C.N.C (COMPUTER NUMERIC CONTROL)

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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:
(i) COMPUTERS
(ii) CONTROL SYSTEM
(iii) DRIVE MOTORS
(iv) TOOL CHANGERS
ACCORDING TO THE CONSTRUCTION OF CNC MACHINE TOOLS, IT WORKS IN THE FOLLOWING (SIMPLIFIED) MANNER:-
1. 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.
2. 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.
3. 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.

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 SOME BODY 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.

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.

DRIVE MOTORS:-THE DRIVE MOTORS CONTROL THE MACHINE SLIDE MOVEMENT ON CNC EQUIPMENT. THEY ARE CLASSIFIED INTO FOUR BASIC TYPES AS FOLLOWS:-

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.

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.

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.

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.

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 CENTRE, 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 CENTRE, 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. 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
1. 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.

2. 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 SOME 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 :-

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:-
1. 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 WORK PIECE BETWEEN WHOLE LOCATIONS. FIGURE ILLUSTRATES THE POINT-TO-POINT TYPE OF CONTROL.

2. 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.

3. 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.

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



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):
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.
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


DRILL:
TOOL/DRILL, D, A, L




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.

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.


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)


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.
1. APPROACH CNC PROGRAMMING IN A LOGICAL AND METHODOLOGICAL WAY.
2. ALWAYS CALCULATE UNKNOWN VALUES – NEVER GUESS.
3. STANDARDIZE A PROGRAMMING STYLE AND ADHERE TO IT.
4. PROGRAM DIMENSIONAL VALUES IN ABSOLUTE MODE WHENEVER POSSIBLE.
5. MAKE A SETUP SHEET AND/OR TOOLING SHEET BEFORE PROGRAMMING, NOT AFTER.
6. PROGRAM AS MANY MACHINING OPERATIONS IN A SINGLE SETUP AS POSSIBLE.
7. USE MINIMUM NUMBERS OF TOOLS FOR MAXIMUM NUMBER OF JOBS- STANDARDIZE.
8. ALWAYS PROGRAM FOR THE SAFETY OF CNC MACHINING.
9. DOCUMENT YOUR WORK AND STORE EVERYTHING RELATING TO THE PROGRAM DEVELOPMENT.
10. WATCH FOR PROGRAMMING ERRORS- SYNTAX AND LOGICAL- ALL ERRORS ARE AVOIDABLE.

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:
ü HIGH ACCURACY AND REPEATABILITY:
ü REDUCED INSPECTION:
ü EASE OF ASSEMBLY AND INTERCHANGEABILITY.
ü LESS SCRAP AND REWORK
ü REDUCTION IN FLOOR SPACE/NUMBER OF MEN/HANDLING, RESULTS IN BETTER MANAGEMENT CONTROL OVER THE PRODUCTION.
ü DEVELOPMENT OF NEW WORK IS DONE FASTER WITH THE USAGE OF CNC MACHINES.
ü SAVING IN JIGS AND FIXTURES AS WELL AS IN DEAD TIME;
ü LESS MATERIAL HANDLING.
ü COST ACCOUNTING AND PRODUCTION CONTROL BECOMES VERY PRECISE.
ü DEPENDENCE ON SKILLED OPERATORS CAN BE DISPENSED WITH.
ü OPTIMUM UTILIZATION OF HORSE POWER OF THE MACHINE.
ü INCREASE EFFECTIVE MACHINE UTILIZATION:
ü REDUCED USAGE OF TOOLS.
ü LESS PAPER WORK.


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