Jigs and fixtures have long been a part of manufacturing. But beyond their benefits, conventional jigs and fixtures also have a few shortcomings. This is especially true when production runs are small and do not repeat on a regular basis. When a company adds modular fixturing to its options, even job-shop-type production can benefit from quality fixturing.
MODULAR FIXTURING’S ROLE IN WORKHOLDING
Modular fixturing is not intended for every workholding operation, but when it is appropriate, it both increases production and reduces fixturing costs. Modular fixturing is not a replacement for permanent fixturing; rather it is an upgrade from “no fixturing,” just a step below permanent fixturing.
Definition of Modular Fixturing
Modular fixturing is a workholding system using a series of standardized components for building specialized workholders. As shown in Figure 6-1, a modular workholder is assembled from a variety of standard off-the-shelf tooling plates, supports, locating elements, clamps, and similar components. The components are assembled with socket-head cap screws and locating screws. A modular-fixturing system may have hundreds of different elements. The components can be assembled in different combinations to build an unlimited variety of workholders.
The assembly process is quite simple. Components are designed to be used together, so each has an identical hole pattern. To build a workholder, the components are simply positioned as required and attached with locating or fastening screws. The simplicity reduces training time and permits technicians to begin building workholders almost immediately. Regardless of the manufacturing operations, a modular-fixturing system can provide workholders for almost any workpiece.
The Hierarchy of Workholding Options
To understand how modular fixtures relate to workholding in general, an awareness of the various forms of workholders is necessary. Workholder forms can be grouped into three general categories, Figure 6-2. The least-complicated tools are the general-purpose workholders. The most-complex and most-detailed workholders are the special-purpose, or permanent, fixtures. Between the two are modular fixtures. Modular fixturing bridges the gap between the general-purpose and special-purpose workholders. Although the three forms of fixturing may seem completely different, each is actually a further development, or refinement, in the workholding process.
General-purpose workholders are the simplest form of fixturing device. This category includes a wide variety of standard clamps, vises, chucks, and similar standard off-the-shelf components. General-purpose components are reusable and normally the least expensive. Although these components represent a smaller initial investment, they are often inadequate or unsatisfactory for complex parts or high-volume production.
Permanent workholders are specifically designed and constructed for a single workpiece or family of parts. These workholders, though usually the most efficient, are also the most expensive. Permanent workholders are built with a variety of standard and custom-made parts to meet specific requirements. These fixtures are the best choice for high-volume or repeated production runs.
Modular workholders, in the most-basic sense, can be described as special-purpose workholders assembled from general-purpose components. Figure 6-3 shows a typical system of modular-fixturing components. The concept of modular fixturing, rather than a departure from conventional fixturing methods, is actually a combination of the best attributes of both special- and general-purpose workholding methods. Modular fixtures are built with the accuracy and detail of special-purpose workholders, but with reusable and universal components, these fixtures compare favorably in cost to general-purpose workholders.
Good Applications for Modular Fixturing
Modular workholders are particularly well suited for one-time jobs, infrequent productions runs, prototype parts, replacement parts, trial fixturing, and temporary tooling.
One-Time Jobs. One-time jobs, found especially in job shops, are ideal for modular fixturing. With modular fixturing, a workholder can be economically built even for a one-part run. In most job shops, each machine tool does a variety of tasks in a single day. With modular fixturing, jobs can be performed with specialized workholders at a cost very competitive with crude machine-table setups. Here, modular fixtures increase quality and accuracy, yet still maintain competitive costs.
Infrequent Production Runs. Jobs that do not repeat on a regular basis are well suited to modular fixturing. Today shorter lead times are quite common. Modular fixturing permits rapid setup of short-notice production
runs. Once again, modular workholders offer many of the benefits of special-purpose tooling at a fraction of the cost.
Prototype Parts. Prototype or experimental parts frequently require special workholders. Since prototype
workpieces are often changed or redesigned, the cost of building jigs or fixtures for each new variation is
prohibitive. Modular fixturing is the best alternative. With modular workholders, each variation of the workpiece
can be quickly fixtured with little or no downtime.
Replacement Parts That Are Made to Order. Replacement parts are an expensive problem in many
companies. In the past, these parts were made in large lots and placed in storage. Some parts were quickly
sold while others were never ordered. Modular fixturing eliminates the need for an inventory of slow-moving
replacement parts. Modular fixturing permits a company to respond to orders as they are received. Instead of
shipping parts from an inventory, parts can be made as needed.
Trial Fixturing Techniques. Trial fixturing is common throughout manufacturing organizations. Before any
product goes into production, workholders must be designed, built, and tested. Assembling a modular workholder for workpiece allows the tool designer to test new tool designs and find problem areas. Modular workholders allow a designer to fine tune tooling ideas before a final production workholder is built.
While Permanent Fixtures Are Built or Repaired. No matter how well a permanent jig or fixture is built,
most require repair or maintenance. A modular workholder can easily be assembled and placed into production while the permanent workholder is repaired. Modular fixtures also buy time for initial building of permanent fixtures without delaying production.
Poor Applications for Modular Fixturing
Just as conventional machine tools are better suited for some tasks than expensive CNC machines, the selection and application of workholders is determined by the work to be performed. Modular workholders are not intended for every job. Two limiting factors with modular workholders are the frequency of production runs and the size of the workholder.
Jobs That Will Repeat Many Times. Recurring production runs are those that repeat on a regular basis. The choice of fixturing method depends more on the frequency of production runs than on the number of parts per run. As shown in Figure 6-4, choosing between modular and permanent fixturing for a particular job depends on how often the job will run, not just on lot size.
Modular workholders are usually disassembled after each job. Each time a job is run, the modular fixture
requires reassembly. Permanent workholders are normally built for a complete product run. If a job repeats on a regular basis, a permanent fixture is the better choice. Modular fixtures could remain assembled between production runs, thus becoming a permanent workholder, but this negates the economy of reusing modular components.
Where Fixture Compactness Is Important. Another factor to consider in the selection of a workholder is
the size of the completed fixture. As a rule, modular workholders tend to be larger than their special-purpose counterparts. Permanent workholders are normally built from custom-made elements and baseplates that permit a smaller, more-compact workholder. Modular components are intended for a variety of applications and, though more universal, are larger than comparable custom-made components. So, when space is limited, such as with multiple-part setups, modular workholders may prove to be too large.