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CNC

CNC

vkrishnaraj
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CNC

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  1. CAM Systems & CNC Machine Overview - Lecture 3 Overview to Computer Aided Manufacturing - ENGR-2963 - Fall 2005 Class Manager - Sam Chiappone

  2. History • 1955 - John Parsons and US Air Force define a need to develop a machine tool capable of machining complex and close tolerance aircraft parts with the same quality time after time (repeatability). MIT is the subcontractor and builds the machine for the project.

  3. History: Continued • 1959 - MIT announces Automatic Programmed Tools (APT) programming language • 1960 - Direct Numerical Control (DNC). This eliminates paper tape punch programs and allows programmers to send files directly to machine tools

  4. History: Continued • 1968 - Kearney & Trecker machine tool builders market first machining center • 1970’s - CNC machine tools & Distributed Numerical Control • 1980’s - Graphics based CAM systems introduced. Unix and PC based systems available

  5. History: Continued • 1990’s - Price drop in CNC technology • 1997 - PC- Windows/NT based “Open Modular Architecture Control (OMAC)” systems introduced to replace “firmware” controllers.

  6. Control Systems • Open-Loop Control • Stepper motor system • Current pulses sent from control unit to motor • Each pulse results in a finite amount of revolution of the motor001” is possible

  7. Control Systems • Open-Loop Limitations • Control unit “assumes” desired position is achieved • No positioning compensation • Typically, a lower torque motor • Open-Loop Advantages • Less complex, Less costly, and lower maintenance costs

  8. Control Systems • Closed-Loop Control • Variable DC motors - Servos • Positioning sensors -Resolvers • Feedback to control unit • Position information compared to target location • Location errors corrected

  9. Control Systems • Closed-Loop Advantages • DC motors have the ability to reverse instantly to adjust for position error • Error compensation allows for greater positional accuracy (.0001”) • DC motors have higher torque ranges vs.. stepper motors • Closed-loop limitations • Cost

  10. Three Basic Categories of Motion Systems • Point to Point - No contouring capability • Straight cut control - one axis motion at a time is controlled for machining • Contouring - multiple axis’s controlled simultaneously

  11. Three Basic Categories of Motion Systems

  12. CNC vs. NC Machine Tools • Computer Numerical Control (CNC) - A numerical control system in which the data handling, control sequences, and response to input is determined by an on-board computer system at the machine tool.

  13. CNC • Advantages • Increased Program storage capability at the machine tool • Program editing at the machine tool • Control systems upgrades possible • Option -resident CAM system at machine tool • Tool path verification

  14. NC • Numerical Control (NC) - A control system which primarily processes numeric input. Limited programming capability at the machine tool. Limited logic beyond direct input. These types of systems are referred to as “hardwire controls” and were popular from the 1950’s to 1970’s.

  15. Machining Centers • A machining center can be defined as a machine tool capable of: • Multiple operation and processes in a single set-up utilizing multiple axis • Typically has an automatic mechanism to change tools

  16. Machining Centers • Machine motion is programmable • Servo motors drive feed mechanisms for tool axis’s • Positioning feedback is provided by resolvers to the control system

  17. Machining Centers • Example - A turning center capable of OD turning, external treading, cross-hole drilling, engraving, and milling. All in machining is accomplished in one “set-up.” Machine may have multiple spindles.

  18. Machining Centers

  19. Programming Methods • Automatically Programmed Tools (APT) • A text based system in which a programmer defines a series of lines, arcs, and points which define the overall part geometry locations. These features are then used to generate a cutter location (CL) file.

  20. Programming Methods-APT • Developed as a joint effort between the aerospace industry, MIT, and the US Airforce • Still used today and accounts for about 5 -10% of all programming in the defense and aerospace industries

  21. Programming Methods-APT • Requires excellent 3D visualization skills • Capable of generating machine code for complicated part programs • 5 axis machine tools

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