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Copyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.

PowerPoint to accompany

Krar Gill Smid

Technology of Machine Tools6th Edition

Computer Numerical

Control

Unit 76


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Objectives

Identify types of systems and controls

used in computer numerical control

List steps required to produce a part by

computer numerical control

Discuss advantages and disadvantagesof computer numerical control


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Numerical Control

Method of accurately controlling operationof a machine tool by series of coded

instructions that the machine control unit(MCU) can understand

Instructions converted into electrical pulses ofcurrent which machine motors and controls

follow

Computer numerical control (CNC)machines minimize human error


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Theory of CNC

Enable industry to consistently produce

parts to accuracies undreamed of a few years

ago

Same part can be reproduced to same degree

of accuracy any number of times with

amazing speed Computer properly programmed

Machine properly set up


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Role of a Computer in CNC

Found many uses in overall manufacturingprocess

Fill three major roles in CNC:1. Almost all machine control units include or

incorporate computer in operation

2. Most of part programming for CNC machinetools done with off-line computer assistance

3. Increasing number of machine tools controlledor supervised by computers that may be inseparate control room (direct numericalcontrol-DNC)


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Two Types of Computers

Analog

Used primarily in scientific research and

problem solving

Replaced in most cases by digital computers

Digital

Accepts input of digital information in

numerical form, processes it and develops

output data


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Three Categories of Computers

and Computer Systems Mainframe

Can be used to do more than one job at a time

Large with huge capacity of storage Company's main computer

Minicomputer Smaller in size and capacity

Dedicated type so performs specific tasks Microcomputer

One chip contains arithmetic-logic and control-logic functions of the central processing unit


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Computer Functions

To receive coded instructions (input data) in

numerical form

Process information

Produce output data that causes machine

tool to function

Most common method to input data is

directly through computer


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CNC Performance

Great advances since NC introduced in mid

1950s

Early machines capable only of point-to-

point positioning and very costly

Cost has continually lowered

Within financial reach of small manufacturing

shops and educational institutions


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CNC Offers Industry Many Advantages

CNC Offers

Accuracy.0001-.0002 in.

Reliability

Repeatability

Productivity


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Advantages of CNC

1. Greater operator safety

2. Greater operator efficiency

3. Reduction of scrap

4. Reduced lead time for production

5. Fewer chances for human error6. Maximum part accuracy and interchange


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7. Complex machining operations

8. Lower tooling costs

9. Increased productivity

10. Minimal spare parts inventory11. Greater machine tool safety

12. Fewer worker hours for inspection

13. Greater machine utilization

14. Reduced space requirements


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Cartesian Coordinates

Allows any specific point on job to bedescribed in mathematical terms in relation toany other point along three perpendicular axes

Machine tool construction based on three axesof motion (X, Y, Z) plus axis of rotation

Example: Vertical milling machine

X axis is horizontal movement (right or left) of table

Y axis is table cross movement (to/away from column)

Z axis is vertical movement of knee or spindle


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Three-Dimensional

Coordinate Planes

X and Y planes are horizontal and represent

horizontal machine table motions

Z plane represents vertical tool motion

Plus and minus signs indicate direction of

movement from zero point along axis Four quadrants formed when X-Y axes cross

are numbered in counterclockwise direction


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Coordinate System

+Y

-Y

-X +X

Origin, orZero Point

X Axis

Two intersecting lines

that form right angles

Quadrant I

(+X, +Y)

Quadrant II

(-X, +Y)

Quadrant III(-X, -Y)

Quadrant I(+X, -Y)


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Three-Dimensional Coordinate Planes

-Z

+Z

+X

-X

+Y

-Y

Copyright The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.


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Guidelines to Follow When Using the

System of Rectangular Coordinates1. Use reference points on part itself

2. Use Cartesian coordinates specifying X, Y, andZ planes to define all part surfaces

3. Establish reference planes along part surfacesthat are parallel to machine axes

4. Establish allowable tolerances at design stage

5. Describe part so that cutter path may be easilydetermined and programmed

6. Dimension part so it is easy to determine shapewithout calculations or guessing


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Machine Axes

Every CNC machine tool has sliding and

rotary controllable axes

Letters (addresses) used to identify each

direction of table or spindle movement

Combined with number to form word

establishes distance axis moves


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Electronics Industries

Association (EIA) Standard

Longest horizontal axis movement is X axis,

Y axis assigned to perpendicular to both X

and Z axes

Secondary axes parallel to X, Y, Z axes

A, B, and C refer to rotary motion axesaround primary axes


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I, J, and K words used for rotary axes when

circular interpolation used for programmingcircles or partial arcs

R word represents radius of circle

U and W words for incremental movementparallel to X and Z primary axes

Chucking and turning centers


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Machines Using CNC

Used on all types of machine tools, fromsimplest to most complex

Two common: chucking center (lathe) andmachining center (milling machine)

1. Chucking centers

Developed in mid-1960s

Operates on two axes

X axis control cross motion of turret head

Z axis control lengthwise travel of turret head


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2. Engine lathe (two axes)

X axis controls cross motion of cutting tool Z axis controls carriage travel toward/away

from headstock

X axis controls table movementleft or right

+X

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3. Machining centers

Developed in 1960s

Allow more operations to be done on part inone setup instead of moving from machine tomachine

Two main types of machining centers Horizontal

Vertical spindle (three axis)

X axis controls table movement left or right

Y axis controls table movement toward or swayfrom column

Z axis controls vertical movement of spindle orknee

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X axis controls table

movement left or right

4. Milling machine (three axis)

Performs operations such as milling, drilling,gear cutting,

contouring

Y axis control table

movement toward or

away from column

Z axis controls

vertical movement

of knee or spindle

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Programming Systems

Two types of programming modes

Incremental system

Absolute system

Most controls on machine tools capable of

handling both by altering code between G90

(absolute) and G91 (incremental) commands

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Incremental System

Program dimensions or positions given from

current point

Disadvantage

If error made in any location, error

automatically carried over to all following

locations

G91 command tells computer and MCU to

be in incremental mode

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Command codes tell machine to move table,

spindle, and knee on vertical milling machine

plus X (+X) causes cutting tool to be located to right of the

last point

minusX

(-X

) causes cutting tool to be located to left of thelast point

plus Y (+Y) causes cutting tool to be located toward column

minus Y (-Y) causes cutting tool to be located away from

column plus Z (+Z) causes cutting tool or spindle to move up or

away from workpiece

minus Z (-Z) moves cutting tool down or into workpiece

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Absolute System

All dimensions or positions given from one

reference point on job or machine

All dimensions given from zero or reference

point

Errors not carried to any other location

G90 command indicates to computer and

MCU that program is to be in absolute mode

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Absolute System Commands

plus X (+X)

causes cutting tool to be located to right of zero point

minus X (-X)

causes cutting tool to be located to left of zero point

plus Y (+Y)

causes cutting tool to be located toward column (above zero)

minus Y (-Y)

causes cutting tool to be located away from column (below zero)

plus Z (+Z)

causes cutting tool to move above program Z0 (top surface of part)

minus Z (-Z)

causes cutting tool to move below the program Z0

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CNC Positioning Systems

Two distinct categories

Point-to-point

Continuous-path

Both can be handled by most control units

Knowledge of both programming methods

necessary to understand what application

each has in CNC

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Point-to-Point Positioning

Consists of any number of programmedpoints joined together by straight lines

Used to accurately locate spindle, orworkpiece mounted on machine table toperform operations

Process of positioning from one coordinate(X-Y) position or location to another,perform the operation, clear tool from work,and move to next location

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Rapid Travel

Point-to-point machining moves from onepoint to another as fast as possible (rapids)while cutting tool above work surface

Used to quickly position cutting toolbetween location points

Rate between 200 and 800 in./min

Both axes (X and Y) move simultaneously

Movement along 45 angle line until one axisreached, then straight line movement to other

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Continuous-Path (Contouring)

Involves work produced on lathe or millingmachine where cutting tool usually in

contact with workpiece as it travels fromone programmed point to next

Ability to control motions on two or moremachine axes simultaneously

Information in CNC program mustaccurately position cutting tool and followpredefined accurate path

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Control Systems

Two main types of control systems

Open loop

Closed loop

Most machine tools manufactured contain

closed loop system

Very accurate and result in better quality work

Open loop systems can still be found on

older NC machines

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Open Loop System

Input data fed into machine control unit

Decoded information sorted until CNC

machining cycle started by operator Program commands converted into electric

pulses Sent to MCU to energize servo control units

which direct servomotors to perform certainfunctions

Amount servomotor moves lead screw dependson number of electric pulses

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Closed Loop System

Similar to open loop system with exception that

feedback unit added to electric circuit

Feedback unit used for absolute position control and/or

velocity feedback

Linear encoder consist of scale mounted to

stationary part of machine

Uses slide mounted to moving part of machine Control unit tells servomotor to adjust until both

signal from control unit and signal from servo unit

equal (one pulse causes .0001 in. movement)

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Input Media

Early media was 1-in. wide, 8-track punched tape

Other types

Magnetic tape, punched cards, magnetic disks, and

manual data input (MDI)

Computer keyboard formatted to American

Standard Code for Information Interchange

(ASCII) standard to input directly to machine

control unit

Microcomputer along with communications software

becoming preferred input method

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Types of Computer Control

Two types of control units

CNC control

Evolved from DNC applications in early 1970s

Generally used to control individual machines

DNC control

Used where six or more CNC machines involved incomplete manufacturing program

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Four Main Parts of Computer

Numerical Control System1. General-purpose computer, which gathers and

stores programmed information

2. Control unit which communicates and directsflow on information between computer and

machine control unit

3. Machine logic, receives information and passes iton to machine control unit

4. Machine control unit which contains servo units,

speed and feed controls, and machine operations

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Computer Numerical Control

Built around powerful minicomputer

Contains large memory capacity

Many features to assist in programming

Microcomputers are now incorporated into

controls

Program stored in computer memory

Main advantage is ability to operate in live mode

Enables program changes at machine so programs can

be tried, corrected, and revised correctly

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Advantages of CNC

Programming More flexible because changes can be made to

program

Can diagnose programs on graphic display screen Can be integrated with DNC systems in complex

manufacturing systems by using communicationsloop

Increases productivity Makes corrections on first part possible

Practical to produce short-run lots (even profitable)

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Direct Numerical Control System

Number of CNC equipped machines

controlled from mainframe computer

Can handle scheduling of work and

download complete program into machine's

memory when new parts required

Equipped with own minicomputer ormicrocomputer

Can operate each machine individually

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Advantages of DNC

Single computer can control many machine

tools at same time

Time saved in eliminating program errors or

revising program

Programming faster, simpler, and more

flexible

Operating costs lower than with NC

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Computer can record any production,

machining, or time data required

Main control unit can be kept in clean

processing room, away from dirty shop

conditions

When three or more machines DNC-

controlled, initial cost lower than for

conventional NC

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Programming Format

Most common type is word address format

Large number of different codes to transfer

program information to machine servos, relays,and micro-switches to carry out machine

movements

Codes then put together in logical sequencecalled block of information

One step of operation

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Word Address Format

Format used on CNC system determined bymachine tool builder

Based on control unit of machine

Uses words

Address character (letter) such as S, X, Y, T, F,or M

Alphabetical character followed by numericaldata used to identify specific function or givedistance, feed rate or speed value

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Codes

Most common CNC programming codes

G-codes: preparatory commands

M-codes: miscellaneous functions

F, S, D, H, P, and T

Used to represent functions: feed, speed, cutterdiameter offset, tool length compensation,

subroutine call, tool number, etc. A (angle) and R (radius) used to locate

points on arcs and circles

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G-Codes

Refer to some action occurring on X, Y,and/or Z axis of machine tool

Grouped into categories with group number

G00 used to rapidly position cutting toolfrom one point to another point

G01, G02, and G03

Move axes at controlled feed rate G01 used for linear interpolation

G02 (clockwise) and G03 (counterclockwise)used for circular interpolation

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G-Codes

Some classified as modal or nonmodal

Modal codes stay in effect in program until

changed by another code from same groupNonmodal codes stay in effect for one operation

only and must be programmed again wheneverrequired

Many of the common G-codes that conformto EIA standards shown on next slide and intext in Fig. 76-28

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GroupG-code Function

01 G00 Rapid positioning

01 G01 Linear interpolation

01 G02 Circular interpolation clockwise (CW)

01 G03 Circular interpolation counterclockwise (CCW)00 G04 Dwell

00 G10 Offset value setting

02 G17 XYplane selection

02 G18 ZXplane selection

02 G19 YZplane selection

06 G20 Inch input (in.)

Commonly Used EIA Preparatory Codes

EIA274-D

Standard

Portion of Figure 76-28

from textbook

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M-Codes

Used to turn either on or off differentfunctions that control certain machine tool

operations (not grouped by categories) M03 turns machine spindle clockwise

M04 turns spindle counterclockwise

M05 turns off spindle All three of the codes above are modal

Common M-codes in text in Fig. 76-29

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M-Code Function

M00 Program stop

M01 Optional stop

M02 End of program

M03 Spindle start (forward CW)

M04 Spindle start (reverse CCW)

M05 Spindle stop

M06 Tool changeM07 Mist coolant on

M08 Flood coolant on

M09 Coolant off

Portion of Figure 76-29

from textbook

Most Common EIA M-codes

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Block of Information

Should contain only enough information to

carry out one step of a machining operation

Example:

Tool moves from one point to another, then to

third point which is two moves (two blocks)

Cannot give first point and third point as one moveso cannot combine blocks

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Interpolation

Generation of data points between given

coordinate position of axes

Interpolator (device within MCU)

Causes drives to move simultaneously from

start of command to completion

Always performed under programmed feedrates

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Types of Interpolation

Linear interpolation for straight-line

machining between two points

Circular interpolation for circles and arcs

Helical interpolation for threads and helical

forms

Parabolic and cubic interpolation used by

industries that manufacture parts having

complex shapes

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Linear Interpolation

Consists of any programmed points joined

together by straight lines

Include horizontal, vertical, or angular lineswhere points may be close together or far

apart

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Circular Interpolation

Make process of programming arc and

circles easy

Basic information required to program circle Position of circle center

Start and end points of arc being cut

Direction of cut Feed rate for tool

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The circle center position, radius, start point, end

point, and direction of cut are required for

circular interpolation.



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Methods Used to

Write Block for Arc

One method uses I and J command to

identify coordinates of center of arc

Simpler method uses R (radius of arc)

command, which MCU uses to calculate

arc center

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Program Planning

Information gathered, analyzed and

calculated before writing program

Consider capabilities of machine

Capacity

Tooling requirements

Programming format

etc.

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Questions Programmer Needs to Ask

for Successfully Programming a Part

1. What are proper cutting speeds and feeds

for type of material being machined?

2. How will part be held? Will clampsinterfere with movement of axes?

3. Are required tools and holders available?

4. Will special coolant be required, or are

present type and concentration correct?

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5. What is the table feed direction?

6. How fast can tool be moved to location:rapid traverse or at feed rate?

7. What will tool do when it reaches its

location for example, drill hole or mill

pocket?

8. Where will the part zero point, or origin,

be located, on part of the machine?

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Tool List

List of all tools required for machiningprocess Complete with correct speeds and feeds for

each tool based Tool material type

Type of material being cut

Depth of cut

Some CNC systems require presetting toollength for purpose of offsets Special gage needed

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Manuscript

Programmer records on prepared form all

instructions that machine tool must have to

complete job

Contains all machine tool movements, cutting

tools, speeds, feeds and any other information

Uniform format and clear as possible

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Manuscript Information

1. Part sketch

2. Zero (or reference) point

3. Work-holding device (include setups)4. Sequence of operations

5. Axes dimensions

6. Tool list and identification7. Speeds and feeds

8. Operator instructions

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