Adding Four-Axis Lathes To The Production Line

Usually thought of as stand-alone machines, they fit right into this automotive stamper's automated production lines.

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Four-axis CNC lathes usually don't figure prominently in stories about firms that make stampings and stamping dies. However, here's a case where a stamping firm incorporated them as key components of two highly automated production lines. The lines enabled the firm to make the giant leap from merely supplying stamped parts to supplying virtually complete assemblies to U.S. automakers.

Koppy Corp. (Orion, Michigan) has been supplying precision stamping dies to the auto industry for more than 60 years. The company specializes in round precision stampings that go into transmission assemblies and torque converters for Ford and GM trucks. Its services don't stop with dies and stampings, however. The firm helps customers engineer and develop new drivetrain products and the production processes for those products. It also manufactures machine tools designed exclusively for the production of products such as clutch cylinders and torque converters, which are primarily assemblies of stamped parts.

Until recently, Koppy Corp.'s Sigma Stamping Division (Auburn Hills, Michigan), a 96,000-square-foot facility with 16 stamping presses ranging in size up to 3,500 tons, was involved almost exclusively in producing stamped components for the automotive companies and their suppliers. However, Sigma wanted to do more than just supply stamped parts. The company's expertise in the design, production and assembly of torque converter components, clutch cylinders and similar products and its equally weighty knowledge of the machines and processes used to produce those products ideally suited it to produce the complete product, not just components. In fact, the company saw such a development as a logical growth track.

Sigma's big opportunity came in the form of a large, multi-year contract to produce a torque converter pump assembly (also known as an impeller) that would be complete except for an ID machining operation that would be done at the automaker's facility. The pump assembly consists of a number of stampings that are mechanically joined and brazed, then welded to a steel hub. After leak testing to confirm the integrity of the weld, the assembly goes to a final machining operation where the hub is machined to the required size and surface finish in preparation for the final operation by the customer.

"The customer wanted a supplier that could make the stampings and do the assembly and machining under the same roof," explains Dennis Goodloe, process engineer for Sigma Stamping. "They wanted to make sure that the entire process was under control."

The stampings consist of an outer shell, multiple blades or vanes, and inner rings. Tabs on the vanes insert in both the outer shell and the inner ring and are rolled over to effect a mechanical assembly. In order to assemble the pumps as efficiently as possible, the entire process is automated. As they come together, the pumps proceed from one station or machine to the next on a conveyor to minimize manual handling. For example, 31 vanes are inserted in slots in the outer shell by an automatic vane loader. An inner ring is fitted over the vanes (photo on facing page), then the assemblygoes to another machine that rolls over the tabs to mechanically secure the vanes. The mechanically joined assemblies are then sent out for brazing.

When the brazed assemblies come back, a steel hub is placed over the main bore of the shell and welded in place. After a helium leak test—the weld must not leak—the assembly is conveyed to one of two four-axis CNC lathes for the final operation, the machining of the hub and embossments.

Demanding Requirements

The CNC lathes chosen for the line had to meet several demanding requirements. First, they had to integrate easily with the line's conveyor system. Second, they had to be fast: To permit a production rate competitive with similar lines in the field, the lathes had to machine the torque converter hub—face the end, single-point turn the hub OD and single-point bore its ID—in one minute, including the part load/unload time. Third, the machines had to be accurate: The customer's tolerance for the hub OD was 0.0012 inch, which meant that Sigma had to hold 0.0008 inch. Finally, the machines had to be reliable: A major breakdown would cripple production.

Sigma brought its list of requirements to Gerotech Inc. (Flat Rock, Michigan), the Mori Seiki distributor in the Detroit area, and asked the firm to turnkey a CNC lathe installation that would get the job done. Gerotech recommended a pair of RL-253 four-axis CNC lathes made by Mori Seiki USA, Inc. (Irving, Texas). Unlike more conventional four-axis lathes with opposing spindles, the RL-253 has parallel (side-by-side) spindles, each served by its own turret. A major feature of the machine is an integral, three-axis gantry loader that takes a part from the loading area at the rear of the machine, delivers it to the machining area, loads it in a chuck in one of the machine's two spindles, and finally removes the machined part and returns it to the unloading area at the rear of the machine.

Although the gantry loader was compatible with the line's conveyor system, it could not accommodate the large (12-inch-diameter) size of the torque converter pump's shell. To accommodate the large part, Gerotech turned to a supplier in the Detroit area that was able to install a freestanding gantry system capable of handling the large part. The gantry takes torque converter pumps from a queue on the conveyor line, loads them in each machine's chucks for machining, unloads them and returns them to the conveyor line for delivery to shipping.

The four-axis CNC lathes provide more flexibility than possible with a machine specially designed for the application. For example, for the machining of the pumps, each machine acts as two independent two-axis CNC lathes, machining two torque converter pump assemblies simultaneously. Because the job is running on both machines, four pump assemblies are machined simultaneously, for a rate of one every 15 seconds.

If Sigma needs to run a more complicated part requiring machining on both sides, the four-axis lathe and gantry can be set up so that the part is delivered to one spindle and then to the other for machining of both ends. Productivity remains high because parts can be machined in both spindles simultaneously. Also, if production requirements change unexpectedly, machines can be added to the line or removed for use elsewhere in the plant.

"Conveying of the torque converter pumps through the line minimizes the number of operators required to staff the line," explains John Walker, vice president of manufacturing for Sigma. "Also, on production lines that produce similar products, manual loading and unloading is commonplace, and operator fatigue is a problem in maintaining consistent accuracy. From the beginning, we attempted to eliminate the fatigue factor. Our automated line has been very successful for us; in fact, it has resulted in the development of two more automated lines at Sigma."

Line No. 2

Where turning is the final operation on the torque converter pump line, a turning operation on another four-axis CNC lathe is the first operation on Sigma's equally automated clutch cylinder line, one of the two new lines mentioned by Mr. Walker. The largest component of the assembly is a drawn shell or cylinder—it looks like a round cake pan—with a large opening in the bottom and closely spaced splines roll-formed into its sidewall (see photos at left and on the following page). A groove for a retaining ring used in the assembly of the unit must be cut in the splines on the ID of the cylinder.

Koppy Corp. makes a slotting machine designed expressly for such applications, and the company expected to use it to produce the retaining ring groove in the cylinder ID. However, the customer specified that the groove had to be machined, so Sigma scrambled to replace the slotter with a CNC lathe. The company again turned to Gerotech for a machine that, as with the first machine, would be compatible with the production line's conveyor system—and fit in the space originally allotted for the slotter. The distributor again recommended the Mori Seiki RL-253 for the application.

Initially, Sigma used both spindles to machine the clutch cylinder. OD operations were performed on the cylinder in the first spindle, then the machine's integral gantry loader transferred the part to the second spindle for machining of the groove in the ID splines.

"We weren't satisfied with the process," Sigma's Mr. Goodloe recalls. "The OD operations were much faster than the grooving operation, and it was taking too long to handle the part in and out of both operations.

One Chucking

"We started looking for a way to completely machine the part in a single chucking," he continues. "As with our first automated line, the part had to be processed at high production rates, with a minimum of operator intervention. The challenge was to make certain that the stamped cylinder was accurately centered and fully seated in the chuck, and that it was gripped firmly enough to ensure machining accuracy, yet not so much as to deform the cylinder.

"We started with a conventional three-jaw chuck, but it deformed the cylinder just enough to affect the accuracy of the turned groove," Mr. Goodloe recalls. "Gerotech took the problem to MP Tool (Roseville, Michigan), a custom workholding supplier, who came up with the idea of a dual collet chuck—a centering collet that centers the cylinder without applying any gripping force, and a holding collet that distributes the holding force along virtually the entire OD of the cylinder to eliminate distortion. The dual collet permits access to the cylinder by all the tools, enabling us to do all of the machining in a single chucking."

Because the gantry load/unload process was completely automatic, Sigma had to minimize the possibility of chips lodging in the chuck and preventing the cylinder from properly seating. To that end, the company sprays the chuck with high-pressure coolant just before loading the part to flush away stray chips. Sensor pads, mounted at three points on the chuck, confirm that the cylinder is properly seated in the chuck. If the sensors determine that the part is not correctly positioned, the gantry arm removes it from the chuck and reinserts it. The procedure is repeated a second and third time. If the problem doesn't correct itself, the turning center stops and signals for operator assistance.

When Sigma first started machining the clutch cylinder using both spindles of the four-axis lathe, it produced about 37 cylinders per hour. By finding a way to do all of the machining in a single chucking, the company was able to use the four-axis lathe as a duplex lathe, with each spindle producing a finished part every 60 seconds. (Actual machining time is 42 seconds; time for the gantry to load and unload the part is another 15-17 seconds.) Sigma is about to add a second four-axis CNC lathe to the line, and the combined output of the four spindles will provide a production rate of 240 cylinders per hour—or one every 15 seconds, Sigma's target figure.

Unexpected Benefit

The four-axis lathes provide consistent accuracy to match their productivity, which benefited the customer in an unexpected way. "When we first started machining the cylinders (both spindles), the customer was using four different snap ring sizes to assemble the clutch packs to the required tolerance," recalls Mr. Walker. "A machine at the customer would gage the part and indicate the proper snap ring size for each assembly. However, since we switched to machining the part complete in one chucking, the part consistency is such that the customer now uses only one snap ring size, which resulted in a cost savings."

Sigma started up the torque converter line 4 years ago. The line convinced visiting customers of the company's ability to produce complex assemblies in large quantities at competitive prices. As a result, Sigma was awarded two long-term, high-volume orders for similar assemblies, and the company installed two more automated production lines. Time has proven the wisdom of Sigma's decision to become more than just a supplier of stamped parts and to provide more value-added services to its customers. In 4 short years, sales of assemblies produced on the company's automated lines have grown to about 50 percent of the company's total sales, and they offer the greatest opportunity for continued growth.

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