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Turnkey strategy meets a set of Tier 1 demands.

Flexible production, ramping up to accommodate a doubling of demand, machining accuracy to less than 25 percent of conventional tolerances, and consistent performance from a complete turnkey solution.

That's what a leading Tier 1 plant was looking for when it took on production of axle assemblies for near-luxury SUVs.

That's what Heller Machine Tools, Troy, MI, was prepared to deliver.

The tolerances on these parts are what you would expect from the automaker--possibly even tighter--according to the plant manager of the new Tier 1 manufacturing facility in the South Carolina piedmont.

"They are a fraction of the tolerance our customers typically require--and we make up to 3 million of this type of assembly a year as the No. 1 independent axle maker. For that reason, we were very careful about the machining supplier we partnered with," the plant manager says.

The automaker, having had experience with Heller in Germany, agreed Heller would be a good partner for designing the machining systems for the carriers and axle parts while the supplier concentrated on the critical assembly process and car testing.

Experience processing similar axle parts to high tolerances was one important qualification in Heller's favor. Another was the ability to operate globally. The Tier 1 supplier produces similar parts in plants in the United Kingdom and South Africa.

Heller's capacity to act as a turnkey system supplier was also vital to this project. The plant manager points out that his company saved itself a great deal of time and manpower resources by using Heller as machining project manager and meeting the quality and volume goals of its customer. Heller managed the robotic material handling, fixturing, tooling, measuring scheme and programming, automation, and chip and fluid management and disposal system.

According to the plant manager, "This last concern was critical because the assembly area in the plant is located between the machining cells and therefore had to be 'hospital clean.' The fluid must be cleaned, chilled, and recycled to support the machining tolerances we must achieve here."

Multiplying cells

Production began two years ago with a cell of five horizontal machining centers producing the front axle housing and cover. Later, the carrier pinion machining cell was installed. Then a second phase was added: It duplicated the two original cells, matching capacity as the business grew. The plant now operates a total of 20 Heller horizontal machining centers in four cells, each automated with a material-handling robot. The plant manager explains:

"Our ramp-up was slow. It took two years to get where we are now. Six months ago it was 50 percent of where we are today. So instead of making one huge investment in a single system--as would be the case with a traditional transfer system--we were able to defer investment until the business grew. Production capacity was added in discrete pieces as volume required, permitting us to manage its cash flow and eliminate risk."

The experience at the facility resulted in a fundamental change in the way the Tier 1 company produces axles parts and assembles them, dramatically reducing variability.

The company wanted to assemble the axle with as little variation as possible and avoid the need for operators to "read" patterns on mating parts. To do that it had to machine the housing and gear sets to extremely tight tolerances to avoid the type of variability operators had to interpret previously.

After each axle part is machined, it is 100 percent measured by a coordinate measuring machine (CMM) within each cell--another part of Heller's turnkey responsibility. Key machined tolerances include axle tube diameters and pinion bore diameter and squareness of these two features to each other. Most plants in the industry check only one part per shift or at some specified interval to monitor quality. The dimensional control data collected in each cell are attached to each part on a barcode. The parts then go directly to the assembly line where data (such as face, pinion diameter, mounting holes) are verified.

Machined carrier pinion housings from the Heller machining centers are matched, according to information on the barcode from the CMM, to gear sets from another plant. Reading this data and data from the ring and pinion gear set, the assembly line control system determines how the two should be put together, eliminating conventional shimming.

Because the carrier tolerances equal the gear set tolerances, the Tier 1 plant avoids the need to make adjustments manually to get the two components to fit acceptably. Both the gear set and the carrier are "one-class"--no classifications or "fits." Until this project, operators in axle plants had to read the patterns and make the final judgment on the acceptability of the axle assembly. At the plant, every carrier and gear set assembly is checked 100 percent on the assembly line with a CMM rather than as production samples in the lab. This confirms that the carrier/gear set assembly has been properly assembled before more parts are added.

According to the plant management, the only way it could have taken operator judgment out of the assembly sequence is to know the data of the carrier and of the gear set. That's why it performs 100 percent measurement on the production floor. The plant now "assembles to position" rather than to a pattern.

Machining solutions

The Tier 1 plant operates two cells each for machining front axle housings and covers and two for machining the carrier pinion housings. If necessary, it can move production to another cell and avoid interruption. Each cell includes five horizontal machining centers, a CMM, and part-loading automation to produce the axle components.

The machines are standard machines, but Heller designed flexible fixturing, tooling, and automation to make the cell responsive to manufacturing challenges.

To produce the three different carrier pinion housings, Heller divides the required five operations among two MCH 250 HMCs and three MC 25s. The larger MCH250, with HSK 100 spindles, handles the roughing operations--boring and milling primarily within a 2min of cycle time each, a total 10min cycle time per part. The MCH250s include a 630x630mm pallet and 800mm travel in each axis. Two smaller MC 25 HMCs with HSK 63 tooling are used for drilling and tapping holes around the pan face and semifinishing. The MC 25s include a 500x630 mm pallet and also offer 800mm axis travels.

In each carrier cell, parts are positioned on a tombstone fixture to present features to the machine tool to minimize total cycle times. A material-handling robot transfers the cast iron carriers between machines and orients the parts in the fixtures in which they are automatically clamped. There are only three fixture positions to complete the part.

Dividing the roughing and finishing between two different size machining centers reduced total machine cost per cell and cut the duplicate tooling needed to only five tools per machine compared with all machines having the same tooling. This also permitted better control of temperature and therefore final tolerances on each part, according to Heller engineers.

Clever processing

Heller's part-processing experience throughout the project helped the Tier 1 plant achieve the strict tolerances its customer required and to meet its goal of "assembling to position." For example, the perpendicularity of the bearing bore to the back face of the carrier was less than 15mu. According to Tony Hayden, Heller vice president of proposal engineering, to have used a boring bar to generate this bore would have had a negative influence on the tolerance. So Heller decided to circular mill the bottom of the face and then finish bore it to achieve the tight tolerances.

In the five-machine cell for the front axle housing and the front axle cover, two pairs of machining centers perform most of the necessary operations on four parts at a time. The cell can produce two different housings and three different covers at a nominal rate of 150,000 of each part per year. In a total of four fixture positions, an MCH250 accomplishes the roughing operations, and an MC25 the drilling, tapping, reaming, and spot facing operations. Again, the Heller process reduced tooling and machine cost by dividing roughing and finishing operations between two different size machining centers and efficient fixturing.

The final operation on the housings and covers is a key to the assembly quality. In this operation, performed on a third MCH250, the fixture is loaded with two front axle housings and two front axle covers so that the machine can semifinish and finish all main bores in the housing and the cover in one clamping. This way, mating parts come off the final machine as a finished, matched pair. A probe checks the external diameter of the cover large bore, which is the locating bore that goes into the case and the internal bore, ensuring those are maintained within a close tolerance. The parts then go directly to assembly.

Global and local

As the Tier 1 was investigating sources for machining systems, its customer pointed out that Heller had experience in similar components and that it would be a technical advantage to work with Heller on this project.

At the about the same time the Tier 1 was installing its Heller cells, a clone of the system was installed in the United Kingdom and in South Africa, with some adjustments for local market requirements. After development of the process by Heller and approval by the Tier 1 and its customer, orders went out to several Heller locations--the United Kingdom, Germany, and the United States, with Heller in Troy taking the lead in coordinating the Heller response to the Tier 1 's needs in each location.

Companies from all over the world have visited the plant in South Carolina to see how it is hitting its tolerances and how its production strategy could benefit them. The next step is to convert some of the machines to machine aluminum carriers alternately with cast iron on the same machine, requiring only fixture changes and proper chip disposal. Heller Machine Tools, www.rslead.scom/608tp-340
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Title Annotation:news & analysis
Publication:Tooling & Production
Date:Aug 1, 2006
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