Introduction to workwholding

INTRODUCTION TO WORKHOLDING

Over the past century, manufacturing has made considerable progress. New machine tools, high-performance cutting tools, and modern manufacturing processes enable today's industries to make parts faster and better than ever before. Although workholding methods have also advanced considerably, the basic principles of clamping and locating are still the same.
HISTORYThe first manufactured products were made one at a time. Early artisans started with little more than raw materials and a rough idea of the finished product. They produced each product piece by piece, making each part individually and fitting the parts into the finished product. This process took time. Moreover, the quality and consistency of products varied from one artisan to the next. As theyworked, early manufacturing pioneers realized the need for better methods and developed new ideas.
Eventually, they found the secret of mass production: standardized parts. Standard parts not only speeded production, they also ensured the interchangeability of parts. The idea may be obvious today, but in its time, it was revolutionary.
These standard parts were the key to enablingless-skilled workers to replicate the skill of the craftsman on a repetitive basis. The original method of achieving consistent part configuration was the template. Templates for layout, sawing, and filing permitted each worker to make parts to a standard design. While early templates were crude, they at least gave skilled workers a standard form to follow for the part. Building on the template idea, workersconstructed other guides and workholders to make their jobs easier and the results more predictable. These guides and workholders were the ancestors of today's jigs and fixtures.
Yesterday's workholders had the same two basic functions as today's: securely holding and accurately locating a workpiece. Early jigs and fixtures may have lacked modern refinements, but they followed many of the sameprinciples as today’s workholder designs.
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, Figure 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, inthis case with a bushing.

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