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Computer Numerical Control (CNC), Machine tools

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    Introduction

    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

    1. 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.
    2. Being reliable, parts made by CNC machines are used in almost all industries like Aerospace, Machinery, Electrical, Fabrication, Automotive, Instrumentation and Mould making.
    3. 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.
    4. A CNC machine typically consists of a drivers unit, a sliding system and the machine control unit (MCU).
    5. 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).
    6. The origin point where the X and Y axes meet is called program zero point or work zero or part zero or program origin.
    7. Machine Geometry is the relationship of distances between a fixed point of the machine and the selectable point of the part.
    8. 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.
    9. Tool Compensation involves the fine tuning of initial settings for accuracy.
    10. The CNC programs are written in G-Codes, M-Codes, T-Codes and F-Codes.
    11. 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.
    12. 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.
    13. 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

    1. 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
    2. 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
    3. Choosing the Right CAM Software. 2008. (online). Available: http://www.americanmachinist.com/304/Issue/Article/False/78582/Issue. Accessed on October 15, 2008
    4. Dr.Karunakaran, K.P. CNC Technology. 2004. (online). Available:http://www.iitb.ac.in/~cep/brochures/cnctech2712.html.  . Accessed on October 15, 2008
    5. Falck, D.  RS274NGC G-CODE PROGRAMMING BASICS. (online). Available:http://www.linuxcnc.org/handbook/gcode/g-code.html. Accessed on October 15, 2008
    6. Geng, H. 2004 . Manufacturing Engineering Handbook. New York:  McGraw-Hill Professional
    7. Gargi. Programming CNC Machines With G-Codes. 2008. (online). Available: http://www.articlealley.com/article_620813_45.html. Accessed on October 15, 2008
    8. G-codes. (online). Available: http://www.cncezpro.com/gcodes.cfm. Accessed on October 15, 2008
    9. Model and prototyping facilities. What is CAD/CAM? . (online). Available:http://www.gsd.harvard.edu/inside/cadcam/whatis.html. . Accessed on October 15, 2008
    10. M-codes.(online). Available: http://www.cncezpro.com/mcodes.cfm. Accessed on October 15, 2008
    11. Mattson, M. 2002  CNC Programming. Florence: Cengage Learning
    12. Smid, P. 2003. CNC Programming Handbook. New York: Industrial Press Inc..
    13. 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.

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