A PLATFORM FOR FLEXIBILITY
In some ways, Muller Martini’s process used to be too flexible. At least the workholding was. The company’s off-the-shelf tombstones offered any number of ways to set up a given part . . . and therefore any number of ways to set it up incorrectly. To avoid these errors, the company used locator pins and written setup diagrams for every job—awkward measures that still allowed a fair amount of human error to slip through. The combined burden of taking these steps to prevent setup error and correcting the error that did occur represented a significant drain on the company’s ability to produce short runs of machined parts with little advance notice. To operate more flexibly, Muller Martini needed workholding components that were more certain and straightforward in the ways they went together.
Muller Martini’s facility in Newport News, Virginia, makes machines for book binding and book stitching—that is, machines that come after the printing press in a publisher’s process for manufacturing books. Though Muller Martini is an international company with multiple locations comparable to this one, the machinery is designed, developed, manufactured and marketed out of this same site.
Central to the site’s manufacturing area is a flexible cell from Makino (Mason, Ohio) with 24 pallets feeding two machining centers designed for a pallet size of 31.5 inches square. This cell is responsible for about 100 part numbers at any given time, which it machines in lot sizes ranging from two to 30 or 40. These small lots are machined as needed to maintain an inventory buffer of about 2 months’ worth of machined parts. In other words, part changeover is frequent.
Accomplishing this changeover without losing efficiency to various forms of setup delay—such as changing fixtures, changing tools and correcting for setup-related error—is partly the result of cellular machining and partly the result of a carefully designed process.
Freedom to develop such a process is one luxury the Newport News plant enjoys. Corporate management in Switzerland encourages the engineers on site to innovate within the various operations they control. Where machining operations are concerned, the two engineers principally involved are Ram Chandran and Jerry Fox. They are the sources for this article.
The process that these men developed and put in place involves electronic read/write tags in the toolholders for carrying cutting tool data to the cell, as well as fixture plates kept in a ceiling-high storage and retrieval system from which they can quickly be obtained.
The process also involves a special mechanism for mounting the fixture plates quickly and repeatably. In place of tombstones featuring a grid of threaded holes, Muller Martini uses specially designed mounting structures called "silos" that employ Ball Lock sockets to achieve simultaneous clamping and location for a given plate. The term "Ball Lock" refers to a mechanical clamping mechanism provided by Jergens (Cleveland, Ohio). Relying on a variety of custom-designed silo configurations that use this mechanism, the Newport News plant has made Ball Lock set up the foundation of its process for responsive, flexible machining.
The Silo Solution
The need to provide written setup instructions for each job was not the only difficulty the shop had with tombstones. The tombstones also imposed a delay between part numbers because they had to be cleaned before a setup could be changed. Holes left uncovered by the first setup would fill with chips during machining. These chips would have to be removed, one hole at a time, before the second setup could be put in place.
There was also a problem of tombstone management. Muller Martini’s range of part sizes often required operators to take different-sized tombstones on and off the machine. Big parts needed wide tombstones, but a wide tombstone wouldn’t do when a much smaller part had to be machined on opposing sides by indexing the pallet. The wide tombstone would block access, and at best only a toolholder extension would let the cutter reach the side of part.
In addition to these inefficiencies, there was also the simple time delay of screwing and unscrewing all of the fasteners to change from one fixture to the next.
The Ball Lock mechanism accomplishes locking and locating somewhat faster. Each individual clamp consists of a locating shank on the fixture plate and a receiver bushing on the mounting surface. One turn of a setscrew drives balls in the locating shank against a tapered surface in the receiver. (See Figure 1.)
Muller Martini bought only the clamps from Jergens. The shop designed its own fixturing system to use them. Fixture plates now conform to standard dimensions, and each plate features locating shanks along the two vertical edges of the plate. These shanks are spaced to let them mate with the palletized workholding structures that have now taken the place of tombstones. These structures—the silos—each feature two vertical rows of receiver bushings that have the receivers spaced 100 mm apart. (See Figure 2.) A tombstone now is used only for prototype machining; a silo is used for any production part. (In the earliest incarnation of its design, the newer workholding structure was built on a tubular metal frame that suggested a silo in appearance. Current silos no longer bear that resemblance, but the name has stuck nonetheless.)
With silos left in place on the cell’s pallets, setting up the work for a short-run job is a fast and simple procedure. The operator obtains the appropriate fixture plate out of a storage and retrieval system from Remstar of Westbrook, Maine. (See Figure 3). The operator mounts that plate on an open silo. Turning the set screw on each Ball Lock clamp is enough to both locate the plate precisely and lock it in place.
The system is not only fast, it also overcomes the tombstone’s problems. Tooling plates cover all of the sockets close to the cut, preventing these holes from filling with chips. And the vertical rows of clamps make it impossible for an operator to locate the plate incorrectly in the horizontal direction. Technically, it is still possible to mislocate the plate in the vertical direction, but the correct vertical location of each plate generally is intuitively clear.
The standard silo’s narrow design provides the side-to-side clearance necessary to machine small parts effectively. For parts requiring a wider clamping area, this same silo’s surface can be expanded by adding one or two "wings." These wings are comparable to the fold-out wings that might increase the tabletop area of a piece of furniture. On the silos, the wings are attachments that also lock in place using Ball Lock clamps. These attachments present receiver bushings for accommodating wider plates. (See Figure 4.)
Another variation the shop developed is a short silo design that has an extra mounting surface on top. Transferring the part from a fixture plate mounted vertically to one mounted horizontally increases the number of faces that can be machined in one cycle. (See Figure 5.)
Muller Martini originally developed the silo workholding for use on its two-pallet machining centers. But the shop saw the streamlined setup times resulting from the silos had made it feasible to set up many jobs quickly in preparation for an unattended shift. The appropriate next step was for the shop to replace its unconnected two-pallet machines with a cell that offered enough pallets to run a steady sequence of jobs without operator intervention.
The Newport News facility now runs its machining cell unattended for 6 to 8 hours per day during periods when customer demand justifies this output. Repeatable jobs are reserved for the unattended shift. Such jobs include work in aluminum where cutter life is so consistent that the right moment to change out a tool can be accurately predicted. Less repeatable jobs are run during a shift that has an operator standing by.
Fast and error-free changeover for the fixturing is only one requirement to let a two-machine cell produce a broad variety of short-run parts efficiently. Another requirement is to change what tools are in the machining centers in a way that is also fast and error-free. The Makino model A99 machines that Muller Martini uses hold 188 tools apiece, but even that capacity is not enough to contain the tools needed. The range of parts calls for somewhere between 400 and 500 different assemblies of tool and toolholder. Only about 140 of these are "standard" tools used across a large number of parts. These 140 stay in place on both machines, but the remaining tool magazine slots are occupied by tools that are taken in and out of the machine as the work requires.
Stocking, storing and transporting all of these tools is difficult enough, but managing the data associated with these tools is the greater challenge. For each tool/toolholder assembly, for example, there is the dimensional data necessary to determine its tool offsets. An on-machine toolsetting probe could be used to capture this information, but this would consume cycle time. Keying in the data obtained at an off-line presetter might also introduce delay, and it would certainly introduce the opportunity for human error.
Muller Martini’s solution is to let electronically stored data follow each toolholder wherever it goes.
Every toolholder used in the Makino cell carries an ID tag for storing electronic data. (See Figure 6.) The tags themselves require no power; they are used in conjunction with special read/write hardware. The complete system of ID tags and read/write units was supplied by Balluff of Florence, Kentucky.
Measurement information obtained in the presetting area is stored on these tags. Other data that the CNC uses while the tool is cutting are also stored there, such as the horsepower limit of the tool and the tool’s expected life, plus its accumulated cutting time so far. All of these data are read into the cell controller whenever a given tool is brought to one of the machines. So long as the tool resides in the machine, the cell controller is the keeper of this information, updating some of the data such as the accumulated cut time. When the tool leaves the machine, current data stored by the cell controller are written to the tag. (See Figure 7.)
This tool management system, combined with cellular machining and quick-change workholding, let Muller Martini realize a process for fast-changeover machining that is simple in the way it operates. The difficult part was putting the process in place.
No one supplier could deliver this process. Some suppliers were particularly accommodating. Machine tool builder Makino, for example, modified pallets to include Ball Lock receivers (similar to the pallet in Figure 8). However, the work of developing, proving and implementing this process could only be performed on-site, by people fully engaged in the work of this plant.
Mr. Chandran, manager of industrial engineering, is the senior engineer involved with the machining process. He points out that manufacturing support engineer Jerry Fox managed and oversaw the transition from the previous method of tool management to the electronic ID system used today. The details were numerous. Not only did the right hardware have to be put in place, but the right procedures had to be put in place also.
Mr. Chandran played a similar role for the quick-change workholding. The silos are products of his designs. Early on, when there was reason to doubt whether the Ball Lock clamps would stand up to machining vibration, he devised the test in which one of the clamps was mounted to a deburring machine to prove that vibration would not shake it loose.
In other words, it took the initiative of these two men to advance the process and put it in place. And the fact that there were only two probably is also significant. Large numbers tend to impede significant change, because details are lost as accountability is divided, and because the larger number multiplies the number of skeptics able to spot reasons to doubt the proposed process and trust the established one.
Compared to other dedicated manufacturing plants within multiple-facility corporations, one striking thing about the Muller Martini plant is an apparent absence—or at least a deficit—of political obstacles. The engineering staff on site is lean, limited to a small number of capable people, and the corporate management is empowering. More than any particular product from any particular supplier, these characteristics of the organization help explain how the company achieved its efficient machining process.
Re-posted with permission. Copyright 2002 by Gardner Publications. Click here for more industry news from Modern Machine Shop.