Processing The Process

Modern Machine Shop, August, 2001 by Mark Albert

The high speed horizontal machining centers in this manufacturing cell represent a very efficient process for machining cost iron transmission cases. How the crew runs and maintains this cell represents a very efficient process for making this cell ever more productive and cost effective.

When this six-machine cell was installed in May 2000, at Eaton Corporation's truck transmission plant in Kings Mountain, North Carolina, its performance was impressive. The 40-hp HMCs were able to finish a family of cast iron transmission cases in about 25 percent less time than previously had been required. Production costs foreach transmission case also dropped substantially on the new cell.

But what is more impressive is how the cell's performance has improved significantly since installation, thanks to a remarkable approach to continuous improvement followed by the operators and programmers who run this cell. They have found ways to reduce tooling costs, boost speeds and feeds, enhance flexibility, add a critical engraving step without sacrificing process time, and exploit the circular interpolation feature of the HMCs. In fact, the team is on the verge of completing some developments that may allow a drastic reduction in the number of different castings required to cover all of the part numbers produced in this cell -- from six castings to two.

The operators and programmers have been systematically re-examining every choice of setup detail, cutting tool, machining parameter, tool path and so on. They have consulted with vendors; they have tweaked and experimented; they have tested and analyzed; they have set goals and time lines; they have been imaginative but disciplined. The results are measured and documented.

According to Lonnie Pettit, product line coordinator, who manages this and several other machining areas in the factory, these improvements have yielded an annualized savings of $340,000. Tooling costs per case have dropped from $8.71 to $6.49, he reports. "Virtually all of these improvements were achieved without the help of manufacturing engineers. The improvements were initiated on the shop floor by shopfloor personnel. That's pretty amazing," Mr. Pettit remarks.

Ron Skibo, who supervises ("coaches," as he puts it) the ten-person team assigned to the line, concurs. "The key has been letting the guys who work with this line take responsibility for its performance. We let them determine the areas where they think improvements can be found and encourage teamwork to go after those improvements."

This processing of the process proves the rule that high speed machining applications are a dynamic, lively affair, even in high-volume, automotive production plants where you might least expect it. Like the innovative aerospace shop or boldly aggressive mold builder, Eaton has been willing to take an unconventional approach by not insisting that a successful runoff of the cell be the last word on its capabilities.

A Big Improvement To Begin With

Before the cell was installed, transmission cases were machined on ten stand-alone conventional machines with each machine dedicated to one operation and requiring a special setup for each operation. At best, cases could be completed in about 70 minutes, although bottlenecks at any one of the machines frequently extended that time.

The new cell consists of six A88 HMCs from Makino (Mason, Ohio). These 40-hp machines have a top spindle speed of 12,000 rpm and a 1,968 ipm rapid traverse rate.

Tools can be changed in 1.7 seconds, with a 4.8-second chip-to-chip time. The machines are lined up three by three on each side of a pallet shuttle system. Workpieces enter and leave the cell via a two-station load and unload facility.

The transmission cases pass through the cell in two fixturings. First, blank castings are manually loaded upright on "stand-up" fixtures mounted on 24- by 24-inch pallets. These castings come in six variations, including one for the innovative Fuller Lightning transmission (see Sidebar). On the stand-up fixture, the castings undergo a series of milling, drilling, tapping and reaming operations. Afterward, the pallet returns to the load and unload facility, where the cases are hoisted off the fixture and refixtured in a "lay-down" position on another pallet for the second pass through the cell. Fourteen pallets circulate through the cell, seven for each fixturing position. Operations for the lay-down fixturing include face milling the front and rear sides of the cases, boring several bearing journals, and some additional drilling and tapping of bolt holes. The boring operations include those for the main shaft bearing, the countershaft reverse idler and the auxiliary countershaft, plus bearing journals on the interior of the case that must be reached with extended boring bars.

Based on a daily production schedule, the cell controller indicates which kind of casting to load. Then the controller routes the pallet to the first available machine after verifying that this machine is ready with all of the appropriate cutting tools. (Each machine is equipped with a 132-station automatic tool changer.) The Lightning transmission case, however, requires a special set of cutters, including several metric sizes, that is maintained on only one of the machines, so these cases are only routed to that machine. This machine is also tooled for any of the other five styles of cases, so it is never idle.