Workholding Definitions

Often the terms "jig" and "fixture" are confused or used interchangeably; however, there are clear distinctions between these two tools. Although many people have their own definitions for a jig or fixture, there is one universal distinction between the two. Both jigs and fixtures hold, support, and locate the workpiece. A jig, however, guides the cutting tool. A fixture references the cutting tool. The differentiation between these types of workholders is in their relation to the cutting tool. As shown in Figure 1-1, jigs use drill bushings to support and guide the tool. Fixtures, Fixture 1-2, use set blocks and thickness, or feeler gages, to locate the tool relative to the workpiece. 

Figure 1-1. A jig guides the cutting tool, in this case with a bushing. 


Figure 1-2. A fixture references the cutting tool, in this case a set block. 


The most common jigs are drill and boring jigs. These tools are fundamentally the same. The difference lies in the size, type, and placement of the drill bushings. Boring jigs usually have larger bushings. These bushings may also have internal oil grooves to keep the boring bar lubricated. Often, boring jigs use more than one bushing to support the boring bar throughout the machining cycle. 

In the shop, drill jigs are the most widely used form of jig. Drill jigs are used for drilling, tapping, reaming, chamfering, counterboring, countersinking, and similar operations. Occasionally, drill jigs are used to perform assembly work also. In these situations, the bushings guide pins, dowels, or other assembly elements. 

Jigs are further identified by their basic construction. The two common forms of jigs are open and closed. Open jigs carry out operations on only one, or sometimes two, sides of a workpiece. Closed jigs, on the other hand, operate on two or more sides. The most common open jigs are template jigs, plate jigs, table jigs, sandwich jigs, and angle plate jigs. Typical examples of closed jigs include box jigs, channel jigs, and leaf jigs. Other forms of jigs rely more on the application of the tool than on their construction for their identity. These include indexing jigs, trunnion jigs, and multi-station jigs. 

Specialized industry applications have led to the development of specialized drill jigs. For example, the need to drill precisely located rivet holes in aircraft fuselages and wings led to the design of large jigs, with bushings and liners installed, contoured to the surface of the aircraft. A portable air-feed drill with a bushing attached to its nose inserted through the liner in the jig and drilling is accomplished in each location. 

Fixtures have a much wider scope of application than jigs. These workholders are designed for applications where the cutting tools cannot be guided as easily as a drill. With fixtures, an edge finder, center, finder, or gage blocks position to the cutter. Many CNC machines have probes, which can establish the cutter position in reference to the workpiece. Examples of the more common fixtures include milling fixtures, lathe fixtures, sawing fixtures, and grinding fixtures. Moreover, a fixture can be used in almost any operation that requires a precise relationship in the position of a tool to a workpiece. 

Fixtures are most often identified by the machine tool where they are used. Examples include mill fixtures or lathe fixtures. But the function of the fixture can also identify a fixture type. So can the basic construction of the tool. Thus, although a tool can be called simply a mill fixture it could also be further defined as a straddle-milling, plate-type mill fixture. Moreover, a lathe fixture could also be defined as a radius-turning, angle-plate lathe fixture. The tool designer usually decides the specific identification of these tools. 

Tool or Tooling
The term "tool" encompasses both jigs and fixtures. Essentially, it is a generic term describing a workholder, which is identified with a part or machine. Sometimes "tool" is used to refer to a cutting tool or a machine tool, so it is important to make clear distinctions. 

Another term, which describes both jigs and fixtures, is "workholder." A broad term, it frequently identifies any device which holds, supports, and locates a workpiece. In addition to jigs and fixtures, vises, collets, clamps, and other similar devices are also workholders. 

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This page contains information originally published in the Jig & Fixture Handbook, 3rd Edition, Copyright 2016, Carr Lane Manufacturing Co., St. Louis, Mo.