Managing runout when holding microtools

Three critical factors must be controlled when holding cutting tools in the spindle for micromachining: runout, runout and runout.

“For very small tools, the amount of runout dramatically affects tool life,” said Alan Miller, engineering manager for BIG Kaiser Precision Tooling Inc. “As tools get smaller, the acceptable amount of runout goes down with them.”

The Hoffman Estates, Ill., company offers a line of collet chucks that accepts a tool shank as small as 0.45mm (0.0177") and another down to 0.25mm (0.0098"). Measured TIR is 3μm (0.00012") at four times the tool diameter for the entire line, according to BIG Kaiser.

The most important factor in ensuring success when micromachining is having a dynamic runout from the spindle face to the tool tip as close to zero as possible, emphasized John Bradford, micromachining R&D manager for Makino Inc., which partners with custom engineering services and products provider Hummingbird Scientific LLC, Lacey, Wash., on micromachining projects. “When I’m trying to educate people in the market, it goes back to that single point,” he said. “When I look at all the important factors, dynamic runout is No. 1.” Mason, Ohio-based Makino builds macro and micro machine tools.

The Tribos-Mini polygonal toolholder from Schunk can accept a tool shank as small as 0.3mm (0.012"). Photo courtesy Schunk.

BIG Kaiser’s Mega Micro collet chuck features a slim nut design, which is 0.394" (10mm) in diameter to provide access into tight locations. That line of toolholders accepts tool shanks down to 0.45mm (0.0177"). The company also offers Mega New Baby collet chucks, which hold shanks as small as 0.25mm (0.0098"). Photo courtesy BIG Kaiser.

The measured TIR for BIG Kaiser’s Mega Micro collet chuck. Photo courtesy BIG Kaiser.

Bradford noted that the smallest collet he uses, which is from BIG Kaiser, holds a 0.5mm (0.0197") shank. Makino also holds microtools in shrink-fit holders, which accept a tool shank down to 3.175mm (1⁄8"). For the smallest microtools used at Makino for R&D and contract manufacturing, those shanks taper down to 10μm (0.0004") at the cutting edges.

Regardless of the type of toolholder, Bradford recommends maximum runout should be from 5 to 7 percent of the tool diameter. For example, runout for a 10μm-dia. cutting tool should be no more than 0.7μm (0.000028"). “We don’t try to generalize so much, but we want the entire assembly at dynamic operation to have a runout of 1µm or below,” he said.

Bradford added that runout isn’t only found in the machine tool spindle and toolholder. “It’s a widely held secret that from the shank of the tool down to the cutting flutes there are, many times, inherent runout issues,” he said.

Makino uses a dynamic device to measure runout at the tool/holder interface and at the tool tip at full rpm. The cutting tools Makino uses generally have an overhang of 10mm (0.394") and are run at a 40,000-rpm maximum spindle speed. Runout for the shrink-fit holders Makino uses is from 0.5μm to 0.7μm. Bradford noted that runout for collet-held tool assemblies is less than 1μm, but slightly more than what the shrink-fit holders provide. “Even for tools we don’t wish to shrink fit or can’t shrink fit, we’re very comfortable using the collet system,” he said.

For its micromachining research, the University of Wisconsin-Madison uses standard collets to hold microtools with 1⁄8" shanks, noted Frank Pfefferkorn, associate professor at the university’s Department of Mechanical Engineering. Those include manually tightened and pneumatically actuated collets, with the latter able to work with an automatic toolchanger. He noted, however, that runout with the manual collets is about 1μm to 3μm, whereas runout is from 3μm to 7μm with the pneumatic models. He added that the pneumatic collets came with the spindle and that Atometric, the machine tool builder, is planning on introducing a collet, which the university will obtain, that halves that runout.

Rather than dynamically measuring runout, UW-Madison determines runout based on measurements of a microscale channel after being cut. In addition to the cutting tool, spindle and holder, he added that other factors impact runout, including the workpiece material and collet maintenance.

“The cleaning and lubrication of the collet is critical to maintaining runout,” Pfefferkorn said. “If you get a little piece of metal in a groove or in between the collet and the taper that it seats on, you’ll be way off on your runout.” He added that a small scratch or other flaw on a collet can also negatively impact runout.

Gary Zurek, president of Kern Precision Inc., Webster, Mass., said the builder of machine tools for micromachining offers two types of HSK toolholders—collet chucks and shrink-fit holders. HSK is the only style of toolholder a Kern machine spindle accepts. He noted that a D-style precision collet chuck is only suitable for roughing and semifinishing because it has more runout than a shrink-fit holder. “When you’re looking at finishing applications, we encourage our customers to incorporate a shrink-fit toolholder,” Zurek said.

Zurek added that a shrink-fit holder may not be appropriate when roughing hardened steel, for example, because that holder doesn’t provide the flexibility a collet does when running microtools at aggressive machining parameters. “If it’s too stiff, then that tool is just going to break,” he said.

Another option for holding microtools is directly chucking them into the spindle, which Kern also offers, especially for its higher spindle-speed offerings from 60,000 to 160,000 rpm. “Then the only runout is in the cutting tool itself,” Zurek said.

He added that chucking tools directly in the spindle is best suited for less aggressive micromachining parameters because the tools aren’t as rigidly held compared to those in toolholders.

In addition, direct chucking tends to enable faster machine setups because there’s no need to arrange a secondary component. “You just load the cutters into, perhaps, a carousel and the toolchanger takes them,” Zurek said. “Whereas with a toolholder, you’re going to have to load the tool into the toolholder then put it into the carousel.”

Zurek noted that every toolholding mechanism eventually wears out, and the same holds true with direct chucking. As the chucking element wears in a spindle, the spindle may need to be replaced, which is costlier than exchanging a toolholder.

An alternative to shrink-fit holders is the Tribos polygonal clamping toolholder, developed by Schunk Inc., Morrisville, N.C., which also offers shrink-fit holders. According to Jeff Keith, product manager for Schunk, clamping extremely small-diameter shanks is not possible using conventional shrink-fit technology.

A shrink-fit holder is heated with an inductive device to expand the holder’s bore so it can accept a tool shank. Then it is chilled to enable the holder to provide 360° of concentric clamping around the shank. In contrast, a polygonal holder’s bore is mechanically reshaped to accept a round shank, with the bore contacting about 70 percent of the shank once clamped and the gap between the holder and shank being less than 1μm for the remaining 30 percent, he noted. Polygonal toolholders are available for clamping a shank as small as 0.3mm (0.012"). “It’s guaranteed to have less than 3μm runout,” Keith said.

Controlling runout isn’t the only issue when placing the smallest microtools into holders. “It’s like threading a needle,” Keith said. “It’s not easy to get a 0.012" shank into a Tribos-Mini holder. You may need a magnifying glass, and you must be careful that you don’t bend, deform or break the tool.” µ

About the author: Alan Richter is senior editor of MICROmanufacturing. Telephone: (847) 714-0175. E-mail: alanr@jwr.com.