Turning fixtures & Collets

1. TURNING FIXTURES

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. 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. 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.

6. 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.

7. 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.

8. 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.

9. 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

10. 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.

11. 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.

20. Collets

21. 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)

22. TYPES OF COLLETS
 Push-out collets
 Pull-in collets
 Dead length collets

23. Push-out collets

24. Pull-in collets

25. Dead length collets

26. Cylindrical Liners

27. Mandrels

28. Mandrels
Mandrels are internal locators used for
concentric machining of outside
diameter of work pieces with finished
bores.

29. Types of Mandrels
 Tapered Mandrels
 Axial clamping Mandrels
 Expanding Mandrels
 Threaded Mandrels

30. Tapered Mandrels

31.  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.

32. 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.

33. Axial Clamping Mandrels (less
accurate for production purpose)

34.  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.

35.  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.

36. Expanding Mandrels

37.  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 .

38.  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.

39. Threaded Mandrels

40. 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.