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CNC Milling_enginerring projectpresent.ppt
- 1. CNC Milling- AnIntroduction Department of Mechanical Engineering Indian Institute of Technology Patna ME 110:Workshop Practice-I
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- 3. Classification of millingmachines Milling is the process of machining using rotary cutter to remove material by advancing a cutter into a workpiece. •According to nature of purpose of use: general purpose; single purpose; special purpose •According to configuration and motion of the work holding table / bed: Knee type; bed type; planar type; rotary table type •According to the orientation of the spindle(s): Plain horizontal knee type; horizontal axis (spindle) and swiveling bed type; vertical spindle type; universal head milling machine •According to mechanization / automation and production rate: Hand mill (milling machine); Planer and rotary table type vertical axis milling machine; Tracer controlled copy milling machine; Milling machines for short thread milling; Computer Numerical Controlled (CNC) milling machine;
- 4. Beginning of CNCmachines • In 1940 John Parson developed first machine able to drill holes at specific coordinates programmed on punch cards. • In 1951 MIT developed servo mechanism ‐ • In 1952 MIT developed first NC machines for milling. • In 1970 First CNC machines came into picture • Now a days modified 1970’s machines are being used. ‐ ‐ Languages of CNC machines While there is a fairly standard set of G and M codes, there is some variation in their application. For example a GO or GOO command is universally regarded as the command for rapid travel. Some older machines do not have a GO command. On these machines, rapid travel is commanded by using the F (feed) word address.
- 5. CNC Machines • flexibilityin automation • change-over (product) time, effort and cost are much less • less or no jigs and fixtures# are needed • complex geometry can be easily machined • high product quality and its consistency • optimum working condition is possible • lesser breakdown and maintenance requirement # Both jigs and fixtures clamp, support and locate the workpiece. A jig also guides the cutting tool. A fixture has a reference point for setting the cutting tool with reference to the workpiece. Replacement of hard or rigid automation by Flexible automation by developing and using CNC has made a great break through since mid seventies in the field of machine tools’ control. The advantageous characteristics of CNC machine tools over conventional ones are : Direct Numerical Control is a system that uses a central computer to control several machines at the same time.
- 6. • Conventionally, anoperator decides and adjusts various machining parameters like feed , depth of cut etc. depending upon the type of job, and controls the slide movements by hand. • In a CNC Machine functions and slide movements are controlled electronically by using computer programs rather than by hands. • The earliest NC machines performed limited functions and movements controlled by punched tape# or punch cards# . • Some of the enhancements that came along with CNC include: canned cycles, sub programming, cutter compensation, work coordinates, coordinate system rotation, automatic corner rounding, chamfering and B–spline interpolation. CNC Machines contd. # Punched tape or perforated paper tape is a form of data storage, consisting of a long strip of paper in which holes are punched to store data. # A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contained either commands for controlling automated machinery or data for data processing applications.
- 7. Basic CNC Principles •All computer controlled machines are able to accurately and repeatedly control motion in various directions. Each of these directions of motion is called an axis. Depending on the machine type there are commonly two to five axes. • Additionally, a CNC axis may be either a linear axis in which movement is in a straight line, or a rotary axis with motion following a circular path. All CNC machining begins with a part program, which is a sequential instructions or coded commands that direct the specific machine functions. • The part program may be manually generated or, more commonly, generated by computer aided part programming systems. • Conversational Control: It lets the programmer use simple descriptive language to program the part. This control then displays a graphical representation of the instructions so that the programmer can verify the tool path.
- 8. CNC Machines- Advantages/Disadvantages Advantages: •High Repeatability and Precision e.g. Aircraft parts • Volume of production is very high • Complex contours/surfaces need to be machined. E.g. Turbines • Flexibility in job change, automatic tool settings, less scrap • More safe, higher productivity, better quality • Less paper work, faster prototype production, reduction in lead times Disadvantages: • Costly setup, skilled operators • Computers, programming knowledge required • Maintenance is difficult
- 9. Basic Length Unit(BLU) • In NC machine, the displacement length per one pulse output from machine is defined as basic length unit (BLU). • In CNC computer each bit (binary digit) represents 1 BLU. Bit = BLU • Example: if one pulse makes a servomotor rotate by 1 degree and the servomotor moves the table by 0.0001mm, 1 BLU will be 0.0001mm Stepper Motor Special type of synchronous motor which is designed to rotate through a specific angle (called step) for each electrical pulse received from the control unit.
- 10. Coordinate systems The machinetool uses Cartesian coordinate system. First identify the z-axis, x- and y- axes will then follow using the Right hand coordinate system.
- 11. Absolute Coordinate SystemIncremental Coordinate System Coordinate systems contd.
- 12. Coordinate systems contd. Machinecoordinate system Work part coordinate system
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- 14. CNC Movement • Eachaxis consists of a mechanical component, such as a slide that moves, a servo drive motor that powers the mechanical movement, and a ball screw to transfer the power from the servo drive motor to the mechanical component. • These components, along with the computer controls that govern them, are referred to as an axis drive system.
- 15. ~ Servo Controller Counter Comparator EncoderA/C Motor Input (converted from analog to digital value) Table Leadscrew
- 16. Encoder • Movement iscontrolled by motors (actuators) • Feedback is provided by sensors (transducers) • An encoder is a device or transducer that converts information from one format or signals to another. • Transducers (such as optical or magnetic encoders) sense position or orientation for use as a reference or active feedback to control position: • A rotary encoder converts rotary position to an analog. • A linear encoder similarly converts linear position to an electronic signal. Such encoders can be either absolute or incremental.
- 17. How CNC Works •Controlled by G and M codes. • These are number values and co-ordinates. • Typed in manually by machine operators or automatically generated by the computer software. • Movement is controlled by a motors (actuators). • Feedback is provided by sensors (transducers) • Tool magazines are used to change tools automatically
- 18. Program Input Different waysof data input are : •MDI : Manual Data Input •PROGRAM PREPARATION WITH CAD/CAM • PROGRAM DATA TRANSFER FROM PC TO CNC M/C •PROGRAM DATA TRANSFER FROM PC TO DNC OPERATIONS Mechanical Clamping Devices
- 19. Automatic Part Programmingby CAD CAM Software programs can automatic generate CNC data Make 3D model Define Tool CNC data Simulate cutting
- 20. CNC programming Important thingsto know: • Coordinate System • Units, incremental or absolute positioning • Coordinates: X,Y,Z, RX,RY,RZ • Feed rate and spindle speed • Coolant Control: On/Off, Flood, Mist • Tool Control: Tool and tool parameters Programming consists of a series of instructions in form of letter codes • Preparatory Codes: G codes- Initial machining setup and establishing operating conditions N codes- specify program line number to executed by the MCU • Axis Codes: X,Y,Z - Used to specify motion of the slide along X, Y, Z direction • Feed and Speed Codes: F and S- Specify feed and spindle speed • Tool codes: T – specify tool number • Miscellaneous codes – M codes For coolant control and other activities
- 21. Programming Key Letters •O - Program number (Used for program identification) • N - Sequence number (Used for line identification) • G - Preparatory function • X - X axis designation • Y - Y axis designation • Z - Z axis designation • R - Radius designation • F – Feed rate designation • S - Spindle speed designation • H - Tool length offset designation • T - Tool Designation • M - Miscellaneous function
- 22. Important G (Preparatory)codes • G00 Rapid Transverse • G01 Linear Interpolation • G02 Circular Interpolation, CW • G03 Circular Interpolation, CCW • G15 Polar Coordinate System off • G16 Polar Coordinate System on • G17 XY Plane,G18 XZ Plane,G19 YZ Plane • G20/G70 Inch units • G21/G71 Metric Units • G28 Return to Reference Point • G40 Cutter compensation cancel • G41 Cutter compensation left • G42 Cutter compensation right • G43 Tool length compensation (plus) • G44 Tool length compensation (minus) • G49 Tool length compensation cancel • G51.1 - X0 X Mirror on • G50.1 - X0 X Mirror off G00 Rapid move G0 X# Y# Z# up to 6 axis or G0 Z# X# G01 Linear feedrate move G1 X# Y# Z# up to 6 axis or G1 Z# X# G02 Clockwise move G03 Counter clockwise move G04 Dwell time G08 Spline smoothing on, optional L# number of blocks to buffer G09 Exact stop check, spline smoothing Off G10 Linear feedrate move with decelerated stop G11 Controlled Decel stop G17 X Y Plane G18 X Z Plane G19 Y Z Plane G28 move to position relative to machine zero G53 Cancel fixture coordinate offsets G54-G59 fixture coordinate offsets 1 through 6
- 23. Important G (Preparatory)codes Contd. • G51.1 - Y0 Y Mirror on • G50.1 - Y0 Y Mirror off • G53 Work Coordinate System off • G54 …..G59 Work Coordinate System 1…6 • G68 Rotation on • G69 Rotation off • G73 Peck Drilling Cycle • G80 Cancel canned cycles • G81 Drilling cycle • G82 Counter boring cycle • G83 Deep hole drilling cycle • G90 Absolute positioning • G91 Incremental positioning • G94 Feed per Minute • G95 Feed per Revolution • G98 Return to Reference point in Drilling Operation • G99 Return to Initial point in Drilling Operation G70 Inch mode G71 Millimeter mode G80 Cancels canned cycles and modal cycles G81 Drill cycle G82 Dwell cycle G83 Peck cycle G84 Tapping cycle G85 Boring cycle 1 bore down, feed out G86 Boring cycle 2 bore down, dwell, feed out G88 Boring cycle 3 bore down, spindle stop, dwell, feed out G89 Boring cycle 4 bore down, spindle stop, dwell, rapid out G90 Absolute mode G91 Incremental mode G92 Home coordinate reset G93 cancel home offsets G98 - G199 User-definable G codes
- 24. Important M (Functional)codes • M00 Program stop • M01 Optional program stop • M02 Program end • M03 Spindle on clockwise • M04 Spindle on counterclockwise • M05 Spindle stop • M06 Tool change • M08 Coolant on • M09 Coolant off • M10 Clamps on • M11 Clamps off • M13 Spindle CW and Coolant On • M14 Spindle CCW and Coolant On • M30 Program stop, reset to start • M98 Sub-Program Call • M99 Sub-Program Exit
- 25. Profile Milling Example •G15 G17 G21 G94 G40 G69 G80 • G91 G28 Z0 • G28 X0 Y0 • M06 T1 • M03 S1200 • G90 G00 G54 X-30 Y-20 • G43 H1 Z10 • M08 • G01 Z-1 F120 • G01 Y20 • G02 X-20 Y30 R10 • G01 X20 • G03 X30 Y20 R10 • G01 Y-20 • G01 X20 Y-30 • G01 X-20 • G03 X-30 Y-20 R10 • G00 Z5 • M09 • G91 G28 Z0 • G28 X0 Y0 • M05 • M30