TYPES OF FIXTURES
Fixtures vary in shape and size according to the job and the nature of operations (machining or welding) to be performed. All devices used to hold and clamp a job with the machine tool table are included in the category of for example, vice with parallel jaws, angle plate, face plate, parallel blocks, etc. are the examples of a fixture. Components of a fixture may be made out of steel sections such as flats, angles, channels and fastened together with bolts or welded. Besides various types of clamping systems, fixtures also comprise mechanisms for indexing and power-operated clamping devices.
Fixtures are designed according to the machine for which they are used, for example, turning fixture, shaper fixture, broaching fixture, milling fixture, grinding fixture, welding fixture and assembling fixtures.
1. A milling fixture may comprise a strong thick cast iron base (about 25 mm thick) which is rigid enough to stand against deflecting forces during milling operations (down-milling and up-milling). Lugs (or feet) are provided with the fixture body for fixing it with the machine tool table. Tenon strips, which are ground and hardened steel strips having width equal to the slots in the base of milling machine and twice longer than their width, are screwed to the bottom face of the fixture (Fig.) for accurately locating the position of the fixture below the tool for any number of uses of the fixture.
Setting blocks are available which are attached to the base of fixture (by screws or dowel pins) to help in setting the fixture relative to the milling cutters. These are made of ground and hardened steel. Tee bolts and clamps of different types are used to clamp the fixture with the machine table [Fig.]). Milling fixture may be for plain milling operations where only one work is milled at a time or for
string or in-line milling where couple of workpieces are milled together at one time when kept parallel to each other in width (Fig.). Other milling fixtures include gang milling fixtures using more than two cutters on the milling machine arbor, profile milling fixtures. indexing in milling fixture for milling a job having a number of surfaces to be milled after successive positioning, etc.
For every setting of the fixture at an appropriate position on the machine table with respect to the milling cutter, the fixture is brought to situation when it touches against the T-bolt locators already fitted in the T-slot of the machine table Clamping of the fixture in that position is done by using T-bolts fitted in other T-slots of the machine table or using flat strap or gooseneck strap or such similar devices.
2. A lathe fixture may comprise turning fixtures or gear cutting fixtures having indexing mechanism or fixtures for any other special work such as cutting of keyway, in a shaft (Fig.).
3. Shaping fixtures are used for performing special machining operations on a shaping machine which are different from the routine work. A typical shaping fixture for cutting keyways in tapered conical sections are shown in [Fig.].
4. Welding fixtures and positioners are used extensively in welding jobs to reduce cost and improve quality of welds. The fixtures are used to bring the job preferably to the down hand welding positions which provide ease in welding and ensure faster welding speeds. Positioners in welding are used to tilt, lift and position the large bulky jobs to down hand welding positions.
There are manipulators, power-operated type, which provide a mount for the welding head that allows it to be raised, lowered, extended, retracted or swung through 366. Power rolls and idlers are convenient means of moving the job when making circumferential welds. Welding fixtures may be for tacking purposes or for regular welding.
A tack welding fixture locates different components of the work in their correct relationship and properly clamped while tack welding of these components is carried out prior to regular welding. Welding fixtures help in locating and clamping the workpieces for complete welding of the tack welded structures. Welding fixtures are stronger than the tacking fixtures as these have to withstand large thermal expansion (hence pressures) due to high temperatures involved during welding.
5. Assembly fixtures are used for assembly work such as joining of various components to form an assembly of a full machine or structure. There are fixtures used for holding the work for mechanical assembling like bolting or riveting by pneumatic riveters. The other types of assembly fixtures are for assembly work which involves joining methods based on heat and thus all welding fixtures come under this category.
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