"Too hard" is just right: a pioneering user of ultrasonic machining seeks out parts that are too difficult to manufacture on other machine tools

Modern Machine Shop, Feb, 2005 by Peter Zelinski

BC Instruments is a job shop big enough to fill three buildings totaling 45,000 square feet. However, those three buildings are located on a side road off Highway 27 in rural Schomberg, Ontario. If you try to drive there before sunrise, you might miss the company in the dark (which I did). Even so, a variety of high-value machining work has found its way to this company because of one capability that makes the shop distinctive in all of North America.

Walking the shop floor won't necessarily reveal this capability. BCI's business units include large-part turning, small-part turning, milling, and also a special projects unit. While the company has always focused on more specialized work (its name is a relic of this), all of those business units use equipment that would be familiar to many shops. The one machine tool that stands apart is segregated into a small, quiet room that houses another business unit unto itself. BCI was the first North American manufacturer to invest in uhrasonic machining, a technology that was developed by Sauer (in Germany) and is now marketed by DMG.

Why BCI bought this machine tool is not entirely clear. That is to say, the market has yet to fully reveal itself--and BCI knew this would be the case. Anthony Pinder, the manager of the new business unit for machining engineered materials, has his work cut out for him. After a period of trial and error in which BCI developed its proficiency with the new technology, Mr. Pinder turned his attention to finding rewarding markets for this expertise.

So far, the ultrasonic machine has been used largely for hard ceramic parts, many of which are used in nuclear power applications. One such component was a part made from a particularly challenging ceramic, magnesia partially stabilized zirconia. While machining this material in its hard state through conventional means is prohibitively difficult, the alternative of machining the material in its softer "green" state is no better, because the material's high rate of shrinkage makes it impossible to hold tolerances. The customer had the material in mind for 10 years, but could never find a way to manufacture a precision part composed of it. Using ultrasonic machining, BCI succeeded in making the part from this material, holding 0.0005-inch tolerances with little difficulty.

This part illustrates why finding markets will take some time. For some of the machining that this technology is capable of, there is little precedent. Mr. Pinder says, "I ask customers and prospects, what is it that you turned down last year because you thought it couldn't be done?" The answers to questions such as this sometimes hold the promise of new work.

20,000 Hertz

In ultrasonic machining, diamond does the cutting. Ultrasonic vibration causes a diamond-covered tool to pulse at a frequency of around 20,000 cycles per second. The rapid expansion and contraction of the tool delivers equally rapid impacts of the diamond particles against the workpiece. The result is a machining process for hard materials that involves relatively little friction or heat. Not only is the machine tool designed specifically for this process, but so are the tool-holder and tool, all of which come from DMG. Mr. Pinder says the combined system can machine any material hardness up to that of the diamond of the tool.

The ultrasonic machine looks very much like a vertical machining center. The machine can also be programmed like a machining center, and in some cases, it even offers a machining center's options. These include fourth and fifth axes beneath the table. The axes are programmable on BCI's machine, and they are capable of full five-axis contouring on other models.

Work even comes from vertical machining centers. For example, BCI previously used CNC milling to produce a complex tooling plate for a nuclear power customer, back when the part was made of aluminum. But now the part is made of ceramic and machined ultrasonically. In providing this part, Mr. Pinder says BCI worked itself out of a source of revenue. The ceramic part doesn't wear out routinely the way the aluminum part did.

Ceramic Machining

In ceramics, BCI sees its ultrasonic machining as a complement to the more traditional machining options that many ceramics providers already have available. BCI is still working with these companies to figure out exactly where this technology fits in, logistically and economically, alongside the traditional options.

Ceramic parts are machined when the quantity of a given part is too small to justify making a mold. A particular part can be machined in its green state or its fired state. While the material is easier to machine green, this state comes before shrinkage. More precise machining has to be performed in the harder state, generally using some variety of grinding machine. The material removal rate in the fired state is low--perhaps only one-tenth that of green machining--and grinding in the fired state also poses the risk of subsurface cracking.