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Turning fixtures

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Turning fixtures & Collets

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Turning fixtures

  1. 1. TURNING FIXTURES
  2. 2. TURNING FIXTURES  COMPONENTS PRODUCED ON LATHE ARE CHEAPER DUE TO LESS MACHINE HOUR RATE COMPARED TO COSTLY MILLING MACHINE AND COSTLY MILLING CUTTER INSERTS.  AVAILABILITY OF LATHE IS MORE COMMON COMPARED TO MILLING MACHINES.  ASYMETRICAL JOBS CAN BE LOCATED USING A FIXTURE WHICH CAN BE DRILLED, TURNED, BORED, REAMED, ON LATHE BY USING SPECIALLY DESIGNED FIXTURE.
  3. 3. SOME OF THE DESIGN CONSIDERATIONS INTURNING FIXTURES  WHILE DESIGNING THE TURNING FIXTURES UTMOST CARE SHOULD BE TAKEN TO AVOID PROJECTIONS FOR THE OPERATOR’S SAFETY  THE ACCURACY OF THE MACHINE TOOL MUST BE PROTECTED BY PLACING NECESSARY BALANCE WEIGHTS IN THE FIXTURE
  4. 4. SOME OF THE DESIGN CONSIDERATIONS INTURNING FIXTURES  THE OVERHANG OF THE FIXTURE SHOULD BE MINIMUM  THE FIXTURE MUST BE PROPERLY LOCATED ON THE FACE PLATE  THE FIXTURE MUST BE DESIGNED CONSIDERING THE STANDARD SLOTS AVAILABLE IN FACE PLATE FOR CLAMPING THE FIXTURE.
  5. 5. WHY & WHEN FACE PLATE IS USED  FACE PLATE IS ACCURATELY TURNED AND LOCATED TO FIT THE NOSE OF THE MAIN SPINDLE.  THE TAPER ON SPINDLE AND TAPER ON THE BORE OF THE FACE PLATE MATCHES EXACTLY.  ASSYMETRICAL WORK PIECES CAN BE MACHINED USING A FACE PLATE.  TIME REQUIRED TO SET THE WORK IS ENORMOUS,LABORIOUS AND LESS SAFE. THIS MAY LEAD THE REJECTION OF WORK PIECE.  IT IS THEREFORE HIGHLY ADVISABLE TO GO FOR A FIXTURE, IF WE CAN JUSTIFY THE COST.
  6. 6. LATHE FIXTURES  THE DESIGNED FIXTURE CAN BE LOCATED ON A FACE PLATE.  THE FIXTURE MUST BE CLAMPED TO THE FACE PLATE SUITABLY.  THE FIXTURE WEIGHT MUST BE AS MINIMUM AS POSSIBLE AND DESIGN SHOULD TAKE CUTTING FORCES EXERTED BY THE TOOL.
  7. 7. GENERAL POINTS FOR A LATHE FIXTURE  GRIP THE ROTATING WORKPIECE SECURELY TO THE FIXTURE TO RESIST TORSIONAL FORCES.  THE FIXTURE SHOULD BE RIGID AND OVERHANG SHOULD BE MINIMUM POSSIBLE  CLAMPING SCREW SIZE SHOULD BE CAREFULLY SELECTED TO RESIST THE CUTTING TORQUE.  LOCATE THE WORKPIECE ON CRITICAL SURFACES FROM WHERE ALL OR MAJOR AND ANGULAR TOLERANCES ARE TAKEN.
  8. 8. GENERAL POINTS FOR A LATHE FIXTURE  PROVIDE ADEQUATE SUPPORT FOR FRAIL SECTION OR SECTIONS UNDER PRESSURE FROM LATHE TOOLS.  FIXTURE SHOULD BE ACCURATELY BALANCED TO AVOID VIBRATIONS AT HIGH SPINDLE SPEEDS.  PILOT BUSHING FOR SUPPORTING TOOLS SHOULD BE PROVIDED WHERE EXTREME ACCURACY IS REQUIRED IN BORING OPERATIONS  REDUCE TIME REQUIRED FOR LOADING AND UNLOADING TO JUSTIFY REDUCED TOTAL COST PER COMPONENT, FOR THE QUANTITY TO BE PRODUCED
  9. 9. HORIZONTAL BORING MACHINES  HORIZONTAL BORING MACHINES CAN ALSO BE USED AS MILLING, DRILLING AND TAPPING MACHINES.  THEY ARE LIMITED TO LOW TO MEDIUM SIZE RUNS.
  10. 10. CHARACTERISTICS OF A HBMS  WORK PIECE /JOB IS KEPT ON MACHINE TABLE BY USING A FIXTURE(WORK HOLDING FIXTURE OR BY DIALLING THE WORK PIECE). NEVER FORGET TO DIAL THE JOB AFTER CLAMPING TO ENSURE THAT THERE IS NO SHIFT IN CLAMPING.  HORIZONTAL SPINDLES THAT ROTATE THE CUTTING TOOLS.  POWER FEED OF THE SPINDLE TO ADVANCE CUTTING TOOLS INTO THE WORKPIECES.  POWER FEED RELATIVE MOTION BETWEEN SPINDLE AND WORKPIECE  POWER SADDLE FEED PARALLEL TO THE SPINDLE AXIS.
  11. 11. Collets
  12. 12. EXTERNAL LOCATORS  COLLETS ARE EXTENSIVELY USED TO LOCATE THE CUTTING TOOLS VERY ACCURATELY IN MILLING / LATHE /  COLLETS ARE MADE OF SPRING STEEL  THE VARIATION OF THE CUTTING TOOL SHOULD NOT BE TOO HIGH ( USUALLY THE TOLERANCE CUTTING TOOL IS h6 / h8)
  13. 13. TYPES OF COLLETS  Push-out collets  Pull-in collets  Dead length collets
  14. 14. Push-out collets
  15. 15. Pull-in collets
  16. 16. Dead length collets
  17. 17. Cylindrical Liners
  18. 18. Mandrels
  19. 19. Mandrels Mandrels are internal locators used for concentric machining of outside diameter of work pieces with finished bores.
  20. 20. Types of Mandrels  Tapered Mandrels  Axial clamping Mandrels  Expanding Mandrels  Threaded Mandrels
  21. 21. Tapered Mandrels
  22. 22.  These are tapered bars with centre at the ends for aligning with the machine axis and a flat for fixing the drive carrier.  Taper generally ranges from 0.4 to 0.5 mm per meter.  This limits the variation in the bore of the work pieces and so, for wide difference in the bores, different mandrels must be used.  The taper facilitates friction drive for the work piece, which is generally pressed on the mandrel with an arbor press.
  23. 23. For repeated use, the mandrels should be hardened and the centers protected by counter bores to prevent damage due to accidental fall. Tapered mandrels are rarely used in mass production as they require longer work piece loading and unloading time.
  24. 24. Axial Clamping Mandrels (less accurate for production purpose)
  25. 25.  These are extensively used for grinding outside diameter of bushes.  The locating diameter must be close push fit with the minimum size of the work piece bore. Consequently, the maximum bore size work piece would be loose on the mandrel by the amount equal to the work piece tolerance.  Thus, the possible concentricity b/w the inside and outside diameter of the work piece would be equal to the tolerance on the bore of the work piece.
  26. 26.  The work piece is clamped axially with a C washer and a hexagonal nut which permit quick loading and unloading.  The drive pin serves as an integral drive carrier. The mandrel is provided with protected centres and is hardened to minimise wear on the locating diameter.
  27. 27. Expanding Mandrels
  28. 28.  For a high degree of concentricity expanding mandrels should be used.  These permit adjustment of the locating diameter to suit the variation in the bore size of the work piece.  In fact, expanding mandrels clamp the work piece on the bore as collet grips the stock on the outside diameter.  Consequently there is no clearance between the mandrel and the work piece which explains the high degree of concentricity .
  29. 29.  The mandrel is split in to three or four pieces and are held against the expanding cone by a retainer spring, the clockwise rotation of the cone pushes the mandrel pieces outwards against the work piece to locate and clamp it.  The rotation of the cone in the opposite direction reduces the distance b/w the mandrel pieces which are pressed inwards by the retainer spring.  this contracts the mandrel to provide clearance b/w the mandrel and work piece for easy loading and unloading.
  30. 30. Threaded Mandrels
  31. 31. For work pieces with internal threads, the locating mandrel should have a clamping collar which has internal threads in the direction opposite to the thread in the work piece. these facilitate tightening of the work piece and the clamping collar against each other ,and prevents the collar from unscrewing during turning operation. for unclamping the work piece, lug on the clamping collar is knocked in an anti-clockwise direction by a hammer.

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