Unit 5 Numerical ControlSections:Fundamentals of NC TechnologyComputer Numerical ControlDistributed Numerical ControlApplications of NCNC Part Programming

Process Planning A manufacturing plan is needed to convert the product design into a physical entity. The activity of developing such a plan is called process planning.Process planning involves determining the sequence of processing and assembly steps that must be accomplished to make the product.

Process ParametersSelecting quality assurance methods

Routing SheetIt is the sheet which describes all the processes used to manufacture the part.Routing sheet is the step of process panning.It is the physical representation of process planning step.

Routing Sheet

Numerical Control (NC) DefinedProgrammable automation in which the mechanical actions of a machine tool are controlled by a program containing coded alphanumeric data that represents relative positions between a work head (e.g., cutting tool) and a work partMachineControl UnitPowerProgramInstructionsTransformationProcess

NC Coordinate SystemsFor flat and prismatic (block-like) parts:Milling and drilling operationsConventional Cartesian coordinate systemRotational axes about each linear axis

For rotational parts:Turning operationsOnly x- and z-axes

Motion Control SystemsPoint-to-Point systemsAlso called position systemsSystem moves to a location and performs an operation at that location (e.g., drilling)Also applicable in robotics

Continuous path systems Also called contouring systems in machiningSystem performs an operation during movement (e.g., milling and turning)

Interpolation MethodsLinear interpolationStraight line between two points in spaceCircular interpolationCircular arc defined by starting point, end point, center or radius, and directionHelical interpolationCircular plus linear motionParabolic and cubic interpolationFree form curves using higher order equations

Absolute vs. Incremental Positioning Absolute positioning Move is: x = 40, y = 50

Incremental positioning Move is: x = 20, y = 30.

Computer Numerical Control (CNC)Storage of more than one part programVarious forms of program inputProgram editing at the machine toolFixed cycles and programming subroutinesInterpolationAcceleration and deceleration computationsCommunications interfaceDiagnostics

Machine Control Unit

DNCDirect numerical control (DNC) control of multiple machine tools by a single (mainframe) computer through direct connection and in real time1960s technologyTwo way communicationDistributed numerical control (DNC) network consisting of central computer connected to machine tool MCUs, which are CNCPresent technologyTwo way communication

Applications of NC

NC Application Characteristics (Machining) Batch and High Volume productionRepeat and/or Repetitive ordersComplex part geometriesMundane operationsMany separate operations on one part

Cost-Benefits of NCCostsHigh investment costHigh maintenance effortNeed for skilled programmersHigh utilization required

BenefitsCycle time reductionNonproductive time reductionGreater accuracy and repeatabilityLower scrap ratesReduced parts inventory and floor spaceOperator skill-level reduced

NC Part ProgrammingManual part programmingManual data inputComputer-assisted part programmingPart programming using CAD/CAM

Manual Part ProgrammingBinary Coded Decimal SystemEach of the ten digits in decimal system (0-9) is coded with four-digit binary numberThe binary numbers are added to give the valueBCD is compatible with 8 bits across tape format, the original storage medium for NC part programsEight bits can also be used for letters and symbols

Types of WordsN - sequence number prefixG - preparatory wordsExample: G00 = PTP rapid traverse moveX, Y, Z - prefixes for x, y, and z-axesF - feed rate prefixS - spindle speedT - tool selectionM - miscellaneous commandExample: M07 = turn cutting fluid on

Example: Word Address FormatN001 G00 X07000 Y03000 M03N002 Y06000

Cutter OffsetCutter path must be offset from actual part outline by a distance equal to the cutter radius

Manual Data InputMachine operator does part programming at machineOperator enters program by responding to prompts and questions by systemMonitor with graphics verifies tool pathUsually for relatively simple partsIdeal for small shop that cannot afford a part programming staffTo minimize changeover time, system should allow programming of next job while current job is running

Computer-Assisted Part ProgrammingWrite machine instructions using natural language type statementsStatements translated into machine code of the MCUAPT (Automatically Programmed Tool) Language

6.1.1 Coordinate System/Cartesian coordinate system: the xyz system is a right-hand systemPositive motion: moving the cutting tool away from the workpiece.

6.1.1 Coordinate System/Z AXISworkpiece-rotating machine: Z is parallel to the spindletool-rotating machine: Z is parallel to the tool axisZZZZ // rotating spindle

6.1.1 Coordinate System/X AXISworkpiece-rotating machine: X is the direction of tool movement horizontal milling machine : X axis is parallel to the table vertical machine : +X axis points to the right when the programmer is facing the machine. ZZZXXXX // table

6.1.5 NC Words/ N, G, X, Y Z, A, B, C, I, J, K, F, S, T, R, MN: specify the sequence number

G: preparatory word to prepare for control functions (the motion of each axis, coordinate system, coordinate plane, cutter radius compensation, tool length offset)

M: miscellaneous word to control miscellaneous functions (spindle on/off, start/stop the machine, turn on/off the coolant, change the tool, and rewind the program tape)

6.1.5 NC Words/

6.1.5 NC Words/M CODES

M00Program stop M06Tool changeM01Optional stopM07Flood coolant onM02End of programM08Mist coolant onM03Spindle CWM09Coolant offM04Spindle CCWM30End of tape

6.1.5 NC Words/N, G, X, Y , Z, A, B, C, I, J, K, F, S, T, R, MF: feed rate of the tool motionS: cutting-speed

T: tool number

G codesUnit selection: G70 (inch), G71(metric)Coordinates selection: G90(absolute), G91(incremental)Working plane selection: G17(X-Y), G18(Z-X), G19(Y-Z)

Set up working coordinateMachine zerois unchangeable and usually is set up with the machineWorking coordinate (is decided by the offset from the machine zero)

Rapid traverse: G00G00: to make the machine move at maximum speed. It is used for positioning motion. G90 G00 X20.0 Y10.0G90: absolute coordinates(0,0)(10,10)(20,10)

Linear interpolation: G01 G01: linear interpolation at feed speed. G91 G0l X200.0 Y100.0 F200.0 G91: incremental coordinates

Circular interpolation: G02, G03 G02, G03: For circular interpolation, the tool destination and the circle center are programmed in one block G02 is clockwise interpolation, G03 is counterclockwise interpolationEnd pointCircle center, radius

Circular interpolation: G02, G03G91 G02 X60.0 Y20.0 R50.0 F300.0G91 G02 X60.0 Y20.0 R-50.0 F300.0Specify R with sign before it:180 +R>180 -R

Return to Machine Zero: G28 G28The machine passes by the specified point, and then move to the machine zero.

G91 G28 X_, Y_, Z_X,Y,Z are the coordinate of specified point that is saved in memory of machine.Usually this instruction is used in the beginning and end of program.G91: incremental coordinates

Return to the Specified point : G29G29 The machine passes from the machine zero by the point that is specified in G28, and then move to the new specified point.

G29 X_, Y_, Z_X, Y, Z are the coordinate of new specified pointG28,G29 are usually used to change tool

Tool CompensationTool-Radius Compensation /Left hand G41 Right hand G42 Cancel tool-radius compensation G40Tool-Height Compensation/Positive G43 Negative G44 Cancel tool-height compensation G49

Tool-Radius CompensationTool-radius compensations make it possible to program directly from the drawing, and thus eliminate the tool-offset calculation

G41 (G42) HH: the radius of tool to compensate is saved in a memory unit that is named HG41/G42 is directly related with direction of tool movement and which side of part is cut.

Tool-Height Compensation

G43 (G44) H

H: specified memory unit used to save height compensation of tool.Positive compensation (G43): real position = specified position + value saved in HNegative compensation (G44): real position = specified position - value saved in H

Tool-Height CompensationExample:N0010 G91 G00 X12.0 Y80.0 N0020 G44 Z-32.0 H02

If we put 0.5mm into H02, real position = -32.0 - 0.5 = -32.5

Cancel tool-height compensation: G49

G91: incremental coordinates

M codemiscellaneous word/miscellaneous functions: turn the spindle on/offstart/stop the machineturn on/off the coolantchange the toolrewind the program (tape)

M codemodal groups/

M00Program stop M06Tool changeM01Optional stopM07Flood coolant onM02End of programM08Mist coolant onM03Spindle CWM09Coolant offM04Spindle CCWM30End of tape

M code: M00, M01M00 and M01 both stop the machine in the middle of a program. M01 is effective only when the optional stop button on the control panel is depressed. The program can be resumed through the control panel.

M code: M03, M04, M05M03 turns on the spindle clockwise. M04 turns the spindle on counterclockwise. (The spindle rpm must be specified in the same line or in a previous line. ) M05 turns off the spindle.

M code: M07, M08, M09M07 and M08 turn on different modes of coolant. M07: flood coolant onM08: mist coolant onM09 turns off the coolant.

M code: M06M06 signals the tool-change operation. On a machine equipped with an automatic tool changer, it stops the spindle, retracts the spindle to the tool-change position, and then changes the tool to the one specified in the T-code.

M code: M02, M30M02 marks the end of the program. M30 marks the end of the tape. It stops the spindle and rewinds the program (tape)

6.2 MANUAL PART PROGRAMMING/ Summary

Adequate for many simply point-to-point processes

Time-consuming Error correction can be cumbersome Errors made by the programmer are often not discovered until the program is tested graphically or on the machine tool

6.4 CAD PART PROGRAMMING/ CAD The modern way ----Creation of NC programs from CAD allow geometry to be described in the form of points, lines, arcs, and so on, just as it is on an engineering drawing, rather than requiring a translation to a text-oriented notation (compared with APT). Use of a graphics display terminal allows the system to display the resulting cutter-path geometry allow earlier verification of a program, which can avoid costly machine setups for program testing.

Several CAD/CAM systems Computervision System CADAM System CATIA System MasterCAM UG

6.4 CAD PART PROGRAMMING/ CAD Procedures

6.4 CAD PART PROGRAMMING/ CAD Part-programming

6.4 CAD PART PROGRAMMING/ CAD rough cutting semi-finishing finishing with end mill to remove cavity

Layer by layer/WorkWorkWorkStraight end millBall end millStraight end millAxial cutting depthAxial cutting depthAxial cutting depth

6.4 CAD PART PROGRAMMING/ CAD Tool path patterns (most popular)direction-parallel millingcontour-parallel milling

6.4 CAD PART PROGRAMMING/ CAD Applications(Zig-Zag) Contour Line

6.4 CAD PART PROGRAMMING/ CAD Tool path patterns (advanced) (extra spiral at corners)Plunge into part)

6.4 CAD PART PROGRAMMING/ CAD Tool path patterns (advanced)

Same roughness

6.4 CAD PART PROGRAMMING/ CAD Part-programming

6.4 CAD PART PROGRAMMING/ CAD Part-programming

6.4 CAD PART PROGRAMMING/ CAD Part-programming

6.4 CAD PART PROGRAMMING/ CAD Approach/Engage/Entry

Departure/Retract/Exit

6.4 CAD PART PROGRAMMING/ CAD Part-programming

6.5 TOOL-PATH VERIFICATION/ Purposes/ to detect geometric error of the cutter pathto detect potential tool interferenceto detect incorrect cutting conditions

6.5 TOOL-PATH VERIFICATION/ /Overcut and undercut

potential problems in sculptured surface machining

6.5 TOOL-PATH VERIFICATION/ Ways to verify a part program/

To make a dry cut on the machine without the workpiece to actually machine a prototype in wax, plastic, wood, foam, or some other soft material.

In a CAD-based system, cutter-path abnormalities can be detected by visual inspectioni.e. the display of an animation sequence that shows the tool moving along its generated path which is superimposed on the part geometry.

6.5 TOOL-PATH VERIFICATION/

*Thumbindex fingermiddle [long] finger ring fingerwedding ringlittle [small] finger*Traverse Circular, dwell, acceleration, deceleration, metric format,*MDI: Multi-Document Interface*correspondingly*CAMCAMR *,,,*Curvature:., *