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Hydraulic Workholding Yields High Accuracy for Machined Castings

May 10, 2012
Flexible fixturing for vertical and horizontal machining centers
Samples of sand-cast aluminum parts in hydraulic fixtures at Southland CNC, to be finish machined for Tier One automotive customers.
Flexible hydraulic fixturing devices from Advanced Machine & Engineering run on vertical and horizontal machining centers at Southland CNC.

Southland CNC of Cornelia, Georgia is a Tier Two supplier of automotive components to domestic and transplant automakers — primarily sand-cast aluminum parts. Established 19 years ago with a single machine in president/ owner Keith Armour’s garage, it now has 30 employees using 21 high-capacity machining centers.

Southland machines various parts, including some production runs of very high volumes, and others of relatively low volume. It uses dedicated hydraulic clamping fixtures on Okuma Howa vertical machining centers for its highvolume production runs, because these fixtures decrease production time significantly, which makes the fixturing highly cost-effective. The dedicated hydraulic fixtures also provide clamping pressure that is consistent and repeatable, which improves machining accuracy.

According to Armour, the typical tolerances are held to 20 microns true position, with critical dimensions to +/-6 microns, which he credits largely to the hydraulic fixturing supplied by Advanced Machine & Engineering of Rockford, IL. Because low-volume production runs do not justify dedicated fixturing, Southland uses manual fixtures, when appropriate, because it is an adaptable approach with a lower cost of acquisition.

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

Southland chose AME to provide its first tombstone hydraulic fixture because its offering was competitively priced and custom designed to the shop’s production needs. Delivery, which was very important to Southland’s scheduling, was significantly better than any other designer/supplier offered, according to Armour. The first AME fixture specifically replaced a customer-supplied fixturing that Southland had been using, which the shop determined was not providing the efficiency needed to meet the cost and schedule targets for the high-volume production run.

The advantages of the AME-designed fixture included reduced load/ unload time, reduced cycle time, reduced scrap rates, and error-free loading. The initial application of the fixture reduced Southland’s cycle time by over 50%, which allowed Southland to meet the high-volume order without additional machines. According to Armour, production of one bearing plate for a supercharger assembly has increased from 50 units to 110 units per day with the same superior tolerances and a 1.67 Cpk, critical to the Six Sigma conformity expected by major automotive customers. All finished parts are verified for accuracy on Southland’s in-house CMM.

The fixture, which provides an automatic clamp and release, is designed with locating dowels to ensure errorfree handling. Each component to be machined is handled only twice: once as it is loaded into the fixture, and a second time as it is unloaded. Because it is a windowed fixture, the component is completely machined on all four sides without additional handling. On the vertical machining centers, typically a second loading pallet is used to mount workpieces, while another fixture is running in the machine.

Armour added that the AME-produced fixturing has been very reliable. One fixture has been in operation 20 hours per day, five days a week for more than seven years, and without a single problem. Because of this reliability, and because of the cooperation and innovation AME has offered, Southland added a second fixture for the vertical machining centers and a fixture dedicated to horizontal machines.

“The fixture configurations we designed for Southland include cast tombstones as well as welded tombstones,” explained Alvin Goellner, manager of AME’s Fixturing Group. “Depending on the intended use for the fixtures, they were provided with external surface mounted hydraulics as well as internally cored hydraulics.”

One particular aluminum casting, a bearing plate for a supercharger, measures 841.5 in. (LWD.) Requiring two machining operations per side, the workpieces are loaded in less than 20 minutes on a second fixture, using the secondary pallet on the VMC. Southland produces approximately 30,000 of these part annually for a Tier One supplier to Jaguar, BMW and Mercedes.

“The fixture was designed specifically for this part, though it’s flexible enough to allow us to use it for other jobs,” according to Keith Armour. “AME had a very short turnaround time, plus their knowledge of workholding and the components they selected were all first-rate.

“We have experienced zero downtime because of the fixturing they supplied us for this particular part,” he continued. “Although we tweak the rest pads for enhanced accuracy, that’s normal with the machining centers we use.”

He also noted 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,” Armour said.