Modular fixturing is certainly not a new idea. People have been building fixtures out of reusable mix-and-match components for more than forty years now. For the right applications, the modular approach can save substantial resources that otherwise would be spent on building and storing dedicated fixtures, plus offer so much more flexibility in the bargain. Yet, particularly in this country, modular fixturing has never caught on nearly as well as it should.

But the benefits are compelling. Most significant are time and flexibility, which make modular fixturing ideal for high mix, low volume production. Fixtures can usually be designed and built in several hours, and modified at a moment's notice. And once built, they can be rebuilt in less time than it takes some companies just to find a stored dedicated fixture. Part design or NC program changes are easily accommodated, as are holding enhancements, the necessity of which only becomes apparent once the first part is cut. Workpiece location can be established quickly and accurately. And because workpiece holding and location are determined essentially off line, machine setup and change-over can become even more efficient.

There are still other benefits for larger shops with more formalized production procurement procedures. For some companies, the process of making a fixture is nearly as complicated as making a part, involving engineering, methods, tool design, purchasing and outside vendors. The paperwork and interdepartmental communications alone can take weeks. With modular, the traditional formalities of fixture design and fabrication are significantly reduced, and sometimes bypassed altogether. To do this, however, requires manufacturers to take a different view of how to manage the process of workholding, and trust broader responsibilities to individuals on the shop floor.

A Case in Point

Wabco's plant near Spartanburg, South Carolina, is a good example of how modular fixturing can work in a medium size shop environment. The 183,600 square foot facility is dedicated to making air brakes, coupling systems and other components for rail transit vehicles. Interestingly, though Wabco--also known as the Westinghouse Air Brake Company--has experienced some changes in corporate parentage over the years, it is the very same company founded by George Westinghouse over 120 years ago to manufacture his then revolutionary new braking system for trains.

In some respects, Wabco had an advantage in applying modular fixturing. It came in when the plant started up just four years ago, so they were not overly burdened with time-hardened shop procedures. They were also spared the formal cost justification procedure, since they believed modular fixturing would be necessary just to get much of the work done. The "problem" was that most of the equipment in the Spartanburg plant was new. Much of the work coming into the plant had been run on manual machines; now it would be machined on CNC equipment, so they would have to find new ways to hold these parts. They had no intention of using modular fixturing on every part, but did expect it to carry the load for the low volume parts and for proving out jobs on their first trip through the plant.

Wabco produces roughly two thirds of their machined parts in the plant, with the rest going to outside vendors. That inside/outside balance is used to deal with fluctuations in the business, smoothing and stabilizing the loading of their own facility. They run some 10,000 different machined parts in the plant, most of which are castings. The prismatic parts are run across one of seven horizontal machining centers or three verticals. They do approximately 250 production setups a month on these machines, with runs averaging 50 parts. They also do some machining for their own maintenance and repair needs. Modular fixturing may be used in as many as thirty of the setups, which means that, on average, they are building about one modular fixture a day.

Given the variety of work that is run, and the number of modular setups that are sometimes in use concurrently, Wabco's investment in modular componentry is substantial. The initial investment approached $200,000 and they have bought more since. They use the Bluco Technik modular fixturing system, supplied by Bluco Corporation (Carol Stream, Illinois).

The basis of this particular system is a set of precision base and angle plates with alternating sets of locating and tapped mounting holes. Fixtures are built by mounting a variety of standard components--risers, edge-locating blocks, supports, clamps, and so on--onto the base plate in the proper configuration to hold a part. Ground dowel pins are used to locate the components on the plate, or relative to each other, and then the mounting is accomplished with standard cap screws. Most of the components have multiple locating surfaces, ground to a tolerance of [+ or -] O.0004 from any one surface to another. The same tolerance holds for any two locating holes on a base plate.