Cnc hardware


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Cnc hardware

  1. 1. CNC Hardware Arvind Deshpande4/10/2012 Arvind Deshpande(VJTI) 1
  2. 2. CNC4/10/2012 Arvind Deshpande(VJTI) 2
  3. 3. CNC Machining Centre4/10/2012 Arvind Deshpande(VJTI) 3
  4. 4. Components of CNC4/10/2012 Arvind Deshpande(VJTI) 4
  5. 5. Structure of Machine Tool Continuous use Higher cutting speeds and feeds results in higher cutting forces High speed of movement High static stiffness - To reduce deflection under heavy load. High stiffness to weight ratio – Better dynamic response4/10/2012 Arvind Deshpande(VJTI) 5
  6. 6. CNC Bed  Cast iron – Damping properties to reduce vibrations.  Heavy ribbing – To improve static stiffness.  Concrete – Low cost and better damping properties4/10/2012 Arvind Deshpande(VJTI) 6
  7. 7. CNC Bed4/10/2012 Arvind Deshpande(VJTI) 7
  8. 8. Spindle Very important element Subjected to higher cutting forces (radial and axial) Deflection to be minimized to get proper surface finish Proper supports with little overhang Proper cooling to remove the heat generated Oil cooling with heat exchanger Spindle motor and gearbox can be isolated to avoid heat transmission to machine tool structure4/10/2012 Arvind Deshpande(VJTI) 8
  9. 9. Spindle4/10/2012 Arvind Deshpande(VJTI) 9
  10. 10. Integral Spindle motor andcooling system4/10/2012 Arvind Deshpande(VJTI) 10
  11. 11. Rigid spindle assembly4/10/2012 Arvind Deshpande(VJTI) 11
  12. 12. Bifurcated Column structure  Used in horizontal machining centre  Increases torsional rigidity of the support structure4/10/2012 Arvind Deshpande(VJTI) 12
  13. 13. Spindle Drives Large power with infinite variation in speed DC motors were very popular with speed control using voltage variation Nowadays AC motors are used with speed control using frequency variation.4/10/2012 Arvind Deshpande(VJTI) 13
  14. 14. Feed Drives Electric motors are by far the most common component to supply mechanical input to a linear motion system. Stepper motors and servo motors are the popular choices in linear motion machinery due to their accuracy and controllability. They exhibit favorable torque-speed characteristics and are relatively inexpensive.1. Stepper motors – Open loop control system2. Servomotor – D. C. Motor with feedback elements. Used in closed loop control system. Used in high capacity CNC machines requiring high torgue. Sturdy, powerful and reliable4/10/2012 Arvind Deshpande(VJTI) 14
  15. 15. Open Loop vs. Closed Loop controls4/10/2012 Arvind Deshpande(VJTI) 15
  16. 16. Open loop systems Programmed instructions are fed into the controller through an input device. These instructions are then converted to electrical pulses (signals) by the controller and sent to the servo amplifier to energize the servo motors. Drawback - No feedback system to check whether the program position and velocity has been achieved. If the system performance is affected by load, temperature, humidity, or lubrication then the actual output could deviate from the desired output. Generally used in point-to-point systems where the accuracy requirements are not critical. Very few continuous-path systems utilize open-loop control.4/10/2012 Arvind Deshpande(VJTI) 16
  17. 17. Closed loop systems Feedback subsystem to monitor the actual output and correct any discrepancy from the programmed input. Use position and velocity feed back. The feedback system could be either analog or digital. The analog systems measure the variation of physical variables such as position and velocity in terms of voltage levels. Digital systems monitor output variations by means of electrical pulses. To control the dynamic behavior and the final position of the machine slides, a variety of position transducers are employed. Majority of CNC systems operate on servo mechanism, a closed loop principle. If a discrepancy is revealed between where the machine element should be and where it actually is, the sensing device signals the driving unit to make an adjustment, bringing the movable component to the required location. Closed-loop systems are very powerful and accurate4/10/2012 Arvind Deshpande(VJTI) 17
  18. 18. Stepper Motors Rotates in steps (fixed angular increments) 200 steps per revolution i.e 1.80 per pulse Permanent magnet rotor Micro step motors – 25000 steps/revAdvantages1. Low cost2. Compact3. Simple in construction4. High reliability5. No maintenance (no brushes)6. Excellent torgue at high speeds7. Stepper motors can easily accelerate a load.8. There is usually no need for clutches and brakes (unless a large external load is acting, such as gravity).4/10/2012 Arvind Deshpande(VJTI) 18
  19. 19. Stepper Motors9. Stepper motors are inherently digital. The number of pulses determines position while the pulse frequency determines velocity.10. Easily and accurately controlled.11. They offer excellent heat dissipationDisadvantages1. The motor can lose position if the load torque exceeds the motors available torque.2. Open loop stepper motor systems should not be used for high- performance or high-load applications.3. Damping may be required when load inertia is very high to prevent motor shaft oscillation at resonance points.4. Stepper motors may perform poorly in high-speed applications.4/10/2012 Arvind Deshpande(VJTI) 19
  20. 20. Stepper Motor4/10/2012 Arvind Deshpande(VJTI) 20
  21. 21. Stepper motor4/10/2012 Arvind Deshpande(VJTI) 21
  22. 22. Stepper motor4/10/2012 Arvind Deshpande(VJTI) 22
  23. 23. 4/10/2012 Arvind Deshpande(VJTI) 23
  24. 24. Stepper Motor1. If a stepper motor has 24 poles on the rotor and 30 poles on the stator, what will be the step angle of this stepper motor?2. A stepper motor with a step angle of 1.60 is attached to the lead screw of machine table. The pitch of the lead screw is 2 mm. What will be the axial distance traveled for an input pulse of 300 to the stepper motor?4/10/2012 Arvind Deshpande(VJTI) 24
  25. 25. Servo Motors Servo motors are more robust than stepper motors, but pose a more difficult control problem. They are primarily used in applications where speed, power, noise level as well as velocity and positional accuracy are important. Servo motors are not functional without sensor feedback. 3 common types The DC brush type servo motors are most commonly found in low- end to mid-range CNC machinery. The DC brushless type are typically applied to high-end CNC machinery. AC servo motors are another variety that offers high-end performance4/10/2012 Arvind Deshpande(VJTI) 25
  26. 26. 4/10/2012 Arvind Deshpande(VJTI) 26
  27. 27. Linear Motors Series of magnets attached to machine base and a set of electrical coils around a steel laminate attached to moving slide No mechanical parts in contact No wear or periodic maintenance required Not limited in travel like ball screws Can produce parts with higher accuracy and close tolerances at higher speeds and feeds4/10/2012 Arvind Deshpande(VJTI) 27
  28. 28. Hydraulic system4/10/2012 Arvind Deshpande(VJTI) 28
  29. 29. Lead Screw To convert rotary motion from drive motor to linear motion of the slides Square threads Sliding contact results in higher friction Low efficiency (35-55%) Greater power requirement Backlash CNC requires high speed of movement where friction will be very high4/10/2012 Arvind Deshpande(VJTI) 29
  30. 30. Recirculating Ball Screw Balls circulate between screw and but Balls are recirculated using deflector or return tube Preloading to reduce backlashAdvantages1. Longer life2. Relatively small wear. Will maintain accuracy through entire life of the screw.3. Rolling motion reduces friction considerably. Can be used for carrying heavier loads at faster speeds.4. High efficiency(85-95%)5. Small power requirement4/10/2012 Arvind Deshpande(VJTI) 30
  31. 31. Recirculating Ball Screw withdeflector4/10/2012 Arvind Deshpande(VJTI) 31
  32. 32. Recirculating Ball Screw withExternal Return Tube4/10/2012 Arvind Deshpande(VJTI) 32
  33. 33. Recirculating Ball Screw4/10/2012 Arvind Deshpande(VJTI) 33
  34. 34. Recirculating Ball Screw A ball screw has lead of 0.125 in/rev. What is the distance the nut will travel if the screw is turned 15 degrees?4/10/2012 Arvind Deshpande(VJTI) 34
  35. 35. Use of Rolling Friction forSlideways4/10/2012 Arvind Deshpande(VJTI) 35
  36. 36. Use of Rolling Friction forSlideways4/10/2012 Arvind Deshpande(VJTI) 36
  37. 37. Closed Loop Control System4/10/2012 Arvind Deshpande(VJTI) 37
  38. 38. Feedback Elements Rotary or linear Absolute or incremental LED Photo transistor or photo diode 4/10/2012 Arvind Deshpande(VJTI) 38
  39. 39. Absolute Encoder  Sensor detects exact position of the shaft  Chances of error whenever 2 bits change  Grey code used to reduce error so that only one bit changes at a time4/10/2012 Arvind Deshpande(VJTI) 39
  40. 40. Absolute Encoder4/10/2012 Arvind Deshpande(VJTI) 40
  41. 41. Incremental Encoder 1 track and 2 sensors More resolution Can measure relative position only An incremental encoder has 240 strips engraved on it. What will be no. of strips to be swept for 300 rotation?4/10/2012 Arvind Deshpande(VJTI) 41
  42. 42. Rotary Feedback Elements4/10/2012 Arvind Deshpande(VJTI) 42
  43. 43. Linear Feedback Elements4/10/2012 Arvind Deshpande(VJTI) 43
  44. 44. Linear Feedback Elements4/10/2012 Arvind Deshpande(VJTI) 44
  45. 45. Mechanical tape reader4/10/2012 Arvind Deshpande(VJTI) 45
  46. 46. 4/10/2012 Arvind Deshpande(VJTI) 46
  47. 47. 4/10/2012 Arvind Deshpande(VJTI) 47
  48. 48. Tooling System4/10/2012 Arvind Deshpande(VJTI) 48
  49. 49. Tool Presetting  CNC programme is prepared for tool tip  Reference can be a fixed point on the tool holder  Measure the distance of tool tip from reference4/10/2012 Arvind Deshpande(VJTI) 49
  50. 50. Tool Turret  Simple method  Spindle combined with tool turret  Tool change with tool change procedure  Tool is identified directly with tool position4/10/2012 Arvind Deshpande(VJTI) 50
  51. 51. Chain type Tool Magazine4/10/2012 Arvind Deshpande(VJTI) 51
  52. 52. Chain type Tool Magazine4/10/2012 Arvind Deshpande(VJTI) 52