Jigs & fixture turning
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Four jaw independent chuck :Used to hold square, rectangular regular & irregular shapes of work pieces.Initially to set theworkpiece it takes more time.This chuck includesfourindependent jawsFour square holesaround of the bodyBy using key , jaw can be taken in or out independently move .
Three jaw universal orself centering chuck :Three jaws and three square slots or holes are provided on the circumference of the body.By inserting the key in any one slot, all jaws can be moved simultaneously.Speedy centering of the work piece.Cannot hold large size of work piece.
Combination chuck :Jaws can be moved in and out simultaneously or independently.Advantages of both the varieties in this chuck, so it is called combination.Scroll plate is used to move the jaws simultaneously.A screw is used for independent movement.
Magnetic chuck :Used to hold thin work pieces which cant hold by chuck.An electro magnet or permanent magnet provides the holding power.In all position flux passes through the work piece and holds.
Collet chuck :In production shop speedy and accurate centering of work piece is necessary.Hence to hold the bar stock collet chuck is used.Outer portion ofcollet is tapered.
Drill chuck :Used for drilling, reaming, tapping operations.It is held in the sleeve of the tail stock or spindle of the headstock.It contains key operated centering jaws for holding the tool.
Lathe centers :Work to be turned between centers must have center hole drilled in each end Provides bearing surface Support during cutting.Most common have. solid Morse taper shank 60 centers, steel with carbide tips.Care to adjust and lubricate occasionally.
Steady rest :The base is made up of cast iron.It is clamp on the lathe bed where the work piece is to be supported.Three jaws are provided.All jaws can be radially moved with independentscrew.
follower rest :It is c type in shape.Two adjustment jaws tosupport the work piece.It is fitted on the rear portion of the carriage by bolt, hence it moves along with the carriage.Used during the finishing operations or for carrying out turning along the entire length of the jaw.
Mandrels :tapered MandrelsAxial clamping MandrelsScrewed Mandrelexpanding Mandrel
* Mandrels are internal locators * Used for external turning* Mandrels is used to hold the hollow/drilled work piece.
Lathe dogs :
Lathe dogs are cast metal devices used to provide a firm connection between the headstock spindle and the work piece mounted between centers.
Lathe face plate :A lathe face plate is a flat round plate that threads to the headstock spindle to the lathe.The faceplate is used for irregularly shaped work piece that cannot successfully held by chucks.
WHY & WHEN FACE PLATE IS USEDface 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.asymmetrical work pieces can be machined using a face plate.long time is required to set the work and its less safe. this may lead to the rejection of work piece.it is therefore highly advisable to go for a fixture, if we can justify the cost.
TURNING FIXTURESComponents 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.Asymmetrical jobs can be located using a fixture which can be drilled, turned, bored, reamed, on lathe by using specially designed fixture.
TURNING FIXTURESThe 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.
SOME OF THE DESIGN CONSIDERATIONS IN TURNING FIXTURESWhile designing the turning fixtures care should be taken to ensure the operators safety .The accuracy of the machine tool must be protected by placing necessary balance weights in the fixture .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.
GENERAL POINTS FOR A LATHE FIXTUREReduce time required for loading and unloading to justify reduced total cost per component, for the quantity to be produced 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
GENERAL POINTS FOR A LATHE FIXTURE
Holding the rotating work piece securely to the fixture to resist tensional forces.The fixture should be rigid and overhang should be minimum possibleClamping screw size should be carefully selected to resist the cutting torque.Locate the work piece on critical surfaces from where all or major and angular tolerances are taken.
Old design for fixture for producing brake drumsThe new design
EXTERNAL LOCATORSCollets are extensively used to locate the cutting tools very accurately in milling / lathe /Collets are made of spring steelThe variation of the cutting tool should not be too high ( usually the tolerance cutting tool is h6 / h8)
TYPES OF COLLETS
Push-out colletsPull-in colletsDead length collets
Dead length collets
Mandrels Mandrels are internal locators used for concentric machining of outside diameter of work pieces with finished bores.
Types of MandrelsTapered MandrelsAxial clamping MandrelsExpanding MandrelsThreaded Mandrels
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.
* 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.
Axial Clamping Mandrels (less accurate for production purpose)
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.
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 centers and is hardened to minimize wear on the locating diameter.
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 .
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.
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.
The successful running of any mass production depends upon the interchangeability to facilitate easy assembly and reduction of unit cost. Mass production methods demand a fast and easy method of positioning work for accurate operations on it. Jigs and fixtures are production tools used to accurately manufacture duplicate and interchangeable parts. Jigs and fixtures are specially designed so that large numbers of components can be machined or assembled identically,