Spindle developments for micromachining
Feb. 3, 2011—To satisfy the requirements for micromachining applications, spindle manufacturers continue to develop products that are faster and more compact, accurate, powerful and thermally stable.
One area of special interest for spindle manufacturers is Swiss-style machines. For example, NSK America Corp. last year introduced the iSpeed3 spindle series for live tooling on Swiss-style automatic lathes. The spindles have an integral electric DC brushless motor and an accuracy of less than 1µm TIR, according to the company. The iSpeed3, which has a maximum length of 103mm, is intended as a replacement for a machine’s standard spindle, noted Dick Garski, senior sales manager for the spindle manufacturer’s U.S. sales office in Schaumburg, Ill.
IBAG’s Micro line for Swiss-style machines includes these 22mm- to 25mm-OD spindles. Photo courtesy IBAG North America.
The series includes 19.05mm- and 20mm-OD spindles with a variable speed from 1,000 to 80,000 rpm and 22mm- and 25mm-OD spindles with a variable speed from 1,000 to 60,000 rpm. “These spindles are designed to increase the speed of the machine,” Garski said. “The standard rpm on most of those Swiss-type machines is 8,000 rpm for live tooling.”
Micropart manufacturers desire high spindle speeds not only to achieve an adequate chip load but also to boost productivity when running microscale cutting tools. “It’s going to take forever to machine a reasonable-size surface unless you’re running at very high speeds,” said Chris Gerrard, R&D manager for Westwind Air Bearings.
The Poole, U.K.-based spindle manufacturer offers units with speeds up to 350,000 rpm for drilling 0.004"-dia. and smaller microholes in printed circuit boards. It also has supplied spindles for micromachining metals, such as brass and aluminum, for the watchmaking industry.
Although Westwind produces air-driven, or turbine-driven, spindles with speeds up to 100,000 rpm, Gerrard noted that direct-drive electric motors are the choice when a high-speed micromachining application requires “real control.” He indicated, however, that turbine spindles are easier to incorporate into a machine, particularly if an end user is doing the upgrade.
“You don’t have to buy a high-speed inverter to get the electric power into the spindle,” Gerrard said. “You just need an air line and a control valve and away you go.” He added that Westwind sells 95 percent of its spindles to machine tool builders.
Bill Popoli, president of IBAG North America, North Haven, Conn., agreed that spindles with air-driven motors are typically not powerful enough. IBAG offers its Micro line of spindles with housing diameters as small as 16mm for Swiss-style machines. The speed range is from 60,000 to 80,000 rpm, and a brushless DC motor delivers the required power and torque, Popoli noted. “For a lot of small-part applications, particularly if they’re in medical, you’re cutting stainless steel, so there’s a need for higher torque,” he said.
The iSpeed3 series electric-powered spindles from NSK America provide speeds up to 80,000 rpm and are for live tooling on Swiss-style automatic lathes. Photo courtesy NSK America.
In addition to spindles for Swiss-style machines, IBAG is building models for 3- and 5-axis machining centers with automatic toolchangers. The company has developed a special rotary coupling to put high-speed spindles on lathes with turrets, which require electrical or pneumatic connections for the spindles.
To illustrate the advantage of an electric spindle compared to a pneumatic one, Garski recalled how a customer near Salt Lake City who was using an NSK air-motor spindle to machine a hexapod drive on a bone screw reduced cycle time from 45 to 15 seconds by switching to an iSpeed3 spindle.
The most critical requirement for virtually all markets, including micromachining, is controlling and minimizing thermal growth of the spindle, according to Bill Howard, product line manager for Makino Inc., Mason, Ohio, a machine tool builder that produces its own spindles. To achieve that, Makino begins by passing temperature-controlled lubricant through the center of the spindle to cool the core. “The technique relies upon long passageways and designed orifices to facilitate oil flow,” he said, adding that the patented process attacks heat generation at its source—the spindle core.
A 1mm, single-crystal-diamond tool on a Westwind 160,000-rpm spindle machines brass samples at Cranfield University. Photo courtesy Westwind Air Bearings.
Effectively lubricating spindle bearings also keeps thermal growth in check, but as spindle bearings spin faster and faster, centrifugal force prevents lubricant from reaching the inner bearing race, where it is needed, Howard explained. Therefore, Makino provides under-race lubrication to ensure lubrication and temperature control. “We’re lubricating the bearings from the inner race out,” he said.
Makino reports that a third method to control thermal growth is to add a cooling jacket that encloses the integral spindle, which means the armature, windings and motor are part of the spindle. The jacket connects to an Oilmatic, or chiller, unit that tightly controls spindle lubrication temperature, relative to the machine’s bed. This ensures that the spindle and bed maintain a constant, controlled relationship, providing a temperature-controlled manufacturing platform, Howard noted.
“Even microns of thermal growth in the spindle are going to cause issues in micromachining,” he said. µ