Technology of Machine Tools

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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 Tools

6
th

Edition

Computer Numerical
Control


Unit 76


76
-
2

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 disadvantages
of computer numerical control

76
-
3

Numerical Control


Method of accurately controlling operation
of a machine tool by series of coded
instructions that the machine control unit
(MCU) can understand


Instructions converted into electrical pulses of
current which machine motors and controls
follow


Computer numerical control (CNC)
machines minimize human error

76
-
4

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

76
-
5

Role of a Computer in CNC


Found many uses in overall manufacturing
process


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 machine
tools done with off
-
line computer assistance

3.
Increasing number of machine tools controlled
or supervised by computers that may be in
separate control room (direct numerical
control
-
DNC)

76
-
6

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

76
-
7

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

76
-
8

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

76
-
9

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

76
-
10

CNC Offers Industry Many Advantages

CNC Offers

Accuracy

.0001
-
.0002 in.

Reliability

Repeatability

Productivity

76
-
11

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 error

6.
Maximum part accuracy and interchange

76
-
12

7.
Complex machining operations

8.
Lower tooling costs

9.
Increased productivity

10.
Minimal spare parts inventory

11.
Greater machine tool safety

12.
Fewer worker hours for inspection

13.
Greater machine utilization

14.
Reduced space requirements

76
-
13

Cartesian Coordinates


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


Machine tool construction based on three axes
of 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

76
-
14

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

76
-
15

Coordinate System

+Y

-
Y

-
X

+X

Origin, or

Zero 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)

76
-
16

Three
-
Dimensional Coordinate Planes

-
Z

+Z

+X

-
X

+Y

-
Y

Copyright © The McGraw
-
Hill Companies, Inc.

Permission required for reproduction or display.

76
-
17

Guidelines to Follow When Using the
System of Rectangular Coordinates

1.
Use reference points on part itself

2.
Use Cartesian coordinates


specifying X, Y, and
Z planes


to define all part surfaces

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

4.
Establish allowable tolerances at design stage

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

6.
Dimension part so it is easy to determine shape
without calculations or guessing

76
-
18

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

76
-
19

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 axes
around primary axes


76
-
20


I, J, and K words used for rotary axes when
circular interpolation used for programming
circles or partial arcs


R word represents radius of circle


U and W words for incremental movement
parallel to X and Z primary axes


Chucking and turning centers

76
-
21

Machines Using CNC


Used on all types of machine tools, from
simplest to most complex


Two common: chucking center (lathe) and
machining 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

76
-
22

Copyright © The McGraw
-
Hill Companies, Inc.

Permission required for reproduction or display.

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 movement

left or right

+X

76
-
23

3.
Machining centers


Developed in 1960s


Allow more operations to be done on part in
one setup instead of moving from machine to
machine


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 sway
from column


Z axis controls vertical movement of spindle or
knee

76
-
24

Copyright © The McGraw
-
Hill Companies, Inc.

Permission required for reproduction or display.

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

76
-
25

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

76
-
26

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

76
-
27

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


“minus
X
” (
-
X
) causes cutting tool to be located to left of the
last 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

76
-
28

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

76
-
29

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

76
-
30

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

76
-
31

Point
-
to
-
Point Positioning


Consists of any number of programmed
points joined together by straight lines


Used to accurately locate spindle, or
workpiece mounted on machine table to
perform 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

76
-
32

Rapid Travel


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


Used to quickly position cutting tool
between location points


Rate between 200 and 800 in./min


Both axes (X and Y) move simultaneously


Movement along 45
º angle line until one axis
reached, then straight line movement to other

76
-
33

Continuous
-
Path (Contouring)


Involves work produced on lathe or milling
machine where cutting tool usually in
contact with workpiece as it travels from
one programmed point to next


Ability to control motions on two or more
machine axes simultaneously


Information in CNC program must
accurately position cutting tool and follow
predefined accurate path

76
-
34

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

76
-
35

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 certain
functions


Amount servomotor moves lead screw depends
on number of electric pulses

76
-
36

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)

76
-
37

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

76
-
38

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 in
complete manufacturing program

76
-
39

Four Main Parts of Computer
Numerical Control System

1.
General
-
purpose computer, which gathers and
stores programmed information

2.
Control unit which communicates and directs
flow on information between computer and
machine control unit

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

4.
Machine control unit which contains servo units,
speed and feed controls, and machine operations

76
-
40

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

76
-
41

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 communications
loop


Increases productivity


Makes corrections on first part possible


Practical to produce short
-
run lots (even profitable)

76
-
42

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 or
microcomputer


Can operate each machine individually

76
-
43

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

76
-
44


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

76
-
45

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 sequence
called block of information


One step of operation

76
-
46

Word Address Format


Format used on CNC system determined by
machine 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 numerical
data used to identify specific function or give
distance, feed rate or speed value

76
-
47

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, cutter
diameter offset, tool length compensation,
subroutine call, tool number, etc.


A (angle) and R (radius) used to locate
points on arcs and circles

76
-
48

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 tool
from 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

76
-
49

G
-
Codes


Some classified as modal or nonmodal


Modal codes stay in effect in program until
changed by another code from same group


Nonmodal codes stay in effect for one operation
only and must be programmed again whenever
required


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

76
-
50

Group
G
-
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

XY

plane selection

02

G18

ZX

plane selection

02

G19

YZ

plane selection

06

G20

Inch input (in.)

Commonly Used EIA Preparatory Codes

EIA

274
-
D

Standard

Portion of Figure 76
-
28

from textbook

76
-
51

M
-
Codes


Used to turn either on or off different
functions 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

76
-
52

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 change

M07

Mist coolant on

M08

Flood coolant on

M09

Coolant off

Portion of Figure 76
-
29

from textbook

Most Common EIA M
-
codes

76
-
53

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 move
so cannot combine blocks

76
-
54

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 feed
rates

76
-
55

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

76
-
56

Linear Interpolation


Consists of any programmed points joined
together by straight lines


Include horizontal, vertical, or angular lines
where points may be close together or far
apart

76
-
57

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

76
-
58

The circle center position, radius, start point, end
point, and direction of cut are required for
circular interpolation.

Copyright © The McGraw
-
Hill Companies, Inc.

Permission required for reproduction or display.

76
-
59

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

76
-
60

Program Planning


Information gathered, analyzed and
calculated before writing program


Consider capabilities of machine


Capacity


Tooling requirements


Programming format


etc.

76
-
61

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 clamps
interfere with movement of axes?

3.
Are required tools and holders available?

4.
Will special coolant be required, or are
present type and concentration correct?

76
-
62

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?

76
-
63

Tool List


List of all tools required for machining
process


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 tool
length for purpose of offsets


Special gage needed

76
-
64

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

76
-
65

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 identification

7.
Speeds and feeds

8.
Operator instructions