Computer Numerical Control (CNC), Machine tools - Computer Essay Example

Computer Numerical Control (CNC), Machine tools

Introduction

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CNC expands to Computer Numerically Controlled. The possibilities of numerically controlled machines were explored in the 1950’s for the US Air Force by the metalworking machine part builders. By the end of the decade they could come out with machines producing complex parts of machines without human interference or inconsistency. A CNC machine has a micro-computer which is programmed to control the machine in terms of its movements and positioning which is more precise and faster than with humans. The basic principles of CNC are mathematics and geometric co-ordinate system. A CNC machine communicates with itself for operation. What makes the CNC machine most outstanding is their ability to move in 3 or more dimensions at once and the co-ordination among the various parts within the machine, all very fast (CNC Defined).

Advantages – The main advantages of CNC Technology are flexibility, accuracy, speed, simplified tools, no skilled workers required since the machine itself is skilled, simple operational skills required and less laborious work for workers (Dr.Karunakaran. 2004) Precision and speed are the most important characteristics of CNC machines. For the same CNC technology are being used in many fields like Machining (2.5D / 3D, Turning – Lathes, Turning Centre, Milling – Machining Centres etc.) and Forming (2D, Plasma and Laser Cutting, Blanking, nibbling and punching, 3D, Rapid Prototyping etc.). The parts made by CNC machines are used in almost all industries like Aerospace, Machinery, Electrical, Fabrication, Automotive, Instrumentation and Mould making, to name a few (CNC technology. 2004)

CNC Technology and CAD/CAM – CAD/CAM stands for Computer-Aided Design and Computer-Aided Manufacturing. CAD part has the drawing / design tools and such design is used by CAM to control the movements of a machine tool to make the exact design drawn. CNC technology uses a universally accepted NC code called APT or Automatically Programmed Tools. CNC and CAD could not be related initially because of the different potentials and the file formats used. CNC was mainly intended for machine tools whereas CAD/CAM softwares were concentrating on product design & development. The advancements in both CNC and CAD/CAM softwares have made them communicate with each other for better design and manufacture of precision tools as well as by architects. CNC technology has contributed significantly to the development of technologies like Computer Integrated Manufacturing (CIM), Computer Aided Process Planning (CAPP), Group Technology (GT) and Cellular Manufacturing, Flexible Manufacturing Systems (FMS) and Just-In-Time Production (JIT) (Model and prototyping facilities).

Let us now have a deeper look into CNC – the technology and how it works. A CNC machine typically consists of a drivers unit, a sliding system and the machine control unit (MCU). The drivers of a CNC machine can be a Hydraulic Actuator which is a high power machine tool, a Stepping motor which is a small machine with limited power and a DC motor which is widely used since it has very good speed regulation and high torque. The sliders or guiding system controls the direction of motion as programmed in the control process ((Dr.Zhou). The CNC process consists of 5 basic steps – the basic idea or concept, physical design using CAD, convert to machine language using CAM, Controlling and Machining. The basic concept is what the designer has which he / she feeds into a system using CAD. CAD makes the design well structured and the design can be used by CAM for processing.

CNC Control – Controlling step is the most important step since without proper control the CAD and CAM as well as the various other parts involved would not be able to co-ordinate. The control system consists of a computer, software and a controller. The control computer need not be a high-end system since it needs to run a very simple program and it will be working under rough conditions such as heavy jolts and / or dust and smoke. The control software executes the program and is usually written in G-Code. There are many types of software available in the market today depending upon the requirement like Mach 3 from Artsoft, Turbo CNC and CNC Pro which is DOS based. These softwares make the control very user friendly. The CNC Controller processes the signal from the software and converts it into motion. They come from various suppliers according to dimensions (2 Axis, 3 Axis, 4 Axis etc.) required for a particular process. Do-It-Yourself (DIY) kits are also easily available in the market that comes as a package with motors and cables (CNC Basics E-Course 5. 2008).

Open & Closed loop control systems – We saw earlier that the CNC machines should be capable of working on their own with very little human interference. This is made possible with the control system. The control system can be open or closed based on the feedback. In an Open looped control system, the input instruction is one-directional i.e. instruction goes from the control software to the particular part and there’s no feedback to check whether the output produced confirms to the instruction received. One example for such open loop control system is water level control in water tank. In a Closed loop control system, a feedback signal is received and this can be checked against the instruction input. A closed loop system is more accurate and consistent with the requirements input (Bawa. 2004). In general, a CNC command is executed through a control program that instructs the drive motor to spin a specific number of times. This in turn rotates the ball screw and the ball screw causes forces the linear axis. The feedback signal from the screwball lets the control confirm whether the requested action was taken or not. The feedback is usually made possible by linear scales or resolvers. Linear Scales are high accuracy glass scales that are read by an optical encoder that tracks the axis movement. A resolver is a complex and costly analog device that feeds back via sine signals according to its movement (Dr.Zhou).

Motion Control – Motion control, as mentioned earlier, is one of the most important aspects of CNC machines. Motion can be single, double or multi-dimensional (axes) which makes CNC very flexible. The axes are accurately and mechanically arranged in the path of movement. Linear Axis is when the movement is in a straight line and it is rotary when in circular path. CNC Control is automated by programming the motion type (whether rotary, rapid or linear), axes, amount of motion and motion rate (feedrate). CNC uses 2 types of coordinate systems to simplify the axis motion – Rectangular co-ordinate System and Polar co-ordinate System. Rectangular coordinate system is commonly used in graphing. Similar to plotting axis is a graph; the physical end points are plotted for axis motions in CNC. Every graph has an origin point where the horizontal and vertical base lines come together. In CNC, this origin point is called program zero point or work zero or part zero or program origin (The Basics Of Computer Numerical Control). Machine Geometry – is the relationship of distances between a fixed point of the machine and the selectable point of the part. The Axis orientation depends on the type of machine that sets the axes. When closely examining the Axis orientation for milling and turning equipments in general. Milling involves 3 axis machines which typically have the X, Y and Z axes. X axis is parallel to the longest dimension of the machine table, Y axis is the shortest dimension and Z axis is the spindle movement. Depending on whether the machine is vertical or horizontal the Y axis is either the saddle cross direction (vertical) or the column direction (horizontal). The horizontal machining centres have an additional indexing B axis. Turning involves 2 axes – X and Z. More axes are possible; a special C axis is designated for milling purposes. CNC lathes have double adjusted XZ axes – there’ll be a front lathe and a rear lathe. The X, Y and Z axes are the primary axes in CNC. U, V and W axes are the secondary axes and are parallel to the primary axes. For spinning or indexing applications, the additional axes are A, B and C being rotated against X, Y and Z axes respectively. Position direction is done by indexing axes. I, J and K are the arc centre vectors which are used in circular interpolation which deals with forming arcs or circles in simple terms (Smid).

CNC Tool Administration – Tool administration involves managing the physical tools and information regarding the tools which in turn is managed by the underlying application or the CNC controller. The term tool includes tool gears and tool assemblies. Further the tools can be perishable or durable. Perishable tools like drills, taps, inserts etc. wear out during use and have to be replaced frequently. Durables like a tool holder are not consumed during usage. Another difference is whether a tool is returnable or assigned to a particular machine or operator. Returnable tools are sent back to the tool crib for reuse. A tool assembly is a combination of perishable and durable tools which need to be assembled and tuned before sent to CNC. The storage and issue of physical tools involves a physical storage system, a unique ID or code attached to each tool for identification and an authorised access to the crib or store. The tool management software (TMS) should be efficient and consistent so that the reports can be reliable. The software should be capable of automated differential inventory control and report the expected purchase / replacement requirements based on the settings. Ideally a good TMS will be one which can be integrated easily with an ERP system for full automation of data (Geng.2004).

Tool Compensation – This involves the fine tuning of initial settings for accuracy. Tool compensation works with offsets or constant numeric values stored in memory location. These values can be input during the initial set up or just before running the machine as the case may be. This enables re-usability of same program in other machine where the offset values are different but functionality is the same. Some areas where offset compensation are useful are tool length compensation, cutter radius compensation, dimensional tool offsets, and tool nose radius compensation (The Basics of Computer Numerical Control).

CNC Programming

CNC machines are dependent on a controller program as we saw in the CNC Controlling section. The CNC programs are written in G-Codes, M-Codes, T-Codes and F-Codes. Hence the machine operator has to understand the codes so that he can set offset parameters and alter the code according to the requirements. G-Codes are the most important and basic part of the CNC programming algorithm. G-Code controls the position and movements of the tool during the working. M-Codes manage the machine, T-Code manages the tools and F-Code manages the tool feed and tool speed controls. CAM helps in compiling these codes (Gargi. 2008). Let us see a suggested sequence of tasks involved in CNC programming:

1.      Study of initial information (drawing and methods)

2.      Material stock evaluation

3.      Machine tool design

4.      Control system

5.      Sequence of operations

6.      Tooling and arrangement of cutting tools

7.      Setup of the part

8.      Setting data (offsets)

9.      Determination of tool path

10.  Working sketches and mathematical calculations

11.  Coding and interpreting

12.  Program testing and debugging

13.  Program documentation (Smid. 2003).

G-Codes – G-Code programs consist of a number of ‘blocks’ that in turn contains one or more ‘words’. A word consists of a ‘letter’ describing a setting to be made, or a function to be performed, followed by a number value. A block can contain a maximum of 256 characters. An example of a program block would be – /N0001 G0 X123.05 where, N is for line number, G0 is for the destination and final position of X axis will be 123.05. Here the line number is optional and blank lines are allowed. Though white spaces are not counted by the interpreter, for better readability it is advised to include a space between words and avoid space within a word. The interpreter is case sensitive. At the same time a line can have only up to 4 G words and 4 M words (counted separately). Two G / M words from the same modal group are not allowed in a line. For legal letters, a line may have only one word beginning with that letter. Let us examine the following code:

n1 x4 – moves from the current x location to x4
n2 y3 – moves from current y location to y1 at x4
n3 z3 – moves from current z location to z3 at x4 and y1

n10 x4 y1 z3 – moves on a single line from current x, y, z to x4 y1 z3

The final position of the first three blocks (n1-n3) and the (n10) block are the same. The first set of blocks might be executed in sequence to move the tool around an obstacle while the path of the tool for the combined block (n10) might run it into the position. Let us now analyse some of the commonly used NC words and what they mean:

NC Word
Functionality
NC Word
Functionality
N
Line Number
X
X-Axis
G
Preparatory Function
Y
Y-Axis
R
Radius
Z
Z-Axis
F
Feedrate
S
Spindle Speed
H
Tool length offset
D
Tool radius offset
T
Tool
M
Miscellaneous function
The preparatory function of the MCU has instruction for a specific mode of operation. The following table has a list of preparatory functions and its functionality for Milling and Turning.

Milling
Turning
G-Code
Functionality
G-Code
Functionality
G00
Positioning in Rapid
G00
Positioning in Rapid
G01
Linear Interpolation
G01
Linear Interpolation
G02
Circular Interpolation (CW)
G02
Circular Interpolation (CW)
G03
Circular Interpolation (CCW)
G03
Circular Interpolation (CCW)
G04
Dwell
G04
Dwell
G07
Imaginary axis designation
G07
Feedrate sine curve control
G09
Exact stop check
.
.
G10
Program parameter input
G10
Data setting
G11
Program parameter input cancel
G11
Data setting cancel
G12
Circle Cutting CW
.
.
G13
Circle Cutting CCW
.
.
G17
XY Plane
G17
XY Plane
G18
XZ Plane
G18
XZ Plane
G19
YZ Plane
G19
YZ Plane
G20
Inch Units
G20
Inch Units
G21
Metric Units
G21
Metric Units
G22
Stored stroke limit ON
G22
Stored stroke check function ON
G23
Stored stroke limit OFF
G23
Stored stroke check function OFF
.
.
G25
Spindle speed fluctuation detection OFF
.
.
G26
Spindle speed fluctuation detection ON
G27
Reference point return check
G27
Reference point return check
G28
Automatic return to reference point
G28
Automatic Zero Return
G29
Automatic return from reference point
G29
Return from Zero Return Position
G30
Return to 2nd, 3rd, 4th reference point
G30
2nd reference point return
G31
Skip function
G31
Skip function
.
.
G32
Thread cutting
G33
Thread cutting
.
.
G34
Bolt hole circle (Canned Cycle)
G34
Variable lead thread cutting
G35
Line at angle (Canned Cycle)
.
.
G36
Arc (Canned Cycle)
G36
Automatic tool compensation
G40
Cutter compensation Cancel
G40
Tool Nose Radius Compensation Cancel
G41
Cutter compensation Left
G41
Tool Nose Radius Compensation Left
G42
Cutter compensation Right
G42
Tool Nose Radius Compensation Right
G43
Tool Length Compensation (Plus)
.
.
G44
Tool Length Compensation (Minus)
.
.
G45
Tool offset increase
.
.
G46
Tool offset decrease
G46
Automatic Tool Nose Radius Compensation
G47
Tool offset double increase
.
.
G48
Tool offset double decrease
.
.
G49
Tool Length Compensation Cancel
.
.
G50
Scaling OFF
G50
Coordinate system setting and maximum rpm
G51
Scaling ON
.
.
G52
Local coordinate system setting
G52
Local coordinate system setting
G53
Machine coordinate system selection
G53
Machine coordinate system setting
G54
Workpiece Coordinate System
G54
Workpiece Coordinate System
G55
Workpiece Coordinate System 2
G55
Workpiece Coordinate System 2
G56
Workpiece Coordinate System 3
G56
Workpiece Coordinate System 3
G57
Workpiece Coordinate System 4
G57
Workpiece Coordinate System 4
G58
Workpiece Coordinate System 5
G58
Workpiece Coordinate System 5
G59
Workpiece Coordinate System 6
G59
Workpiece Coordinate System 6
G60
Single direction positioning
.
.
G61
Exact stop check mode
G61
Exact stop check mode
G62
Automatic corner override
G62
Automatic corner override
G63
Tapping mode
G63
Tapping mode
G64
Cutting mode
G64
Cutting mode
G65
Custom macro simple call
G65
User macro simple call
G66
Custom macro modal call
G66
User macro modal call
G67
Custom macro modal call cancel
G67
User macro modal call cancel
G68
Coordinate system rotation ON
G68
Mirror image for double turrets ON
G69
Coordinate system rotation OFF
G69
Mirror image for double turrets OFF
G70
Inch Units
G70
Finishing Cycle
G71
Metric Units
G71
Turning Cycle
G72
User canned cycle
G72
Facing Cycle
G73
High-Speed Peck Drilling Cycle
G73
Pattern repeating
G74
Counter tapping cycle
G74
Peck Drilling Cycle
G75
User canned cycle
G75
Grooving Cycle
G76
Fine boring cycle
G76
Threading Cycle
G77
User canned cycle
.
.
G78
User canned cycle
.
.
G79
User canned cycle
.
.
G80
Cancel Canned Cycles
G80
Canned cycle for drilling cancel
G81
Drilling Cycle
.
.
G82
Counter Boring Cycle
.
.
G83
Deep Hole Drilling Cycle
G83
Face Drilling Cycle
G84
Tapping cycle
G84
Face Tapping Cycle
G85
Boring Cycle
.
.
G86
Boring Cycle
G86
Face Boring Cycle
G87
Back Boring Cycle
G87
Side Drilling Cycle
G88
Boring Cycle
G88
Side Tapping Cycle
G89
Boring Cycle
G89
Side Boring Cycle
G90
Absolute Positioning
G90
Absolute Programming
G91
Incremental Positioning
G91
Incremental Programming
G92
Reposition Origin Point
G92
Thread Cutting Cycle
G93
Inverse time feed
.
.
G94
Per minute feed
G94
Endface Turning Cycle
G95
Per revolution feed
.
.
G96
Constant surface speed control
G96
Constant surface speed control
G97
Constant surface speed control cancel
G97
Constant surface speed control cancel
G98
Set Initial Plane default
G98
Linear Feedrate Per Time
G99
Return to Retract (Rapid) Plane
G99
Feedrate Per Revolution
.
.
G107
Cylindrical Interpolation
.
.
G112
Polar coordinate interpolation mode
.
.
G113
Polar coordinate interpolation mode cancel
.
.
G250
Polygonal turning mode cancel
.
.
G251
Polygonal turning mode
(g-codes)

M-Codes – are miscellaneous functions that can be used to control the input and output of the program. They also contain some functions for switching on and off the spindle, coolant etc. M-Codes can stop or end a program or subroutine. They are more related to the control of program / machine rather than movement of tool ((Falck).

Milling
Turning
M-Code
Functionality
M-Code
Functionality
M00
Program Stop
M00
Program Stop
M01
Optional Program Stop
M01
Optional Program Stop
M02
Program End
M02
Program End
M03
Spindle On Clockwise
M03
Spindle On Clockwise
M04
Spindle On Counter-Clockwise
M04
Spindle On Counter-Clockwise
M05
Spindle Stop
M05
Spindle Stop
M06
Tool Change
.
.
.
.
M07
Coolant 1 On
M08
Coolant On
M08
Coolant 2 On
M09
Coolant Off
M09
Coolant Off
M10
Clamps On
.
.
M11
Clamps Off
.
.
M30
End of Program, Reset to Start
M30
End of Program, Reset to Start
M98
Call Subroutine command
M98
Subprogram call
M99
Return from subroutine command
M99
Return from subprogram
(m-codes)

Differences between G-Code and M-Code – The main difference between G-Code and M-Code is that G-Code is preparatory function that controls the position and movement of tools whereas M-Code controls the entire program or machine. G-Codes start with G and takes parameters for position, axes and motion. M-Codes start with M and takes parameters to control the program like spindle on/off, coolant on/off, program start/stop etc.

Some examples of code for some specific tools are as follows:

Drilling Cycle –           G81 X1.0 Y1.0 Z-.75 F4.0 (DRILL A HOLE .75 DEEP)

X2.0 Y1.0 (SUBSEQUENT HOLE LOCATIONS)

X3.0 Y1.0

Drilling Cycle with Dwell – G82 X1.0 Y1.0 Z-.75 R.2 P.5 F4.0

Peck Drilling cycle – G83 X1.0 Y1.0 Z-.75 Q.25 R.2 F4.0

Tapping cycle –           M03 S100

G84 X1.0 Y1.0 Z-.75 F5.00

Boring cycle – G85 X1.0 Y1.0 Z-.75 F4.0  (Mattson. 2002)

Modal Commands change the mode of the machine till it is explicitly changed by another modal command. G-Code motion commands are modal. If a G1 command is given on a particular line, the motion remains straight till it is changed by another motion command later in the program. Modal commands are arranged in groups of similar function – for example all modal commands for motion belong to the same group. This makes sure that only one command from a group is effective at any given time.

G-Code Modal groups:

group 1 = {G0, G1, G2, G3, G80, G81, G82, G83, G84, G85, G86, G87, G88, G89} – motion
group 2 = {G17, G18, G19} – plane selection
group 3 = {G90, G91} – distance mode
group 5 = {G93, G94} – spindle speed mode
group 6 = {G20, G21} – units
group 7 = {G40, G41, G42} – cutter diameter compensation
group 8 = {G43, G49} – tool length offset
group 10 = {G98, G99} – return mode in canned cycles

group12 = {G54, G55, G56, G57, G58, G59, G59.1, G59.2, G59.3} coordinate system selection.

M-Code Modal groups:

group 2 = {M26, M27} – axis clamping
group 4 = {M0, M1, M2, M30, M60} – stopping
group 6 = {M6} – tool change
group 7 = {M3, M4, M5} – spindle turning
group 8 = {M7, M8, M9} – coolant
group 9 = {M48, M49} – feed and speed override bypass (Falck).

CAM – Computer Aided Manufacturing & CNC

CAM or Computer-Aided Manufacturing has simplified the machining work through its 2D/3D visualization which gives the machine user a better idea about his settings. It has improved the productivity and accuracy since the design is automatically transformed into a program which controls the machine instead of a manual control program. The co-ordinates can be set more accurately due to visualization and the motion can also be set more precisely. This has reduced the time taken for a manual machine geometrics and control program which need not be so accurate. The advancements made in CAM softwares have made it more reliable and accurate and hence it is being used widely for automation design and development. More and more sophisticated machines are now relying on CAM. There are about 40 CAM applications available in the market. The DFM product line from Geometric Software Solutions can suggest the avoidable features of a model fed into the application by the designer. This can help the designer to alter such parts and cut down on the running cost. Another CAD / CAM company, SolidWorks has worked with other software developers to integrate their applications with SolidWorks so that required data exchange is possible without having to re-input them. Other examples of high end CAD/CAM softwares are Unigraphics, Cimatron, Desault’s Catia and Delcam’s PowerX (Choosing the Right CAM Software. 2008).

Summary

·         A CNC (Computer Numerically Controlled) machine has a micro-computer which is programmed to control the machine in terms of its movements and positioning which is more precise and faster than with humans. The motion is possible in more than 3 dimensions.

·         Being reliable, parts made by CNC machines are used in almost all industries like Aerospace, Machinery, Electrical, Fabrication, Automotive, Instrumentation and Mould making.

·         CAD/CAM stands for Computer-Aided Design and Computer-Aided Manufacturing. CAD part has the drawing / design tools and such design is used by CAM to control the movements of a machine tool to make the exact design drawn. Advancements made in CNC programming and CAD / CAM softwares have made it communicate with each other for better design and control of sophisticated machines.

·         A CNC machine typically consists of a drivers unit, a sliding system and the machine control unit (MCU).

·         CNC Control system consists of a computer, software and a controller. The control system can be open or closed based on the feedback. The feedback is usually made possible by linear scales or resolvers. CNC Control is automated by programming the motion type (whether rotary, rapid or linear), axes, amount of motion and motion rate (feedrate).

·         The origin point where the X and Y axes meet is called program zero point or work zero or part zero or program origin.

·         Machine Geometry is the relationship of distances between a fixed point of the machine and the selectable point of the part.

·         Tool administration involves managing the physical tools and information regarding the tools which in turn is managed by the underlying application or the CNC controller.

·         Tool Compensation involves the fine tuning of initial settings for accuracy.

·         The CNC programs are written in G-Codes, M-Codes, T-Codes and F-Codes.

·         G-Code controls the position and movements of the tool during the working. M-Codes manage the machine, T-Code manages the tools and F-Code manages the tool feed and tool speed controls. CAM helps in compiling these codes.

·         The main difference between G-Code and M-Code is that G-Code is preparatory function that controls the position and movement of tools whereas M-Code controls the entire program or machine.

·         Many CAD / CAM softwares are developed in such a way that they can be easily integrated and this makes them more popular.

Conclusion

CNC is a very complex and upgrading technology which is making tooling and machining, in general machine automation, much easier and reliable. The advancement made in CNC has made sophisticated machine / tool making much simpler. Integration of CAD / CAM softwares has made design and development of machine automation very easy today. With more research and development happening in the field we can expect CNC / CAD / CAM simplify new areas of automation and make technology advance faster.

References

Bawa, H.S. 2004. Manufacturing Processes. Noida: Tata McGraw-Hill. The Basics Of Computer Numerical Control. Key concept number one: Fundamentals Of CNC . Available:http://www.cncci.com/resources/articles/CNC%20basics%201.htm. Accessed on October 15, 2008

CNC technology and CNC programming. 2004. (online). Available:www.me.metu.edu.tr/me445/Assets/lecture%20notes/ch4%20CNC%20machine%20tools.ppt. Accessed on October 15, 2008

CNC Basics E-Course 5. 2008. (online). Available:http://revver.com/video/594746/cnc-basics-e-course-5-cnc-control-learn-cnc-control-video-cadcam-tips-learn-machining-cnc-design-cnc-process-cnc-basics-cnc-lessons/. Accessed on October 15, 2008

Choosing the Right CAM Software. 2008. (online). Available: http://www.americanmachinist.com/304/Issue/Article/False/78582/Issue. Accessed on October 15, 2008

Dr.Karunakaran, K.P. CNC Technology. 2004. (online). Available:http://www.iitb.ac.in/~cep/brochures/cnctech2712.html.  . Accessed on October 15, 2008

Dr.Zhou, J.M..Introduction to CNC. (online). Available:http://www.iea.lth.se/mek/Mekatronikkursen%202006/Production%20Tech/Introduction%20to%20CNC%20machine.pdf. Accessed on October 15, 2008

 Falck, D.  RS274NGC G-CODE PROGRAMMING BASICS. (online). Available:http://www.linuxcnc.org/handbook/gcode/g-code.html. Accessed on October 15, 2008

Geng, H. 2004 . Manufacturing Engineering Handbook. New York:  McGraw-Hill Professional

Gargi. Programming CNC Machines With G-Codes. 2008. (online). Available: http://www.articlealley.com/article_620813_45.html. Accessed on October 15, 2008

G-codes. (online). Available: http://www.cncezpro.com/gcodes.cfm. Accessed on October 15, 2008

Model and prototyping facilities. What is CAD/CAM? . (online). Available:http://www.gsd.harvard.edu/inside/cadcam/whatis.html. . Accessed on October 15, 2008

M-codes.(online). Available: http://www.cncezpro.com/mcodes.cfm. Accessed on October 15, 2008

Mattson, M. 2002  CNC Programming. Florence: Cengage Learning

Smid, P. 2003. CNC Programming Handbook. New York: Industrial Press Inc..

The Basics of Computer Numerical Control. Key concept number three: You must Understand the forms of compensation . (online). Available:http://ase.tufts.edu/mechanical/shop/classes/me126/cnc3.html. . Accessed on October 15, 2008

What is CNC Technology?. CNC Defined. (online). Available:http://www.komo.com/CNC%20Routers/Machine%20Construction/cnc_technology.htm. Accessed on October 15, 2008

 

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