The following points highlight the four main devices used in turret lathe. The devices are: 1. Collet 2. Arbors 3. Chucks 4. Fixtures.
Device # 1. Collet:
When components are turned and parted off from a bar fed through the hollow spindle of the machine, a collet is used. The bar is generally of round, square or hexagonal shape. Collet chucking equipment may be hand or air operated. A sectional view of hand operated collet chuck is shown in Fig. 32.7.
When the handle is moved to the closed position, the sliding sleeve S is caused to rotate and, as the grooves which accommodate pads P are cut on a helix, the sleeve is forced to move to the left. In doing this, it forces the ball operating sleeve C to the left and this causes the right hand ring of balls held in the ball cage B to move inwards. This causes the sliding cone sleeve R to compress on the collet and hence closes it.
ADVERTISEMENTS:
Moving the lever to the opposite direction reverses the action and left hand ring of balls in B moves sleeve R to release the collet. In the position as shown in the figure, the collet is closed. M is the machine spindle and H is the housing bolted to the head-stock. The knurled cap K adjusts the collet for variations in the size of bar and retains it in a fixed lengthwise position which is unaffected during opening and closing of chuck.
For large stock, collets of parallel closing type are sometimes used, but generally collets of spring type are recommended. Pneumatically operated chucks reduce operator fatigue.
Device # 2. Arbors:
Expanded or threaded arbors are used to hold short pieces of stock having previously machined accurate hole. The action of holding the work is very much similar to that of collet chuck.
Device # 3. Chucks:
As the turning of work between centres finds no place in turret lathe, work is held at the spindle in some form of chuck or fixtures. Essential qualities are rigidity and speed of operation and for the second reason, nearly all the chucks used are of the self-centering type.
ADVERTISEMENTS:
In three-jaw self-centering type chuck, the jaws are in two parts, a base jaw that always remains in the chuck body, and two removable gripping soft jaws. These are unhardened steel blanks specially designed to hold irregularly shaped components.
In addition to the standard chucks, several other types of chucks are also used for holding the irregular shaped work pieces. One of them is known as a two jaw box chuck which is designed to hold work with parallel flat sides. Both the jaws arranged 180° opposite to each other, move in and out simultaneously.
When a compressed air supply is available, pneumatic chucks are used. This type of chuck reduces chucking time, relieves the operator of manual fatigue and if air valves are foot operated, leaves him with both hands free to manipulate the work during chucking. These chucks are operated at a pressure of 5.5 kg/cm2.
Device # 4. Fixtures:
As much of the work turned on turret and capstan lathes is of special or irregular form, it must be located in some type of fixture. As a matter of fact, there is no limit to the variety of such fixtures, yet standard face plates with different kinds of holding devices are frequently used.