Lathe Chuck

  1. Three Jaw Chuck
  2. Four Jaw Chuck
  3. Two Jaw Concentric Chuck
  4. Combination Chuck
  5. Collect Chuck
    1. Push out chucks
    2. Draw-in chucks
    3. Dead length bar chucks
  6. Magnetic Chuck
  7. Hydraulic Chuck or Air operated Chuck

1. Three Jaw Chuck

The 3 jaw chuck is also known as self-centering chuck. The majority of the chucks have two sets of jaws for holding internal and external diameters. Only perfectly round work, or work with equally spaced flats, divisible by three, should be held in a 3 jaw chuck.

The construction of a 3 jaw chuck shows that the scroll not only clamps a component in place but also locates the component. This is fundamentally a bad practice, since any wear in the scroll and / or the jaws impairs the accuracy of location. Further, there is no means of adjustment possible to compensate for this wear.

The jaws of this type of chuck are not reversible, and separate internal and external jaws have to be used.

Parts of a 3 jaw chuck

  1. Back Plate
  2. Body
  3. Jaws
  4. Crown Wheel
  5. Pinion
Three Jaw Chuck
Three Jaw Chuck

1. Back Plate

The back plate is fastened at the back of the body by means of allen screws. It is made out of cast iron. Its bore is tapered to suit the taper of the spindle nose. It has a keyway which will fit into the key provided on the spindle nose. There is a step in the front on which the thread is cut. The threaded collar, which is mounted on the spindle, locks the chuck by means of the thread, and locates by means of the taper and the key.

2. Body

The body is made out of cast steel, and the face is hardened. The body has three openings – 120° apart to assemble the jaws and operate them. Three pinions are fixed on the periphery of the body to operate the jaws by means of a chuck key. The body is hollow in cross-section. The crown wheel is housed inside the body.

3. Jaws

The jaws are made out of high carbon steel, hardened and tempered, which slide on the openings of the body. Generally there are two sets of jaws, viz. external jaws and internal jaws. External jaws are used for holding solid
works. Internal jaws are used for holding hollow works. The steps on the jaws increase the clamping range. The back side of the jaws are cut out of scroll thread. Each jaw is numbered in a sequential manner, which will help in fixing the jaws in the corresponding numbered slots.

4. Crown wheel

The crown wheel is made out of alloy steel, hardened and tempered. On one side of the crown wheel a scroll thread is cut to operate the jaws and the other side is tapered on which bevel gear teeth are cut to mesh the pinion. When the pinion is rotated by means of the chuck key, the crown wheel rotates, thus causing the jaws to move inward or outward depending upon the rotation.

5. Pinion

The pinion is made out of high carbon steel, hardened and tempered. It is fitted on the periphery of the body. On the top of the pinion, a square slot is provided to accommodate the chuck key. It has a tapered portion on which the bevel gear teeth are cut, which match with the crown wheel.

2. Four Jaw Chuck

The four jaw chuck is also called as independent chuck, since each jaw can be adjusted independently; work can be trued to within 0.001″ or 0.02 mm accuracy.

This type of chuck is much more heavily constructed than
the self-centering chuck, and has much greater holding power. Each jaw is moved independently by a square thread screw, and is reversible.

The independent 4 jaw chuck has four jaws, each working independently of the others in its own slot in the chuck body and actuated by its own separate square thread screw. By suitable adjustment of the jaws, a workpiece can be set to run either true or eccentric as required. ‘T’ slots are provided on the face of the chuck to accommodate ‘T’ bolts for clamping irregular works or for assembling balance weights.

To set the job for the second time it can be trued with the help of a dial test indicator. The check on the workpiece should be carried out near the chuck and repeated as far from it as the workpiece permits, to ensure that the work is not held in the chuck at an angle to the axis of rotation.

The independent adjustment also provides the facility of deliberately setting the work off-centre to produce an eccentric workpiece.

The parts of a 4 jaw chuck are:

  1. Back Plate
  2. Body
  3. Jaws
  4. Screw Shaft
Four Jaw Chuck
Four Jaw Chuck

1. Back Plate

The back plate is fastened to the back of the body by means of Allen screws. It is made out of cast iron/steel. Its bore is tapered to suit the taper of the spindle nose. It has a keyway which fits into the key provided on the spindle nose. There is a step in front on which the thread is cut. A threaded collar which is mounted on the spindle locks the chuck by means of the thread, and locates by means of the taper and key. Some chucks do not have back plates.

2. Body

The body is made out of cast iron/cast steel and the face is flame-hardened. It has four openings at 90° apart to assemble the jaws and operate them. Four screw shafts are fixed on the periphery of the body by means of finger pins. The screw is rotated by means of a chuck key. The body, hollow in the cross-section, has equi-spaced circular rings provided on the face, which are marked by numerical numbers. Number 1 starts in the middle and increases towards the periphery.

3. Jaws

Jaws are made out of high carbon steel. hardened and tempered, which slide on the openings of the body. These jaws are reversible for holding hollow work. The back side of the jaws are square-threaded which will help in fixing the jaws with the operating screws.

4. Screw Shaft

Screw shaft is made out of high carbon steel, hardened, tempered and ground. The top portion of the screw shaft is provided with a square slot to accommodate the chuck key. On the body portion, a left hand square thread is cut. In the middle of the screw shaft, a narrow step is made and held by means of finger pins. The finger pins permit the screws to rotate but not to advance.

3. Two Jaw Concentric Chuck

Two Jaw Concentric Chuck
Two Jaw Concentric Chuck

The constructional features of this chuck are similar to those of 3 jaw and 4 jaw chucks. Each jaw is an adjustable jaw which can be operated independently. In addition to this feature, both jaws may be operated concentric to the centre. Irregular shaped works can be held. The jaws may be specially machined to hold a particular type of job.

Two Jaw Concentric Chuck is mainly employed to hold an irregularly shaped job. As the chuck is designed with two jaws, it can be used as a turning fixture.

4. Combination Chuck

The combination chuck is normally a four jaw chuck in which the jaws may be adjusted either independently as done in a 4 jaw chuck, or together, as done in a 3 jaw universal chuck. This kind of chuck is used in places where duplicate workpieces are to be machined. One piece is accurately set as done in a 4 jaw chuck, and the subsequent jobs
are held by operating the centering arrangement.

Combination chuck may be used both as a universal 3 jaw chuck and as a 4 jaw independent chuck. This chuck is very useful where duplicate workpieces are involved in the turning.

5. Collect Chuck

A collet is a hardened steel sleeve having slits cut partly along its length. It is held by a draw-bar which can be drawn in or out in the lathe spindle. The collet is guided in the collet sleeve, and held with the nose cap. It is possible
to change the collet for different cross-sections depending on the cross-section of the raw material.

Collet Chuck
Collet Chuck

There are three most commonly used types of collet chucks.
• Push-out chucks
• Draw-in chucks
• Dead length bar chucks

The operation of collet chucks may be manual, pneumatic, hydraulic or electrical. They are mainly used to hold round, square, hexagonal or cast profile bars.

Collet chuck is mainly used for holding jobs within a comparatively small diameter. The main advantage of collets lies in their ability to centre work automatically and maintain accuracy for long periods. It also facilitates to hold the bar work.

A. Push out chucks

Push Out Chucks
Push Out Chucks

The collet closes on the workpiece in a forward direction and consequently an end-wise movement of the work results. The cutting pressure tends to reduce the grip of the collet on the workpiece.

B. Draw-in Chuck

Draw-in chuck
Draw-in chuck

The collet closes on the workpiece in a backward direction and movement of the work. Take special care to avoid increases the grip of the collet on the workpiece.

C. Dead length bar chucks

Dead Length Bar Chuck
Dead Length Bar Chuck

Dead Length bar chucks are widely used in modern machines as they provide an accurate end-wise location of the workpiece. The chuck does not move end-wise during gripping or closing operation. These chucks are made to
hold round, hexagonal or square bars, and when they are not gripping, they maintain contact with the core thus preventing swarf and chips collecting between the collet and the core.

The disadvantage with dead length bar chucks is that each collet cannot be made to grip bars which vary by more than about 0.08 mm without adjustment.

6. Magnetic Chuck

Magnetic Chuck
Magnetic Chuck

Magnetic chuck is designed to hold the job by means of magnetic force. The face of the chuck may be magnetized by inserting a key in the chuck and turning it to 180°. The amount of magnetic force may be controlled by reducing the angle of the key. The truing is done with a light magnetic force, and then the job is held firmly by using the full magnetic force.

Magnetic chuck is mainly used for holding thin jobs which cannot be held in an ordinary chuck. These are suitable for works where a light cut can be taken on the job.

7. Hydraulic Chuck or Air operated Chuck or

Air Operated Chuck
Air Operated Chuck

Hydraulic chucks or Air operated chucks are mainly used for getting a very effective grip over the job. This mechanism consists of a hydraulic or an air cylinder which is mounted at the rear end of the headstock spindle, rotating along with it. In the case of a hydraulically operated chuck the fluid pressure is transmitted to the cylinder by operaring the valves. This mechanism may be operated manually or by power. The movement of the piston is transmitted to the jaws by means of connecting rods and links which enable them to provide a grip on the job.

Hydraulic chucks or Pneumatic chucks are mainly used in mass production because of their speedy and effective gripping capacity.