January 2009 / Volume 61 / Issue 1|
HSM: What's right for you?
By George Weimer
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.”
Vilgertshofer added that advances in bearing technology allow greater speeds, loads and precision. “The maximum speed is essentially determined by the OD of the spindle and the bearings,” he said. “Physics still limits the maximum; the bigger the slower. There is a trend to increase speeds to achieve finer finishes and higher feed rates. The limits are usually related to the maximum speeds that a material will tolerate or, possibly, by the existing motor spindle’s maximum output power and torque at a given range.”
Vilgertshofer warned that expertise in tool selection for any given process is “vital” but often not sufficiently covered or ignored altogether.
NSK Americas, Ann Arbor, Mich., offers what it calls “the world’s fastest oil-lubricated spindle at 3.8 million DmN (pitch diameter times speed), running at 50,000 rpm.” A machine tool demonstrated at the JIMTOF trade show in Japan incorporated a 60mm bore-bearing set based on a “super lean oil lubrication design,” said Bimal Nathwani, the company’s project manager, machine tool/linear motion.
According to Nathwani, bearing design improvements that enhance lubrication performance should keep speeds going up and costs coming down. However, the limitations as to what most bearings can achieve in terms of spindle speeds are “matters of friction and other forces that apply on the rolling elements,” he said.
Perhaps it’s not high speed itself that should be the goal, but rather the most productive combination of speeds and feeds.
Make sure your machine tool supplier is current on the latest developments and trends, as well as the research being conducted at universities and organizations like the National Institute for Standards and Technology.
More importantly, end users must recognize that a machine tool is only a part—albeit a crucial part—of the total operation. The fastest spindle in the world will not live up to its potential if the material handling or another element of the machining system isn’t properly coordinated. CTEAbout the Author: George Weimer, a freelance writer based in Lakewood, Ohio, has an extensive background in the metalworking industry’s business press. Contact him by e-mail at email@example.com.
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