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Rev it up: High-speed spindles and micromachining

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July/August 2013 Volume 6 Issue 4

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By Kip Hanson

Guest Author

(520) 548-7328

kipatron@msn.com

When shopping for a micromilling machine, one of the first things you’ll need to consider is spindle speed—how fast is fast enough? The answer depends on a number of factors, including cutter diameter, workpiece material, required feed rate and the money in your bank account.

Some in the industry say spindle speeds of 10,000 rpm or less are good enough. Probably not, said Bill Popoli, president of IBAG North America, North Haven, Conn. “The aerospace and medical industries, for example, are frequently milling titanium and stainless steel, using cutters measuring 0.02" in diameter or smaller. Do the calculations with regard to surface speed and you’ll find that high-rpm spindles are very necessary for this type of work.”

DATRON_M10_3kW_Spindle_40000rpm.tif

High-speed milling of an aluminum component on a Datron machining center equipped with a 40,000-rpm spindle. Image courtesy Datron Dynamics.

Given the cutter diameter and material’s recommended cutting speed, any machinist worth his salt can do the calculation. I’ve remembered it simply as rpm = 4 × sfm/diameter ever since the day Mr. Schipansky, my vo-tech instructor, used a grease pencil to write it on my forehead. Thanks, Teach.

Now, let’s say you have to mill a 0.01" slot in a stainless steel gas turbine component. You grab your Machinery’s Handbook and look up the cutting speed in surface feet per minute. (For these purposes, assume 200 sfm.) Using Mr. Schipansky’s formula, you’re looking at 80,000 rpm. Do you really need to spin a tool at this seemingly very high speed? The answer is a resounding maybe.

Permac Industries Inc., an ISO-certified job shop in Burnsville, Minn., successfully micromachines using a 15,000-rpm Okuma MC-V4020 vertical machining center. “It was considered high speed when we bought it,” said Permac Director of Operations Mike Bartizal.

Permac manufactures aerospace and medical parts, some with features as small as 0.015". Bartizal described a micromilling operation where the shop saw more than 100 minutes of tool life, even though it was using a spindle with less than adequate rpm. “It was a sleeve made of 300-series stainless. We milled a slot 0.022" wide using a 2-flute cutter at around a 25-ipm feed rate. The spindle was running wide open, but we still had very good results.”

Greg Nottoli, senior product manager for Hoffman Estates, Ill.-based NSK America Corp., cited an application for a medical product manufacturer. “They mill titanium in the 30,000- to 38,000-rpm range with a coated 2-flute endmill, 0.5mm in diameter, and a chip load of a couple tenths per tooth. They wouldn’t be able to do it without the right spindle.”

Retrofits can be a good way to garner the benefits of high-speed spindles, according to Nottoli. For example, another medical manufacturer was operating 40 Swiss-style machines from various manufacturers, all with compressed air-driven spindles. After assessing the application, the manufacturer asked NSK to retrofit the machines with NSK’s iSpeed 3 series and E-3000 series high-speed electric spindles. Each machine received four electric spindles (two front-working and two back-working).

Part cycle time decreased 65 percent, surface finish improved and tool life tripled, according to Nottoli. Also, the need for compressed air was eliminated, further reducing operating and maintenance costs. The end user calculated a 16-month payback on the retrofit.Image.tif

Dr. Michael Kreis, director of development and technology for machine builder Datron AG, Darmstadt, Germany, said a high-speed spindle is “absolutely needed” for micromilling. “That doesn’t necessarily mean 100,000 rpm, but those who suggest you can use a commodity spindle for micromilling are wrong. In my experience, insufficient speed produces chip loads too excessive for the very small tools needed for such work. It’s just a matter of calculation.”

All about the chips

The sleepless people who machine truly tiny parts use endmills a few thousandths of an inch in diameter and smaller. In this situation, Mr. Schipansky would set down his Machinery’s Handbook, tell you to do the best you can and head over to the teacher’s lounge. There’s simply no chance of achieving adequate speed with a tool this small.

Here lies the biggest opportunity for high-speed spindles. Do the math: If you define micromilling as the use of any cutter smaller than 1mm in diameter, you’ll rarely go below 10,000 rpm, except in the nastiest of materials, and most of your machining will be done at spindle speeds substantially higher than this.

There’s far more to this than speed, however. It is not simply a matter of buying a fast spindle. As Datron’s Kreis explained, higher speeds won’t do you much good without a machine tool that can achieve the correct feed rate relative to the spindle speeds needed for micromachining.

“Of course, you will use a very low chip load, as these tiny tools are delicate and prone to breakage. But even so, you should achieve at least a 5µm to 10µm depth of cut in most materials,” Kreis said. “This is very reasonable with microtools, and is sufficient for proper chip formation.”

There’s no mystery here. Micromilling, like any machining operation, requires the proper cutting parameters. A high-speed spindle will provide sufficient speed, and a properly made cutting tool will deliver the goods. So the question remains: Can your machine’s servosystem stay on course while driving through the intricate twists and turns of a microtool’s path? This is often the equivalent of navigating the Starship Enterprise through an asteroid belt at warp speed. More power, Scotty.

John Bradford, micromachining R&D team leader at Makino Inc., Auburn Hills, Mich., said that feed and speed calculations assume a constant chip load and static inertial forces on the cutter.

Micro%20Med%20Staple.tif

This medical staple mold was milled with cutters as small as 100µm in diameter. Machining like this is not possible without a rigid machine platform, high-speed spindles, proper thermal management and advanced motion control, according to Makino. Image courtesy Makino.

“Unfortunately, this is almost never the case unless the machining toolpath is a simple, straight line, with a single-axis direction of feed. In actual micro applications, it is common to encounter geometry where the movement requires 3-axis, simultaneous interpolation, with short linear or circular movements. In this case, the machine tool is never reaching a full programmed feed rate.”

The result is poor chip formation. Heat buildup, rubbing and unexpected cutting forces can spell certain death for a microtool. In this case, the best advice is to lower the speed somewhat, but not so much that it falls below the critical threshold of surface speed. It’s a delicate dance, one that becomes increasingly difficult to perform as cutter size decreases.

Brian Balfrey, general manager of Nanophorm LLC, a job shop in Incline Village, Nev., said the only reason to use a lower speed than the maximum allowed would be because of an inadequate machine tool.

“If the user is unable to balance the tool and spindle, then the upper useful speed will be limited. On a high-performance machine, one that is rigid and accurate, the faster the better, whereas lower-performance machines are limited by vibration and error motion. At some point, if the machine is not very good, micromachining just doesn’t work well.”

Balancing act

Datron’s Dr. Kreis is a member of the ISO 16084 committee looking to standardize the requirements for balancing rotating tool systems. He said tool unbalance is one of the primary causes of vibration in high-speed machining.

“It’s very difficult to achieve proper balancing using standard tool systems,” he said. “You must consider an HSK-style toolholder, and preferably an HSK E-series, which is smaller. This will be absolutely fine for micromachining applications up to 60,000 rpm. We also recommend a collet mechanism over, say, a shrink-fit or hydraulic clamping device, as high-quality collets provide better concentricity. This helps with tool life, surface finish and part accuracy.”

Even then, extremely fine micro-machining is no cakewalk. Kreis said: “If you’re talking about a chip load of 5µm and you have runout or imbalance equal to that, one cutting edge will do double the work while the other cuts air. This can’t be ignored. This is why some spindle builders have introduced magnetically balanced spindles, as these allow you to balance the tool while it’s rotating. Using the best-quality cutting tools is also important, as the endmill itself can be a cause of imbalance at very high speeds.”

NSK%20E4000%20series%20and%20HPS%20series%20spindle%20dual%20mounted%20within%20machine.tif

NSK America’s model E4000 electric spindle, capable of programmed speeds up to 40,000 rpm (left), mounted alongside one of the company’s HPS pneumatic spindles and associated toolchanger, capable of speeds of up to 50,000 rpm. Image courtesy NSK America.

IBAG’s Popoli agreed. “If you have a 0.01"-dia. tool with a few tenths TIR (total indicator runout), it puts a lot of force on the tool, and tools that size don’t have much structural strength,” he said. “The result will be breakage.”

Despite the challenges, Popoli said many shops are seeing success. “We sell a ton of spindles that run 40,000 rpm or higher to machine builders and end users alike.” Many of these go into Swiss-style lathes, where milling of microfeatures in titanium and medical-grade materials is the norm.

But these things aren’t cheap. The list price on a 60,000-rpm spindle is around $8,000. How does a shop afford to replace four or more live tools on a Swiss lathe with accessories this costly? “If you can reduce your cycle time by 70 percent or more, they pay for themselves very quickly,” Popoli said.

As proof, he pointed to a medical shop in Ohio that makes bone screws. “Using the driven tools that came with [the shop’s] Swiss machine, top milling speed was 8,000 rpm,” Popoli said. “Because of this limitation, most of the 30-minute cycle time went toward interpolating the Torx socket on the screw head. We retrofitted one of [the shop’s] machines with a pair of high-speed spindles, one for roughing the socket and one for finishing, and cycle time dropped to just 3 minutes. Now the shop has 16 of our spindles.”

Ready to go faster? For the right application, investing in a high-speed spindle will turn nickels into dollars. Just make sure you have a good machine, good tooling and people who understand Mr. Schipansky’s feed and speed formulas. You’ll be revving into higher profits in no time. µ


a/r = f

Rahman Mustafizur, professor at the National University of Singapore and founder of Singapore-based Mikrotools Pte. Ltd., throws a microwrench into the well-established rules of machining.

“If you increase the feed rate to match the desired cutting speed without first considering the depth of cut and tool edge radius, you may end up plowing the material rather than machining it,” he said.

Mustafizur and his colleagues at the university performed an extensive series of titanium machining tests. They found that practitioners of micromachining must consider something most of us pay little attention to: edge sharpness. At the microscale, the slight radius found on any cutting tool edge becomes large relative to the tool and the DOC itself. Take too light of a cut and this radius may be the only part of the tool that engages the material. Unfortunately, a tiny tool can only take light cuts. That’s why it’s critical to understand the edge radius effect and its impact on chip formation.

Mustafizur explained that when the ratio of “a” (chip thickness) and “r” (edge radius) is large, good shearing takes place but cutting forces may be excessive, leading to broken tools. When a/r is very small, the opposite is true and plowing takes place: the chips pass below the cutting edge, almost no material is removed and the surface becomes very rough. However, when the ratio of a/r is around 0.2, proper chip formation occurs and surface finish remains smooth, despite what some would consider to be inadequate surface speed.

“Essentially, it is not the cutting speed, but rather the cutting mechanism which will decide an efficient machining process,” Mustafizur said. That’s a mouthful to be sure, but the bottom line for efficient micromachining goes like this: a) DOC must be greater than the radius of the cutting edge; b) the cutting speed must be high enough to assure proper chip formation, but not so high that you break tools; and c) the feed rate must be sufficient to prevent rubbing while still maintaining surface finish.

Just think of micromachining as a dance: If you’re too aggressive, you’ll break your partner’s toes; hesitate and you’ll go home alone. Better get some good dancing shoes.

—K. Hanson


Contributors

Datron Dynamics Inc.
(888) 262-2833
www.datron.com

IBAG North America
(203) 407-0397
www.ibagna.com

Makino Inc.
(800) 552-3288
www.makino.com

Mikrotools Pte. Ltd.
+65 6776 9013
www.mikrotools.com

Nanophorm LLC
(888) 755-1425
www.nanophorm.com

NSK America Corp.
(800) 585-4675
www.nskamericacorp.com

Permac Industries Inc.
(952) 894-7231
www.permacindustries.com

FtrAuthorAbout

Kip Hanson is a manufacturing consultant and freelance writer. telephone: (520) 548-7328. E-mail: khanson@jwr.com. Telephone: (520) 548-7328. E-mail:  kipatron@msn.com.