APPLICATIONS FOR JIGS AND FIXTURES

Typically, the jigs and fixtures found in a machine shop are for machining operations. Other operations, however, such as assembly, inspection, testing, and layout, are also areas where workholding devices are well suited. Figure 1-7 shows a list of the more-common classifications and applications of jigs and fixtures used for manufacturing. There are many distinct variations within each general classification, and many workholders are actually combinations of two or more of the classifications shown.
EXTERNAL-MACHINING APPLICATIONS:
Flat-Surface Machining • Milling fixtures • Surface-grinding fixtures • Planing fixtures • Shaping fixturesCylindrical-Surface Machining • Lathe fixtures • Cylindrical-grinding fixturesIrregular-Surface Machining • Band-sawing fixtures • External-broaching fixtures


INTERNAL-MACHINING APPLICATIONS:
Cylindrical- and Irregular-Hole Machining • Drill jigs • Boring jigs • Electrical-discharge-machining fixtures • Punching fixtures • Internal-broaching fixtures

NON-MACHINING APPLICATIONS:
Assembly • Welding fixtures • Mechanical-assembly fixtures (Riveting, stapling, stitching, pinning, etc.) • Soldering fixtures
Inspection •Mechanical-inspection fixtures •Optical-inspection fixtures • Electronic-inspection fixtures
Finishing • Painting fixtures • Plating fixtures • Polishing fixtures • Lapping fixtures • Honing fixtures
Miscellaneous • Layout templates • Testing fixtures • Heat-treating fixtures
Jigs and fixtures are production tools used to accurately manufacture duplicate and interchangeable parts. Jigs and fixtures are specially designed so that large numbers of components can be machined or assembled identically, and to ensure interchangeability of components.
The economical production of engineering components is greatly facilitated by the provision of jigs and fixtures. The use of a jig or fixture makes a fairly simple operation out of one which would otherwise require a lot of skill and time.
Both jigs and fixtures position components accurately; and hold components rigid and prevent movement during working in order to impart greater productivity and part accuracy. Jigs and fixtures hold or grip a work piece in the predetermined manner of firmness and location, to perform on the work piece a manufacturing operation.
A jig or fixture is designed and built to hold, support and locate every component (part) to ensure that each is drilled or machined within the specified limits.
The correct relationship and alignment between the tool and the work piece is maintained. Jigs and fixtures may be large (air plane fuselages are built on picture frame fixtures) or very small (as in watch making). Their use is limited only by job requirements and the imagination of the designer.
The jigs and fixtures must. be accurately made and the material used must' be able to withstand wear and the operational (cutting) forces experienced during metal cutting
Jigs and fixtures must be clean, undamaged and free from swarf and grit Components must not be forced into a jig or fixture.
Jigs and fixtures are precision tools. They are expensive to produce because they are made to fine limits from materials with good resistance to wear. They must be properly stored or isolated to prevent accidental damage, and they must be numbered for identification for future use.
igs and fixtures form an important category of equipment that go a long way in achieving productivity, says Narayan Joglekar
One of the prime concerns of any manufacturing engineer is productivity. He has to ensure maximum quantity of production, adhering to the specified or better quality standards. In fact, this is the main objective in any industrial activity meant for generating wealth. And in mechanical production, jigs and fixtures form an important category of equipment that goes a long way in achieving productivity.
Jigs and fixtures are appliances used in manufacturing or assembly and various other activities for facilitating the operation to which they are applied. A fixture is a device used on machine tools to hold and locate a work-piece during a manufacturing operation. A jig not only holds and locates a work piece but also guides the cutting tool, especially a drill. Both are primarily used for the following objectives:
To facilitate the holding and support of an awkward or frail article for a machining operation. If a component is to be given heavy machining cuts on the milling machine and it does not have proper flat surfaces parallel with one another, it is difficult to hold it. If the component is weak at some sections, cutting pressures may deform it. A fixture eliminates such a problem.
To position a component and to guide the drill or cutter so that every component will be uniform. In machining we perform operations and create surfaces, flat or curved or round holes, which must bear some definite relations in terms of distance or angle with others. In mass production, this must be essentially uniform. This uniformity is achieved by use of jigs and fixtures.
To accommodate several components at one setting in order to take advantage of multiple machining. Instead of making a hole in one thin flat piece at a time, 10 such pieces can be stacked in a jig and the hole can be drilled in one go.
To hold a component which could not be held conveniently without a fixture. This means convenience and saves human effort.
Mostly the use of a jig or fixture would achieve more than one of the objectives.
Elements
Locating elements
Functionally, these elements are provided so that the operation being performed or surface being generated by that operation, should bear desired positional relationship with some other surface on the component. For example, when a flat piece is held tight on the magnetic table of the surface grinder, the top surface generated by the grinding wheel is perfectly parallel to the bottom surface, which was held flat against the magnet.
A perfect V-block ensures that the cross-centre line of any perfectly round job passes through its own centre line (dividing the included angle of the V). For locating on holes, locating pins, plungers or mandrels are used.
Clamping elements
The purpose of clamping elements is to provide an invariably stable position of the work piece as located in the jig or fixture. For this, displacement is prevented by tightly holding the work piece against the work holding support to create frictional forces.
Strap or plate type clamps in conjunction with screw or studs are most simple and common devices. Faulty application of clamping may spoil a satisfactory method of location. The common faults are:
(a)The component gets lifted away from the surface.
(b) Due to poor design considerations, the swarf has a tendency to get accumulated or clogged on the locating surface which results in errors and rejections.
(c) Due to excessive clamping force, the clamp or even the work support or location may get temporarily or permanently deformed. These faults are apparent in some poorly designed or manufactured fixtures after usage over months or over hundreds of pieces.
There are different devices for operating the clamping elements – in conventional machine shops these could be manual as they are simple to design and make. But manual clamping has inherent disadvantages like difficulty in determining the required force for reliable clamping and operator fatigue due to repeated exertion. Also, the time required in manual clamping is much larger than that required in power driven devices where very less time is taken to switch on or set an actuating lever.
Tool guiding(jig) or cutter setting(fixture)
Most common tool guiding elements are drill bushings. If you want to make one or many holes per piece on a batch of jobs, you can use a drill plate type jig. This type incorporates many drill bushes with particular internal diameters and perfect centre distances made on jig boring machine, to yield drilled holes with accurate diameters and correct axis and concentricity.
For milling slots, we may have cutter-setting pieces. Cutter without power movement can be inserted and manually rotated in the slot on setting piece to ensure correct cutter position and then taking onto the actual job, production runs can be kept on.
Indexing Devices
These are for accurately changing the position of the work piece in the jig or fixture so that same or different operations can be performed with positional accuracy on various sides of a job. For example, if four holes are to be made on all sides of a square piece or even a round piece, an indexing device has to be used.
In the simplest form, it could be an index plate well hardened and ground, and having four equidistant holes made to highest accuracy in terms of size, position and axiality. It is connected to the job directly or indirectly but with definite rigidity. It is used in conjunction with a plunger of diameter perfectly suiting the holes on the plate.
Auxiliary elements
Various guards and coolant flow guiding ducts, especially for multiple jobs or difficult-to-access jobs, may come under this category. Also arrangements made to avoid swarf falling and clogging at the critical location surfaces may come under this category.