August 2008 Edition

WORKHOLDING

Getting the Right Grip on Workholding

Using specially-designed fixturing for high-volume production runs, an automotive supplier more than doubled its output

It is somehow appropriate that what became a successful supplier to the automotive industry began in a garage. Keith Armour, founder, owner, and president of Southland CNC, Cornelia, GA, began the company with a single machine 19 years ago and has grown it to a 21-machine company employing 30 people.

The company is an automotive industry supplier of machined aluminum sand-cast components. It machines parts that include both high- and low-level production runs. The company uses dedicated hydraulic clamping fixtures on Okuma Howa vertical machining centers for its high-volume work because the fixtures speed production time, making the fixturing cost-effective. The dedicated hydraulic fixtures also have consistent and repeatable clamping pressure which improves machining accuracy.

Typical tolerances are held to 20µ true position, with critical dimensions to ±6µ. Armour said he credits the tolerances to the hydraulic fixturing supplied by Advanced Machine & Engineering, Rockford, IL. Because low-volume production runs do not justify dedicated fixturing, Southland uses manual fixturing because of its adaptability and lower cost of acquisition.

Southland has four horizontal machining centers and three vertical machining centers that each use hydraulic tombstone fixturing, all designed and built by AME. The fixtures are dedicated to high-volume production of single parts or single families of parts.

The company chose AME to provide its first hydraulic tombstone because the fixturing solution was competitively priced and designed to fit Southland’s production needs. Delivery, which was important to production scheduling, was significantly better than any other supplier, Armour said.

A Replacement Brings Results

The first AME fixture replaced the customer-supplied fixturing Southland was using, which the shop determined was not providing the efficiency needed to meet cost and schedule goals for the production run.

The advantages of the AME-designed fixture included reduced load and unload time, reduced cycle time, reduced scrap rates, and error-free loading. The initial application of the fixture reduced cycle time by more than 50 percent, which let the shop meet its production goals without additional machines.

On a bearing plate for a supercharger assembly, production has gone from 50 units to 110 units per day with the same tolerances and a 1.67 Cpk, critical to the Six Sigma conformity for its automotive customers.

The fixture, which provides automatic clamp and release, is designed with locating dowels for error-free handling. Each component to be machined is touched only twice: once as it is loaded into the fixture, and once as it is unloaded. Because it is a windowed fixture, the component is machined on all four sides without additional handling. On the vertical machining centers, a second loading pallet is used to mount workpieces while another fixture is running in the machine. This enhances Southland’s throughput.

"The AME-produced fixturing has been very reliable," Armour said. "One fixture has been in operation 20 hours a day, five days a week for more than seven years without a single problem."

Because of this reliability, and AME’s cooperation and innovation, Southland added a second fixture for the vertical machining centers, plus a fixture dedicated to horizontal machines.

"The fixture configurations we designed include cast tombstones as well as welded tombstones," Alvin Goellner, AME fixturing group manager, said. "Depending on the intended use for the fixtures, they had external surface mounted hydraulics as well as internally cored hydraulics."

A "Supercharged" Choice

One particular part – a bearing plate for a supercharger – is aluminum sand-cast and measures 8"×4"×1-1/2". A two-operation part per side, the workpieces are loaded in less than 20 minutes on a second fixture, using the secondary pallet on the VMC. The shop supplies these parts to Jaguar, BMW, and Mercedes.

"The fixture was designed for this part, though it’s flexible enough to let us use it for other jobs," Armour said. "AME had a very short turnaround time, plus its knowledge of workholding, and components selection were all first-rate. There’s been zero downtime due to fixturing, although we tweak the rest pads for enhanced accuracy. That’s normal with the machining centers we use."

Armour said the design and development work was done via CAD drawings and the completion of the entire project was done on schedule and at the quoted price.

"When we began to go from 50 parts per day to 110 or better, with absolutely no loss of accuracy and finish quality, we knew we’d made a wise choice," he said. Advanced Machine & Engineering

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