From Cutting Tool Engineering

Meeting the workholding challenge: Turning Performance

In my August column, I described several common applications for workholding when turning, where the cutting tool is static and the workpiece spins. There are several standard workholders for turning, which were illustrated.

December 15, 2012By Joe Mason

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However, for clamped parts that are cut with a rotating tool, or parts held in position for specific processes, such as welding or broaching, many workholding applications are custom; you could probably create an encyclopedia with examples of workholders designed and built for specific parts. These unique workholding methods are often required to ensure part integrity is the same coming out as it was going in.

In addition, workholder intricacy varies. Workholding for milling machines can be as simple as clamps on a table or as complex as a hydraulically plumbed behemoth weighing thousands of pounds. Workholding for stamping presses comes from the die tool itself, which must support the tons of pressure applied to the material to achieve the desired form.

Other types of workholding for static parts require different approaches. For example:

Broach fixtures require self-aligning features in order to correctly remove metal in a shearing motion.
Workholders for weld cells require space to allow the welder to move between the weld points and weld at the correct angle.
Assembly machines must hold parts accurately so other programmed motions are executed correctly.

The science behind any workholding technique is based on locating techniques outlined in my previous columns. The form may change, but the principles remain the same.

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Workholding, of course, is still subject to limitations and encumbrances. Workholding designers must ask themselves, “Can we make it to the desired configuration on time and within budget?” More and more, part makers are being asked to both reduce manufacturing costs and justify their existing tooling budgets.

Workholder designers and builders are increasingly asked to answer these difficult questions. Can we:

Get away with fewer clamps?
Put more parts on the fixture and not exceed the machine tool’s load capacity?
Avoid buying additional fixtures if we push cutting tools harder?
Guarantee uptime with lower-cost fixtures and their components?
Purchase workholding fixtures from an outside supplier and make that supplier responsible for workholder performance?
Use fixtures to accommodate additional stresses not originally designed for?
Buy pre-engineered locators and rest pads and still accurately locate the part while not worrying these components will quickly wear?

Manufacturing costs have been driven down by low-labor-cost countries so much that suppliers have been forced to manufacture products that may function correctly—but not necessarily well.

That puts more pressure on workholder designers and suppliers to develop novel solutions. Experience is often the best teacher. The art of designing and building functional and robust workholding is learned. That requires knowledge about materials, machine tools, heat-treat processes, welding, grinding, milling and other machining processes. All those elements must come together to create a fully functional workholder that will do what you ask of it.

One of my bosses, after 20 years of designing fixtures, said he saw himself as only a good beginner. I feel that same humility in my own machining experiences. Making lower-cost products that meet or exceed expectations is the challenge of our time, and we’ll have to use all our skills to meet that challenge.