manufacturing technology-ii unit 5

Unit 5 - CNC MACHINE TOOLS AND PART PROGRAMMING

G.Ravisankar, Asst Prof , Mechanical, Sri Eshwar college of Engineering , Coimbatore .

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UNIT V - CNC MACHINE TOOLS AND PART PROGRAMMING

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Numerical Control (NC) machine tools – CNC types,constructional details, special features, machiningcentre, part programming fundamentals CNC – manualpart programming – micromachining – wafermachining

1. Hajra Choudhury, "Elements of Workshop Technology", Vol.II., Media Promoters

2. Rao. P.N “Manufacturing Technology - Metal Cutting and Machine Tools", Tata McGraw-Hill, New Delhi, 2003.

An Introduction to -Computer Numerical Control

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Introduction

• CNC: Computer Numerical Control

• Production of machined parts whose production is controlled by a computer.

• Computer uses a controller to drive each axis of the machine tool. (X,Y,Z)

• Controls direction, speed, and length of time motor rotates.

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Introduction

• A programmed path is loaded into the computer and then executed.

• Program consists of numeric point data (X,Y,Z), along with machine control and function commands.

• Numerical Control (NC) & Computer Numerical Control (CNC) mean the same.

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Introduction

• A major manufacturing development in past 60 years.

• Resulted in:▫ new manufacturing techniques

▫ higher production levels

▫ higher quality

▫ stabilization of manufacturing costs

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Evolution of CNC

• Single items produced by crafts people

• Interchangeable Parts ▫ Eli Whitney (Cotton Gin)

▫ Manual labor was still the most cost effective method.

• WW II manufacturers could not maintain quantity & quality parts.

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Evolution of CNC

• Machinists could produce superior quality but not at high volume that was required.

• As quantity increased, quality decreased due to human factors

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Evolution of CNC

• ENIAC – developed by the United States Army Ballistic Research Lab & University of Pennsylvania

• First digital computer.• Vacuum tube technology. (30,000)• Used to calculate artillery tables.• Programming involved setting hundreds of

switches and cables.

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ENIAC Electronic Numerical Integrator And Computer

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ENIAC

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CNC & WW II

• Need to manufacture large amount of products for the war.

• Need for quantity and quality.• U.S. Air Force set up companies to develop and

produce NC systems to handle volume and repeatability.

• Repeatability: the ability to perform the same operation over & over within specified parameters.

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Specific Goals

• Increase production

• Improve quality & accuracy of machined parts.

• Stabilize manufacturing costs.

• Speed up production & assembly operations.

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NC Timeline

• 1949 - First contract awarded for NC machine.

• 1951 - servo system for machines developed.

• 1952 - tape-fed machine was created.

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History

• Development of G codes - Punch tape input (Cartesian Coordinate System)

• 1970’s Development of computer chips▫ Cheaper processing power

▫ Smaller computers

▫ More reliable

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Paper Tape Control

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Paper Tape Control

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• Strip of paper tape with holes in it.

• Machine read pattern of holes and performed the required operation.

Paper Tape Control

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Paper Tape Control

• Disadvantages▫ Difficult to identify parts of program.

▫ Programs could be quite large.

▫ Stored on large bulky reels.

▫ Fragile, could rip easily.

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CNC

• Further developments in the computer allowed it to be used to control the machine instead of the paper tape.

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Definitions• NC - A method of accurately controlling the

operation of a machine tool by a series of coded instructions, consisting of numbers, letters of the alphabet, and symbols that the machine control unit can understand

• MCU - Machine Control Unit - decodes NC codes to drive and monitor servo motor movements.

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Definition• CNC - Computer Numeric Control - computer

provides machine codes to the MCU.

• Control Systems▫ Open loop system - servo motor driven by pulses

without feed back encoders.

▫ Closed loop system - servo motor is driven by electrical pulses. An encoder provides feedback to verify machine movements.

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History of CNC

1949US Air Force asks MIT to develop a "numerically controlled" machine.

1952Prototype NC machine demonstrated (punched tape input)

1980-CNC machines (computer used to link directly to controller)

1990-DNC: external computer “drip feeds” control programmer to machine tool controller

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CNC Advantages vs. NC

• Programs could be stored in computer memory.• Easier to edit.• More complex parts could be manufactured.• Use of 3d geometry.• Networking/file sharing / other computers.

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Advantages of CNC

• Increased productivity after programming is completed

• Reliability - reduces human error• Often eliminates need for special jigs and

fixtures• Reduces location of part features• Makes possible the machining of complex

shapes requiring simultaneous 3 axis motion

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Advantages

• Single part and production runs can be programmed and machined with minimum effort and cost.

• Programs can readily be altered and re-run• Reduced inspection costs (more reliable)• Once programming, setup and verified the

equipment can be operated by a less skilled operator.

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Disadvantages

• Initial cost of CNC machine tools

• Servicing of equipment

• Larger machines require more space

• Personnel must be trained in the programming and operation of this equipment.

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Conventional milling machines

Vertical milling machine

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Vertical Milling machine architecture


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Horizontal Milling machine architecture


How does the table move along X- Y- and Z- axes ?

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NC machines

Motion control is done by: servo-controlled motors

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Servo Controller

Counter Comparator

Encoder A/C Motor

Input (converted from analog to digital value)

TableLeadscrew

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NC SYSTEM ELEMENTS

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CNC SYSTEM ELEMENTS

A typical CNC system consists of the following six elements

• Part program • Program input device • Machine control unit • Drive system • Machine tool • Feedback system

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OPERATIONAL FEATURES of CNC MACHINES

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CNC terminology

BLU: basic length unit smallest programmable move of each axis.

Controller: (Machine Control Unit, MCU) Electronic and computerized interface between operator and m/c

Controller components:

1. Data Processing Unit (DPU)

2. Control-Loops Unit (CLU)

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Controller components

Data Processing Unit:

Input device [RS-232 port/ Tape Reader/ Punched Tape Reader]

Data Reading Circuits and Parity Checking Circuits

Decoders to distribute data to the axes controllers.

Control Loops Unit:

Interpolator to supply machine-motion commands between

data points

Position control loop hardware for each axis of motion

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SAMPLECNC MACHINES

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CNC TURNING

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CNC MILLING

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CNC LASER CUTTING

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CNC PLASMA CUTTING

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CNC PRESS

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CNC RAPID PROTOTYPING

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Industrıes Most Benefited by CNC

• Aerospace

• Machinery

• Electrical

• Fabrication

• Automotive

• Instrumentation

• Mold making

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SAMPLE PRODUCTSOF

CNC MANUFACTURING

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AUTOMOTIVE INDUSTRY

Engine Block Different Products

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AEROSPACE INDUSTRY

Aircraft Turbine Machined by 5-Axis CNC Milling Machine

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CNC MOLD MAKING

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ELECTRONIC INDUSTRY

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RAPID PROTOTYPING PRODUCTS

Types of CNC machines

Based on Motion Type:Point-to-Point or Continuous path

Based on Control Loops:

Open loop or Closed loop

Based on Power Supply:

Electric or Hydraulic or Pneumatic

Based on Positioning System

Incremental or Absolute

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Open Loop vs. Closed Loop controls

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Open loop control of a Point-to-Point NC drilling machine

NOTE: this machine uses stepper motor control

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Components of Servo-motor controlled CNC

Motor speed control

Two types of encoder configurations

Motor lead screw rotation table moves

position sensed by encoderfeedback

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Motion Control and feedback

Encoder outputs: electrical pulses (e.g. 500 pulses per revolution)

Rotation of the motor linear motion of the table: by the leadscrew

The pitch of the leadscrew: horizontal distance between successive threads

One thread in a screw single start screw: Dist moved in 1 rev = pitch

Two threads in screw double start screw: Dist moved in 1 rev = 2* pitch

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Guide Ways and Slide Ways

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Slide Ways

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Slide Ways

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Slide Ways

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Slide Ways

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Slide Ways

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Tool holding and work holding devices

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ATC

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ATC

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Manual NC programming

Part program: A computer program to specify

- Which tool should be loaded on the machine spindle;

- What are the cutting conditions (speed, feed, coolant

ON/OFF etc)

- The start point and end point of a motion segment

- how to move the tool with respect to the machine.

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Part program

The RS274-D is a word address format

Each line of program == 1 block

Each block is composed of several instructions, or (words)

Sequence and format of words:

N3 G2 X+1.4 Y+1.4 Z+1.4 I1.4 J1.4 K1.4 F3.2 S4 T4 M2

sequence no

preparatory function

destination coordinates dist to center of circle

feed rate spindle speed

tool

miscellaneous function

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Manual Part Programming Example

Tool size = 0.25 inch,Feed rate = 6 inch per minute,Cutting speed = 300 rpm,Tool start position: 2.0, 2.0Programming in inches

(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

Motion of tool:p0 p1 p2 p3 p4 p5 p1 p0

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Spindle CCW

(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

1. Set up the programming parameters

N010 G70 G90 G94 G97 M04

Programming in inches

Use absolute coordinates

Spindle speed in rpm

Feed in ipm

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Flood coolant ON

(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

2. Set up the machining conditions

N020 G17 G75 F6.0 S300 T1001 M08

Machine moves in XY-plane

Feed rate

Tool no.

Spindle speed

Use full-circle interpolation

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(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

3. Move tool from p0 to p1 in straight line

N030 G01 X3.875 Y3.698

Linear interpolation

target coordinates

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(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

4. Cut profile from p1 to p2

N040 G01 X3.875 Y9.125


target coordinates

N040 G01 Y9.125

X-coordinate does not change no need to program it

or

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(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

5. Cut profile from p2 to p3

N050 G01 X5.634 Y9.125


target coordinates

1”

p3

.125

(x, y)

(6.5, 9)

y = 9 + 0.125 = 9.125

(6.5 - x)2 + 0.1252 = (1 - 0.125)2

x = 5.634

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coordinates of center of circle(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

6. Cut along circle from p3 to p4

N060 G03 X7.366 Y9.125 I6.5 J9.0

circular interpolation, CCW motion

target coordinates

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(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

7. Cut from p4 to p5

N070 G01 X9.302

target coordinates (Y is unchanged)


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(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

8. Cut from p5 to p1

N080 G01 X3.875 Y3.698

target coordinates (see step 3)


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(4, 4)

(2, 2)

5”

p0

p1

p2

5”

2.5”

1”

45°

p3p4

p5

9. Return to home position, stop program

N090 G01 X2.0 Y2.0 M30

end of data

target coordinates (see step 3)


N100 M00

program stop

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PART PROGRAMMING • Part program is a sequence of instructions, which

describe the work, which has to be done on a part, in theform required by a computer under the control of anumerical control computer program

• Programming is where all the machining data arecompiled and where the data are translated into alanguage which can be understood by the control systemof the machine tool.

The machining data is as follows :(a) Machining sequence classification of process, tool start up

point, cutting depth, tool path, etc.

(b) Cutting conditions, spindle speed, feed rate, coolant, etc.

(c) Selection of cutting tools.

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PART PROGRAMMING While preparing a part program, need to perform the

following steps : (a) Determine the startup procedure, which includes the

extraction of dimensional data from part drawings and data regarding surface quality requirements on the machined component.

(b) Select the tool and determine the tool offset.

(c) Set up the zero position for the work piece.

(d) Select the speed and rotation of the spindle.

(e) Set up the tool motions according to the profile required.

(f) Return the cutting tool to the reference point after completion of work.

(g) End the program by stopping the spindle and coolant

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PART PROGRAMMING Methods of part programming can be of two types

depending upon the two techniques as below :(a) Manual part programming,

(b) Computer aided part programming

Manual Part Programming

• The programmer first prepares the programmanuscript in a standard format.

• Manuscripts are typed with a device known as flexowriter, which is also used to type the programinstructions.

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PART PROGRAMMING Computer Aided Part Programming

• Complex-shaped component requires calculations toproduce the component are done by theprogramming software contained in the computer.

• The programmer communicates with this systemthrough the system language, which is based onwords.

• There are various programming languagesdeveloped in the recent past, such as APT(Automatically Programmed Tools), ADAPT,AUTOSPOT, COMPAT-II, 2CL, ROMANCE, SPLIT

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PART PROGRAMMING Computer Aided Part Programming

A translator known as compiler program is used to translate it in a form acceptable to MCU.

The programmer has to do only following things (a) Define the work part geometry.

(b) Defining the repetition work.

(c) Specifying the operation sequence.

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Computer Aided Part Programming

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Terminology

• NC – Numerical Control

• CNC – Computer Numerical Control

• DNC – Direct Numerical Control

• APT – Automatic Programmed Tool

• CAD – Computer Aided Design

• CAM – Computer Aided Manufacturing

• CIM – Computer Integrated Manufacturing

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Direct Numerical Control (DNC)• Direct numerical simultaneously control the operations of a

group of NC machine tools using a shared computer. Programming, editing part programs and downloading part programs to NC machines are main responsibilities of the computers in a NC system.

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G - Code Programming

• G – Code Programming

• Originally called the “Word Address” programming format.

• Processed one line at a time sequentially.

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Common Format of a Block

Sequence #

Preparatory Function

Dimension Words

Feed Rate

Spindle Function

Tool Function

Misc. Function

N50 G90 G01 X1.40Y2.25 F10 S1500 T01 M03

Individual Words

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Word Address 1

• N – Sequence or line number• A tag that identifies the beginning of a block of code.

It is used by operators to locate specific lines of aprogram when entering data or verifying theprogram operation.

• G – Preparatory function• G words specify the mode in which the milling

machine is to move along its programmed axes.

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Word Address 2

• Dimension WordsX – Distance or position in X directionY – Distance or position in Y directionZ – Distance or position in Z direction

• M – Miscellaneous functions• M words specify CNC machine functions not related to

dimensions or axial movements.

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• F – Feed rate (inches per minute or millimetersper minute)

• Rate at which cutting tool moves along an axis.

• S – Spindle speed (rpm – revolutions per minute)• Controls spindle rotation speed.

• T – Tool number• Specifies tool to be selected.

Word Address 3

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• I – Circular cutting reference for x axis

• J – Circular cutting reference for y axis

• K – Circular cutting reference for z axis

Word Address 4

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G Word• G words or codes tell the machine to perform

certain functions. Most G words are modal which means they remain in effect until replaced by another modal G code.

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Common G Codes• G00 – Rapid positioning mode• Tool is moved along the shortest route to

programmed X,Y,Z position. • Usually NOT used for cutting.

• G01 – Linear Interpolation mode• Tool is moved along a straight-line path at

programmed rate of speed.

• G02 – Circular motion clockwise (cw)• G03 – Circular motion counter clockwise

(ccw)

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Common G Codes, con.,

• G17 – XY plane

• G18 – XZ plane

• G19 – YZ plane

• G20 – Inch Mode

• G21 – Metric Mode

• G28 – Return to axis machine Zero (Home)

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G Codes: G90, G91

G90 – Absolute Coordinate ReferenceReferences the next position from an absolute zero point which is set once for the entire program.

G91 – Incremental Coordinate ReferenceReferences the next position from the previous position.

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G Codes: Canned Cycles

• G80 – Cancel canned cycle• G81 – Drilling cycle• G83 – Peck drilling cycle• G84 – Tapping cycle• G85 – Boring cycle• G86 – Boring cycle

• NOTE: A canned cycle stays in effect until cancelled by a G80.

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Canned Cycles: G81

• G81 – Drilling Cycle• Feed to depth, rapid return

Example of program code:• N35 G81 X.500 Y.500 Z-1.000 R.100 F1.50• N36 X1.000 Y1.500• N37 X1.500 Y2.000• N38 G80

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Canned Cycles: G83, G84

• G83 – Peck Drilling Cycle• Feed to an intermediate depth, rapid out, rapid back

to just above previous depth, feed to next depth, rapid out, repeat until reaching full depth.

• G84 – Tapping Cycle• This cycle creates internal threads in an existing

hole. • NOTE: One cannot over-ride the feed rate.

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Canned Cycles: G85, G86

• G85 - Boring Cycle• Feed to depth, feed back out.

• G86 – Boring Cycle• Feed to depth, rapid out.

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G Codes: Cutter Compensation

• G40 – Cancel cutter diameter compensation.

• G41 – Cutter compensation left.

• G42 – Cutter compensation right.

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Table of Important G codesG00 Rapid TransverseG01 Linear InterpolationG02 Circular Interpolation, CWG03 Circular Interpolation, CCWG17 XY Plane, G18 XZ Plane, G19 YZ PlaneG20/G70 Inch unitsG21/G71 Metric UnitsG40 Cutter compensation cancelG41 Cutter compensation leftG42 Cutter compensation rightG43 Tool length compensation (plus)G43 Tool length compensation (plus)G44 Tool length compensation (minus)G49 Tool length compensation cancel

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Table of Important G codesG80 Cancel canned cyclesG81 Drilling cycleG82 Counter boring cycleG83 Deep hole drilling cycleG90 Absolute positioningG91 Incremental positioning

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M Word

• M words tell the machine to perform certain machine related functions, such as: turn spindle on/off, coolant on/off, or stop/end program.

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Common M words

• M00 – Programmed pause• Automatically stops machine until operator pushes a button

to resume program.

• M01 – Optional stop• A stop acted upon by the machine when operator has

signaled this command by pushing a button.

• M02 – End of program• Stops program when all lines of code are completed. Must be

last command in program.

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• M03 – Turn spindle on• In clockwise direction

• M04 – Turn spindle on• In counter clockwise direction

• M05 – Stop spindle• Usually used prior to tool change or at end of program.

• M06 – Tool change• Stops program and calls for a tool change, either

automatically or manually.

Common M words

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• M08 – Turns Accessory 1 on.

• M09 – Turns Accessory 1 off.

• M10 – Turns Accessory 2 on.

• M11 – Turns Accessory 2 off.

• M30 – End of program• Similar to M02 but M30 will also “rewind” the program. Must

be last statement in program. If used, DO NOT use M02.

Common M words

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Zero Points

• Part Zero▫ Used for absolute programming mode.▫ Usually a position on the part that all absolute

coordinates are referenced to. ▫ Changes with different parts and programs.

• Machine Zero or Machine Home Position ▫ Fixed for each machine from the manufacturer.▫ Not changeable.

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Cutter Path Generation

• Cutter path is generated by moving the tool from point to point. The points are previously defined from the part drawing dimensions.

• Each line of code will show the destination point of where the tool will go to.

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Interpolation

• Method of determining intermediate points along a cutting path.

• Two methods:• Linear interpolation – cut a path along a specified

angle at a specified feed rate.

• Circular interpolation – cut a path along an arc or circle at a specified feed rate.

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Interpolation

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Absolute System

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Incremental System

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TYPES OF CNC MACHINES In every aspects of manufacturing CNC machines are

used. It can be mainly classified in eight classes. ▫ Mills and Machining centers

▫ Lathes and Turning centers

▫ EDM Machines

▫ Grinding machines

▫ Cutting Machines

▫ Fabrication Machines

▫ Welding Machines

▫ Coordinate Measuring Machines

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CNC• CNC – Turning Center • CNC – Machining Center

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CNC – Turning Center

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CNC – Machining Center • It is a machine tool capable of multiple machining

operations on a work part in one setup under NC program control.

Classification

• Machining centres are classified as vertical, horizontal, or universal.

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CNC – Machining Center

• Vertical MC • Horizontal MC

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• Horizontal Boring Mill

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5 Axis - Vertical Axis Machining Center

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• Reference points and axis on a Milling Machine

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Automatic Part Programming

Software programs can automatic generation of CNC data

Make 3D model

Define Tool

CNC data

Simulatecutting

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Automatic part programming and DNC

Very complex part shapes very large NC program

NC controller memory may not handle HUGE part program

computer feeds few blocks ofNC program to controller

When almost all blocks executed,controller requests more blocks

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Summary

CNC machines allow precise and repeatable control in machining

CNC lathes, Milling machines, etc. are all controlled by NC programs

NC programs can be generated manually, automatically

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