Accessories

The devices employed for holding and supporting the work and the tool on the lathe are called accerssries. They incluedethe device like chucks, driving plat ,dogs,toolholder,andposts,centers,mandrels,jigs and fixtures ,etc the selection of the accessories is governed by the type of the job to be made.

1-Workpiece holding accessories:

A) Chucks:
1-Three-jaw:


A three-jaw chuck is a rotating clamp which uses three or 'jaws', usually interconnected via a scroll gear (scroll plate), to hold onto a tool or work piece. Three-jaw chucks are usually self-centering (as a result of the jaws' meshing with the scroll plate) and are best suited to grip circular or hexagonal cross sections when very fast, reasonably accurate (±.005" TIR) centering is desired. Independent-jaw versions can be obtained.
The image shows a three-jaw chuck and key with one jaw removed and inverted showing the teeth that engage in the scroll plate. The scroll plate is rotated within the chuck body by the key, the scroll engages the teeth on the underside of the jaws which moves the three jaws in unison, to tighten or release the workpiece.

2-Four-jaw:

A four-jaw chuck is similar to a three-jaw chuck, but with four jaws, each of which can be moved independently. This makes them ideal for (a) gripping non-circular cross sections and (b) gripping circular cross sections with extreme precision (when the last few hundredths of a millimeter [or thousandths of an inch] of runout must be manually eliminated). The non-self-centering action of the independent jaws makes centering highly controllable (for an experienced user), but at the expense of speed and ease. Four-jaw chucks are almost never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads.
The image shows a four-jaw chuck with the jaws independently set. The key is used to adjust each jaw separately.

3-Multi jaw:
For special purposes, and also the holding of fragile materials, chucks are available with six or eight jaws. These are invariably of the self-centering design, and are built to very high standards of accuracy.
Two jaw chucks are available and can be used with soft jaws (typically an aluminum alloy) that can be machined to conform to a particular workpiece. Many chucks have removable jaws, which allows the user to replace them with new jaws, specialized jaws, or soft jaws.

4-Self-centering four jaw:
A four jaw chuck with a mechanism for centering the work piece. Sometimes used to refer to chucks where the jaws are moved in interconnected pairs.

5-Magnetic:
Used for holding ferromagnetic work pieces, a magnetic chuck consists of an accurately centered permanent magent face. Electromagnets or permanent magnets are brought into contact with fixed ferrous plates, or 'pole pieces', contained within a housing. These pole pieces are usually flush with the housing surface. The part or 'work piece' to be held forms the closing of the magnetic loop or path, onto those fixed plates, providing a secure anchor for the work piece.

6-Face plates:
For irregular shapes, a face plate can be used. The face plate shown in Fig.2 has racially placed slots which allow the workpiece to clamp to it by means of bolts.

B) lathe centers:

For accurate turning operation, or in cases where the work surface is not truly cylindrical, the workpiece can be turned between centers. This form of work holding is show in fig.3 initially the workpiece has a conical hole drilled at each end to provide location for the lathe centers.

1- Dead center:

A dead center (one that does not turn freely, ie: - dead) may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece. Lubrication is therefore required between the center and workpiece to prevent friction welding from occurring. Additionally the tip of the center may have an insert of carbide which will reduce the friction slightly and allow for faster speeds. Dead centers may also be fully hardened to prevent damage to the important mating surfaces taper of the taper and to preserve the 60 ° nose taper.

2-Soft center:
Soft centers are identical to dead centers except the nose is deliberately left soft (unhardened) so that it may be readily machined to the correct angle prior to usage. This operation is performed on the headstock center to ensure that the centers axis is aligned with the spindles axis.

3-Live or revolving center:
A live center or revolving center is constructed so that the 60 ° center runs in its own bearings and is used at the non driven or tailstock end of a machine. It allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures. They are used almost exclusively in CNC lathes as well as for general machining operations.
The term live center may also refer to a dead center when mounted in the spindle of the machine, where it is considered to be live by virtue of the spindle bearings rather than its own bearings.

4-Pipe center:
A pipe center has a larger diameter at the 60 ° taper end. This allows the center to be used in the bore of a pipe (or similar workpiece). While a pipe center ensures the workpiece remains concentric, its main advantage is that it supports the workpiece securely. Thin walled material such as pipes easily collapse if excessive pressures are used at the chuck end.

5-Cup center:
The cup center is a variation of the live center and is used in woodworking to support the softer material around the actual center and prevent the material splitting.

6-Drive center:
A drive center is used in the driving end of the machine (headstock). It consists of a dead center surrounded by hardened teeth. These teeth bite into the softer workpiece allowing the workpiece to be driven directly by the center. This allows the full diameter of the workpiece to be machined in a single operation, this contrasts with the usual requirement where a carrier is attached to the workpiece at the driven end. They are often used in woodworking or where softer materials are machined

C) Mandrel:

It is a device for holding and rotation a hollow piece of work that has been previously drilled or bored. The work revolves with the mandrel which is mounted between two centers. The mandrel should be true with accurate centre holes for machining outer surface of the workpiece, concentric with the bore. To avoid distortion and wear, it is made of high carbon steel. For different sizes of holes in workpiece, different mandrels.

D) rests:

When very long job is to be turned between centers on lather, due to its own weight it

provides a springing action and carries a lot of bending moment. The result is that turning tool is spoiled very soon, and may even break sometimes. To avoid this, such jobs are always supported on an attachment known as "steady rest" .this prevent the deflection of the job and at the same time enables the operator to take heavy cuts.fig.5 shows the steady rest and follower rests.