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.
The 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 they worked, 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 enabling less-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, workers constructed 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 same principles as today’s workholder designs.
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.
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 is 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 the cutter. 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.
PERMANENT AND TEMPORARY WORKHOLDERS
Jigs and fixtures are most often found where parts are produced in large quantities, or produced to complex specifications for a moderate quantity. With the same design principles and logic, workholding devices can be adapted for limited-production applications. The major difference between permanent and temporary workholders is the cost/benefit relationship between the workholder and the process. Some applications require jigs and fixtures solely for speed; others require less speed and higher precision. The requirements of the application have a direct impact on the type of jig or fixture built and, consequently, the cost.
Permanent Jigs and Fixtures
Workholders for high-volume production are usually permanent tools. These permanent jigs and fixtures are most often intended for a single operation on one particular part. The increased complexity of permanent workholders yields benefits in improved productivity and reduced operator decision-making, which result in the tool having a lower average cost per unit or per run. Therefore, more time and money can be justified for these workholders.
In the case of hydraulic or pneumatic fixtures, inherent design advantages can dramatically improve productivity and, hence, reduce per-unit costs even further, even though the initial cost to construct these fixtures is the most expensive of all fixture alternatives. In some cases, where machine-loading considerations are paramount, such as a pallet-changing machining center, even duplicate permanent fixtures may be justified.
Permanent jigs and fixtures are typically constructed from standard tooling components and custom-made parts. Figure 1-3 shows a typical permanent workholder for a drilling operation.
Low-volume runs and ones with fewer critical dimensions are often produced with throwaway jigs and fixtures. These tools would typically be one-time-use items constructed from basic materials at hand and discarded after production is complete. Although throwaway jigs and fixtures are technically permanent workholders, in effect they are actually temporary.
In many instances, the shape of the part and the machining to be performed allow for the use of a general-purpose workholder such as a vise, collet, or chuck. These workholders are adaptable to different machines and many different parts.
Since they are not part-specific, their versatility allows for repeated use on a variety of different or limited-production runs. The cost of these workholders would usually be averaged over years and might not even be a factor in job-cost calculations. The general-purpose nature of these workholders necessitates a higher level of operator care and attention to maintain consistency and accuracy. For these reasons, general-purpose workholders are not preferred for lengthy production runs.
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.
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.
Figure 1-5 shows how modular fixturing fits into the hierarchy of workholding options, ranking below permanent fixturing yet above general-purpose workholders. Virtually every manufacturer has good applications for each of these three options at one time or another.
The principal considerations when choosing among workholder varieties fall into three general categories: tooling cost, tooling details, and tooling operation. Although each of these categories is separated here, in practice they are interdependent. The following are some design differences and considerations for permanent, general-purpose, and modular workholders.
The total cost of any jig or fixture is frequently the major area of consideration in many workholder designs. Although initial cost is a major element, it should not be the basis for accepting or rejecting any tooling option.
A more-proper economic evaluation of the workholder design takes into consideration many other factors. As discussed previously, permanent fixtures have distinct advantages in the production of high-volume and high-precision parts. They also typically reduce machine setup time, machine cycle time, and the level of operator skill required to produce satisfactory quality output. Over a long production run, or a series of runs in the life of a tool, the average cost of the tool per piece produced can be quite low.
General-purpose workholders are more expensive than temporary tools in most cases, but their utility and flexibility often allow these workholders to be regarded as a capital cost to be amortized over a period of time without regard to actual usage. Similarly, modular fixturing is typically a capital investment to be amortized over a set lifespan, with an average cost assigned to usage for each anticipated job.
Another cost to be considered is workholder disposition. Permanent fixtures require storage and maintenance to keep them available for their next use. General-purpose tools are reused extensively, but still incur some costs for maintenance and storage. Similarly, modular fixtures will be disassembled, and the components maintained, stored, and reused frequently.
Tooling details are the overall construction characteristics and special features incorporated into the jig or fixture. Permanent workholders are designed and built to last longer than temporary workholders. So, permanent jigs and fixtures usually contain more-elaborate parts and features than temporary workholders.
There are several other differences between permanent and temporary workholders in this area. These include the type and complexity of the individual tooling elements, the extent of secondary machining and finishing operations on the tool, the tool-design process, and the amount of detail in the workholder drawings. Since the elements for modular workholders are usually part of a complete set, or system, only rarely will additional custom components need to be made.
Permanent workholders contain different commercial tooling components based on expected tool usage. Permanent jigs intended for a high-volume drilling operation, for example, often use a renewable bushing and liner bushing together. A throwaway jig for a smaller production run often uses a simple press-fit bushing.
The secondary operations normally associated with tooling include hardening, grinding, and similar operations to finish the workholder. Usually, permanent workholders are hardened and ground to assure their accuracy over a long production run. Since they are intended only for short production runs, throwaway jigs and fixtures do not require these operations. Another secondary operation frequently performed on permanent tools, but not temporary tools, is applying a protective finish, such as black oxide, chrome plating, or enamel paint.
In designing a permanent workholder, the designer often makes detailed engineering drawings to show the toolroom exactly what must be done to build the workholder. With temporary workholders, the design drawings are often sent to the toolroom as simple freehand sketches.
Permanent tools are normally designed for long-term use. This being the case, the drawings and engineering data for the permanent jig or fixture then become a permanent record. With modular workholders, the designer may either construct drawings or specify building the workholder directly around the part. Here only a parts list and photographs or video tape are kept as a permanent record.
Certain workholding applications require special fixture characteristics. For example, a particularly corrosive environment may require stainless steel components and clamps to deliver a satisfactory life cycle. In other cases, variable workpiece dimensions, as in a casting, necessitate clamping devices which can compensate for these variations. Appearance of a finished part might require the use of nylon, plastic, or rubber contact points to protect the part.
Similarly, the selection of tooling details can enhance the productivity of some permanent tools. For example, utilizing small hydraulic clamps may allow loading many parts on a workholder due to the compactness of the design. This would enhance productivity by reducing load/unload time as a percentage of total cycle time. Duplicate fixtures are sometimes justified for machining centers because they allow loading of parts on one pallet during the machining cycle on the other pallet.
The performance of any workholder is critical to the complete usefulness of the tool. If the workholder cannot perform the functions desired in the manner intended, it is completely useless, regardless of the cost or the extent of the detail. As the performance of a permanent, modular, or general-purpose workholder is considered, several factors about the machine tools must be known. These factors include the type, size, and number of machine tools needed for the intended operations.
Workholders are sometimes designed to serve multiple functions. For example, it is possible to have a workholder that acts both as a drill jig and a milling fixture. These tools are called combination tools or multiple-function workholders. Figure 1-6 shows a typical temporary workholder for drilling and milling operations on the same part. In this example, since the workholder has provisions for both milling and drilling, it is classified as both a drill jig and milling fixture.
Other machine considerations may come into play as well. On numerically controlled machines, for example, care must be taken in fixture design to position clamps out of the cutting tool's path. Pallet machines require different fixtures than other machines. Obviously, vertical mills would be tooled differently than horizontal mills. Likewise, the way parts are loaded onto the fixture has implications for fixture design.
Despite the workholder design or the size of the production run, every jig or fixture must meet certain criteria to be useful. These criteria include accuracy, durability, and safety. Accuracy, with regard to jigs and fixtures, is the ability of a workholder to produce the desired result, within the required limits and specifications, part after part, throughout the production run.
To perform to this minimum level of accuracy, the workholder must also be durable. So, the jig or fixture must be designed and built to maintain the required accuracy throughout the expected part production. If part production is continuous, year after year, the jig or fixture must be more durable than is necessary for only one production run.
The final consideration, safety, is actually the most important. No matter how good the design or construction, or how well it produces the desired accuracy, if the workholder is not safe, it is useless. Safety is a primary concern in the design of any workholder.
Safety, as well as speed and reliability of part loading, can often be improved by the use of power clamping, either pneumatic or hydraulic. Once set, power clamps will repeatedly clamp with the identical force. This is not always true with manual clamps, which depend on operator diligence for the proper application of clamping force. In addition, power-clamping systems can have interlocks to the machine control which will shut the machine down if the system loses power—a clear safety advantage for both operator and machine tool.
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.