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High speed machining techniques do not require any particular spindle speed. Even if the spindle is relatively slow, some of the benefits of high speed machining can still be realized by increasing the feed rate and decreasing the depth of cut. Shops adept at high speed machining know there is no particular minimum rpm value at which this capability begins.

By the same token, there is no set rpm value at which conventional machining ends. Depending on the application and the tool, even a spindle running at 30,000 rpm may be suitable only for conventional cutting.

Macro Technologies Inc. (Kirkland, Washington) makes 30,000-rpm spindles that are routinely used for conventional cutting. These spindles are driven by shop air. Among their applications are milling and drilling with small tools. For a tool smaller than 1/8 inch in diameter, says Macro president Ed Huncovsky, a speed of 30,000 rpm should not be considered "high speed." With a tool that small, the speed is necessary simply to realize normal cutting.

Consider a 1/32-inch tool, he says. At 10,000 rpm, this tool achieves just 82 sfm. That's below the efficient speed for almost all materials. The same tool gets up to 246 sfm at 30,000 rpm. Only now does the cutting speed enter the range that a handbook would suggest for stainless steels, for example.

Macro's shop-air-driven spindles are intended to let shops achieve this high rpm value on existing equipment. An air-driven spindles with a taper like a toolholder can ride in a machining center's tool magazine.