Holding challenging parts: 5-Axis Machining

Some workpieces, like those that are oddly shaped, large or delicate, can be a real pain in the neck. Delicate geometries, thin walls and shapes more flexible than a ballerina—what’s the best way to securely grab hold of them? Gerard Vacio, workholding systems specialist at BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill., said the economics of machining these parts means spending more time and money up front to figure out the best way to fixture them.

“Nobody wants to do that,” he said. “Designing and building the fixture often ends up eating all the profits from the job. That’s why a lot of shops turn down this kind of work. Unless you have some influence over part design, where you can add features or tweak geometry to make a part easier to hold, it’s often a losing proposition. Even companies that build their own products don’t always realize the possibilities of lobbying the engineering department to make parts more manufacturable.”

Design for manufacturability aside, some companies thrive on making complex parts. Vacio said these shops use their creativity and engineering brilliance to find the most economical solution. “No one has yet invented a tractor beam that ‘s going to solve workholding problems. The result is that people end up tackling each element of a difficult workpiece individually.”

Examples include sandbags placed on the part to reduce vibration, or the use of specialty glue or low-melt potting compound to support delicate areas during machining. Often, you can use part features, such as a threaded boss or dowel pin holes, to locate and clamp a part. “There’s no one solution. It’s a matter of utilizing whatever’s available,” Vacio said.

A landing gear steering lever for the Viking Twin Otter aircraft begins as a 10 “×4 “×1.5 ” block of 7075 aluminum and is mostly completed in one operation. Image courtesy Mitee-Bite Products.

He recommends careful planning, because the first operation can make or break the ability to grip difficult-to-hold parts later on. This is one job 5-axis machining centers excel at, he explained. A part can be gripped once, when it has the most material available, then tipped and rotated until the machining is nearly completed. With the right strategy and a little luck, all that remains is refixturing the part and a quick saw cut or a slice with a wire EDM and the part drops off complete.

Even without a 5-axis machine, this use of sacrificial material is a common practice with oddly shaped workpieces. David Bishop, general manager of workholding manufacturer Mitee-Bite Products LLC, Center Ossipee, N.H., said the carrier method is an easy solution for oddballs. “It’s a fun challenge to place as many parts as possible in the machining envelope and complete the part in two operations or less. Even an extra ¹/₁₆ ” of material on the bottom of the workpiece is often enough to hang on to. By gripping low, you can machine the top and sides, then nest the part in machinable jaws or a quick-change fixture of some kind to mill away the gripping surface and at the same time finish the workpiece.”

One excellent example of this is an aluminum landing gear component made by one of Mitee-Bite’s customers, Straightline Precision Industries Inc., Sidney, B.C. (see photo above). Here, the part was machined nearly complete on a tilt-rotary table—all that remains is to grip the center section and remove the carrier. Another approach is locating on internal part features. A pair of adjacent holes makes a nifty place to grab a part, which is why Mitee-Bite and other workholding providers carry a range of expansion clamps and internal collets to grip a ³/₁₆ ” diameter or smaller. Bishop pointed out a recent example of this with Haas Automation, where a rotary table was used to machine the part in several operations, the final one locating off a pair of 1.375 “-dia. bores (see photo below).

The final operation on this part uses a Haas HRC210 rotary table and locates two 1³/₈ ” diameter bores to hold the part while several holes and slots are machined on the profile. Image courtesy Mitee-Bite Products.