Work that cannot be readily mounted between the lathe centres is usually held in a chuck. Many machining operations can be performed on work held in a chuck, such as threading, boring, turning etc.
Independent (4-jaw) chuck of lathe is used more than other types because it can hold practically any shape of work and can be adjusted to any degree of accuracy desired.
For very accurate central location of workpiece, dial indicator is used. The workpiece is first centred approximately and dial indicator is mounted in the tool post and jaws are adjusted until the indicator shows no change in reading when the workpiece is rotated. After the piece has been centred, all the jaws are tightened securely. The chuck key must be removed before switching on the power to machine.
The reversing feature of the jaws in the 4-jaw independent chuck for lathe also makes it possible to turn the work with extreme differences in diameter [Refer Figs. 12.37 (a) and (b)].
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The universal chuck is the best for round and hexagonal work for quick chucking purposes because all three jaws move simultaneously and automatically centre the work within close tolerances.
Usually the 3 jaw universal chucks are supplied with two sets of jaws. One set of jaws is used to hold large diameter work (Refer Fig. 12.38 (a) and (b)). In universal chucks, the jaws as well as slots are numbered 1, 2 and 3 and they must be fitted accordingly for proper centering.
Since the jaws are made for a particular chuck (and are not interchangeable), it should be ensured that chuck and jaws carry the same serial number, otherwise centering cannot be guaranteed.
The chuck jaws are installed after cleaning the jaws, jaw slots and scroll. The scroll is turned till the first thread of the spiral does not quite show in jaw slot No. 1. The slide jaw No. 1 is then slid into the slot as far as it would go and the scroll is turned until the spiral engages with the first tooth on the bottom of the jaw. The same operation is repeated for installing jaws Nos. 2 and 3.
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The accuracy of universal chuck jaws is affected by:
(i) Wear and tear of scroll plate,
(ii) Gripping work very close to the front of jaws, and
(iii) Applying too much pressure to mount the work.
Draw-in-collet chuck is the most accurate one and is used for high precision work such as making small tools and manufacturing small parts for watches. The collets are made for round, square and other shapes of work.
Collets are small accurately made sleeves. Their one end is split into 3 even sections by cutting slots running slightly more than half the length. Thus the jaws are able to spring in and clamp the work. The other end is threaded to enable their mounting.
Thus the collets are able to centre the work automatically. They maintain their accuracy over long periods of hard usage. However a separate collet is needed for each different size or stock shape and their initial cost is high.
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Fig. 12.39 shows a collet chuck used for holding base stock for mass production where quick setting and accurate centering are required.
It is important that the chuck is installed correctly on the spindle nose, otherwise the accuracy of the chuck will be affected. While mounting and removing the chuck on spindle nose, a board should be placed on the machine ways under the chuck for protection of hands and the ways. Careful cleaning of spindle and use of a few drops of spindle oil on spindle is desirable before mounting of chuck.
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For facing a stock held in chuck, use of round nose cutting tool in a straight tool holder mounted on a compound rest pivoted at 30° to the right and mounting the tool holder at less than 90° to the face of the work as shown in Fig. 12.40 is recommended. It is essential that the cutter bit be located exactly on centre, otherwise some piece at the centre would be left un-machined. For facing, it is preferable to start cut from centre and feed outward.
Jacobs chuck is used for turning small diameter work such as screws, pins, etc.
The headstock spindle chuck is similar to Jacobs chuck but is hollow and can be fitted directly on the spindle nose. It does not interfere with the compound rest because of its small size and thus makes it possible to work very close to the chuck.
Combination chuck has the features of both self- centering and independent chuck. The jaws can be operated either individually by separate screws or simultaneously by the scroll disc. (Refer Fig. 12.41).
Sometimes magnetic chuck is used for holding thin flat workpiece which may get distorted under pressure of the jaws of chuck. The magnetic flux radiating from the electro-magnet/permanent magnets housed in the chuck decides the holding power of the chuck. A face plate placed over magnetic chuck consists of alternate strips of magnetic and non-magnetic materials.
When the magnetic strips are over magnets, magnetic lines of flux pass through workpiece and it is held with the chuck. When face plate is displaced so that non-magnetic strips are over magnets then magnetic lines of force are not able to pass over the work and it is held loosely (Refer Fig. 12.42).
Fig. 12.43 shows an air or hydraulically operated chuck used for mass production work. It is fast in operation and has high effective gripping capacity. It houses a power cylinder which is mounted at the back end of the headstock spindle and rotates with it. Fluid pressure is communicated to the cylinder by operating a valve with a lever. The movement of the piston is transmitted to the jaws by a connecting rod and links, and the jaws grip the workpiece securely.
