HSM: What’s right for you?

When machining, one goal has always been to increase the spindle speed as much as possible because making parts faster increases profits. But is there a machining speed limit? Not theoretically, but in practice speed is limited by the need to control chatter and tool wear and the spindle speed the workpiece material can tolerate. Also, a higher speed is not always a good idea. A better idea might be to consider the optimal speed for a given operation.

There are primary issues to consider when high-speed machining, said Dr. Tony L. Schmitz of the University of Florida’s Machine Tool Research Center. “It is important to consider the process dynamics,” he said. “There are particular spindle speeds that enable a higher axial depth without producing chatter. These speeds are directly related to the system’s natural frequency. That corresponds to the most flexible vibration mode.”

This relationship between the ideal spindle speed and axial depth is demonstrated in stability lobe diagrams. The diagrams plot regions of stability and regions of chatter as functions of spindle speed and DOC. Stable regions are called lobes, and they may be increasingly pronounced at higher speeds.

Interestingly, academic research and industrial trial-and-error efforts have shown that some chatter, particularly “regenerative chatter,” where a cutting tool leaves a distorting pattern that causes vibration, can be overcome by increasing the spindle speed. Traditionally, the solution was to reduce the speed and increase the feed. With HSM, that approach can worsen the problem.

Of course, spindle bearings are crucial to any machine tool’s operation — especially at higher speeds. There has been some work on hydrostatic bearing technology where a pressurized fluid supports the spindle shaft. However, most bearing and machine tool manufacturers achieve gains by improving contact-type rolling element bearings.

Spindle manufacturer IBAG Switzerland AG, for example, offers various bearing technologies, ranging from hybrid ceramic angular contact ball bearings to air bearings and active magnetic and fluid (oil and water) bearing systems. IBAG North America, North Haven, Conn., produces IBAG spindles in the U.S.

HSM remains a moving target. “An increase from 10,000 to 20,000 rpm could be considered going to high speed,” said Erich Vilgertshofer, general manager, HPT Drive Systems Inc., Newport Beach, Calif. “Our highest speed spindles are up to 100,000 rpm with life-lubricated bearings and 140,000 rpm with oil-air lubrication.”