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 work holding 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 work holders 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 fixtures

Cylindrical-Surface Machining

   • Lathe fixtures

   • Cylindrical-grinding fixtures

Irregular-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

Modular Fixtures

Modular fixtures achieve many of the advantages of a permanent tool using only a temporary setup. Depicted in Figure 1-4, these workholders combine ideas and elements of permanent and general-purpose workholding.

Figure 1-4. Modular workholders combine ideas and elements of both permanent and temporary workholding to make inexpensive-yet-durable workholders.

The primary advantage of modular fixtures is that a tool with the benefits of permanent tooling (setup reduction, durability, productivity improvements, and reduced operator decision-making) can be built from a set of standard components. The fixture can be disassembled when the run is complete, to allow the reuse of the components in a different fixture. At a later time the original can be readily reconstructed from drawings, instructions, and photographic records. This reuse enables the construction of a complex, high-precision tool without requiring the corresponding dedication of the fixture components.

Arc Welding:

The arc welding is a fusion welding process in which the welding heat is obtained from an electric arc struck between the work(or base metal) and an electrode. The temperature of the heat produced by the electric arc is of the order of 6000°C to 7000°C. Both the direct current (D.C) and alternating current(A.C) may be used for arc welding, but the direct current is preferred for most purposes. When the work is connected to the positive terminal of the D.C welding machine and the negative terminal to an electrode holder, the welding set up is said to have straight polarity. On the other hand, when work is connected to negative and the electrode to a positive terminal, then the welding set up is said to have reversed polarity. The straight polarity is preferable for some welds while for other welds reversed polarity should be used.

Following are the two types of arc welding depending upon the type of electrode:

(A): Un shielded arc welding:

When a large electrode or filler rod is used for welding, it is said to be un- shielded arc welding.

(B): Shielded arc welding:

When the welding rods coated with fluxing material are used, then it is called shielded arc welding.

Arc welding Processes:

The following are the various welding processes commonly used in engineering practice.

1: Carbon arc welding

In carbon arc welding, the welding heat is obtained from an electric arc between a carbon electrode and the work. In welding heavy plants, the additional metal is deposited in the weld from a filler rod.

2: Metal arc welding

In metal arc welding , the arc is produced between the metal electrode(also called filler rod) and the work piece. During the welding process, the metal electrode is melted by the heat of the arc and fused with the work piece. The temperature produced by the heat is about 2400° C to 2700° C.

3: Metallic inert gas (MIG) Arc welding:

In MIG welding , the electrode is consumable, the filler metal is deposited by the arc which is completely surrounded by an inert gas.

4: Tungsten inert gas(TIG) arc welding

In TIG welding, the heat is produced from an arc between the non consumable tungsten electrode and the work piece. The welding zone is shielded by an atmosphere of inert gas(such as helium or argon) supplied from a suitable source. The direct current with a straight polarity is used for welding copper alloys and stainless steel. Whereas the reversed polarity is used for magnesium. The alternating current is more versatile in welding for steel, cast iron, aluminum and magnesium.

5: Atomic hydrogen welding

In atomic hydrogen welding, the arc is obtained between two tungsten electrodes (non consumable) while a stream of hydrogen passes by the arc and envelopes the welding zone.

6: Stud arc welding

It is a direct current arc welding process, and is used for welding metal studs to the flat metal surfaces.

7: Submerged arc welding

In submerged welding, the arc is produced between a bare metal electrode and the work piece. The submerged arc welding is mostly done on low carbon and alloy steels, but it may be used on many of the non-ferrous metals.

8: Thermit welding

In this welding, a mixture of iron oxide and aluminum known as thermit, is used. The mixture is ignited only at a temperature of about 1500°C. A major advantage of the thermit welding is that all parts of the weld section are molten at the same time and the weld cools almost uniformly. This results in a minimum problem with internal residual stresses. The thermit welding is often used in joining iron and steel parts that are too large to be manufactured, such as rails, trucks frames, locomotive frames, other large sections used on steam and rail roads, for stern frames, rubber frames etc. In steel mills, Thermit electric welding is employed to replace broken gear teeth, to weld new necks on rolls and pinions and to repair broken shears.

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