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