Power Workholding For High-Flexibility Production

By combining creative fixturing with power workholding, Carr Lane Mfg. Co. has achieved dramatic reductions on the parts setup time, handling time, and transport time between machines. To achieve these results the company selected horizontal machining centers so that the multiple part faces could be machined without

Case Study From: 6/1/1997 Modern Machine Shop

By combining creative fixturing with power workholding, Carr Lane Mfg. Co. has achieved dramatic reductions on the parts setup time, handling time, and transport time between machines. To achieve these results the company selected horizontal machining centers so that the multiple part faces could be machined without refixturing, simply by indexing the rotary table. In order to meet their needs, they chose Cincinnati Machine's Maxim 500 CNC horizontal machining centers (HMCs) to gain fullest advantage of power workholding strategies in its own manufacturing operations. Carr Lane ordered the two Maxims with Cincinnati Machine's quick connect/disconnect power workholding option—two continuous 7500 psi hydraulic lines attached to each pallet in the revolving dual-pallet changer.

"We use the Maxims for relatively short runs with frequent changeovers. One machine operator can easily handle four different jobs, even short cycle time jobs, thanks to the use of power workholding," says vice-president Ray Okolischan.

Carr Lane Mfg. Co. uses Swiftsure and Hilma modular power workholding devices, produced by its own joint venture subsidiary, Carr Lane Roemheld Mfg., Co., to create varied fixturing configurations with quick-change capabilities from off-the-shelf components. "We're getting the productivity advantages of power workholding while avoiding high investments in custom, dedicated fixtures," says Mr. Okolischan.

Those economies and flexibility are essential to Carr Lane, he stresses, as the company produces hundreds of different parts in a range of sizes, families and steel chemistries—low-carbon, high-carbon, stainless and alloy.

"Our fixtures and machine tools have to be multi-purpose," says Mr. Okolischan. The Maxims provide that capability with XYZ machining volume of 29.5 by 27.6 by 29.5 inches (or 750 by 700 by 750 millimeters) and 33 hp (25 kW) No. 50 taper spindle. The generous machining volume accommodates a range of fixture configurations, especially multi-face fixtures that allow multiple quantities and designs of parts to be run in a single operation. The Maxim delivers spindle torque of 526.8 foot-pounds. This enables aggressive cutting and faster cycle times on the various grades of tool steel, which can range up to Rc 44 in hardness.

Carr Lane Mfg. Co. used its many contacts with machining customers to observe and evaluate HMCs before making a purchase decision. "We learned about the Maxims from some early machine purchasers," says Mr. Okolischan. "They used the Maxims extensively with excellent results," he says.

"Cincinnati Machine's approach to power workholding, a continuous fluid feed to each pallet, was a vast improvement over more time-consuming coupling/decoupling methods," he stresses. By combining creative fixturing with power workholding, the company has achieved dramatic reductions on the parts setup time, handling time, and transport time between machines.

The HMCs were selected so that multiple part faces could be machined without refixturing, simply by indexing the rotary table. "Most of the parts processed on the Maxims are rectangular and require accurate machining on two or three sides," he notes. "We even machine some parts on all six sides."

Power workholding adds a variety of benefits. "First, power workholding reduces load/unload time to almost zero in most cases," says Mr. Okolischan, "especially with a continuous hydraulic hookup to the pallets. Second, speeds and feeds can be increased significantly with the every-time-is-the-same security of power clamps and the added part rigidity provided by hydraulic work supports. Third, the high-precision vises enable many parts to be fixtured and machined in one operation."

As an example of the productivity gains, Mr. Okolischan points to the machining of one type of precision locating pad that required seven separate operations before. Now, he says, the part is completed in one setup. "You can imagine how much this improves our start to finish delivery time," he adds.

Carr Lane builds fixtures using its own standard tooling blocks (mainly, four-sided types) with a standard modular fixturing grid. A medium size modular system is used for the 500 mm pallet (19.7 square inches) Maxims. Multipurpose mounting holes are located in a two inch (5.08 cm) grid pattern, with each hole having a 0.5000-inch (12.7 mm) precision bushing on top and a 1/2-13 thread insert below it.

A two-passage hydraulic rotary coupling is mounted at the top of each tooling block, making continuous hydraulic connection possible on the rotating machining center pallets. The coupling's bottom portion is connected to the pallet, while its two port connections branch out to each clamping valve.

"The Maxim design of two separate hydraulic power units for the two-pallet machining center provides great application flexibility," stresses Mr. Okolischan. "The two hydraulic lines to each pallet on the Maxim—one pressure line and one return line—allow the installation of any number of separate clamping valves on the pallet."

Carr Lane Mfg. Co. uses hydraulically powered vise-jaw fixtures for most of the workholding. "Using heavy-duty, high precision vises mounted on tooling blocks offers two big advantages," states Mr. Okolischan. "For a relatively low cost, an entire tooling program can be nearly complete in one day using off-the-shelf items. This type of setup allows true quick-change fixturing by simply switching vise-jaw fixtures or swapping vises for quick change fixture plates."

Hilma high precision vises are called by Mr. Okolischan, "our all-in-the-family choice" for power workholding. Business ties notwithstanding, Carr Lane favors Hilma vises for quick fixturing, he says. The vises have extremely accurate repeatability when mounted on modular tooling plates and blocks. Fixed jaw position and jaw height are accurate to ±0.0004-inch (±0.001 mm). Both the fixed and sliding jaws are precision ground—±0.0004-inch on top, with keyslots, to provide an accurate base for custom tooling or top step jaws. Custom fixed jaws with built-in side locators can be mounted with a center locating pin for absolute three-axis accuracy.

Fixtures can be single-acting (spring-return clamps) or double-acting (power-return clamps), notes Mr. Okolischan. For single-acting fixtures, a three-way clamping valve is installed on each tooling-block face; for double-acting fixtures, a four-way clamping valve is used.

While the standard fixture design pressure is 5,000 psi, occasionally a lower pressure might be required on one pallet to prevent part deformation or a higher pressure might better secure a difficult-to-machine part. MMS

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Self centering vise is a cost effective and universal workholding method for five sided machining.
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