Study of Operational Concepts of Conventional and CNC Machines - Lathe & Milling Pratik Basu Final year B.E student Lingaya’s Inst. Of Mgmt. & Tech.
Overview <ul><li>I had the opportunity to study both conventional and CNC machines, but have given more stress on the CNC machines, owing to their emerging importance. A general description of conventional lathe and milling machines has been given. The concepts of Computer Numerically Controlled machines have been explained in greater detail. </li></ul>
ABOUT NPL <ul><li>National Physical Laboratory (NPL), New Delhi is the premier research laboratory in India in the field of physical sciences. It is under the Council of Scientific and Industrial research (CSIR) located at the PUSA campus on Dr. K.S.Krishanan Marg, New Delhi-110012. </li></ul><ul><li>MAIN ACTIVITIES OF NPL </li></ul><ul><li>Research and Development </li></ul><ul><li>Consultancy </li></ul><ul><li>Sponsored and contact research </li></ul><ul><li>Calibration and Testing </li></ul><ul><li>Ever since its inception in 1947, NPL has made several fruitful contributions to the nation, society and industry as well as to building knowledge base in its specific areas of strength in physics. </li></ul>
Workshop <ul><li>Electroplating </li></ul><ul><li>Designing </li></ul><ul><li>Sheet Metal working </li></ul><ul><li>Carpentry </li></ul><ul><li>Cutting and welding </li></ul><ul><li>Conventional machines section </li></ul><ul><li>CNC machines section </li></ul>NPL has a large, fully furnished Central Workshop consisting of various kinds of automated as well as manually controlled machines along with divisions like: The workshop staff consists of skilled professional staff for producing Ultra High Precision parts. One of the most important jobs performed by the workshop is to provide the various R&D departments with the spare parts of machineries that are not easily available. This leads to saving of adequate amount of time and money.
Conventional machines section <ul><li>Some of the machines in this section are : </li></ul><ul><li>Bandsaw - The bandsaw is useful for cutting stock to size and roughing out shapes. It cuts curved shapes very well. </li></ul><ul><li>Belt sander – A belt sander is used to remove rough edges. It makes use of an abrasive riding belt. Effective on wood, plastic and most metals. </li></ul><ul><li>Drilling machine – The drill press is mostly used for drilling holes, reaming, boring, countersinking etc. </li></ul><ul><li>Power hacksaw – A type of hacksaw powered either by it’s own electric motor or connected to an engine. </li></ul>Drilling machine Power hacksaw
And several other machines for sheet metal work, electroplating etc. , as well as callipers, micrometers etc. for measurement and testing. <ul><li>Surface grinder – It is a machine tool used to provide high precision surface finish. The grinding wheel rotates in the spindle head, it’s height adjustable. The workpiece is fed by a horizontal table positioned below the grinding wheel. </li></ul><ul><li>Slotting machine - Similar to a vertical shaper. The most common use is to machine straight, flat surfaces. </li></ul>Surface grinder Slotting machine
Lathe Machine The 2 principal conventional machines on which we are focusing are the lathe and milling machines. Lathes are designed for precisely machining relatively hard materials. With their inherent versatility, they are used in a wide range of applications, and can machine a broad range of materials. These lathe machine removes material from a rotating work piece via the linear movements of various cutting tools, such as tool bits and drill bits.
Operations performed on a lathe <ul><li>Turning – The diameter of a part can be reduced to desired dimension. </li></ul><ul><li>Facing - A lathe can be used to create a smooth, flat, face very accurately </li></ul><ul><li>perpendicular to the axis of a cylindrical part. </li></ul><ul><li>Parting - Deeper and narrower than a turning tool. It is designed for making narrow grooves and for cutting off parts. </li></ul><ul><li>Drilling - A lathe can also be used to drill holes accurately concentric with the centerline of a cylindrical part. </li></ul><ul><li>Boring - Boring is an operation in which a hole is enlarged with a single point cutting tool. </li></ul><ul><li>Threading - External threads can be cut with a die and internal threads can be cut with a tap. </li></ul>
Types of lathe <ul><li>Centre lathe / bench lathe / engine lathe - The most basic type of lathe. </li></ul><ul><li>Toolroom Lathe - Lathe optimized for toolroom work. It has all of the best optional features that may be omitted from less expensive models, such as a collet closer, taper attachment, and others. </li></ul><ul><li>Turret lathes and capstan lathes - Used for repetitive production of duplicate parts. </li></ul><ul><li>Multispindle lathe - Multispindle lathes have more than one spindle and automated control. </li></ul><ul><li>CNC lathe - CNC lathes are rapidly replacing the older production lathes due to their ease of setting and operation. The part may be designed by the Computer-aided manufacturing (CAM) process, the resulting file uploaded to the machine, and once set and trialled the machine will continue to turn out parts under the occasional supervision of an operator. </li></ul>
Milling machine A milling machine is a machine tool used for the shaping of metal and other solid materials. Its basic form is that of a rotating cutter, which rotates about the spindle axis (similar to a drill), and a table to which the workpiece is affixed. In contrast to the lathe machine, in the milling machine the workpiece moves longitudinally against the rotating cutter. Milling machines may be operated manually or by CNC.
Types of milling cutters In vertical mills , milling cutters with solid shafts are usually used. End mills are designed for cutting slots, keyways and pockets. Two fluted end mills can be used to plunge into work like a drill. Ball end mills can produce a fillet.
Computer and Numerically Controlled Machines Numerically Controlled (NC) NC is the operation of M/c tool by a series of coded instructions consisting of numbers, letters of the alphabets and symbols, which the MCU (Machine Control Unit) can understand. Computer numerically controlled (CNC) When numerical control is performed under computer supervision, it is called computer numerical control (CNC). Computers are the control units of CNC machines. A programmer enters some information in the program, but the computer calculates all necessary data to get the job done. For both NC and CNC systems, working principles are the same. Only the way in which the execution is controlled is different. Normally, new systems are faster, more powerful, and more versatile.
Various popular CNC control systems <ul><li>ECS </li></ul><ul><li>LECS </li></ul><ul><li>NUM </li></ul><ul><li>SELCA </li></ul><ul><li>MARPOSS </li></ul><ul><li>Z-16 </li></ul><ul><li>FANUC </li></ul><ul><li>FAGOR </li></ul><ul><li>FIDIA </li></ul><ul><li>DECKEL </li></ul><ul><li>SINUMERIC </li></ul><ul><li>HINUMERIC </li></ul><ul><li>HEIDENHAIN </li></ul><ul><li>GILDEMEISTER </li></ul>A CNC Milling center by DECKEL
Important terms related to CNC machining <ul><li>Machine Zero - Machine zero is a point at the origin of the machine’s coordinate measuring system. All the Axis movements and other dimensions are measured from this point. It is similar to the origin of coordinate measuring system. </li></ul><ul><li>Machine reference point - It refers to the initial point of return for the purpose of measuring/feedback systems. Whenever a CNC machine is switched on the feedback system has to be initialized by referring this point on every axis. </li></ul><ul><li>Work Zero - This is the origin for the measuring of dimensions of workpiece. The programmer is free to select it anywhere on the drawing. </li></ul><ul><li>Absolute measuring system - In this measuring system all the dimensions are made from the work zero, which defined. The machine control uses work zero as the reference point to position the tool during program execution. The main advantage of programming in absolute system is that any point can be readily changed without affecting subsequent dimensions. </li></ul>
<ul><li>Incremental measuring system - The movements are based on the change in position between two successive points. It expresses the relative distance between the current location and the next position. This type of measuring system is called Incremental Measuring system. The main advantage of this system is that sum of the dimensions must always be zero if start point and finishing point is same at the end of programming which makes it easy to check a program. </li></ul><ul><li>Axis designation (conventions) - Axis designation for each type of machine tool is suggested in the EIA (Electronic Industries Association) RS 274-B standard. This conforms to ISO Recommendations R831. The nomenclature of the three main axes (X, Y AND Z) is based on the “Left hand rule”. The thumb indicates the orientation of the X-axis; the index finger indicates the Y-Axis, and the middle finger points in the direction of the Z-axis. </li></ul><ul><li>Tool and tool offset - The T function is used to call the particular tool and tool offset in the program. The tool offset is used to correct the values entered in the coordinate system preset block. Using the tool offsets, it is easy to set up the tools and to make adjustments in part size. </li></ul>
<ul><li>Spindle speed - The spindle speed is the rotational frequency of the spindle of the machine, measured in revolutions per minute (RPM). The preferred speed is determined based on the material being cut. Using the correct spindle speed for the material and tools will greatly affect tool life and the quality of the surface finish. </li></ul><ul><li>Feed rate - Feed rate is the velocity at which the cutter is fed, that is, advanced against the workpiece. It is expressed in units of distance per revolution for turning and boring (millimeters per revolution). For milling it is expressed in units of distance per time for milling (millimeters per minute). </li></ul><ul><li>Cutting Speed - Cutting speed may be defined as the rate (or speed) that the material moves past the cutting edge of the tool , irrespective of the machining operation used — the surface speed. </li></ul>
<ul><li>Tool Chart - </li></ul><ul><li>Process Chart - </li></ul>Tool insert specification Tool material Tool radius Tool length Tool description Process no. S.no. remark tool time M/C process Process no. S.no.
The construction of CNC machines <ul><li>1. Elements of motion transmission </li></ul><ul><li>Ball-screws and nut assembly </li></ul><ul><li>In a CNC machine, the connection between the screw and the nut is through an endless stream of re-circulating steel balls, replacing sliding friction threads with rolling friction. Advantages are higher efficiency, reversibility and reduction in wear and tear. </li></ul><ul><li>Hydrostatic slideways </li></ul><ul><li>In the hydrostatic slideways air or oil is pumped into small pockets machined into slides which are in contact with the slideway. </li></ul>2. Computers CNC machines use an on-board computer that allows the operator to read, analyze, and edit programmed information. In CNC machine, computer works on a binary principle, 0 for information and 1 for processing. Special built-in software compiles the user entered program (in code language) into machine language and the machine moves the tool by its servomotors.
<ul><li>3. Control systems </li></ul><ul><li>There are two types of control systems on CNC machines: </li></ul><ul><li>Open loop (less accurate) </li></ul><ul><li>Closed loop (more accurate) </li></ul><ul><li>The open loop control system does not provide positioning feedback to the control unit. Since this control system only counts tool movement pulses and cannot identify discrepancies in positioning, it is slightly inaccurate. </li></ul><ul><li>In closed loop control system , the electronic movement pulses are sent from the control to the servomotor, enabling the motor movement. Movements are detected by a feedback device (transducer), which can send a signal to the control for checking after each step. </li></ul><ul><li>4. Drive motors </li></ul><ul><li>The drive motors control machine slide movement on CNC equipment. Types used : </li></ul><ul><li>Stepper motors (convert a digital pulse into a small rotation, mostly used in applications where low torque is required ) </li></ul><ul><li>DC servo motors (rotate in response to the applied voltage, used to drive lead screw and gear mechanisms, provide higher-torque output) </li></ul>
5. Tool changers Several different cutting tools are used to produce a part. The tools must be replaced quickly for the next machining operation. For this reason, the majority of NC/CNC machine tools are equipped with automatic tool changers. They allow tool changing without the intervention of the operator. An automatic tool changer grips the tool in the spindle, pulls it out, and replaces it with another tool. Tool changers are equipped for either random or sequential selection. In random tool selection there is no specific pattern of tool selection. In sequential tool selection , the tools must be loaded in the exact order in which they are called for in the program. 3. AC servo motors (controlled by varying the voltage frequency to control speed, more power than a DC servo, used to drive a lead screw and gear mechanism ) 4. Fluid servo motors (variable speed motors, produce more power, in the case of pneumatic motors, than electric servomotors)
<ul><li>1. Modes of Operation </li></ul><ul><li>Automatic operation </li></ul><ul><li>1. Memory operation – The required program is already registered in the CNC memory. We can just select the program and start the operations </li></ul><ul><li>2. MDI Operation - In the MDI mode, program can be inputted in same format as normal programs and executed from the MDI panel. Mostly used for simple test operation. </li></ul><ul><li>3. Program restart - Restarting of a program for automatic operation from an intermediate point, a Sequence No. is assigned to a block. MDI also usable as High Speed Program Check Function. </li></ul><ul><li>4. Manual handle interruption - Movement by manual handle operation can be done by overlapping it with the movement by automatic operation </li></ul><ul><li>5. Sequence number search - Function is used to search for a sequence number within a program and to start or rescue the program from the block having that sequence number. </li></ul>
<ul><li>Manual operation </li></ul><ul><li>1. Jog feed </li></ul><ul><li>In the jog mode, a feed axis and direction selection switch on the machine operator’s panel moves the tool along the selected direction. The jog feed rate can be adjusted with the jog feed-dial rate. </li></ul><ul><li>2. Incremental feed </li></ul><ul><li>In the incremental (STEP) mode, pressing a feed axis and direction selection switch on the machine operator’s panel moves the tool one step along the selected axis in the selected direction. Each step can be 10, 100, 1000 times he least input increment ( minimum distance moved by tool). </li></ul><ul><li>3. Manual handle feed </li></ul><ul><li>In the handle mode, rotating the manual pulse generator on the machine operator’s panel can move the tool. The minimum distance the tool is moved when the manual pulse generator is rotated by one graduation. </li></ul><ul><li>4. Manual absolute on and off </li></ul><ul><li>When the switch is turned on, the distance the tool is moved by manual operation is added to the current coordinates. </li></ul>
2. Processing of CNC programs <ul><li>Determination of the sequence of operations required in machining process ( involves examining the shape features of the components to be produced) . </li></ul><ul><li>Selection of cutting tools, order of use corresponding to the sequence of operations and determination of feeds and cutting speeds for each operation. </li></ul><ul><li>Settings of program auxiliary functions, including tool changes, spindle starts and stops, coolant on and off and so on. </li></ul><ul><li>Analyses of drawing for CNC programming </li></ul><ul><li>3.1 STUDY OF DRAWING </li></ul><ul><li>Verification of drawing quadrant/angle~ 1st and 3 rd </li></ul><ul><li>Unit of measurement: mm and inch </li></ul><ul><li>Raw material size </li></ul><ul><li>Drawing zero/work piece zero </li></ul><ul><li>Marking of all the turning points (where profile is changing) & working points (for drilling, boring etc.) </li></ul>
<ul><li>Section of measuring system-Absolute, Incremental </li></ul><ul><li>Making of co-ordinates for all turning points and working points. </li></ul><ul><li>Study of tolerances and important dimensions. </li></ul>3.2 PART PROGRAM STRUCTURE <ul><li>Name of the program </li></ul><ul><li>Selection of Working plane, Measuring system ( Absolute or incremental ), unit of </li></ul><ul><li>measurement (mm or inch) </li></ul><ul><li>3. Defining and calling work origin </li></ul><ul><li>4. Tool changing position (Remote area away from work piece), Tool call & Tool change </li></ul><ul><li>5. First position (Movement in working Plane) and Second positioning (Movement in spindle axis) for working, Spindle start & coolant on </li></ul><ul><li>6. Third positioning for working (for mechanizing, tool movement with tool radius compensation) </li></ul><ul><li>7. Depth of cut (in feed only) </li></ul><ul><li>8. Definition of geometry/preparation of profile (feeding of CNC drawing data) </li></ul><ul><li>9. Return to second position, spindle stop & coolant off </li></ul><ul><li>10. Cancellation of fixed cycles, Macro instructions, Special commands, Tool Radius compensation </li></ul><ul><li>11. Return to tool change position </li></ul><ul><li>12. Movement in spindle axis and in working plane </li></ul><ul><li>13. If required, repeat step Nos. to 16 as required. </li></ul><ul><li>14. End of part program </li></ul>
FUNDAMENTALS OF PROGRAMMING <ul><li>Numerical control machine are programmed by means of a series of coded instructions, commonly entered into the controller using manual programming. Each set of instruction codes that machine acts upon is called NC block and is terminated on the program by means of an end character (EOB) that corresponds to the carriage return. A typical block of instruction is of the following form: </li></ul><ul><li>N001 G80 X75 Y65 Z10 F60 S160 T01M03 EOB </li></ul><ul><li>CODE TYPES </li></ul><ul><li>Sequence Number (N-Codes) </li></ul><ul><li>This is an identification number for each block of instructions and increases sequentially through the program. </li></ul><ul><li>Preparatory Function (G-Codes) </li></ul><ul><li>The G-codes are the codes that position the tool and do the actual work . These codes are largely standardized and can be seen in the provided G-code table. Many of the preparatory functions can indicate canned cycles. </li></ul>
Co-ordinate codes These indicate the co-ordinates for the tool movement. Four to five axis machines can have the following axes. Feed rate (F-code) This specifies the feed rate for the operation. The units may be mm per minute or mm per revolution (indicated by the G-code used), with the decimal point implied at a fixed position from the right. Spindle speed (s-code) This specifies the spindle speed to be used for the operation. Tool Number This indicates to the controller which tool is to be used for the operation. In case of tool adaptors with multiple tool slots, the machine just switches to the next tool without removing the earlier one form the adaptor. J B Y K C Z I A X Circular Interpolation about Cartesian axes Angular positions for the Cartesian axes Cartesian Co-ordinates
Miscellaneous Code (M-code) These codes program various auxiliary functions on the machine tool. The miscellaneous function may be acted upon at the start or the end of the motion described by a block of instruction. With word address format, information need not be repeated in successive blocks if it is to remain the same for subsequent blocks. ISO MISCELLANEOUS FUNCTIONS ( M-Codes ) CODE FUNCTION M00 Program stop, spindle and coolant off M01 Optional programmable stop M02 End of program-often interchangeable with M30 M04 Spindle on CCW M05 Spindle stop M06 Tool change M07 Coolant supply No. 1 on M08 Coolant supply No. 2 on M09 Coolant off M10 Clamp M11 Unclamp M13 Spindle on, CW + Coolant on
M14 Spindle on, CCW + Coolant on M 20 – 29 Unassigned M30 Program stops at end of tape+ tape rewind M31 Interlock by-pass M40-M45 Gear Changes; otherwise unassigned M90 Reserved for user M99 (Subroutine call) Reserved for user G - Codes (Fanuc Machine Control) G CODE FUNCTION G00 Positioning (Rapid traverse) G01 Linear Interpolation (Cutting feed) G02 Circular interpolation /Helical cutting CW G03 Circular interpolation/Helical cutting CCW G04 Dwell Time G17 XY plane selection G18 ZX plane selection G19 YZ plane selection G20- 21 Input in inch and mm respectively.
G28 Return to reference point G40 Cutter compensation cancel G41 Cutter compensation left G42 Cutter compensation right G43 Tool length compensation + direction G44 Tool length compensation – direction G53 Machine coordinates system selection G54 - 59 Work co-ordinate system 1 -6 selection G80 Canned cycle cancel G81 Drilling cycle, spot boring G82 Drilling cycle, counter boring G83 Peck drilling cycle G84 Tapping cycle G85 Rough Boring cycle G86 Finish Boring cycle G90 Absolute command G91 Incremental command G92 Programming of absolute zero point G94/98 Feed per minute G95/99 Feed per rotation G96 Constant surface speed control
G97 Constant surface speed controls cancel G98 Return to initial point in canned cycle G99 Return to R point in canned cycle Sample Code G00 X 5.0 Y 5.0 ; Here we rapid tool movement to co-ordinates (5,5). “Air cutting” is minimized by G00 and tool need move along a straight line only. G41(Radius compensation on the left side of the profile) G42(Radius compensation on the right side of the profile)
<ul><li>Fixed or canned cycles </li></ul><ul><li>Many NC machines can be programmed using fixed or canned cycles. </li></ul><ul><li>Drilling Cycle, Spot drilling (G81) </li></ul><ul><li>This cycle, is used for normal drilling. Drilling is performed at cutting feed to the bottom of the hole. The tool is then retracted from the bottom of the hole to retraction height in rapid traverse. </li></ul><ul><li>G81 X_ Y_ Z_ R_ F_ K_ ; </li></ul><ul><li>Drilling Cycle, counter boring drilling (G82) </li></ul><ul><li>This cycle, is used for counter boring, similar to G81. At the bottom, a dwell is performed, then tool retraction takes place. More accurate drilling w.r.t depth. </li></ul><ul><li>G82 X_ Y_ Z_ R_ P_ F_ K_ ; </li></ul><ul><li>Peck drilling Cycle (G83) </li></ul><ul><li>This cycle performs peck drilling. Drilling is performed at intermittent cutting feed to the bottom of hole while removing chips from the hole. </li></ul><ul><li>G83_ X_ Y_ Z_ R_ Q_ F_ K_ ; </li></ul>Tapping cycle (G84) This cycle is used for tapping. Spindle rotated clockwise, when the bottom of the hole reached the spindle is rotated in the reverse direction for retraction. G84_ X_ Y_ Z_ R_ F_ K_;
Rough Boring cycle (G85) This cycle, is used to bore a hole. G85 X_ Y_ Z_ R_ F_ K_ ; Finish Boring cycle (G86) This cycle, is used to bore a hole, a better surface finish is obtained. G86 X_ Y_ Z_ R_ F_ K_ ; In all of the above canned cycles, the following parameters need to be provided X_ Y_ : Hole position data Z_ : The distance from point R to bottom of the hole R_ : The distance from the initial level to point R level F_ : Cutting feed rate K_ : Number of repeats <ul><li>Multiple repetitive Cycles </li></ul><ul><li>Stock Removal in Facing (G71) </li></ul><ul><li>G71 U(Δd) R(e) </li></ul><ul><li>G71 P(Ns) Q(Nf) U(Δu) W(Δw) F(t) S(s) T(t) ; </li></ul>
<ul><li>Stock Removal in Facing (G72) </li></ul><ul><li>G72 W(Δd) R(e) </li></ul><ul><li>G72 P(Ns) Q(Nf) U(Δu) W(Δw) F(t) S(s) T(t) ; </li></ul><ul><li>Cutting is made by operation parallel to Z-axis. </li></ul><ul><li>Stock Removal in Facing (G73) </li></ul><ul><li>This function permits cutting a fixed pattern repeatedly. </li></ul><ul><li>G73 U(Δi) W(Δk) R(d) </li></ul><ul><li>G73 P(Ns) Q(Nf) U(Δu) W(Δw) F(t) S(s) T(t) ; </li></ul><ul><li>Finishing Cycle (G70) </li></ul><ul><li>G70 P(Ns) Q(Nf) U(Δu) W(Δw) ; </li></ul><ul><li>In all the above cycles, </li></ul><ul><li>Δd : depth of cut (radius designation) (designation without sign) in x direction </li></ul><ul><li>e : escaping amount/retraction </li></ul><ul><li>Ns : sequence number of first block for programming of finishing shape </li></ul><ul><li>Nf : sequence number of last block for programming of finishing shape </li></ul><ul><li>Δu : distance and direction of finishing allowance in X direction (dia/radius) </li></ul><ul><li>w : distance and direction of finishing allowance in Z direction </li></ul><ul><li>F, S, T : Any F, S, T function contained on block Ns to Nf in the cycle is ignored </li></ul><ul><li>and the F, S, T function in this G71 block is effective. </li></ul>
CONCLUSION The importance of lathes and milling machines even if they are conventional cannot be undermined. These machines have played a real important role in bringing about industrial revolution and have laid the foundations. But the bringing about of the new technology in the present era is very important. The conventional machines are required in small quantities whereas the CNC machines must be increased to improve the quantity and quality of production.
Points of difference b/w CNC and Conventional Machines <ul><li>One person cannot operate more than one conventional machine at a time </li></ul><ul><li>One person can supervise several CNC machines, and they can be left to work themselves. </li></ul><ul><li>Chances for major improvement in the same conventional machines are very less. </li></ul><ul><li>CNC machines can be updated by using improved software to drive the machines. </li></ul><ul><li>Highly skilled operators are required to work on conventional machines </li></ul><ul><li>Less skilled/trained persons can operate the CNC machines. </li></ul><ul><li>Resemblance of the products formed depends on the skill of the operator. Visible differences may occur </li></ul><ul><li>CNC machines can produce several products, all similar and dead accurate </li></ul><ul><li>Conventional machines are difficult operate for several hours at a stretch. The operator has to continuously focus on the job at hand. </li></ul><ul><li>CNC machines can be used continuously for long intervals and only need to be switched off for occasional maintenance. </li></ul>Conventional Machine CNC Machine
<ul><li>REFERENCES </li></ul><ul><li>www.nplindia.org </li></ul><ul><li>www.wikpedia.org </li></ul><ul><li>www.fortune.com </li></ul><ul><li>www.headland.com </li></ul><ul><li>www.machinetoolhelp.com </li></ul><ul><li>www.cncezpro.com </li></ul><ul><li>I.C.GROVER, “CAD/CAM”, published by Macmillon </li></ul><ul><li>NARANG, “Introduction to Numerical Control” </li></ul><ul><li>P.S.GILL and KRAR, “Fundamental of Numeric Control” </li></ul><ul><li>STEVE KRAR & ARTHUR GILL, “CNC Technology and Programming”, published by McGraw Hill </li></ul><ul><li>Members of the Mechanical Workshop, National Physical Laboratory, New Delhi </li></ul>
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