Meeting micromachining’s need for speed

Microscale parts and features present many challenges when machining, not least of which are the high spindle speeds required to reach acceptable cutting speeds and cut rather than rub workpieces or break tools. A lot of shops have robust, accurate equipment that has served them well through the years but lacks adequate spindle speed.

Many machines are built for torque, not speed. A solution might be a specialized micromachine or a premium machine with a high-speed spindle, but a spindle speeder is another option.

Spindle speeders amplify speeds for many jobs that need small-diameter tools, often less than 6 mm (0.24″). Speeders multiply spindle speeds by several times while maintaining the torque of a machine.

“The biggest problem with (using) small tools is that the shop can’t run them fast enough,” said Steve Bryan, owner of Bryan Machine Service Inc., Huntington, Indiana. “Spindle speeders can take an older machine and increase that up to 15,000 to 40,000 rpm. The older machines have a new life.”

Spindle speeders are suitable for myriad shank types, including small tapers that are lightweight and precisely produced for balance and reduced total indicator runout.

The MO spindle speeder for micromachining applications can increase spindle speeds to 35,000 rpm. Image courtesy of OMG North America

Because higher-end spindle speeders are compatible with automatic tool changers, it is easy to do multiple setups on one machine. For example, in addition to producing and selling its line of VRT spindle speeders, Bryan Machine Service operates a machine shop. Sitting amid a variety of higher-tech equipment are two 50-taper, 7,000-rpm machines. One job required machining runners on a large plate, as well as microfeatures.

“With the spindle speeder,” Bryan said, “we can put on a 1⁄8″- (3.18 mm) or a 1⁄16″-dia. (1.59 mm) tool on that great big machine and run it at 30,000 rpm.”

Need for Speed

Faster spindle speeds can help impart fine surface finishes and reduce burrs, chatter marks and other cosmetic blemishes. Carbide cutting tools also require higher spindle speeds for best results.

The trick is to run at the right surface feet per minute, which also can be expressed in surface meters per minute. Essentially, the faster a spindle turns or the larger the diameter of a tool is, the higher the sfm is based on the formula in which sfm equals (rpm times tool diameter) divided by 3.82. For instance, a 0.5″-dia. (12.7 mm) endmill running at 1,146 rpm achieves a cutting speed of 150 sfm (45.72 m/min.) while a 0.05″ (1.27 mm) tool must run at 11,459 rpm to cut at 150 sfm, and a 0.005″ (0.127 mm) tool needs an 114,592-rpm spindle speed to reach 150 sfm.

But those numbers are just the start. Different materials cut best at specific speeds, and each tool supplier suggests cutting speeds for its tools.

“When machining material like aluminum, the tool manufacturers might recommend a surface speed as high as 350 sfm (106.68 m/min.), and if you wanted to run a 1 mm (0.039″) carbide drill, you would need 35,000 rpm to reach that surface speed recommendation,” said Mark Johnson, manager of OMG North America, Post Falls, Idaho.

The company offers the MO series of spindle speeders, including the MO10.HS for micromachining applications with a maximum output speed of 35,000 rpm.

Meeting Challenges

If a machine can’t deliver the spindle speed to meet the required cutting speed, mechanical or cosmetic issues may occur, such as chatter marks.

Even something as common as clogged flutes is problematic, said John Young, president of Eltool Corp., a Mansfield, Ohio-based supplier of spindle speeders, right-angle spindle heads and live tooling.

A coolant-driven spindle speeder is displayed. Image courtesy of Eltool

“People who run aluminum at lower rpm just weld the whole chunk of aluminum right onto the cutter,” he said. “That is why it is so critical to get the surface feed up, so the tool doesn’t stay in the material long enough to stick to the tool.”

High speeds are key but challenging to achieve. A machine running at or near its maximum rpm can have balance and runout issues, generate friction-caused thermal expansion that burns out bearings and gears and potentially distort tools during operation.

Spindle speeder suppliers, however, have developed methods to overcome many of these problems. For example, when Bryan made his spindle speeder turbine system, one of his must-haves was a short distance between a spindle and workpiece.