape reader, data buffer, signal

input/output channels, feedback channels and sequence control,

The tape reader has an electromechanical device used to read and wind the

tape The data buffer then interprets the program of instructions and also stores the

instructions in logical blocks of information From here signals are sent to through the signal output channels which are

connected to the servomotor and other controls in the machine tools.

Feedback signals are provided for ensuring proper execution of the given instructions

The sequence control coordinates the activities of the other elements of the control unit

3. Processing Equipment

It is the component that performs useful work The machine tool consists of worktable and spindle to hold tools, motors and

controls necessary to drive them.


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NC Procedure 1. Process planning 2. Part programming

3. Tape preparation 4. Tape verification

5. Production Process Planning The engineering drawing of the workpart must be interpreted in terms of the

manufacturing processes to be used It consists of preparation of route sheet

Part Programming A part programmer plans for the portions of the job to be accomplished by NC The part programmers are responsible for planning the sequence of operations

to be performed by NC There are two ways to program for NC

Manual part programming Computer-assisted part programming

Manual part programming

In this, the machining instructions are prepared on a form called a part program manuscript

The manuscript is a listing of the relative cutter/ work piece positions which must be followed to machine the part.

Computer assisted part programming

In this , much of the tedious computational work required in manual part programming is transferred to the computer.

This is particularly appropriate for complex work piece geometries and jobs with many machining steps

It also saves part programming time

Tape Preparation A punched tape is prepared from the part programmer‟s NC process plan

In manual part programming, the punched tape is prepared from the part program manuscript or a typewriter like device equipped with tape punching capability

In Computer assisted part programming, the computer interprets the list of part programming instructions, performs the necessary calculations to covert this

into detailed set of machine tool commands, and then controls a tape punched device to prepare tape for specific NC machine

To check the accuracy of punched tape some methods are used

Sometimes the tape is checked by running it through a computer program which plots the various tool movements (or table movements) on paper. In this

major errors can be discovered The “Acid Test” of the tape involves trying it on the machine tool to make the

part.

A foam or plastic material is sometimes used for this purpose. Production

This involves ordering the raw workparts, specifying and preparing the tooling and any special fixturing that may be required.

The Operator‟s function is to load the workpart in the machine and establish

the starting position of the cutting tool relative to the workpiece.


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The NC system then takes over and machines the part according to the

instructions on tape.

NC coordinate system

In order to plan positions and movement of cutting tool relative to the workpiece,

a standard system is developed to specify relative positions.

NC axis system for milling and drilling

Two axis, x and y, are defined in the plane of the table. z axis is perpendicular to this and movement in the z axis is controlled by the

vertical motion of the spindle.

Three rotational axis defined in NC : the a, b and c axes. These axis specify angles about x, y and z axis respectively

To distinguish positive motion from negative motions, the right hand rule can be used.

Using the right hand with the thumb pointed in the positive linear direction (x,

y or z), the fingers of the right hand are curled to point in the positive rotational direction.

NC axis system for Turing

For turning operation, two axis are normally required to command the

movement of the tool relative to the rotating workpiece. The z-axis is the axis of rotation of the work part, and x-axis defines the radial

location of the cutting tool.

Fixed zero and floating zero

The programmer must define the position of the tool relative to the origin

(zero) point of the coordinate system. There are two method for specifying zero point 1. Fixed point zero


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2. Floating zero

Fixed zero

In this the origin is always located at the same position on the machine table.

Usually, that position is the southwest (lower left hand corner) of the table and all tool locations will be defined by positive x and y coordinates.

Floating zero

In this machine operator can set zero point at any point on the machine table. The part programmer decides the zero point.

The decision based on the part programming convenience e.g. in case of symmetric part the zero point can be located at the centre of the

symmetry.

The position of the zero point is communicated to the machine operator. At the start the operator moves the under manual control to some “Target

point” on the table. When the tool has been positioned to the target point, the machine operator

pressed a “zero” button on the machine tool

Absolute and incremental positioning

Absolute positioning means that the tool locations are always defined in

relation to the zero point. Incremental positioning means that the next tool location must be defined with

reference to the previous tool location.

Absolute and incremental positioning

NC Systems There are three types of motion control used in Numerical control

1. Point to point

2. Straight cut 3. Contouring

Point to point NC

Point to point (PTP) is also called a positioning system. In PTP, the objective of the machine control unit is to move the cutting tool to

a predefined location The speed or path by which this movements is accomplished is not important

in point to point NC Once the tool reaches the desired location , the machining operation is

performed at that position

Example of this system is NC drill press. Positioning system are the simplest machine tool control systems.

This system is least expensive of the three types


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Straight-cut NC

Straight cut control systems are capable of moving the cutting tool parallel to one of the major axis at a controlled rate suitable for machining

It is appropriate for performing milling operations on work pieces of rectangular shape.

Angular cut would not be possible. An NC tool capable of straight cur movements is also capable of point to point

movements

Contouring NC

(Continuous path NC ) Contouring is most complex, the most flexible and most expensive type of

machine tool control. It is capable of performing both PTP and straight cut operations In addition, it can control more than one axis movement of the machine tool.

The path of the cutter is continuously controlled to generate desired geometry of the machine tool.

Turing and milling are common examples

In order to machine a curved path in a NC system the direction of feed rate is

to be changed continuously to follow desired path.


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This is achieved by breaking the curved path into very short straight line

segments.

Applications

• Milling

• Drilling and related processes

• Boring

• Turning

• Grinding

• Sawing Other applications

• Press working machine tools

• Welding machines

• Inspection machines

• Automatic drafting

• Assembly machines

• Tube bending

• Flame cutting

• Plasma arc cutting

• Laser beam processes

• Cloth cutting

• Automatic riveting NC machines are suitable for

1. Parts are processed frequently and in small lot sizes 2. The part geometry is complex 3. Many operations must be performed on the part in its processing

4. Much metal needs to be removed


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5. Engineering design changes are likely

6. Close tolerance must be held on the part 7. It is an expensive part where mistakes in processing would be costly

8. The parts require 100 % inspection. Advantages of NC over conventional systems:

• Flexibility with accuracy, repeatability, reduced scrap, high production rates, good quality

• Reduced tooling costs

• Easy machine adjustments

• More operations per setup, reduced leadtime, accommodate design change, reduced inventory

• Rapid programming and program recall, less paperwork

• Faster prototype production

• Less-skilled operator , multi-work possible

• Reduced fixturing Limitations of NC

• Relatively high initial cost of equipment

• Need for part programming

• Special maintenance requirements

• More costly breakdowns

• Finding and/or training NC personnel The Punched tape in NC

• The part program is converted into a sequence of machine tool actions by means of the input medium, which contains the program, the controller unit,

which interprets the input medium.

• The controller unit and input medium which must be compatible.

• Input medium uses symbols which represent part program and the controller unit must be capable of reading it.

• The most common input medium is Punched Tape

• The punched tape used in NC is 1 inch wide.

• The tape has eight tracks or coding channels and a column of smaller holes for feeding the tape.

• The coding of tape is provided by either the present or absence of a hole in various positions

• This system of coding is called as binary system

• The NC tape coding system is used to code not only numbers, but also alphabetical letters and other symbols


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• There are two methods for preparing punched tape

• The first method associated with manual part programming and involves the use of typewriter like device.

• The operator writes directly from the part programmer‟s hand written list of coded instructions.

• This produces a typed copy of the program as well as the punched tape.

• The second method used with Computer assisted part programming.

• By this approach, the tape is prepared directly by the computer using a device called a tape punch.

• During production, the tape id fed through the tape reader once for each

workpiece.

• It is advanced through the tape reader one instruction at a time.

• While the machine tool is performing one instruction, the next instruction is being read into the controller unit‟s data buffer.

• After last instruction has been read into the controller, the tape is rewound back to the start of the program to ready for next workpart.

How instructions are formed?

• A binary digit is called as bit.

• It has value of 0 or 1 depending on the absence or presence of hole in a certain

row or column position on the tape (columns of hole positions run lengthwise along the tape. Row positions run across the tape.)

• Out of row of bits, a character is made

• A character is a combination of bits, which represent a letter, number or other

symbol

• A word is a collection of characters to form part of an instruction. Examples of NC words are x position, y position, cutting speed and so on.

• Out of collection of words, a block is formed.

• A block of words is a complete NC instruction

• E.g. in NC drilling a block might contain information on the x and y coordinates of the hole location, the speed and feed at which the cut should be taken.

• To separate a block , an end-of block (EOB) symbol is used.

• The tape reader feeds the data from the tape into the buffer in blocks


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• That is, it reads in a complete instruction at a time NC words

Following are the different types of words in formation of block. Not every NC uses all the words

• Sequence number (n-words): This is used to identify the block

• Preparatory words (g-words): This word is used to prepare the controller for instructions that are to follow e.g. g02 is used for circular interpolation along an arc in the clockwise direction . The preparatory words are needed so that

the controller can correctly interpret that follow it in the block

• Coordinates (x-, y-, and z-words): This give the coordinate positions of the tool. In two axis system o9nly two of the words would be used. In four or five

axis, additional a-words and/or b-words would specify the angular positions

• Feed rate (f-words) : this specifies the feed in the machining operation

• Cutting speed (s-words) : This specifies the cutting speed of the process, the rate at which the spindle rotates.

• Tool selection (t-word) : This control would needed only for machines with a tool turret or automatic tool changer. The t-word specifies which tool is to be used in the operation.

• Miscellaneous words (m-words): The m-word is used to specify certain

miscellaneous or auxiliary functions which may be available on machine tool

• E.g. m03 used to start spindle rotation. This word is last in the block.

Programming Formats

• Fixed block format

• Tab sequential format

• Word address format Fixed block format

• In this number of characters in every block is the same

• The words in each block must follow a fixed sequence e.g. sequence number followed by x dimension, followed by y dimension, followed by end of block

statement

• In case of floating zero coordinate system, every coordinate value must be preceded by its sign.

• This is not necessary in case of fixed zero coordinate system. Tab sequential format

• The tab character is used to separate the words in a block

• The words follow the fixed sequence as like fixed block format

• In this repeated words (dimension) need not be punched and may be passed

over by pressing the tab character. Word address format

• The words in block need not follow fixed sequence, as each displacement information is preceded by an identifying word (X or Y) which acts as the

address

• In this the X and Y characters must be punched

• This makes program longer at the same time offers greater flexibility to the programmer.


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Program: Point to Point

1. Drilling a hole Φ6 at point 1.

2. Drilling a hole Φ6 at point 2, and 3. Making a threaded hole M12 at point 3;machining hole M12 involves drilling

a hole Φ10.3 and then tapping (Taps are come in two sets)

• The machine tool format is N3.G2.X 3.3. Y 3.3. M2.EOB

• In this format. represents tab-sequential format Additional features to be considered:

• Machine tool has provision for absolute as well as incremental dimensioning, i/p of dimensions in mm or inch, and PTP and continuous path controls,

• The machine has full range zero shift

• The machine zero is shifted to the setup point, which is the lower left corner of

the part

• The machine control unit has provision for suppressing trailing zeros

• To avoid trouble in loading and unloading of the workpiece and tool changing must be positioned at point 4 (X=100, Y = 100) during these operations

Coordinates of programmed points

Point X Y

On

drawing On tape

On

drawing On tape

1 5 005 5 005

2 5 005 45 045

3 55 055 25 025

4 100 1 100 1

Program


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Preparatory functions used for Programme

G90 prepares the machine control unit for the absolute dimension input G71 prepares machine tool control unit to accept the dimensions in mm G81 prepares machine tool control unit for drilling cycle, the cycle includes rapid

traverse to the X and y coordinates, rapid quill movement to the gauge height, feed to the required depth and retracting the quill

G80 Instructs the machine tool control unit that the cycle stands cancel G82 Prepares the machine control unit for the thread tapping cycle.; the cycle is similar to drilling cycle

Miscellaneous functions used for Programme M03 spindle start , clockwise direction

M05 spindle stop M06 Tool change M02 End of program, machine stop, tape rewind

Program: Positioning-cum-straight-cut problem

• The program is to be prepared for drilling, two 10-mm holes 1 and 2 and milling 10 mm wide milling slot

• The machine tool format is

N3, G2, X 3.3, Y 3.3, M2, M2, EOB

• The format is fixed block format has two M2 statements. In the two

miscellaneous functions may be coded in a single block

• The following are the constraints for programs: 1. The part is to be made on a point-to-point type vertical milling machine with

full floating zero and absolute dimensions in mm only,


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2. The machine zero is to be selected at the center of the workpiece,

3. The groove is machined with a 10-mm end mill cutter 4. The machine tool has provision for suppressing trailing zeros,

5. During loading and unloading the spindle is stationed at point 5(X=65, Y=70). 6. For milling the groove, the cutter must start from point 3 and stop at point 4 Coordinates of programmed points

Point X Y

On

drawing On tape

On

drawing On tape

1 -25 -025 -20 -02

2 25 025 20 02

3 -42 -042 0 0

4 42 042 0 0

5 65 065 70 07

Program: Positioning-cum-straight-cut problem

Preparatory functions used for Programme G00 Point to point control G81 Drilling cycle

G90 prepares the machine control unit for the absolute dimension input G71 prepares machine tool control unit to accept the dimensions in mm


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G80 Cancel fixed cycle

G83 Milling cycle; the cycle performs rapid traverse to the programmed position in X and Y coordinates, lowers the cutter to gauge height and stops the machine

G66 Instructs the machine to initiate milling operation to the end of the cut specified by the programmed point. Miscellaneous functions used for Programme

M06 Tool change M13 Spindle start

M05 Spindle stop M09 Coolant off M17 No miscellaneous function required for this block

M02 End of program, machine stop, tape rewind Program

Program

Word address Format

N001 G00 G71 G90 M06 EOB

N002 X50 Y12 EOB N003 M00 EOB N004 X30 Y35 EOB

N005 M00 EOB N006 X12 Y55 EOB

N007 M00 EOB N008 X120 Y120 EOB N009 M30

M00 Program stop. Program may be initiated by the operator by pushing a

button

Program

Word address Format

• Three holes to be drilled

• The depth of hole is 10 mm

• Z=00 at the surface of the workpiece

• The cutting tool is positioned

above the workpiece surface


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• Spindle speed and feed is to be given in the program . Speed is 1000 rpm, feed rate is 200 mm/min

Program N001 G71 G90 G94 EOB

Metric mode, Absolute system and feed in mm/rev

N002 M03 F200 S1000 EOB Spindle start CW at 1000 rpm and feed rate 200 mm/rev

N003 G00 X10.00 Y10.00 EOB Move in rapid to point P(10,10)

N004 G00 Z2.00 EOB Move in rapid to a point to point 2 mm above w/p surface

N005 G01 Z-10.00 EOB

Drill hole

N006 G00 Z 2.00 EOB

Move in rapid to a point to point 2 mm above w/p surface

N007 G00 X50.00 EOB Move in rapid to x=50

N008 G01 Z-10.00 EOB Drill hole


N010 G00 Y30.00 EOB

Move to point Y30

N011 G01 Z-10.00 EOB

Drill hole


N013 G00 X00 Y00 EOB Move in rapid to (X0,Y0)

N014 M02 EOB Programmed end Program

• Machining at AB and BC

• Feed 200 mm/rev and speed 2000 r.p.m.

Z=0 is 50 mm above the surface of the workpiece and depth of cut is 5 mm

Program N001 G71 G90 G94 EOB

N002 F200 S2000 EOB Set feed 200 mm/min. and speed 200 r.p.m.

N003 M03 M08 EOB


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Spindle on clockwise, coolant on

N004 G00 Z2.00 EOB Spindle moves Z=2 in rapid mode

N005 X100.0 Y0.00 EOB Move to X=100, Y =0

N006 Z-55.00 EOB

Spindle down to required depth of cut N007 G01 X200.00 EOB

Linear interpretation to X=200.00

N008 G01 Y150.0 EOB Move to Y=150.00

N009 G00 Z10 M09 EOB Rapid spindle retract to z=10 and coolant off

N010 G00 X250.00 Y250.00 EOB Rapid to X=250 and Y=250

N011 M02 EOB

I – Distance to arc center parallel to X J- Distance to arc center parallel to Y K- Distance to arc center parallel to Z

• In the component , it is assumed that the pocket is through and hence only outside is to be machined as a finish cut of the pocket

• The tool to be used is a 20 mm diameter slot drill .

• If an end mill is to be used the program should be modified with a hole to be

drilled at B first before the end mill is used

• The setting is done with point A as reference (0,0,0) N001 G92 X0 Y0 Z0 EOB Absolute presetting at A

N002 G90 G94 EOB Absolute programming

N003 G00 X25.00 Y25.00 Z2.00 EOB


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Tool brought rapidly at B, 2 mm above XY plane

N004 G01 Z-12.00 F120 S1000 M03 EOB Tool goes down to full depth

N005 Y75.00 EOB Proceeds to C N006 X65.00 EOB

Proceeds towards right to D N007 G02 Y25.00 I0 J-35.00 EOB

Cuts curved profile till E N008 G01 X25.00 EOB Proceeds to B

N009 Z2.00 EOB Tool moves 2 mm above XY plane

N010 G00 Z50.00 M05 EOB Spindle stops and rapidly moves up N011 X0 Y0 EOB

Rapid move to start position 0,0 N012 M30 EOB

End of program and tape rewind

CUTTER RADIUS COMPENSATION

• In contouring operations, it becomes necessary to calculate the tool path for preparing the program by offsetting the contour by amount equal to the radius of the cutter

• Apart from the problem of calculation, whenever cutter size changes, the program would need editing

• However, if a compensation equal to the radius of the cutter is entered and stored in the control system, then the program could be written for the

component profile and thus no change in the program would be required.

• It is as if the program is written with a cutter of zero radius

• It is necessary to indicate whether compensation is to be made to the right or to the left of the tool when machining

• The following three G-codes are used for cutter compensation radius 1. G-41 - It is used when cutter is on left of the programmed path when looking

in the direction of the tool movement

2. G-42 - It is used when cutter is on right of the programmed path when looking in the direction of the tool movement

3. G-40- Cancel cutter radius compensation Program Word address Format


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• Part programming for the part shown in the figure using cutter radius

compensation is given below:

• Programmed cutter diameter is 0 mm i.e. programming is done as per the part drawing

• Diameter of the cutter available = 28 mm

• The difference in radius (0-28)/2 = -14 mm is stored in the memory for cutter radius compensation under address D 01.

Program Word address Format

N001 G71 G94 EOB N002 G00 X-20.00 Y-20.00 Z20.00 EOB N003 G01 Z-5.00 EOB

N004 G01 G41 D01 X20.00 Y85.00 F400 S1000 M03 EOB N005 G01 X60.00 Y115.00 EOB

N006 G02 X75.00 Y100.00 I0.00 J-15.00 EOB N007 G01 X105.00 EOB N008 G02 X130.00 Y115.00 I15.00 J0.00 EOB

N009 G01 X120.00 Y85.00 EOB N010 G01 X130.00 Y55.00 EOB

N011 G01 X110.00 EOB N012 G01 X80.00 Y20.00 EOB N013 G01 X35.00 Y55.00 EOB

N014 G01 X20.00 Y85.00 EOB N015 G00 G40 X0.00 Y0.00 EOB

N016 M02 EOB Program Word address Format

• Cutter radius compensation is stored on D02

• Z = 0 is at the top of the surface of the work piece Feed = 65 mm/min Speed = 1000 rpm

depth of cut = 10 mm


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Program

N001 G71 G90 G94 EOB N002 G00 X-20.00 EOB

N003 G00 Z-10.00 EOB N004 G01 G42 D02 X0 Y0 F200 S1000 M03 EOB N005 G01 X80.00 EOB

N006 G01 X95.00 Y15.00 I0 J15.00 EOB N007 G01 Y50.00 EOB

N008 G01 X15.00 EOB N009 G01 X0 Y35.00 EOB N010 G01 X0 Y0 EOB

N011 G40 EOB N012 G00 X-20.00 Z20.00 EOB

N013 M02 EOB Lathe Operations

• In case of CNC lathe operations , only two axes (X-axis and Z-axis ) are involved

• The Z-axis is the axis of the spindle and X-axis is the direction of transverse motion of the tool post.

• To develop CNC part programme for lathe operations the following procedure is adopted.

1. Move the cutting tool to a point near the job in the rapid mode(G00) 2. Set linear interpolation(G01) and move to the required depth of cut in X-

direction 3. Move along Z-axis to the required length of the job as per drawing 4. Set rapid mode (G00) and retract the tool along X-axis.

5. Move to start point in G00 mode. For the component shown below perform the following operations

• Facing operation

• To reduce the diameter from 30 mm to 26 mm

N001 G71 G90 G94 EOB

N002 T01 F200 M03 S800 EOB (Tool No. 1, Feed rate 200 mm/min and speed 800 rpm ) N003 G00 X22.00 Z1.00 EOB

(In rapid mode, move to point X22, Z=1) N004 G00 X0 EOB

(Move to X=0, Z remaining constant) N005 G01 Z0 EOB (Go to Z =0 in G01 mode)

N006 X30.00 EOB N007 Z-60.00 EOB

N008 G00 X32.00 EOB (Withdraw the tool by 1 mm so that when tool is taken back to Z=0, it does not leave scratch mark on the job)


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N009 G00 Z0 EOB

N010 G01 X26.00 EOB N011 G01 Z-60.00 EOB

N012 G00 X32.00 EOB N013 Z20.00 Z20.00 N014 M02 EOB

Taper Turning Programming The raw material size is 20 mm dia bar, the operations involved

1. Facing 2. Turn upto 15 mm dia. over 15 mm length 3. Taper turning

N001 G71 G90 G94 EOB N002 T01 S1000 M03 EOB

N003 G00 X22 Z0.5 EOB N004 G01 X0.00 F200 EOB N005 Z0.00 EOB

N006 X20.00 EOB N007 X15.00 EOB

N008 Z-15.00 EOB N009 X20.00 Z-20.00 EOB N010 Z-35.00 EOB

N011 G00 X25.00 Z20.00 EOB N012 M02 EOB

Taper Turning Programming (Incremental) Starting Position (X=0, Z=1) N001 G71 G91 G94 EOB

N002 T01 S1000 M03 EOB N003 G00 X11.00 Z-0.5 EOB

N004 G01 X-11.00 F200 EOB N005 Z-0.5 EOB N006 X11.00 EOB

N007 X-3.5 EOB N008 Z-15.00 EOB

N009 X2.5 Z-5.00 EOB N010 Z-15.00 EOB N011 G00 X2.5 Z55.00 EOB

N012 M02 EOB Multi Pass Turning

• Turn to 30 mm diameter

• Raw material available : 40 mm

• Rough cut : 2 to 3 mm


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• Finish cut : 0.75 to 1 mm

N001 G90 G71 G94 F500 S1000 T01 EOB N002 G00 X41.00 Z1.00 M03 EOB N003 G01 X37.00 EOB

N004 G01 X-30.00 EOB N005 G00 X41.00 Z1.00 EOB

N006 G01 X34.00 EOB N007 G01 Z-30.00 EOB N008 G00 X41.00 Z1.00 EOB

N009 G01 X31.00 EOB N010 G01 X-30.00 EOB

N011 G00 X41.00 Z1.00 EOB N012 G01 X30.00 EOB N013 G01 Z-30.00 EOB

N014 G01 X41.00 EOB N015 G00 Z25.00 EOB

Computer aided part programming

• The programs for machining simple parts are small and do not require much calculations.

• Such programs may be prepared manually.

• However, for complicated profiles , especially free formed surfaces, a lot of calculations have to be done and the program also is often long

• Manual part program is a labour oriented task and needs skilled programmers who should have through knowledge of the various machining processes ,

materials, speeds and feeds, part programming codes, capabilities of various machine tools etc.

• All the problems of manual part programming have been overcome and part programming has been considerably simplified with the use of computer aided part programming

• In this, the computer generates the part Programme required to machine the component

• The process of generating part programmes is partly done by part programmer and partly by the computer

• Part programmer is to define the geometry of the component from the component drawing.

• The geometry or shape of the component is split into simple elements like points, lines , arcs, full circles, distances and directions and these elements are

assigned specific numbers to identify their position. Advantages

1. Part programming is considerably simplified


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2. The part programmes generated are accurate and efficient

3. All arithmetic operations are done by the computer , resulting in saving in time and elimination of errors

4. Such system can deal with many axis for simultaneous movement

Part Programmer‟s Job

• Defining work part geometry

• Specifying operations sequence and tool path The Computer‟s Job

Input translation : It converts the coded instructions contained in the program into computer usable

form, preparatory to further processing Arithmetic calculations : The unit of the system consist of a comprehensive set of subroutines for solving

the mathematics required to generate the part surface.

• These subroutines are called by various part programming language statements

• The units frees the programmer from the time consuming geometry and trigonometry calculations, to concentrate on workpart processing

Cutter offset calculations :

• The actual tool path is different from the part outline because the tool path is defined as the path taken by the center of the cutter.

• The purpose of cutter offset computations is to offset the tool path from the desired part surface by the radius of the cutter.

• This means that the programmer can define the exact part outline in the geometry statements.

Post processor:

• NC machine tools are different

• They have different features and capabilities. They use different NC formats

• Nearly all the programming languages including APT, are designed to be general purpose languages, not limited to one or two machine tool types.

• Therefore the final task of the computer in computer assisted part programming is to take the general instructions and make them specific to a

particular machine tool system.


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• The unit that performs this task is called as Postprocessor. APT (Automatically programmed Tools)

• This is most widely used and most comprehensive part programming language available.

• APT is a three- dimensional system which can be used to control upto five

axes. • In programming , it is assumed that the workpiece remains stationary and

cutting tool does all the movements. • APT consists of four types of statements 1. Geometry statements: These are also called definition statements and are

used to define geometric elements like point, circle. Arc, plane, etc. 2. Motion statements : The motion statements are used to define cutter path

3. Post processor statements : These statements are applicable to specific machine tools and are used to define machining parameters like feed, speed, coolant on/off, etc.

4. Auxiliary statements : These are miscellaneous statements used to identify the part, tools, tolerances, etc.

1. GEOMETRY STATEMENTS : The geometry statements is defined with the

help of points, lines, circles, arcs and planes, etc. The general form of APT is :

Symbol = geometry type/descriptive data Point

P1 = POINT/X, Y, Z (Point with coordinates X, Y, Z) P2 = POINT/ CENTER C1

(Point at center of circle 1 ) P3 = POINT/ INTOF,L1,L2

(Point at intersection of lines L1 and L2 ) P4 = POINT/XLARGE, INTOF, L1,C1 (Point at intersection of line L1 and circle C1, where x coordinates has higher

value) P5 = POINT/YSMALL, INTOF,C1, C2

(Point at intersection of circle C1 and circle C2, where y coordinates has smaller value) Here it has been assumed that lines and circles have already been defined in

the geometry statements LINE

In APT line can be defined in many ways, some of the definitions of a straight line are: L1 = LINE/P1,P2

(Line passing through points P1 and P2) L2 = LINE/P2.PARLEL,L1

(Line passing through point P2 and parallel to line L1) L3 = LINE/P1. ATANGL,60,L2 (Line passing through point P1 and at an angle 60 degrees to line L2)

L4 = LINE/ P1, ATANGL,-120,L2 (Line passing through point P1 and at angle -120 degree to line L2)

L5 = LINE/PARLEL,L31,YLARGE,10 (Line parallel to line L31 and offset by 10 mm towards Y large coordinates) CIRCLE

C1 = CIRCLE/CENTER,P1,RADIUS, 10


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(Circle with center point P1 and radius 10 mm)

C2 = CIRCLE/CENTER, P1, TANTO, L1 (Circle with center at P1 and tangent to line L1)

C3 = CIRCLE/P1, P2, P3 (Circle passing through points P1, P2,P3) C4 = CIRCLE/ XSMALL, L1, YSMALL, L2, RADIUS,5

(Circle passing through the intersection of lines L1 and L2 and radius 5 mm (with X small on L1 and Y small on L2)

PLANE

PL1 = PLANE/P1,P2,P4 (Plane passing through three points )

2. MOTION STATEMENTS :

• Motion statements are also called as machine control instructions

• The motion statements define the tool pat as per the part geometry specified in the program

• The general format for APT motion statement is Motion command/descriptive data e.g. GOTO/P2 or FROM/P1

• If the movement is in incremental mode, GODLTA command is used e.g. GODLTA/5.0, 4.0, 0.0 Instructs the machine tool to move 5 mm in X direction ,

4 mm in Y direction and Z directions remains same some of the motion statements are : GO/PAST

GO LFT GO FWD

GO BACK GO UP GO DOWN

3. POST PROCESSOR STATEMENTS

• Post processor statements are machine specific and control the operation of the machine spindle, feed rate and other features like tool change etc

some of the post processor statements are : COOLNT/ON

FEDRAT/250 (feed rate 200 mm/min) MACHIN/ TURNING CENTER 1 (machine tool to be used for the operation) MACHIN/PD,1 (postprocessor identification)

RAPID/ SPINDL/1000 (spindle speed is 1000 rpm)

SPINDL/ON SPINDL/OFF TURRET

CYCLE/ is used to execute a particular cycle e.g. drilling, tapping cycle


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e.g. CYCLE/DRILL, RAPID, TO, -10, FEDTO, -15, MMPM, 150

(drilling cycle, direct the tool to move rapidly to the Z coordinates -10 mm, and then move further at a feed rate of 150 mm/min to the z coordinate -15 )

END/ STOP/ FINI/

4. AUXILIARY STATEMENTS

• Auxiliary statements are used for part identification, cutter size definition, defining dimensional tolerances, and other functions required to prepare the

control system to accept and execute the part Programme.

• Some of the auxiliary APT words are : PART NO/ (part number for identification of the component)

CUTTER/ (cutter size specification) INTOL (inside tolerances ) OUTTOL (outside tolerances)

CLPRINT (print cutter location data) REMARK

APT Example

• Write a Programme for drilling holes 1 to 5 in the flat plate shown in the figure.

• Feed = 5 mm/rev

• Drill = 15 mm

• Speed = 600 rpm PARTNO FLAT PLATE NO 1 MACHIN/PD,11

SETPT = POINT/75,50,50 P1 = POINT/25,25,5 P2 = POINT/50,25,5

P3 = POINT/50,75,5 P4 = POINT/25,75,5

P5 = POINT/30,50,5 REMARK/ SET,FEDRAT,0.5 MMPR REMARK/SET,SPINDL,600 RPM

REMARK/DRILL, 15 MM SPINDL/ON

COOLNT/ON FROM/SETPT RAPID

GOTO/P1 CYCLE/DRILL,FEDTO,-15

GODLTA/0,0,20 GOTO/P2 CYCLE/DRILL,FEDTO,-15

GODLTA/0,0,20 GOTO/P3

CYCLE/DRILL,FEDTO,-15 GODLTA/0,0,20 GOTO/P4

CYCLE/DRILL,FEDTO,-15 GODLTA/0,0,20 GOTO/P5


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CYCLE/DRILL,FEDTO,-15

GODLTA/0,0,20 COOLNT/OFF

SPINDL/OFF RAPID GOTO/SETPT

REWIND FINI

APT MOTION STATEMENTS FOR CONTINUOUS PATH PROGRAMMING

• In this the cutter is guided in its path by three planes: 1. Part surface (PS or PSURF): The plane which guides the bottom surface of the

tool is known as the part surface

2. Drive surface (DS or DSURF): The plane guiding the side of the cutting tool. i.e. the plane along which tool side moves is known as drive surface

3. Check surface (CS or CSURF) :The plane used to stop the cutter or change its direction is known as check surface.

APT Example

Write a Programme for the following component it is required to write the Programme for finishing cut

Cutter geometry


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APT Example

PARTNO/SAMPLEJOB 1 MACHIN/MILLING 1

CLPRINT P0 = POINT/0,-20, 0 P1 = POINT/0, 0, 0

P2= POINT/150.0, 0, 0 P3= POINT/150.0, 0, 0

P4 = POINT/ 100.0, 50.0, 0 P5 = POINT/ 100.0, 100.0, 0 P6 = POINT/50, 100.0, 0

P7 = POINT/50.0, 50.0, 0 P8 = POINT/0, 50.0, 0

L1 = LINE/ P1,P2 L2 = LINE/ P2,P3 L3 = LINE/ P3,P4

L4 = LINE/ P4,P5 L5 = LINE/ P5,P6

L6 = LINE/ P6,P7 L7 = LINE/ P7,P8 L8 = LINE/ P8,P1

PL1= PLANE/P1,P2,P8 CUTTER/10.0

SPINDL/500 FEDRAT/80 COOLNT/ON

FROM/P0 GO/TO, L1,TO, PL1, TO, L8

GORGT/L1,PAST,L2 GOUP/L2,PAST,L3 GOLFT/L3,TO,L4

GOUP/L4,PAST,L5 GOLFT/L5,PAST,L6

GODOWN/L6,TO,L7 GOLFT/L7,PAST,L8 GODWON/L8,PAST,L1

RAPID GOTO/P0

COOLNT/OFF FINI APT Example


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PARTNO

MACHIN/ABM,8 UNITS/MM

CLPRINT SETPT =POINT/0,0,0 P1 = POINT/50,25,0

P2 = POINT/150,25,0 P3 = POINT/150, 40, 0

P4 = POINT/130,50,0 P5 = POINT/50,100,0 L1 = LINE/P1, P2

L2 = LINE/P2, P3 L3 = LINE/P3, P4

C1=CIRCLE/80,100,30 L5 = LINE/P1, LEFT, TANTO.C1 L4 = LINE/P4,RIGHT, TANTO, C1

PL1 = PLANE/P1,P2,P5 CUTTER/10

FEDRAT/0.1, MMPR REMARK/SET, SPINDL,1000,RPM INTOL/0.01

OUTTOL/0.01 FROM/SETPT

GO/TO, L1,PL1,TO,L5 SPINDL/ON COOLNT/ON

GORGT/L1,PAST,L2 GOLFT/L2,PAST,L3 GOLFT/L3,TO,L4

GORGT/L4,TANTO,C1 GOFWD/C1, TANTO,L5

GOFWD/L5,PAST,L1 SPINDL/OFF COOLNT/OFF


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RAPID

GOTO/SETPT FINI

APT Example Speed = 700 rpm Feed = 80 mm/rev

Cutter = 10 mm dia Endmill

APT Example

• Post processor ID=MACHIN/NC,1

• Feed rate = 100mm/min

Cutter geometry


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CNC & DNC

Problems with conventional NC

Part Programming mistakes Syntax or numerical errors

Nonoptimal speeds and feeds Punched tape

Paper tape is fragile and its susceptibility to wear and tear causes it to

be unreliable NC component for repeated use in NC Tape reader

It is the most unreliable component of machine Controller

Control features cannot easily altered to incorporate improvements into

unit. Management Information

It is not equipped to provide timely information on operational performance to management.

Function of CNC 1. Machine tool control • This involves conversion of part program instruction into machine tool

motions through the computer system and servo system.

• The CNC system is capable to incorporate variety of control features into soft wired controller unit.

• A few functions such as circular interpolation can be accomplished efficiently

on a hard wired NC system than on a computer. • Hybrid CNC Controller

• It is a combination of the hardwired logic circuit and soft wired computer. • The logic circuit is aimed to perform functions such as circular interpolation

and feed rate generation which they do best.

• The computer performs the remaining control functions which are out of the scope of the logic circuit.

• Straight CNC Controller • The controller uses a computer to perform to perform all NC functions


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• The only hardwired elements are those required to interface the computer with

the machine tool and the operator‟s console • Interpolation, tool position feedback, and all other functions are performed by

computer software. • The advantage of this configuration is the additional flexibility. • It is possible to make alternations in the interpolation program whereas the

logic contained in the hard-wired circuits of hybrid CNC cannot be altered.

2. In-Process Compensation a. This involves dynamic correction of the tool motions for changes

or errors which occur during processing. E.g.

b. Adjustments for errors sensed by in-process inspection probes and gauges.

c. Recomputation of axis positions when the inspection probe is used to

locate the datum reference on a work part. d. Offset adjustments for tool radius and length

e. Computations of predicted tool life and selection of alternative tooling when indicated.

3. Improved programming and Operating features a. The flexibility of soft-wired control has permitted the introduction of

many convenient programming and operating features. E.g. b. Editing of part programs at the machine. c. Graphic display of the part to verify the tape.

d. Various types of interpolation e. Use of specially written subroutines

f. Local storage of more than one program g. Manual Data Input (MDI)

4. Diagnostics • CNC machines are equipped with a diagnostics feature capability to assist

in maintaining and repairing the system.

• Diagnostic subsystem would accomplish some functions 1. The subsystem would be able to identify the reason for a downtime

occurrence so that the maintenance personal could make repairs more quickly

2. The subsystem would be alert signs that indicate the imminent failure of a

certain component. • Hence maintenance personnel could replace the faulty part during

scheduled downtime, thus avoiding unplanned interruption of production • Third function associated with CNC system to contain a certain amount of

redundancy of components.


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• When one of these component fails, the diagnosis subsystem would

automatically disconnect the faulty component and activate the redundant component

Advantages of CNC • The part program tape and tape reader are used only once to enter program

into computer memory.

• Tape editing at the machine site • Metric conversion

• Greater flexibility • User written programs • Total manufacturing system

Components of DNC 1. Central computer

2. Bulk memory

3. Telecommunication lines

4. Machine Tools

• The computer calls the part program instructions from bulk storage and

sends them to the individual machines as the need arises. • It also receives data back from the machines • This two way information flow occurs in real time, which means that

each machine‟s request for instructions must satisfied instantaneously. • Depending upon no. of machines and computational requirements that

are imposed on the computer, it is sometimes necessary to use of satellites computers.

• Each satellite computer controls several machines

• Groups of part program instructions are received from the central computer and stored in buffers.

• Feedback data from the machines are also stored in the satellite‟s buffer before being collected at the central computer

Two types of DNC

1. Behind The Tape Reader (BTR) System • In this the computer is directly linked to the regular NC controller unit.

• In this arrangement the tape reader is replaced by telecommunication lines to the DNC Computer.

• Except for the source of command instructions , the operation of the system is

similar to conventional NC. • The controller unit uses two temporary storage buffers to receive blocks of

instructions from the DNC computer and convert them into machine actions • While one buffer is receiving a block of a data , the other is providing control

instructions to the machine tool.

2. Special Machine Control Unit • In this NC controller is replaced by a special MCU and forms a link between

computer and machine tool. • The special MCU achieves a superior balance between accuracy of machining

and faster material removal rate.

• This unit is soft wired so more flexibility can be achieved and alterations can be made easily.

• It is much difficult to make changes in the regular NC controller because rewiring is required.


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Functions of DNC

1. NC without punched tape

• One of the original objectives in DNC was to eliminate the use of punched

tape. • Relatively unreliable tape reader • The fragile nature of paper tape


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• The difficulties in making corrections

• Changes in the program contained on punched tape, etc. 2. NC part program storage

• The second important function of DNC is concerned with storing the part programs.

• The program storage subsystem must be structured to satisfy several purpose.

• First the must be made available for downloading to NC m/c tool. • Second subsystem must allow for new program to be entered, old program to

be deleted & existing program to be edited as the need arises. • Third the DNC software must accomplish the post processing function. The

part program in DNC is stored in CLFILE. The CLFILE must be converted

into instructions for a particular m/c tool. • Fourth the storage subsystem must be structured to perform certain data

processing & management functions such as file security, display of program, manipulation of data& so on.

• The DNC program storage subsystem usually consists of an active storage &

secondary storage. 3. Data collection, processing and reporting

• DNC involves a two way data transfer, • The basic purpose behind the data collection, processing reporting function of

DNC is to monitor production in the factory.

• Data are collected on production pieces are counts, tool usage, m/c utilization & other factors that measure performance in the shop.

• This data must be processed by the DNC computer, & reports are prepared to provide management with information necessary for running the plant.

4. Communications

• A communication network is required to accomplish the previous three functions of DNC.

• Communication among the various subsystems is a function that is central to the operation of any DNC system.

• The essential communication link DNC are between the following

components , Central computer & M/c tool

Central computer & NC part programmer terminal Central computer & bulk memory, which stores NC program.

ADVANTAGES OF DNC • Elimination of punched tape & tape reader.

• Convenient storage of NC part program in computer file. • Program stored as CLFILE (Cutter path data). • Reporting of shop performance.

• Establishes a frame work for the evaluation of future computer automated factory.


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CHAPTER 10. Group Technology

Introduction

It is a manufacturing philosophy in which similar parts are identified and groped together to take advantage of their similarities in manufacturing and design.

For example a plant producing 10000 different part numbers may be able may be able to group the vast majority of these parts into 50 or 60 distinct families.

Each family would posses similar design and manufacturing characteristics.

Hence, the processing of each member of a family would be similar results in mfg. efficiencies

These efficiencies are achieved in the form of reduced setup times, lower in process inventories, better scheduling , improved tool control, and the use of standardized process plans.

Part families A part family is a collection of parts which are similar either because of

geometric shape and size or because similar processing steps are required in their manufacture.

Part family concept

The part family concept can be explained as follows Figure 1 shows a process type layout for batch production in machine shop.

The various machine tools are arranged according to function. During machining of a given part, the work piece must be moved in different

sections, with perhaps the same section visited several times.

This results in a significant amount of material handling, a large in-process inventory, usually more setups than necessary, long mfg. lead times, and high

cost.

Figure 1 PROCESS-TYPE LAYOUT

Figure 2 shows a production of equivalent capacity, but with machines

arranged into cells. Each cell is organized to specialize in the manufacture of a particular part

family.

Lath

e

Millin

g

Drillin

g

Grindin

g

Assembl

y

Receiving and

Shipping

L

L L

L

L

L

L

L M

M M

M M

M

A A

A A

D

D D

D

G

G

G

G G

G


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Advantage get in the form of reduced workpiece handling, lower setup times,

less in-process inventory, less floor space, and shorter lead times.

Figure 2 GT LAYOUT Grouping part into part families

Visual inspection Production flow analysis Parts classification and coding system.

Visual inspection It is simplest and cheapest method

It involves classification of parts into families by looking at either the physical parts or photographs and arranging them into similar grouping.

This method is considered to be least accurate of three.

Production flow analysis It is a method of identifying part families and associated machine tool

grouping by analyzing the route sheets for parts produced in a given shop. It groups together the parts that have similar operation sequences and machine

routings.

The major drawback of this system is that it does not consider the actual flow of material

Parts classification and coding It involves an examination of the individual design and/or mfg attributes of

actual part.

The attributes of the part are uniquely identified by means of a code no. This classification and coding may be carried out on the entire list of active

parts in the firm or a sampling process may be used to establish the part families.

Parts classification and coding system divide themselves into one of three

categories


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1. System based on part design attributes

2. System based on part manufacturing attributes 3. System based on both design and manufacturing attributes.

Design and manufacturing attributes

Part design attributes

Basic external shape Major dimensions

Basic internal shape Minor dimensions

Length/diameter ratio Tolerances

Material type Surface finish

Part function

Manufacturing attributes

Major process Operation sequence

Major operations Production time

Major dimensions Batch size

Length/ diameter ratio Annual production

Surface finish Fixtures needed

Machine tool Cutting fluids

Coding System structure A parts coding scheme consists of a sequence of symbols that identify part‟s

design and/or mfg. attributes.

The symbols in the code can be all numeric, all alphabetic, or a combination of both types.

Most of the classification and coding systems use number digits only. There are basic three code structures used in GT:

1. Hierarchical structure

2. Chain type structure 3. Hybrid (combination of above)

Hierarchical structure (Monocode or Tree structure) In this, the interpretation of each succeeding symbol depends on the value of

the preceding symbols.

It provides compact structure which conveys much information with limited number of digits.

Chain type structure (Polycode) In this, the interpretation of each symbol in the sequence is fixed and does not

depend on the value of the preceding digits.

These codes are relatively long. The use of polycode allows for convenient identification of specific part

attributes. This can be helpful in recognizing parts with similar processing requirements.

Hybrid type structure (combination of Monocode and Polycode)

These are constructed as a series of short polycodes. Within each of these shorter chains, the digits are independent, but one or

more symbols in the complete code number is used to classify the part population into groups, as in the hierarchical structure.

These system serve the need of both design and production.

Parts classification and coding system 1. Optiz system

2. MICLASS system 3. CODE system


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OPTIZ SYSTEM

MICLASS System (Metal Institute Classification System) The MICLASS system was developed to help automate and standardize a

number of design, production and management functions. These includes : Standardization of engineering drawings

Retrieval of drawing according to classification number. Standardization of process routings Automated process planning

Selection of parts for processing on particular groups of machine tools. Machine tool investment analysis.


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The MICLASS classification no. can range from 12 to 30 digits.

The first 12 are universal codes that can be applied to any part. Upto 18 additional digits can be used to code data that are specific to the

particular company or industry. e.g. lot size, cost data and operation sequence might be included in the 18 supplementary digits

The workpart coded in the first 12 digits of the MICLASS no. are as follows:

1st digit main shape 2nd and 3rd digit shape elements

4th digit position of shape elements 5th and 6th digit main dimensions 7th digit dimension ratio

8th digit auxiliary dimension 9th and 10th digits Tolerance codes

11th and 12th digits material codes In this parts can be coded using computer interactively. The number of questions depends upon complexity of the part.

The question may vary from 7 to 20. Based on the response to these questions computer assigns a code number to

the part.

The CODE system

Its most universal application is in design engineering for retrieval of part design data, but also has applications in mfg. process planning, purchasing, tool design, and inventory control.

The CODE number has eight digits . For each digit there are 16 possible values (zero to 9 and A through F) which

are used to describe part‟s design and mfg attributes. The initial digit indicates the major geometry of the part and is called as major

division of the code system.

The second and third digit provide additional information concerning the basic geometry and principal mfg. process for the part.

Digits 4,5 and 6 specify secondary mfg. processes such as threads, grooves, slot and so forth.

Digits 7 and 8 are used to indicate the overall size of the part (e.g. diameter

and length for turned part.)


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Benefits Product design benefits: 10 % reduction in no. of drawings can be expected

through standardization with GT Tooling and setup: GT tends to promote standardization of several areas of

mfg.

Material handling: GT machine layout lend themselves to efficient flow of materials through shop.

Production and inventory control Employee satisfaction Process planning procedures.


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COMPUTER AIDED PROCESS PLANNING

What is process planning?

Process planning is concerned with determining the sequence of individual manufacturing operation needed to produce a given product.

The resulting operation sequence is documented on a form called as operation sheet.

Manufacturing planning, process planning, material processing, process

engineering machine routing are contents under process planning. Activities

Selection of processes and tools Selection of m/c tools and equipments Sequencing the operations

Grouping of operations Selection of work piece holding devices

Selection of inspection instruments Determining the tolerances Determining proper cutting conditions

Determining the machining time and non machining time Editing the process sheets

Need of CAPP

In conventional planning the plan is created by planners who have their own ideas and opinions about best routing.

Thus it has different plans by the different planners for the same product process.

It thus requires experienced planner for efficient planning. It requires lot of time

Introduction to CAPP

In recent years attempts have been made to capture logic, judgment and experience required and incorporated them in computer.

Based on characteristics of the product automatically generates the sequence of manufacturing operations.

The automation provides opportunity to generate production routings which

are rational, consistent and perhaps even optimal. Requirements for CAPP

The input to system may be engineering drawing or CAD database. The other requisites are:- Part list

Annual demand / batch size Accuracy and surface finish requirement

Equipment details Data on cutting fluids, tools, jigs and fixtures, gauges. Standard available stock size

Machining data, data on handling and setup C. A. P. P. Can be classified as :-

1.variant approach 2. Generative approach

VARIANT PROCESS PLANNING


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A variant process planning system uses similarity amongst component to

retrieve the existing process plan. The process plan that can be used by family of components is called as

standard plans. Following are the sequences in the design of variant process planning system

1. Family formation

2. Database structure design. 3. Search algorithm development & implementation.

4. Plan editing. 5. Updating.

1. Family formation

Components requiring similar processes are grouped into same family. Then a standard process plan can be shared by entire family. Minimum modification

is required on standard plan for such family members. 2. Database Structure Design Database structure contains all the necessary information to apply and can be accessed

by several program for specific application. 3. Search Procedure

The search for a process plan is based on search of a part family to which component belongs when the part family is found the associated standard plan can be retrieved. 4. Plan Editing

Before plan is issued some modification is required & some parameters are to be added. There are two types of plan editing

1. Editing the standard plan. 2. Editing the plan for component itself.

5. Updating

Data in the process parameter file are link so that we can go through the tree to find the speed & feed for an operation. The file can be integrated to select the

parameters automatically


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GENERATIVE PROCESS PLANNING SYSTEMS Generative process planning involves the use of the computer to create an

individual process plan from scratch, automatically and without human assistance.

Generative CAPP system synthesizes the design of the optimum process sequence based on an analysis of part geometry material and other factors which would influence manufacturing decisions.

In ideal generative process planning package, any part design could presented to the system for creation of the optimal plan.

Generative CAPP systems are developed for somewhat limited range of manufacturing process.

Benefits of CAPP

Process rationalization Increased productivity of process planners. Reduced turnaround time.


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Improved legibility.

Incorporation of other application programs.