Making The Most of Modular Fixturing

For the last 20 years or so, Ingersoll Milling Machines (Rockford, Illinois) has used modular fixturing to hold for machining the many one- and two-of-a-kind components that go into its custom machine tools. As is the case with most machine tool builders during this economic slump, Ingersoll's machine tool orders have declined. However, the company is keeping busy with a rapidly growing contract machining business, and its modular fixturing system is proving just as useful for machining the larger quantities required by the contract work as it is for onesies and twosies.

Modular fixturing is a system of interchangeable components designed to securely and accurately hold a workpiece for machining, primarily on holemaking machines such as horizontal and vertical machining centers. It provides an alternative to dedicated workholding fixtures, which are costly, require weeks to design and make, take up a lot of storage space when not being used, and usually cannot be used for anything except the job for which they were designed. Unlike dedicated workholding fixtures, a modular workholding fixture can be disassembled into its component parts at the conclusion of a job and reassembled with other components of the system to secure a workpiece with a completely different configuration.

One of the major advantages of modular fixtures is the cost savings. A modular fixturing system is initially more expensive than a dedicated fixture; however it can be used over and over to hold a variety of parts, making it less expensive in the long run. Payback periods are typically short—months instead of years—depending on frequency of use. Also, where engineering changes can result in scrapping of or expensive changes to a dedicated fixture, they can usually be accommodated with only minor adjustments to a modular fixture, typically by just replacing or moving a component. Similarly, where a machine tool crash might ruin a dedicated fixture, the damage to a modular fixture is usually confined to one or more modular components that can be quickly, easily and inexpensively replaced.

Time savings is another important advantage. Where it can take days to weeks to design and build a dedicated workholding fixture, a modular fixture can be designed and assembled from available components in minutes to hours, depending on the complexity of the job. That provides a tremendous competitive advantage in terms of delivery: Job orders received today can be set up tonight and machined tomorrow.

Dedicated fixtures must be stored against the time when the job repeats. They take up a lot of storage space when idle, yet despite their size and weight, they manage to disappear at the precise time that they are needed. By contrast, when machining jobs are done, modular fixtures are broken down to their individual components, which are stored in space-saving, conveniently located, appropriately labeled, drawers or cabinets, ready for the next job.

Good management practice provides a permanent location for the modular components and provides that all components be returned to their location when the job is completed. That not only eliminates the time wasted searching for missing components, but it also helps ensure that needed components will be available so that the setup person can assemble the fixture as quickly as possible.

Ingersoll uses the Bluco modular fixturing system supplied by Bluco Corp., Naperville, Illinois. The foundation of the Bluco system is a cast iron base plate drilled at regular intervals with holes that are alternately tapped or fitted with steel bushings that receive dowel pins. Precise location of the bushings in the dowel pin holes is the key to the accuracy of the system. Each bushing is positioned in its bore in the base plate to within ± 0.0004 inch using a master gage. While the sleeve is held in place, an epoxy is poured into the space between it and the base plate bore. When the epoxy cures, the sleeve is accurately located to receive its dowel pin.

Other modular components of the system that are used to secure the workpiece to the base plate, such as angle towers, adapter blocks and support angles, are similarly drilled, and tapped or fitted with steel bushings. Dowel pins precisely locate and position the components relative to the base plate and to each other. Once positioned, the modular components are joined with socket head cap screws. Various clamps and holders secure the workpiece to the base plate and the other modular components required to support it.

The Bluco system is sold in three separate sizes to accommodate small, medium or large parts. For those with a broad range of part configurations, starter packages are offered in each system size. Base plates from as small as 150 mm by 150 mm to as large as 1,250 mm by 1,250 mm are available. The user can purchase individual components to more closely match specific workholding requirements, as well as to replace components worn or damaged in use.

For Its FMS

Ingersoll acquired the Bluco system for a flexible machining system (FMS), consisting of seven (now five) 40-hp Bohle four-axis horizontal machining centers with pallet changers, originally installed to machine relatively small machine components in lot sizes of one and two. The FMS is arranged in a straight line, headed by a staging area with five setup stands (photos on pages 82 and 83). Each stand holds a pallet on which a base plate for the modular fixturing system is mounted. The workpiece is fixtured on the pallet using the modular components.

When a setup is complete, an automated guided vehicle (AGV) removes the pallet holding the setup and delivers it to the input stand of the HMC scheduled to machine that particular part. If the HMC has just completed machining a workpiece and has shuttled it to its output stand, the AGV will remove the pallet with the machined part and return it to the setup stand where the trip began. At the setup stand, the setup person removes the machined workpiece from the modular fixture and either loads another identical workpiece or starts on a different setup. There will be more about setup procedures a bit later in this article.

A daily schedule is prepared for each of the machines in the FMS so that the setup person knows what job to set up next. In setting up the workpiece using the modular fixturing, the setup person works from a CAD/CAM department-prepared print that includes a list of the modular fixturing components required for the setup. A scaled drawing shows where each of the components must be installed—right down to the hole locations for each dowel pin and cap screw.

The Programmer Does It

The person who prepares the machining program for the part also designs the modular workholding fixture for the part. Making the programmer responsible for both tasks reduces the danger of the setup person causing a wreck by unknowingly installing a clamp or some other component in the path of the cutter. Making the programmer, or someone else in the department, responsible for modular fixture design may be the single most important step that can be taken to help ensure the successful implementation of a modular system.

"When we were exclusively making parts for Ingersoll machine tools, most of our jobs involved one or two pieces," recalls Chris Linenfelser, FMS manager at Ingersoll. "We were doing between 30 and 35 setups per day, constantly building a new setup for a new part, running one or two pieces, tearing down the fixture, and starting all over again. We were thrilled when we'd get an order for four of anything. The modular workpiece fixturing system is ideal for such jobs; with it, setups go together quickly and easily.

"Now that we're also doing contract machining, our job runs are usually larger, however we are using the modular fixturing to process them as well," Mr. Linenfelser continues. "The biggest difference between the Ingersoll work and the contract work is that the part quantities are usually higher for the contract work and setup changes are less frequent. Instead of tearing apart the setup when removing machined parts from the modular fixture, the emphasis shifts to removing the fewest components that will permit getting the finished part out and loading the next workpiece."

Both of the jobs that were running on the FMS during a recent visit were contract jobs. The rock drilling tool body (shown being fixtured on page 82) is rotated 360 degrees to machine holes for mounting the replaceable cutting inserts.

Mix And Match

The second job, a part dubbed the wedge (shown on page 85) that is used in the X-ray field, demonstrates the flexibility of modular fixturing. In this case, Ingersoll incorporates a specially machined support in an otherwise totally modular workholding fixture to permit milling of slots vertically instead of at a slight angle, to speed machining time. Had the run consisted of only one or two pieces, the special support could not have been justified. However, the initial order calls for 40 pieces, and the job is expected to repeat one or more times for the same number of parts each time. Thus, the time savings per part more than justified incorporating the dedicated part in the modular assembly.

Mr. Linenfelser explains why the fixture designers decided to add the custom component to the holding fixture: "The workpiece is a casting, and each of its two long sides has a 2 ½-degree taper, giving the part the appearance of a wedge," he explains. "Wide slots are milled at intervals in the long sides of the part. We could have written the machining program to have the tool go in at a 2 ½-degree angle to compensate for the tapered surface. However, we realized that if we could incorporate a support with an opposing 2 ½-degree angle, we could present the side of the wedge perpendicular to the cutting tool and simply mill straight up and down. That would not only simplify the programming but also speed up the milling operation.

"Obviously, we could not have justified spending hundreds of dollars for a dedicated part if the job involved just one or two parts," Mr. Linenfelser continues. "But the time that we calculated we would save in the machining of 40 parts more than justified the cost of the dedicated support." He also makes the point that while the special support provides some of the advantages of a dedicated fixture, it only takes up a small fraction of the storage that a dedicated fixture would require.

Incorporating special components in a modular workholding fixture has turned out to be an acceptable compromise for some in the plant who are used to and feel more comfortable with the more traditional dedicated fixtures and are still a little skeptical of modular fixturing's capabilities: "We tell our people that the modular fixturing is here to stay, but if they feel that they have to, say, mill a soft pad to orient the part a certain way for machining—as was the case with the wedge—we give them the green light," adds Steve Anderson, manager—light machining for Ingersoll Contract Manufacturing Co. "Just don't ask for a $2,000 dedicated fixture!"

Over the years, Ingersoll's fixture setup people have streamlined the modular fixturing process. In particular, they have learned to take advantage of similarities when scheduling parts for machining to minimize fixturing changes from one job to the next. "We may need to run 10 different parts on each machining center in a given period," Mr. Anderson explains. "If no one job has a priority over the others, we allow our setup people to decide the order in which they set up the jobs to take advantage of those part similarities.

"By doing so, we can usually avoid having to completely disassemble the fixtures," he continues. "The setup for part A may be so similar to that for B that we can get away with just moving a few components around. That intelligent fixturing is one of the reasons we can do so many setups in a day. When we're busy it is not uncommon for us to do 30 setups a day (over three shifts). If we had to use dedicated fixtures for those jobs instead of modular fixturing, we'd be up to our elbows in fixtures."

Modular fixturing is saving Ingersoll a lot more than space, however. Mr. Anderson estimates that the company saves about $150,000 annually by using the modular fixturing system instead of dedicated fixtures. That's a total savings of over $2.5 million for the last 17 years. The plant is still using its original modular fixturing system. Costs incurred by using the system are limited to the occasional replacement of a worn or damaged component.

The ability to machine parts quickly using the modular fixturing system may be more important than the cost savings, particularly in light of Ingersoll's aggressive efforts to grow its contract machining business. Contract work currently accounts for about 70 percent of the Fulton Avenue plant's activity, and the number could go higher. "The modular fixturing system gives us a 1 ½- to 2-week advantage over competitors who still use dedicated fixtures," Mr. Anderson stresses. "If we were to use a dedicated fixture, it would take a week to have it fabricated, a day or two to get it stressed, and another three to five days to get it through our shop. That's two weeks before we would be able to begin machining the customer's parts. But with the modular fixturing, we can get jobs in today, program them tonight, and literally be making parts tomorrow. That's the kind of turnaround that's going to get us more customers."