Efficient CNC Flexible System Transfer Saves Time And Money

Since its organization in 1994, American Axle & Manufacturing (AAM) (Detroit, Michigan) has been proving to the industry that a U.S. manufacturer can be competitive on cost and quality. The company attributes its success to its investment in manufacturing technology and training.

To meet quality and volume requirements from a customer who hoped to increase the torque and load-bearing capabilities of its full-size pickup trucks and SUVs, AAM engineers developed a process to produce a new rear differential gear carrier. It was the largest the company had ever produced, at 118 pounds.

The process initially used a series of large horizontal machining centers (HMCs) to meet the demand for the 11.5-inch carrier. Then, using the same process, fixtures and tooling, AAM ramped up using a CNC Flexible System Transfer (FST) line from Heller Machine Tools (Troy, Michigan) when volumes went from 35,000 to 240,000 parts annually. The process and the equipment permitted the plant to respond quickly and cost-effectively to the change in demand while consistently achieving quality goals.

In developing the carrier design, AAM paid special attention to details that would positively affect the part's perceived noise, vibration or harshness characteristics, and would thereby provide an advantage to customers in the pickup truck and SUV market.

Once the parts were designed, the company conducted simultaneous engineering programs with three machine tool companies to evaluate each vendor's approach to its manufacturing challenge. In each case, AAM was interested in flexible manufacturing cells to handle relatively low initial production volumes. The company also looked for machine tool vendors that used Hirth couplings on their rotary tables, which AAM felt would help it meet critical tolerances. The purpose was to prove that the critical dimensions could be consistent on a machining center.

Carrier housings and differential case housings are two important axle components, which AAM machines and supplies to its customers. These parts have been produced by a number of different machining processes; for example, carrier housings were machined on single-spindle horizontal machining centers and dedicated transfer lines, as well as CNC lift-and-carry transfer lines. The differential case housing was produced on single-spindle HMCs and lathes.

Rather than using 11 to 12 HMCs to produce the housings, AAM invested in a single Heller FST. In the experience of its Three Rivers facility, the FST has proven to be a reliable high production system and to be more cost-effective than HMCs. According to AAM, the FST has the benefits of using less floor space (in this case, about 30 percent less) and has capital costs of up to 25 percent less than conventional transfer lines.

The facility uses the 14-station FST to machine the 11.5-inch carrier housings. These operations include face milling, drilling pan holes, and machining pads for end-cap seating and rough-boring the axle tube bores. The FST then completes the boring process, maintaining the perpendicularity of bores in the carrier. Each bore is completed in three passes. Once the part is finished, it is unloaded and put on a conveyor to a robot-loaded machining center cell, where the remaining holes are drilled and tapped in the carriers. Empty fixtures in the FST are returned via an overhead conveyor to a washer and then back to the load station.

Six Heller MCP-H250 HMCs are also incorporated at Three Rivers. All the machines have HSK 100 spindles, making it possible to use the same tooling in both types of machines when necessary. A hydraulic workholding system uses swivel pads and locator points on the casting to ensure repeatable clamping. The HMCs are also capable of being retooled and refixtured for other components or to supplement production of the FST.

The FST concept uses predesigned modular units mounted on standardized bases with independently controlled stations. Three basic pre-engineered machine sizes cover component dimension, power and technology applications. System components can be combined to create the system best suited to the application. Because all interfaces between the units are standardized, engineering time is reduced and assembly is accomplished quickly, the company says.

The FST was fully assembled and run off on the Heller assembly floor in Troy prior to the delivery to the company. This allowed the FST to be fully operational and on the plant floor in a third less time than a transfer line usually demands.

Because each of the 14 FST stations is CNC, the company can easily accommodate different part geometry as well as change the speeds and feeds of the machining heads through programming. The FST also uses in-line gaging to feed back size compensations (for automatic adjustment) to the finishing tool. This ensures that critical part features are maintained within print tolerance and provides SPC trend data. After initial operations are completed, one part per hour is inspected off-line.

The FST control system consists of one central console with identical station controls in an open communication network. This has allowed process engineering to further optimize feeds and speeds and accommodate a change in insert rake from positive to negative, resulting in an increase in tool life from 30 to 200 pieces.