CNC MACHINE TOOLS - CHRONOLOGICAL DEVELOPMENT

CHRONOLOGICAL DEVELOPMENT

The highly sophisticated CNC machine tools of today, in a vast and diverse range, found throughout the field of manufacturing processing, started from very humble beginnings in a number of major industrialized countries. Some of the earliest research and development works in this field were completed in U.S.A.

A major problem occurred just after World War II, in which progress in all areas of military and commercial development has been so rapid that the levels of automation and accuracy required by the modern industrialized world could not be attained from the labour intensive machines in use at that time. To overcome this disadvantage of conventional plant, the earliest work into numerical control was the study commissioned in 1946 by the US government. The Study’s conclusion was that the metal cutting industry throughout the country could not cope with the demands of the American Air Force. Therefore, they contracted the Parsons Corporation and the Massachusetts Institute of Technology (MIT) during the period 1949-1953 and jointly developed the first NC system in 1953, which could be adapted to the wide range of machine tools. The Cincinatti Machine Tool Company converted one of their milling machines to a three-axis automatic milling machine for this contract. This machine made use of a servo-mechanism for the drive system on the axis, which controlled the table positioning, cross-slide and spindle head.

In the mid-1960s, a UK company, Molins, introduced their unique “system 24”, a machine which worked for 24 hours a day. It could be-+ thought of as a “machining complex” which allowed a series of NC single-purpose machine tools to be linked by a computerized conveyor system. This conveyor allowed the work pieces to be balletised and then directed to each machine tool as necessary. This was an early attempt to initialize the concept of flexible manufacturing system, but was unfortunately doomed to failure.

C.N.C (Computer Numeric Control)

Many of the achievement in computer Numeric Control have a common origin in Numerical Control (abbreviated NC). The conceptual framework established during the development of numerical control is still undergoing further refinement and enhancement. Modern NC systems rely heavily on computer technology.

NC (Numerical Control)

Numerical control can be defined as a form of programmable automation in which the process is controlled by numbers, letters, and symbols. Numerical Control is the combination of Mechanical, Electrical and Electronic devices, controlled by numerical data. In NC, the numbers form a program of instructions designed for a particular work part or job. When the job changes, the program of instructions is changed. This capability to change the program for each new job is what gives NC its flexibility. It is much easier to write new programs than to make major changes in the production equipment.

NC technology has been applied to a wide variety of operations, including drafting, assembly, inspection, sheet metal press working, and spot welding. However numerical control finds its principal applications in metal machining processes. The machined work parts are designed in various sizes and shapes, and most machined parts produced in industry today are made in small to medium-size batches. To produce each part, a sequence of drilling operations may be required, or a series of turning or milling operations. The suitability of NC or these kinds of jobs is the reason for the tremendous growth of numerical control in the metal working industry over the last 25 years.

Constructional Details of C.N.C Machines: -

In general, a CNC machine tool consists of the following units:

·         Computers

·         Control System

·         Drive Motors

·         Tool Changers

According to the construction of CNC machine tools, it works in the following (simplified) manner

a.       The CNC machine controlled by the computer reads the program and translates it into machine language, which is a programming language of binary notation used on computers, not on CNC machines.

b.       When the operator starts the execution cycle, the computer translates the binary codes into electronic pulses which are automatically sent to the machine’s power units. The control unit compares the number of pulses sent and received.

c.        When the motor receives each pulse, they automatically transform the pulses into rotations that drive the spindle and lead screw, causing the spindle to rotate and slide or move the table. The part on the milling machine table or the tool in the lathe turret is driven to a position specified by the program.

Computers:

CNC machines introduced in the late 1970s were less dependent on hardware and more dependent on software. These machines store a program into memory when it is first read in. This Facilitates faster operation when producing number of identical parts, since the program can be recalled from memory repeatedly without having to read it again. CNC machines use an on-board computer that allows the operator to read, analyze and edit programmed instruction, while NC machines require operators to make a new tape to alter a program. In essence, the computer distinguishes CNC from NC.

As with all computers, the CNC machines computer also works on a binary principle using only two characters, 1 and 0 (Machine Language) for information processing. When creating the program, the programmer does not care about the machine language, instead he or she simply uses a list of codes i.e. G&M codes and keys in meaningful information. Special built-in software compiles the program into machine language and the machine moves the tools by servomotors. However, the ability to program the machine is dependent on whether there is a computer in the machine control.

Modern CNC machines use 32-bit processors in their computers to allow fast and accurate processing of information. This results in considerable saving of machining time.

Control System:

There are two types of control systems on NC/CNC machines. The overall accuracy of the machine is determined by the type of control loop used.

        I.            Open loop: -

The open loop control system does not provide positioning feedback to the control unit. The movement pulses are sent out by the control unit and are received by a special type of servomotor called a stepper motor. The stepper motor then proceeds with the next movement command. Since this control system only counts pulses and cannot identify discrepancies in positioning, the control has no way of knowing whether the tool has reached the proper location or not.  The machine will continue this inaccuracy until somebody finds the error.

The open loop control can be used in applications in which there is no change in load conditions, such as the CNC drilling machine. The advantage of the open loop control system is that it is less expensive, since it does not require the additional hardware and electronics needed for positioning feedback. The disadvantage is the difficulty of detecting positioning error.

      II.            Closed loop: -

In the closed loop control system, the electronic movement pulses are sent from the control to the servomotor, enabling the motor to rotate with each pulse. The pulses are detected and counted by a feedback device called a transducer. With each step of movement, a transducer sends a signal back to the control, which compares the current position of the driven axis with the programmed position. When the number of pulses sent and received match, the control starts sending out pulses for the next movement.

Closed loop systems are very accurate. Most have an automatic compensation for error, since the feedback device indicates the error and the control makes the necessary adjustments to bring the slide back to its position. They use AC, DC or hydraulic servomotors.

    III.            Drive Motors

The drive motors control the machine slide movement on CNC equipment. They are classified into four basic types as follows: -

    IV.            Stepper Motor:

These convert a digital pulse, generated by the microcomputer unit (MCU i.e. machine control unit) into small step rotation. Stepper motors have a certain number of steps that they can travel. The number of pulses that the MCU sends to the stepper motor controls the amount of rotation of the motor. Stepper motors are mostly used in applications where low torque is required.

Stepper Motors are used in open loop control system, while AC, DC or hydraulic servomotors are used in closed loop control systems.

      V.            DC Servomotor: - 

These are variable speed motors that rotate in response to the applied voltage. They are used to drive a lead screw and gear mechanism. DC servos provide high torque output than stepper motors.

    VI.            AC Servomotor: -

These are controlled by varying the voltage frequency to control the speed. They can develop more power than a DC servo. They are also used to drive a lead screw and gear mechanism.

  VII.            Fluid Servomotor: -

It is also a variable speed motor. They are able to produce more power or more speed in the case of pneumatic motors than electric servomotors. The hydraulic pump provides energy to valves which are controlled by the MCU.

VIII.            Tool Changers:

Most of the time, different cutting tools are used to produce one part of a machine. The tools have to be replaced quickly for the next machining operation. Owing to this reason, the majority of CNC machine tools are equipped with automatic tool changers, such as magazines on machining centers and turrets on turning centers fig. They allow tool changing without the intervention of the operator. Typically, an automatic tool changer grips the tool in the spindle, pulls it out, and replaces it with another tool. On most machines with automatic tool changers, the turret or magazine can rotate in either forward or reverse direction.

Tool changers may be equipped for either random or sequential tool selection. In random tool selection, there is no specific pattern of tool selection on the machining center, when the program calls for the tool, it is automatically indexed into waiting position, where it can be retrieved by the tool handling device. On the turning center, the turret automatically rotates, bringing tools into position.

In sequential tool selection, the tools must be loaded in the exact order in which they are called for in the program (Fig.). Even if the tools are not in the correct order, the next tool is automatically selected, whether it is suitable or not for the next machining operation. When it is necessary to use a tool twice, the operator must load another tool with the same purpose.

The advantage of sequential tool selection is that less time is needed for indexing the tool into the waiting position. The disadvantage is that more time is needed for setup when switching to a job with a different order of tools. This means that although the same tools are to be used, they have to be preloaded (rearranged) because of a different order in the program. The majority of modern machines are able to return the tool to the magazine and search for the next tool during the program execution. This eliminates the time advantage of sequential tool selection, making random tool selection a standard feature on today’s CNC machine tools.

CNC Coordinate Systems:

·         For milling: -

In order for the part programmer to plan the sequence of positions and movements of the cutting tool relative to the work piece, it is necessary to establish a standard axis system by which the relative positions can be specified. However, to make things easier for the programmer, we adopt the view point that the work piece is stationary while the drill bit is moved relative to table. Accordingly, the coordinate system of axes is established with respect to the machine table.

Two axes, x and y, are defined in the plane of the table, as shown in Figure. The z axis is perpendicular to this plane and movement in the z direction is controlled by the vertical motion of the spindle. The positive and negative directions of motion of tool relative to table along these axes are as shown in Figure. CNC drill presses are classified as either two- axes or three- axes machines, depending on whether or not they have the capability to control the z axis.

A numerical control milling machine and similar machine tools (boring mill, for example) use an axis system similar to that of the drill press. However, in addition to the three linear axes, these machines may possess the capacity to control one or more rotational axes. Three rotational axis axes are defined in CNC: the, b, and c axis. These axes specify angle about the x, y, and z axes, respectively. To distinguish positive from negative angular motions, the “right-hand rule” can be used. Using the right hand with the thumb pointing in the positive linear axis direction (x, y, or z), the fingers of the hand is curled to point in the positive rotational direction.

·         For turning

For turning operations, two axes are normally all that are required to command the movement of the tool relative to the rotating work piece. The z axis is the axis of rotation of the work part, and x axis defines the radial location of the cutting tool. This arrangement is illustrated in Figure.

The purpose of the coordinate system is to provide a means of locating the tool in relation to the work piece. Depending on the CNC machine, the part programmer may have several different options available for specifying this location.

Positioning of machine origin

·         Fixed Zero: -

The programmer must determine the position of the tool relative to the origin (zero point) of the coordinate system. CNC machines have either of two methods for specifying the zero point. The first possibility is for the machine to have a fixed zero. In this case, the origin is always located at the same position on the machine table. Usually, that position is the southwest corner (lower left-hand corner) of the table and all tool locations will be defined by positive x and y coordinates.

·         Floating Zero: -

The second and more common feature on modern CNC machines allows the machine operator to set the zero point at any position on the machine table. This feature is called floating zero. The part programmer is the one who decides where the zero point should be located. The decision is based on part programming convenience. For example, the work part may be symmetrical and the zero point should be established at the center of symmetry. The location of the zero point is communicated to the machine operator. At the beginning of the job, the operator moves the tool under manual control to some “target point” on the table. The target point is some convenient place on the work piece or table for the operator to position the tool. For example, it might be a predrilled hole in the work piece. The target point has been referenced to the zero point by the part programmer. In fact, the programmer may have selected the target point as the zero point for tool positioning. When the tool has been positioned at the target point, the machine operator presses a “zero” button on the machine tool console, which tells the machine where the origin is located for subsequent tool movements.

MODE OF POSITIONING:

·         Absolute positioning: -

Another option sometimes available to the part programmer is to use either an absolute system of tool positioning or an incremental system. Absolute positioning means that the tool locations are always defined in relation to the zero point. If a hole is to be drilled at a spot that is 8 in. above the x axis and 6 in. to the right of the y axis, the coordinate location of the hole would be specified as x=+6.000 and y=. +8.000.

·         Incremental positioning: -

Incremental positioning means that the next tool location must be defined with reference to the previous tool location must be defined with reference to the previous tool location. If in our drilling example, suppose that the previous hole had been drilled at an absolute position of x=+4.000 and y=+5.000.

Accordingly, the incremental position instructions would be specified as x=+2.000 and y=+3.000 in order to move the drill to the desired spot. Figure illustrates the difference between absolute and incremental positioning.

CNC motion control systems: -

In order to accomplish the machining process, the cutting tool and work piece must be moved relative to each other. In CNC, there are three basis types of motion control systems: -

·         Point- to- point CNC

Point-to-point (PTP) is also sometimes called a positioning system. In PTP, the objective of the machine tool control system is to move the cutting tool to a predefined location. The speed or path by which this movement is accomplished is not important in point-to-point CNC.

Once the tool reaches the desired location, the machining operation is performed at that position. CNC drill presses are a good example of PTP systems. The spindle must first be positioned at a particular location on the work piece. This is done under PTP control. Then the drilling of the hole is performed at the location, and so forth. Since no cutting is performed between holes, there is no need for controlling the relative motion of the tool and workpiece between whole locations. Figure illustrates the point-to-point type of control.

·         Straight-cut CNC: -

Straight-cut control systems are capable of moving the cutting tool parallel to one of the major axes at a controlled rate suitable for machining. It is therefore appropriate for performing milling operations to fabricate work pieces of rectangular configurations. With this type of CNC system it is not possible to combine movements in more than a single axis direction. Therefore, angular cuts on the work piece would not be possible. An example of a straight-cut operation is shown in figure. A CNC machine capable of straight cut movements is also capable of PTP movements.

·         Contouring CNC:-

Contouring is the most complex, the most flexible, and the most expensive type of machine tool control. It is capable of performing both PTP and straight-cut operations. In addition, the distinguishing feature of contouring CNC systems is their capacity for simultaneous control of more than one axis movement of the machine tool. The path of the cutter is continuously controlled to generate the desired geometry of the work piece. For this reason, contouring systems are also called continuous-path CNC systems. Straight or plane surfaces at any orientation, circular paths, conical shapes, or most any other mathematically definable form are possible under contouring control. Figure illustrates the versatility of continuous path CNC. Milling and turning operations are common examples of the use of contouring control.

In order to machine a curved path in a numerical control contouring system, the direction of the feed rate must continuously be changed so as to follow the path. This is accomplished by breaking the curved path into very short straight-line segments that approximate the curve. Then the tool is commanded to machine each segment in succession.

What results is a machined outline that closely approaches the desired shape. The maximum error between the two can be controlled by the length of the individual line segments, as illustrated in Figure.

G codes

Code	Function
G00	Point to point positioning mode of control(Rapid Transverse)
G01	Linear interpolation mode of control(Linear Transverse)
G02	Circular interpolation Arc Clockwise(Normal Dimension)
G03	Circular interpolation Arc counter clockwise(Used for normal dimension)
G04	Dwell-A predetermined time delay before executing (Current block instructions.)
G05	HOLD-An infinite delay before executing current block instructions terminated only by operator or interlock switch.
G06	Unassigned-May acquire standard use.
G07	Avoid Acceleration
G08	Reacceleration
G09	Linear interpolation used for long dimensions
G10	Linear interpolation used for short dimension
G11	3-D-interpolation
G12 to 16	Axis Selection
G17	XY Plane selection
G18	ZX Plane Selection
G19	YZ Plane Selection
G20	Circular interpolation Arc CW(inches mode)(used for long Dimensions)
G21	Circular interpolation Arc CW for (mm) mode (used for short Dimensions)
G22	Coupled motion-
G23	Coupled motion-
G24	Unsigned
G25	Start of sub routine
G26	End of sub routine
G27 to 29	Unassigned
G30	Reserved for contouring CCW(long distance)
G31	Reserved for contouring CCW (Short distance)
G32	Unassigned
G33	Thread cutting (constant lead)
G34	Thread cutting (increasing lead)
G35	Thread cutting (Decreasing lead)
G36	Used for control purpose only
G37	Calling of subroutine
G38
G39	Permanently unassigned
G40	Cutter compensation (Cancel)
G41	Cutter compensation (Left)
G42	Cutter compensation (Right)
G43	Cutter compensation (Positive)
G44	Cutter compensation (Negative)
G45 to 51	Unassigned
G52	Unassigned and reserved for adaptive Control
G53	Linear shift can el
G54	Linear shift (x)
G55	Linear Shift(y)
G56	Linear Shift(z)
G57	Linear Shift(xy)
G58	Linear Shift(xz)
G59	Linear Shift(yz)
G60 to 61	Unassigned
G62	Positioning fast
G63	Tapping
G64	Change of rate
G65	Cassette load
G66	Cassette save
G67	Cassette Search
G68 to 69	Unassigned
G70	Inch programming on CNC tools which accept dimensions in inches as well as millimeters
G71	Metric programming
G72 to 77	Unassigned
G78	Mill cycle
G79	Mill cycle
G80	Fixed cycle cancel
G81	Repeat function-Fixed Turning Cycle/Drilling cycle.
G82	Circular cycle/Drill Dwell
G83	Drilling cycle
G84	Rectangular cycles(Threading cycle)
G85 to 89	Unassigned
G90	Absolute dimension programming
G91	Incremental Dimension Programming
G92	Position preset
G93	Unassigned
G94	Feet rate in mm/min(inches/mm)
G95	Feet rate in mm/rev(inches/rev)
G96	Constant surface speed (mm/min)
G97	Speed (Rev/min)
G98	Speed (rev/Min)
G99	Floating Datum

M Codes

M00	Program stop
M01	Optional (planned) stop
M02	End of Program
M03	Spindle start in clockwise direction.
M04	Spindle start in ACW direction.
M05	Spindle stop
M06	Tool change
M07	Coolant on (Type 2-fluid Cooling)
M08	Coolant on (Type 1-Mist cooling)
M09	Coolant Off
M10	Clamp
M11	Unclamp
M12	Unassigned
M13	CW spindle start-coolant ON
M14	ACW spindle start + Coolant ON
M15	Motion + ve
M16	Motion – ve
M17	Unassigned
M18
M19	Oriented spindle stop
M20	Auxiliaries.
M21	Input
M22 to 29	Unassigned
M30	End of tap, similar to M02 except that it must include rewinding of tape to end of record, thus ready for next work piece.
M31	Interlock by-pass
M32 to 35	Constant cutting speed (used with turning)
M36	Feed Range 1
M37	Feed Range 2
M38	Spindle speed range 1
M39	Spindle speed range 2
M40 to 47	Gear Change
M48	Cancel M49
M49	Bypass override
M50	Coolant No.3 ON
M51	Coolant No.4 ON
M52 to 54	Unassigned
M55	Linear tool shift position 1
M56	Linear tool shift position 2
M57 to 59	Unassigned
M60	Work piece change
M61	Linear work piece shift position 1
M62	Linear work piece shift position 2
M63 to 67	Unassigned
M68	Clamp work piece
M69	Unclamp work piece
M70	Unassigned
M71	Angular work piece shift position 1
M72	Angular work piece shift position 2
M73 to 77	Unassigned
M78	Clamp slide
M79	Unclamp slide
M80 to 89	Unassigned

 CUT VIEWER:

Cut Viewer Turn V3.1 is an easy to use program that graphically displays the material removal process for turning operations in 2 axes. Based on the stock statements and tool definitions, Cut Viewer will show you exactly what material will be removed from a raw stock.

HARDWARE REQUIREMENT:

Cut Viewer is a 32-bit application. It runs in Windows95/98/ME/NT/2000/XP. The Hardware minimums include a Pentium processor, 32 MB of RAM, 20 MB of hard disk space, and a video graphics card with at least 2 MB of video RAM.

STOCK DEFINATION

L - Stock Length

D1 - Cylinder Diameter

D2 - Hole Diameter

Z - Origin Z

The stock reference point (origin) is the center of the right face. Origin Z indicates the z position of the program origin relative to the stock origin.

If the Stock has more complicated shape than cylinder it may be defined by sequence of G-code lines prefixed with the character your CNC controller uses for a comment line.

This sequence must be placed between STOCK/BEGIN and STOCK/END commands.

TOOL DEFINITIONS:

Standard OD tool: -

TOOL/STANDARD, BA, A, R, IC, ITP

Standard ID Tool has the same definition (Back angle instruct Cut Viewer to the orientation of the tool):

TOOL/STANDARD, BA, A, R, IC, ITP

Note: The IC (Inside Circle) is the diameter for which the tool insert geometry is created about. The IC is an industry standard term used by all insert manufactures. The ITP (Imaginary Tool Point) is the intersection of the vertical and horizontal edges of the tool and this point often is used for Tool Path programming.

The ITP is a value indicating the tip position of the Imaginary Tool Point with respect to the Tool Nose Radius Center Point as illustrated below.

ITP=0 if the Tool Nose Radius Center Point is used for Tool Path programming.

Button OD Tool: -

TOOL/BUTTON, R, L, W, OA, ITP

OA=90 (orient angle)

Button ID Tool:

TOOL/BUTTON, R, L, W, OA, ITP

OA=270

Grooving OD tool:

TOOL/GROOVE, R1, R2, L, W, A1, A2, OA, ITP

OA=90

Grooving ID tool:

TOOL/GROOVE, R1, R2, L, W, A1, A2, OA, ITP

OA=270

For Face tool OA=0

Note: To change control point (left or right tool corner) simply changes sign of W value.

Threading OD tool:

TOOL/THREAD, A, L, W, OA

Threading ID tool:

TOOL/THREAD, A, L, W, OA

Drill:

TOOL/DRILL, D, A, L

Note: You can insert a new tool into the NC file at the needed position.

Part Programming codes that are available with this machine are according to the latest ISO standards and adopted by the Industrial CNC controller manufacturers worldwide. Following G, M, F, T, S commands are included in this controller.

Part Programming:

The programmer carefully converts the sequence of operations to a set of instructions, i.e., (part program).

Part programming consists of sequence of blocks. Each block has a specific function to perform. Machine read one block & commands the tool or other slides to perform that operation. After this controller shifts to the next block.

In this way complete machining is performed which consists of small step operation define by each block. Let us take example of some blocks.

1) Format: G02 X__ Z__ I__ K__ F__ (I, K Format)

OR G02 X__ Z__ R__ F__

Here in first syntax,

I = Distance between start point & center point of arc along X-axis.

K= Distance between start point & center point of arc along Z-axis.

& in second syntax

R = Radius of the arc.

The G02 command is utilized to move the tool in the circular arc profile. With G02 the movement will be in the Clockwise direction. The movement taken will be at the programmed feed rate.

  

Example:

To machine the radius of 10mm on diameter =20mm, starting from center point i.e. X=0, Z=0 up to the length of 10mm.Following program code is used

G02 X20 Z-10 I0 K-10 F30

OR G02 X20 Z-10 R10 F30

2) Format: G03 X__ Z__ I__ K__ F__

G03 X__ Z__ R__ F__

Here,

I = Distance between start point & center point of arc along X-axis.

K= Distance between start point& center point of arc along Z-axis.

Example: To machine the radius of 25mm on diameter =50mm, starting from X=0

Z=0, following program code is used.

A MANUAL PART PROGRAM:

The program contains G and M codes. G codes are called preparatory Codes. They prepare the machine for cutting operation e.g. linear interpolation, circular interpolation, rapid etc. M codes are called miscellaneous codes. They perform all other operation except for cutting like spindle ON/OFF, coolant ON/OFF, tools changing etc. The manual part program looks like the following statement.

N10 G90 G00 X + 100 Y – 100 Z + 50; (Single Block)

EXAMPLE OF SOME MANUAL PART PROGRAMS:

1.       AUTOCAD DRAWING FOR G02: -

(TOOL/STANDARD, 40, 40, 0, 10, 3)

(COLOR, 255,255,255)

(STOCK/40, 24, 0, 0)

• N01 M03 S2000 -------------Spindle Start Clockwise with Spindle Speed 2000
• N02 G00 X25 Z2 -----------Rapid Positioning Up to the Reference Point (i.e.=25, Z=2)
• N06 G02 X24 Z-1 R1 F80--Circular Interpolation Clockwise with radius 1mm & Feed rate 80mm/min
• N08 G00 Z0---------------- Rapid Transverse upto reference point (Z=0)
• N10 G01 X20 F80---------- Linear interpolation upto specified point (X= 20) with Feed 80mm/min
• N12 G02 X24 Z-2 R2 F80- Circular Interpolation Clockwise with radius 2 mm & Feed rate 80mm/min
• N14 G00 Z0----------------- Rapid Transverse upto reference point (Z=0)
• N16 G01 X18 F80-----------Linear interpolation upto specified point (X= 18) With feed 80mm/min.
• N18 G02 X24 Z-3 R3 F80-- Circular Interpolation Clockwise with radius 3 mm & Feed rate 80mm/min
• N20 G00 Z0 -------------- -- Rapid Transverse upto reference point (Z=0)
• N22 G01 X16 F80--------- Linear interpolation upto specified point (X= 16) With feed 80mm/min.
• N24 G02 X24 Z-4 R4 F80-- Circular Interpolation Clockwise with radius 4mm & Feed rate 80mm/min
• N26 G00 Z0--------------- Rapid Transverse upto reference point (Z=0)
• N28 G01 X14 F80---------- Linear interpolation upto specified point (X= 14) With feed 80mm/min
• N30 G02 X24 Z-5 R5 F80-- Circular Interpolation Clockwise with radius 5 mm & Feed rate 80mm/min
• N32 G00 Z0----------------- Rapid Transverse upto reference point (Z=0)
• N34 G01 X12 F80-----------Linear interpolation upto specified point (X= 12) With feed 80mm/min
• N36 G02 X24 Z-6 R6 F80— Circular Interpolation Clockwise with radius 6 mm & feed rate 80mm/min
• N38 G00 Z0----------------- Rapid Transverse upto reference point (Z=0)
• N40 G01 X10 F80-----------Linear interpolation upto specified point (X= 10) With feed 80mm/min
• N42 G02 X24 Z-7 R7 F80-- Circular Interpolation Clockwise with radius 7 mm & Feed rate 80mm/min
• N44 G00 Z0--------------- Rapid Transverse upto reference point (Z=0)
• N46 G01 X8 F80------------ Linear interpolation upto specified point (X= 8) With feed 80mm/min
• N48 G02 X24 Z-8 R8 F80-- Circular Interpolation Clockwise with radius 8 mm & Feed rate 80mm/min
• N50 G00 Z0---------------- Rapid Transverse upto reference point (Z=0)
• N52 G01 X6 F80-------------Linear interpolation upto specified point (X= 6) With feed 80mm/min
• N54 G02 X24 Z-9 R9 F80-- Circular Interpolation Clockwise with radius 9mm & Feed rate 80mm/min
• N56 G00 Z0------------- -- Rapid Transverse upto reference point (Z=0)
• N58 G01 X4 F80-------------Linear interpolation upto specified point (X= 4) With feed 80mm/min
• N60 G02 X24 Z-10 R10 F80-- Circular Interpolation Clockwise with radius 10 mm & feed rate 80mm/min
• N62 G00 Z0------------------Rapid Transverse up to reference point (Z=0)
• N64 G01 X2 F80-------------Linear interpolation up to specified point (X= 2) With feed 80mm/min
• N66 G02 X24 Z-11R11 F80-- Circular Interpolation Clockwise with radius 11 mm & feed rate 80mm/min
• N68 G00 Z0----------------- Rapid Transverse up to reference point (Z=0)
• N70 G01 X0 F80----------- Linear interpolation up to specified point (X= 0) With feed 80mm/min
• N72 G02 X24 Z-12 R12 F80-- Circular Interpolation Clockwise with radius 12 mm & feed rate 80mm/min
• N74 G00 X25 Z2------Rapid Transverse up to reference point (X=25, Z=2)
• N76 M30---------------Program End & Rewind

The simulation for the above example is shown below:

TOOL PATH

2.       AUTOCAD DRAWING FOR G71: -

SAMPLE PROGRAM: -

(TOOL/STANDARD, 40, 40, 0, 10, 3)

(COLOR, 255,255,255)

(STOCK/55, 25, 0, 0)

• N10 M03 S2000------------- Spindle Start Clockwise with Spindle Speed 2000
• N20 G00 X30 Z5------------ Rapid Positioning Up to the Reference Point (i.e.(Diameter)X=30, (Length)Z=5)

• N30 G71 U1.0 R0.5---------Stock Removal Operation with depth of Cut 1mm & Escaping Amount 0.5mm

• N40 G71 P50 Q120 U0.5 W0.5 F80 S2000--------- Stock Removal Operation for Turning Start of Block (P)=50 & End of Block (Q)=120 with Finishing Allowance 0.5mm on each axis & Feed rate 80 mm/min

• N50 G01 X10
• N60 G01 Z-8 F80
• N70 G01 X15 Z-12 Programmed Block
• N80 G01 Z-21
• N90 G01 X23 Z-33
• N100 G01 Z-38
• N110 G01 X25 Z-40
• N120 G01 Z-45

• N130 G70 P50 Q120 -----------Finishing Cycle for the previously Defined Block (P)=50 & (Q) =120

• N140 G00 X30 Z5------------- Rapid Positioning Up to the Reference Point (i.e.(Diameter)X=30, (Length)Z=5)

• N150 M30------------------------ Program End & Rewind

The simulation for the above example is shown below:

TOOL PATH

PROGRAMMING TIPS

Programming is just like any other work- with good knowledge and appositive attitude; it can be done right and with first class results. Here are some tips to get the best result from any programming effort.

•  Approach CNC programming in a logical and methodological way.
• Always calculate unknown values – never guess.
• Check the actual size of the blank material – do not count on the paper dimensions.
• Standardize a programming style and adhere to it.
• Program dimensional values in absolute mode whenever possible.
• Make a setup sheet and/or tooling sheet before programming, not after.
• Program as many machining operations in a single setup as possible.
• Use minimum numbers of tools for maximum number of jobs- standardize.
• Always program for the safety of CNC machining.
• Document your work and store everything relating to the program development.
• Use cutter radius compensation for contouring, if possible.
• Use any suitable built in cycles the CNC system offer.
• Watch for programming errors- syntax and logical- all errors are avoidable.
• Use a microcomputer and text editor to write and print the program hard copy.
• Apply the use of subprograms to prevent errors caused by repetition.
• Make sketches for calculation simple, clear and always in scale.
• Place comments to the CNC operator in the program printed copy.
• Do not forget ‘small’ items – such as coolant and spindle stop.
• The CNC operator can be an excellent resource of valuable information –communicate.
• Avoid programming excessive clearances or dwells.
• Keep the program under your control – not the operators.
• Admit an error if you made one – do not blame other people unfairly.
• Always write the program for the convenience of the CNC operator – not yours.
• Check, double check and triple check the program.

·         Write as much of the program in one sitting as possible

 ADVANTAGES OF CNC MACHINE

Most of the advantages derived from CNC technology are due to the high level of automation, high flexibility of CNC machines and their ability to combine multifunction machining requirements in minimum number of workstations and setups.

The significant advantages are as follows:

• High Accuracy and Repeatability
• Reduced Inspection
• Ease of Assembly and Interchangeability
• Less Scrap and Rework
• Reduction in floor space/number of men/handling, results in better management control over the production.
• Development of new work is done faster with the usage of CNC machines
• Saving in Jigs and Fixtures as well as in dead time
• Less Material Handling
• Cost Accounting and Production control becomes very precise
• Dependence on skilled operators can be dispensed with
• Optimum utilization of Horse Power of the machine.
• Increase effective machine utilization
• 
  
• Reduced Usage of tools
• Machine can switch over to different jobs, as setup times are very low. Tool setting is done on the presetting devices and switching off the machine. Cutting speeds, feeds, depths of cut, dimensions etc. are stored in the form of tape
• Less paper work
• In process inventory gets reduced due to reduction in lead time and faster setup time
• Ability to combine operations on CNC machines makes the production faster and eliminates waiting time of components in between machines and stage inspection
• Change in design can be easily incorporated as it means only change of tape

            Ability for higher levels of integration such as

a.       Distributed Numerical Control (DNC)

b.       Flexible Manufacturing System (FMS)

c.        Adaptive Control System (ACS)

d.       Computer Aided Design(CAD)

e.       Computer Aided Manufacturing (CAM)

f.         Computer Integrated Manufacturing (CIM)