Work Holding Devices Used During Operations on Machine

When cutting operations are performed on the machine, lots of forces are generated. To counter these forces the work and the tool have to be held rigidly so that during cutting operation there should not be any vibration or jerk. The tool is held rigidly in the tool post with the help of bolts. The work is held in the various types of work holding devices like three jaw chuck, four jaw chuck, combination chuck, magnetic chuck, hydraulic chuck, face plate, driving plate, angle plate and lathe carriers.

Types of Work Holding Devices

The following are the various work holding devices used on lathe machine:

• Chuck
• Face Plate
• Driving Plate
• Angle Plate
• Lathe carrier or lathe dog

Chucks

Chucks are efficient and true devices for holding the work on the lathe during the operation. The most common types of chucks are:

(a) Three jaw chuck

(b) Four jaw chuck

(c) Magnetic chuck

(d) Combination chuck

(e) Collet chuck               

(a) Three Jaw Chuck. It is also known as three jaw self centering chuck. It consists of a cylindrical body having three jaws fixed radially at its front. Further, it carries a through hole at its centre to enable the long job to project backward in the spindle. It consists of a circular disc having spiral scroll at its front and a rack at its back. The rear face of jaws is provided with slots corresponding to the scroll serrations. The bevel pinions are fitted radially at the back of the disc and they mesh with the rack at its back. Top of these pinions are matching with the outer surface of the chuck and have square slots to accommodate the chuck key. For operating the chuck anyone of these pinion can be rotated by means of the chuck key which, intern, revolves the scroll disc. This causes all the jaws to move in the radial direction simultaneously. Normally two sets of jaws are provided with each chuck, one for gripping on the outside surface & other for inside.

(b) Four Jaw Chuck. In outside appearance, It is very much similar to the three jaw chuck except that it has four jaws, but its internal mechanism differs totally. The rear portion of its jaws is threaded & is engaged with separate adjusting screws. With the result all the jaws can be moved separately and adjusted at desired distance from the centre of the chuck. Due to this it is also known as independent jaw chuck. This enables the chuck to successfully hold irregular or eccentric jobs in addition to the regular cylindrical shaped jobs. It is possible to reverse the same jaws so that the work can be gripped from inside surfaces also.

(c) Magnetic Chuck. It holds the job by magnetic force. The following are the two types of magnetic chucks. These are explained in the subsequent paragraphs:

(i) Permanent Magnetic Chuck. It is generally used on lathes and surface grinders. For this no electric current is needed. When the operating lever brings the magnet to ‘ON’ position the flux created inside. The magnetic flux passes through the work and hence holds it. It can hold light and thin jobs without any clamp. The complicated jobs, which are difficult to be clamped in other chucks, can be held on it without use of clamping. These are made either in rectangular or circular shape. The following are different parts of permanent magnetic chuck:

• Body. It is made of cast iron. There is a provision in the body for keeping the magnetic plates. The body of the round chuck is connected with a back plate, which is threaded to be screwed on the nose of the spindle.
• Base. It is also made of cast iron. In rectangular chuck the base is used for mounting on the table. In round chuck the base plate is used for mounting on the lathe spindle.
• Magnetic  Plates. These are magnetic plates of high strength, which are kept in the body.
• Non-Ferrous Metal Plate. A non-ferrous metal plate such as brass or aluminium is kept around the magnetic plates. This plate does not allow having any magnetic effect in the body of chuck.
• Lever or Handle. A lever is provided in the chuck, which converts the magnetic field, and the magnet is "on".

(ii) Electro Magnetic Chucks. These are also called temporary captivating chucks. It implies the use of electric current for developing a strong electromagnet, which holds the job centrally in chuck.

Precautions

Following precautions to be observed while handling the magnetic chucks:

• Light works should be clamped on magnetic chucks.
• Always take light cuts.
• When the job is to be machined on lathe, the speed and feed should be kept low.
• Small packing should be provided around the work.
• The surface of the job and the chuck should be properly cleaned from oil etc.

(d) Combination Chuck. As the name implies, a combination chuck may be used as a self centering or as an independent chuck to take the advantages of both types. The jaws may be operated individually by separate screws or simultaneously by the scroll disc. The screws mounted on the frame have teeth cut on its underside which messes with the scroll and all the jaws together with the screws move radially when the scroll is made to rotate by a pinion.

(e) Collet Chuck. It fits in to the spindle nose of the headstock. It can be used on a centre lathe, Capstan lathe, or turret lathe for producing items form bar stock. It is constructed  with a hollow body having internal threads for screwing on the spindle nose. The slits in collets provides it with springing action to allow the bar for easy passing. These are of two types:

• Draw in type
• Push out

Face Plate

It is usually a circular cast iron disc having threaded hole at its centre so that it can be screw to the threaded nose of the spindle. It consists of number of holes and slots by means of which the work can be secured.

Driving Plate

It is a cast circular disk having a projected boss at its rear. The boss carries internal threads so that it can be screwed on spindle nose. It also carries a hole to accommodate a pin which engages with the tail of a lathe dog or carriers.

Angle Plate

It is employed for holding odd shape work in conjunction with a faceplate. When the shape of the work is such that it is not possible to mount it directly on the face plate it can be mounted on angle plate.

Lathe Carriers or Lathe Dogs

These are used in conjunction with the driving plate. The work to be inserted in the ‘V’ shaped hole of the carrier and then firmly secured in position by means of a screw, Lathe dogs have two types of tails:

(a) Straight tail

(b) Bent tail