lecture 9 - cnc technology and economics of machining-w15

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February 3rd , 2015

MECH 360

Principles of Manufacturing

Lecture 9: CNC technology

Economics of machining processes

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Outline

1. Introduction

2. NC coordinate and motion control systems

3. NC interpolation methods and positioning system

4. NC programming

5. Tolerance and surface finish

6. Selection of cutting parameters

7. Product design considerations in machining

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Introduction

Numerical Control (NC) Defined

Form of programmable automation in which the

mechanical actions of a machine tool or other

equipment are controlled by a program containing

coded alphanumeric data

The alphanumeric data represent relative positions

between a workhead (e.g., cutting tool) and a

workpart

When the current job is completed, a new program

can be entered for the next job

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Introduction

Basic components of an NC system 1. Program of instructions part program in machining

detailed set of commands to be followed by the

processing equipment

2. Machine control unit (MCU) controls the process

microcomputer that stores and executes the program by

converting each command into actions by the

processing equipment, one command at a time

3. Processing equipment performs the process

accomplishes the sequence of processing steps to

transform the starting workpart into completed part

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NC Coordinate System

Consists of three linear axes (x, y, z) of Cartesian

coordinate system, plus three rotational axes (a, b, c)

Rotational axes are used to orient workpart or workhead

to access different surfaces for machining

Most NC systems do not require all six axes

(a) for flat and prismatic work (b) for rotational work

NC Motion Control Systems

Two types:

1. Point-to-point

Also called position systems

System moves to a location and performs an operation at that

location (e.g., drilling)

Also applicable in

robotics

Drilling of Three Holes in

Flat Plate

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NC Motion Control Systems

Two types:

2. Continuous path

Also called contouring systems in machining

System performs an operation during movement (e.g., milling

and turning)

Profile Milling of Part Outline

NC Interpolation Methods

1. Linear interpolation

Straight line between two points in space

2. Circular interpolation

Circular arc defined by starting point, end point,

center or radius, and direction

3. Helical interpolation

Circular plus linear motion

4. Parabolic and cubic interpolation

Free form curves using higher order equations

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Circular Interpolation

Approximation of a curved

path in NC by a series of

straight line segments, where

tolerance is defined on

inside,

outside, or

inside and outside

of the nominal curve

NC Interpolation Methods

NC Positioning System

Motor and leadscrew arrangement in a numerical control positioning

system

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Absolute and Incremental Positioning

Absolute positioning

Locations defined relative to origin of axis system

Incremental positioning

Locations defined relative to previous position

Example: drilling



Converts the coordinates specified in the NC part

program into relative positions and velocities between

tool and workpiece

Leadscrew pitch p - table is moved a distance equal to the pitch

for each revolution

Table velocity (e.g., feed rate in machining) is set by the RPM

of leadscrew

To provide x-y capability, a single-axis system is

piggybacked on top of a second perpendicular axis

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Two Basic Types of Control in Numerical Control

Open loop system

Operates without verifying that the actual position is equal to

the specified position

Closed loop control system

Uses feedback measurement

to verify that the actual position

is equal to the specified location


Open loop system


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Open loop system



EXAMPLE

Open loop positioning

A stepping motor has 48 step angles. Its output shaft is coupled to a

leadscrew with a 4:1 gear reduction (four turns of the motor shaft

for each turn of the leadscrew). The leadscrew pitch = 5.0mm.

The worktable of a positioning system is driven by the

leadscrew. The table must move a distance of 75.0mm from its

current position at a travel speed of 400mm/min. Determine: a)

how many pulses are required to move the table the specified

distance, b) motor speed, and c) pulse frequency required to

achieve the desired table speed.

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Operation of an Optical Encoder in closed loop

control system


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Precision in NC Positioning

Three measures of precision:

1. Control resolution - distance separating two adjacent

addressable points in the axis movement

2. Accuracy - maximum possible error that can occur

between the desired target point and the actual

position taken by the system

3. Repeatability - defined as 3 of the mechanical error distribution associated with the axis


Control Resolution (CR)

Defined as the distance between two adjacent control

points in the axis movement

Control points are locations along the axis to which the

worktable can be directed to go

CR depends on:

Electromechanical components of positioning system

Number of bits used by controller to define axis coordinate

location


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Control resolution in a open loop system

Control resolution of the electromechanical

components:

Control resolution of the computer control system:

The control resolution of the positioning system is

maximum of the two values:

Its generally desirable for

Control Resolution (CR)

gsrn

pCR 1

122

B

LCR

21,CRCRMaxCR

12 CRCR


Accuracy of NC positioning system When a positioning system is directed to move to a

given control point, the movement to that point is

limited by mechanical errors

Maximum possible error that can occur between desired

target point and actual position taken by system

For one axis:

Accuracy = 0.5 CR + 3

where CR = control resolution; and = standard deviation of the error distribution


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Repeatability Capability of a positioning system to return to a given

control point that has been previously programmed

Repeatability of any given axis of a positioning system

can be defined as the range of mechanical errors

associated with the axis

Repeatability = 3


CNC Part Programming Techniques

1. Manual part programming

2. Computer-assisted part programming

3. CAD/CAM-assisted part programming

4. Manual data input

Common features:

Points, lines, and surfaces of workpart must be defined

relative to NC axis system

Movement of cutting tool must be defined relative to

these part features

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Manual Part Programming

Uses basic numerical data and special alphanumeric

codes to define the steps in the process

Suited to simple point-to-point machining jobs, such as

drilling operations

Example command for drilling operation:

n010 x70.0 y85.5 f175 s500

Complete part program consists of a sequence of

commands


Computer-Assisted Part Programming

Uses high-level programming language

Suited to programming of more complex parts

First NC part programming language was APT = Automatically Programmed Tooling

In APT, part programming is divided into two basic steps:

1. Definition of part geometry

2. Specification of tool path and operation sequence


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APT Geometry Statements

Part programmer defines geometry of workpart by constructing it of basic geometric elements such as points, lines, planes, and circles

Examples:

P1 = POINT/25.0, 150.0

L1 = LINE/P1, P2

Similar statements are used to define circles, cylinders, and other geometry elements


APT Motion Statements: Point-To-Point

Specification of tool path accomplished with APT

motion statements

Example for point-to-point operation:

GOTO/P1

Moves from current location to location P1

P1 has been defined by a previous APT geometry

statement


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APT Motion Statements: Continuous Path

Previously defined geometry elements such as lines,

circles, and planes are used to direct tool

Example for contouring operation:

GORGT/L3, PAST, L4

Directs tool to go right (GORGT) along line L3 until

it is positioned just past line L4

L4 must be a line that intersects L3


CAD/CAM-Assisted Part Programming

In computer-assisted part programming, the part

program is entered into the computer for processing

Programming errors may not be detected until computer

processing

Using CAD/CAM, programmer receives visual

verification as each statement is entered

If part design is prepared on CAD, then the CAD

model can be used in part programming, so that

geometric definition is unnecessary


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Tolerances and surface finish

Tolerances

Machining provides high accuracy relative to most

other shape-making processes

Closer tolerances usually mean higher costs

Surface roughness in machining is determined by:

1. Geometric factors of the operation

2. Work material factors

3. Vibration and machine tool factors

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Geometric factors

Machining parameters that determine surface

geometry:

Type of machining operation, e.g., milling vs. turning

Tool geometry, especially nose radius

Feed

Surface geometry that results from only these factors

is the "ideal" or "theoretical" surface roughness

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Geometric factors

Effect of (a) nose radius, (b) feed, and (c) ECEA

Tool geometry and tool-path combine to form the surface

geometry

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Ideal surface roughness

where Ri = theoretical arithmetic average surface

roughness;

f = feed;

NR = nose radius

NRfR

i 32

2

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Work material factors

Built-up edge effects

Damage to surface caused by chip

Tearing of surface when machining

ductile materials

Cracks in surface when machining

brittle materials

Friction between tool flank and new work surface

(commonly due to tool flank wear)

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Effect of work material factors

iaia RrR

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EXAMPLE

Actual and ideal surface roughness

A turning operation is performed on C1008 steel (a ductile steel)

using a tool with a nose radius = 1.2mm. Cutting speed = 100

m/min and feed 0.25 mm/rev.

Compute an estimate of the surface roughness in this operation.

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Solution:

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Vibration and machine tool factors

Related to machine tool, tooling, and setup:

Chatter (vibration) in machine tool or cutting tool

Deflections of fixtures

Backlash in feed mechanism

If chatter can be eliminated, then surface roughness

is determined by geometric and work material

factors

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How to avoid chatter

Add stiffness and/or damping to setup

Operate at speeds that avoid cyclical forces with

frequencies close to natural frequency of machine tool

system

Reduce feeds and depths to reduce forces

Change cutter design to reduce forces

Use a cutting fluid

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Selection of cutting parameters

One of the tasks in process planning

For each operation, decisions must be made about

machine tool, cutting tool(s), and cutting conditions

Cutting conditions: depth of cut, feed, speed, and

cutting fluid

These decisions must give due consideration to

workpiece machinability, part geometry, surface

finish, and so forth

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Selecting depth of cut

Depth of cut is often predetermined by workpiece

geometry and operation sequence

In roughing, depth is made as large as possible to

maximize material removal rate, subject to limitations of

horsepower, machine tool and setup rigidity, and strength

of cutting tool

In finishing, depth is set to achieve final part dimensions

and surface finish

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Determining feed

Select feed first, speed second

Determining feed rate depends on:

Tooling harder tool materials require lower feeds

Roughing or finishing?

In roughing high feeds, limits on feed are imposed by

forces, setup rigidity, and maybe horsepower maximize

MRR

In finishing low feeds, select feed to achieve desired

finish

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Optimizing cutting speed

Select cutting speed to achieve a balance between

high metal removal rate and suitably long tool life

Mathematical formulas available to determine

optimal speed

Two alternative objectives in these formulas:

Maximum production rate

Minimum unit cost

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Optimizing cutting speed for maximum production rate

Maximizing production rate is equivalent to minimizing

cutting time per unit

In turning, total production cycle time for one part

consists of:

1. Part handling time per part = Th

2. Machining time per part = Tm

3. Tool change time per part = Tt/np,

where np = number of pieces cut in one tool life

Total time per unit product for operation:

Tc = Th + Tm + Tt/np

Cycle time Tc is a function of cutting speed V

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Optimizing cutting speed for maximum production rate

where n and C are the parameters in Taylor tool life equation

Cycle Time vs. Cutting Speed

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Optimizing cutting speed for minimum unit cost

In turning, total production cycle cost for one part

consists of:

1. Cost of part handling time = CoTh ,

where Co = cost rate for operator and machine

1. Cost of machining time = CoTm

2. Cost of tool change time = CoTt/np

3. Tooling cost = Ct/np , where Ct = cost per cutting edge

Total cost per unit product for operation:

Cc = CoTh + CoTm + CoTt/np + Ct/np

Again, unit cost is a function of cutting speed, just as Tc

is a function of V

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Optimizing cutting speed for minimum cost per unit

where n and C are the parameters in Taylor tool life equation

Unit Cost vs. Cutting Speed

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EXAMPLE

Determining cutting speeds in machining economics

A turning operation is to be performed with HSS tooling on mile steel

with Taylor tool life parameters n=0.125, C=70m/min. Workpiece

length of 500mm and diameter of 100mm. Feed is 0.25 mm/rev.

Handling time per piece is 5.0min and tool change time is 2.0

min. Cost of machine and operator is $30/hr and tooling cost is $3

per cutting edge.

Calculate cutting speed for maximum production rate and for

minimum cost, respectively.

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Solution:

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EXAMPLE (continued)

Production rate and cost in machining economics

Determine the hourly production rate and cost per piece for the two

cutting speeds computed in the previous example.

Solution:

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Solution:

Cc

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Comments on machining economics

As C and n increase in Taylor tool life equation,

optimum cutting speed increases

The harder the cutting tool is, the higher cutting speed

can be applied.

Vmax is always greater than Vmin

As tool change time Tt and/or tooling cost Ct

increase, cutting speed should be reduced

Tools should not be changed too often if either tool cost

or tool change time is high

Disposable inserts have an advantage over regrindable

tools because tool change time is lower

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Product Design Guidelines for

Machining

Reasons why machining may be required:

Close tolerances

Good surface finish

Special geometric features:

Threads

Accurate holes

Accurate cylindrical sections

Flat and/or straight surfaces

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Tolerances should be specified to satisfy functional

requirements, but process capabilities should also be

considered

Very close tolerances add cost but may not add value to part

As tolerances become tighter, costs generally increase due to

additional processing, fixturing, inspection, sortation, rework,

and scrap


Machining

Surface finish should be specified to meet functional

and/or aesthetic requirements

However, better surface finish generally increases processing

cost by requiring additional operations such as grinding or

lapping

Machined features such as sharp corners, edges, and

points should be avoided

They are difficult to machine

Sharp internal corners require pointed cutting tools that tend to

break during machining

Sharp corners and edges tend to create burrs and are dangerous

to handle


Machining

Machined parts should be designed so they can be

produced from standard stock sizes

Example: rotational parts with outside diameters equal

to standard bar stock diameter

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Avoid undercuts as in (a)

Additional setups, operations, and

often special tooling are required

for undercuts as in (b)

Design parts with features that can

be produced in a minimum

number of setups

Machined parts should be

designed with features that can

be achieved with standard

cutting tools


Machining

Applications of

Computer Numerical Control

Operating principle of NC applies to many processes

Many industrial operations require the position of a

workhead to be controlled relative to the part or product

being processed

Two categories of NC applications:

1. Machine tool applications

2. Non-machine tool applications

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Computer Numerical Control (CNC)

Additional Features

Storage of more than one part program

Various forms of program input

Program editing at the machine tool

Fixed cycles and programming subroutines

Interpolation

Acceleration and deceleration computations

Communications interface

Diagnostics

Configuration of

CNC Machine Control Unit

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DNC

Direct numerical control (DNC) control of multiple

machine tools by a single (mainframe) computer

through direct connection and in real time

1960s technology

Two way communication

Distributed numerical control (DNC) network

consisting of central computer connected to machine

tool MCUs, which are CNC

Present technology

Two way communication

General Configuration of a

Direct Numerical Control System

Connection to MCU is behind the tape reader (BTR). In distributed NC, entire

programs are downloaded to each MCU, which is CNC rather than

conventional NC

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Distributed Numerical Control

Configurations

Local area network (LAN)

lecture 9 - cnc technology and economics of machining-w15

Documents

nc systems

nc coordinate system

nc programming

machining system

position systems system

helical interpolation

processing equipment

linear motion