1. Computer Numerical Control
Unit 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

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

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

5. Role of a Computer in CNC
Found many uses in overall manufacturing process
Fill three major roles in CNC:
Almost all machine control units include or incorporate computer in operation
Most of part programming for CNC machine tools done with off-line computer assistance
Increasing number of machine tools controlled or supervised by computers that may be in separate control room (direct numerical control-DNC)

6. Two Types of Computers Analog Digital
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

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

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

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

10. CNC Offers Industry Many Advantages
Accuracy
in.
Reliability
Repeatability
Productivity

11. Advantages of CNC Greater operator safety Greater operator efficiency
Reduction of scrap
Reduced lead time for production
Fewer chances for human error
Maximum part accuracy and interchange

12. Complex machining operations
Lower tooling costs
Increased productivity
Minimal spare parts inventory
Greater machine tool safety
Fewer worker hours for inspection
Greater machine utilization
Reduced space requirements

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

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

15. Two intersecting lines that form right angles
Coordinate System +Y
Quadrant II (-X, +Y)
Quadrant I (+X, +Y)
Y Axis -X
Origin, or Zero Point +X
X Axis
Quadrant III (-X, -Y)
Quadrant I (+X, -Y) -Y
Two intersecting lines that form right angles

16. Three-Dimensional Coordinate Planes-Z +Z +X -X +Y -Y
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17. Guidelines to Follow When Using the System of Rectangular Coordinates
Use reference points on part itself
Use Cartesian coordinates – specifying X, Y, and Z planes – to define all part surfaces
Establish reference planes along part surfaces that are parallel to machine axes
Establish allowable tolerances at design stage
Describe part so that cutter path may be easily determined and programmed
Dimension part so it is easy to determine shape without calculations or guessing

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

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

20. R word represents radius of circle
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

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)
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

22. X axis controls table movement left or right
Engine lathe (two axes)
X axis controls cross motion of cutting tool
Z axis controls carriage travel toward/away from headstock +X
X axis controls table movement left or right
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

23. 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

24. Milling machine (three axis)
Performs operations such as milling, drilling, gear cutting, contouring
Z axis controls vertical movement of knee or spindle
X axis controls table movement left or right
Y axis control table movement toward or away from column
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

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

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

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

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

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

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

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

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

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

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 in. movement)

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

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

39. Four Main Parts of Computer Numerical Control System
General-purpose computer, which gathers and stores programmed information
Control unit which communicates and directs flow on information between computer and machine control unit
Machine logic, receives information and passes it on to machine control unit
Machine control unit which contains servo units, speed and feed controls, and machine operations

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

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)

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

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

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

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

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

47. Codes Most common CNC programming codes F, S, D, H, P, and T
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

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

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

50. Commonly Used EIA Preparatory Codes
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.)
EIA—274-D
Standard
Portion of Figure 76-28
from textbook

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

52. Most Common EIA M-codes
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

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

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

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

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

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

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.

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

60. Program Planning
Information gathered, analyzed and calculated before writing program
Consider capabilities of machine
Capacity
Tooling requirements
Programming format
etc.

61. Questions Programmer Needs to Ask for Successfully Programming a Part
What are proper cutting speeds and feeds for type of material being machined?
How will part be held? Will clamps interfere with movement of axes?
Are required tools and holders available?
Will special coolant be required, or are present type and concentration correct?

62. What is the table feed direction?
How fast can tool be moved to location: rapid traverse or at feed rate?
What will tool do when it reaches its location – for example, drill hole or mill pocket?
Where will the part zero point, or origin, be located, on part of the machine?

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

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

65. Manuscript Information
Part sketch
Zero (or reference) point
Work-holding device (include setups)
Sequence of operations
Axes dimensions
Tool list and identification
Speeds and feeds
Operator instructions