Lecture 3 CNC Machining.pdf

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Curtin University
Mechanical Engineering
Alokesh Pramanik

Lecture 3: CNC Machining 26 October 2013 12:32 PM • Introduction • CNC Machine Tool Axis Conventions • Absolute vs. Incremental Positioning • Reference Coordinates • CNC MotionControl system • CNC Part Program • Sample Program • DVD on ComputerNumerical Control • CNC (ComputerNumerical Control):form of NC system whose Machine Control Unit (MCU) is based on dedicated microcomputerrather than a hardwires controller • NC (Numerical Control): form of programmableautomationin which the process is controlled by numbers, letters and symbols. CNC Machine • Storage of morethan one part program • Various forms of program input • Program editing at the machine tool • Fixed cycles and programming subroutines • Interpolation • Positioning features for setup • Cutter length and size compensation • Communicationsinterface • Diagnostics • Automatic tool change facility • On machine measurementfacility • Convenient in workpieceswhere: ○ Parts are processed frequently and in small lot sizes ○ Part geometryis complex ○ Many operationsmust be performedon the part in its processing ○ Much metal needs to be removed ○ Engineering design changes are likely ○ Expensive part where mistakes in processing are costly ○ Requires 100%inspection • Reduced non-productive time which results in shorter cycle times. • Lower manufacturing lead time • Simpler fixtures • Greater manufacturing flexibility • Improved accuracy • Reduced inventory • Reduced human error • Higher investmentcost • Higher maintenance cost • Finding/training NC personnel Coordinate Systems • Axis: any direction of motionthat is totally controlled by specific commands • Most commonmachine tools have 5-axes • Programmedin a Cartesian coordinate system • Z-axis always aligned with direction of spindle • X-axis normally parallel to longest dimension of primary machine table • Y-axis normally parallel to shortest dimension of primary machine table • Y-axis normally parallel to shortest dimension of primary machine table • A, B, C designation angular motions around X, Y, Z axes respectively • Axes definition sown indicate machine slide movements • In case of a turning centre these movementsare identical to movementin relation to the work • On machining centres, where is it the table and not the cutting tool which moves,this is not the case • On a vertical machining centre, in order to achieve a tool movementin relation to the workpiece in the positive direction it is necessary to program a machine slide movementin the X-negative direction • NC Positioning system:convertsthe coordinates specified in the NC part program into relative positions and velocitiesbetween tool and workpart • Leadscrew pitch (p): table is movea distance equal to the pitch for each revolution • Table Velocity (F): set by the RPM of the leadscrew • To provide x-y capability, a single-axis system is piggybacked on top of a second perpendicular axis • Absolute positioning: No accumulation of positioning errors, faster programming, easier to debug and trace • Incremental positioning: take advantage of macros and other subroutines, continued programming errors, cumulative errors • Reference coordinates:used to calculate other points on the workpiece;only used in absolute coordinates,referred to as zero points (machine reference point, reference return point, program reference point, work reference point) • Machine reference point: machine home; used for setup and calibration; initial setup of the machine, reference point for other coordinate points, tool change point • Reference return point: fixed position in which the machine tool builder uses a physical limit switch to obtain; only position on the machine that can be found after powering off, hardwire limit switch; most work coordinate systems(G54-G59)and work reference points are measured from the reference return point. • Program reference point: machine-toolcoordinates are usually not practical for part programming;allow the programmer to define a convenient origin point in order to create geometry;known as 'set point', must be defined before any programming can occur • Work reference points: work zeroes are the origin of the workpiece coordinatesystem that can be placed anywhere on the machine table; defined relati
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