Editor's note: This article first appeared in the debut issue of MICROmanufacturing magazine, Fall 2008.
As the trend of parts miniaturization continues, more machine tool builders are designing machines specifically for micromachining applications. The offerings vary in size, cost and accuracy to suit an ever-expanding array of production requirements, from R&D and prototyping to high-volume runs.
Schultz-Creehan LLC, Blacksburg, Va., is one company that recently purchased a machine for producing microsize parts for the aerospace, defense and medical device markets. The company did some cursory searching, according to Jeff Schultz, chief technology officer, then chose a
Model 2007-TC CNC micromachining center from Cameron Micro Drill Presses, Sonora, Calif., in part because of Schultz-Creehan’s experience with one of Cameron’s manual machines.
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| A component produced on a Vibra-Free machine from Compumachine. Photo courtesy Compumachine. |
“I’ve just been really happy with the manual product they have,” he said, adding that the manual machine functions as a complement to the engineering firm’s small-hole EDM. Production runs are from 10 to 30 parts.
Schultz-Creehan does some milling, but primarily uses the CNC machine to drill 0.020" holes about ½" deep—a 25:1 depth-to-diameter ratio. After spot drilling, peck drilling is performed with a 14,000-rpm spindle speed and a 6-ipm feed before counterboring.
“I was a little scared about those holes, but once we got the speeds and feeds right, it was no problem,” Schultz said.
Because three tools are applied for most holes, the micromachine’s four-position automatic toolchanger is beneficial, even if it slightly reduces the working envelope of 6" for the Xaxis, 4" for the Y-axis and 6" for the Z-axis. “The toolchanger interferes a little bit with how high it can go,” Schultz noted. Nonetheless, he added that having the ATC was a big selling point.
The base model costs $28,900 and can be ordered with various options, ranging from a programmable electrical circuit for $200 to a process-inspection camera and light system for $2,900.
Schultz-Creehan ordered the optional $1,500 VEF 100 video edge finder to make setups easier, according to Schultz. To mate cross-holes at a 90° angle, the company ordered the optional 4th-axis unit, which sells for $2,500 and includes a rotary table, 3-jaw chuck, right-angle attachment and T-slot table. The company made sure the machine was a good fit. “We bounced some parts off Cameron to see if we could make them with their CNC machine,” Schultz said.
“The tipping point was some [of our] parts require a fourth axis.”
Although
Schultz-Creehan drills mainly at 14,000 rpm, the Model 2007-TC’s maximum spindle speed is 50,000 rpm. Schultz explained that hole diameter tolerance isn’t necessarily critical because generally gas is flowing through the holes in the parts the company makes, but the specifications for the Cameron micromachining center, which has a 18"×20" footprint, indicate a high level of accuracy. Axis resolution is 0.0001", axis repeatability is 0.00019" and spindle runout is less than 1μm.
“Our core engineering skills for design and development are enhanced by the greater fabrication abilities this machinery brings,” said
Schultz-Creehan CEO Nanci Hardwick. “Our customers in the aerospace and biomedical industries seek smaller and smaller components and this center allows us to meet their demands with improved quality.”
Built for small
Another CNC machine designed for micromachining applications is available from Chicago-based Microlution Inc. Its 363-S 3-axis micromilling machine offers a positioning accuracy of 0.000080" and 0.000008" axis repeatability in a 21/2" work envelope. The machine has a 36- pocket ATC and comes standard with a 50,000-rpm spindle; other spindles are available to accommodate different applications, said Andy Phillip, president of Microlution.
Larger machines are available that can achieve accuracies similar to Microlution’s machine, but they tend to cost more. Phillip said the price for a 363-S, which has noncontact linear motors on all its stages and granite substructures, is less than $130,000, including the ATC, a tool sensor, touch screen and workpiece pallet system. “There’s a lot of savings you can obtain by picking a machine rightsized for the job,” he said.
In addition, floor space limitations make a machine with a smaller footprint attractive, especially for crowded shops looking to add micromachining capabilities without removing existing equipment or expanding their physical facility. “There can be a big advantage with a machine that only takes up 2'°—2' on the ground,” Phillip said.
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| Schultz-Creehan purchased a Model 2007-TC micromachining center from Cameron Micro Drill Presses. The manufacturer uses the 2007-TC to machine parts like its Omniprobe stainless steel pressure sensing probes (shown left and right). Photos courtesy Schultz-Creehan. |
One option that Microlution doesn’t offer is a rotary table. “Based on the design work we’ve done, to make a machine that’s very well tailored for micromachining operations, you need to start with a clean sheet if you want a 4- or 5-axis machine,” Phillip said, adding that the company is working on developing such a machine. “Your optimal performance is not achieved by taking, fundamentally, a 3-axis machine and just adding rotary stages.” That’s because micromachining often has higher acceleration requirements than macroscale machining to maintain a minimum chip thickness at a given spindle speed, and stacking stages arbitrarily can be detrimental to performance, according to Phillip.
He noted that the 363-S is appropriate for prototyping and production runs. For production environments, the number of machines needed is driven by the number of parts required over a period of time and the cycle time to produce each part, which can vary from 10 to 20 seconds for a simple part to hours for intricate ones. “It depends on the amount of material you need to remove and how small a tool you have to use to remove that material,” Phillip said. “Multiple machines can definitely be required to hit your production requirements.”
Off to the office
Just because a machine tool wasn’t developed for micromachining doesn’t mean it isn’t appropriate. That’s true of the Office Mills from Haas Automation Inc., Oxnard, Calif.
“The original premise was to make them so they could fit through a door. That’s where the ‘office’ term got coined,” said Dave Hayes, Haas’ product manager for Office machines. “We didn’t design the machine for micromachining, per se. The whole idea was high precision, production and [the ability to] fit through a door.”
Hayes noted that the initial OM-1 and OM-2 models, which do not have an ATC, were discontinued this spring, but the OM-1A and OM-2A with a 20- pocket, ISO 20-taper ATC continue to sell well, especially in Europe, where the line was introduced 2 years ago. “They just seem to eat it up over there,” he said.
Hayes noted that the initial OM-1 and OM-2 models, which do not have an ATC, were discontinued this spring, but the OM-1A and OM-2A with a 20- pocket, ISO 20-taper ATC continue to sell well, especially in Europe, where the line was introduced 2 years ago. “They just seem to eat it up over there,” he said.
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| An intricate feature generated on Compumachine's Vibra-Free machine. Photos courtesy Compumachine. |
The machines provide a 0.0002" positioning accuracy, 0.0001" repeatability and 0.0000061" resolution. The printed specification lists a 30,000-rpm spindle, but it’s able to run at 40,000 rpm, according to Hayes.
Tools as small as about 0.003" in diameter can be run in the machine. “Not that 30,000 or 40,000 rpm is enough for that size tool, but it’s all we have in most cases and people make due, depending on what they’re machining,” Hayes said. When a higher spindle speed is required, end users can mount an electric or pneumatic spindle into the machine’s spindle or mount an auxiliary spindle. For example, Micro Precision Parts Manufacturing Ltd., Vancouver, B.C., mounted a 200,000-rpm NSK pneumatic spindle in the standard OM-2A spindle to rotate tools as small as 0.016" in diameter fast enough to cut efficiently when making miniature parts, such as gears and gearboxes for the robotics industry.
To avoid additional part handling, Hayes said it’s common to add a 4thaxis rotary table to the machine or even a miniature 5-axis trunnion setup.
The base price for the OM-1A machine is $44,999, and the OM-2A costs $48,999. Haas also offers its Office Lathe, starting at $34,999. Hayes said some end users, such as those in the jewelry industry, balk at the price because they’re more accustomed to the lower prices for benchtop machines, and the Office line may be a larger machine than they desire. “Most people doing micromachining just don’t want a big machine,” he said.
Nonetheless, Hayes and others feel bench-top machines can compromise precision. “There are some really lowbudget ones that are almost toys and then there are some pretty good ones,” he said. “There is nothing wrong with a lot of those machines, but typically the complaints are that they’re not super accurate, they can’t take much of a cut in metal, and the controllers leave much to be desired. If it’s for prototyping and the accuracy is not so critical, maybe a bench top will work.”
In one instance, Jesse Chen, product manager of the Vibra-Free line of machines built by Compumachine Inc., Danvers, Mass., related how one customer migrated from a bench-top machine to a larger Vibra-Free machine to produce 3"-dia. dies that have about 3,000 0.003" holes. “They were doing them on a bench-top machine before but the accuracy was not as tight as what we can provide,” he said, adding that cycle time went from days to 10 hours.
Bridge construction
According to Chen, the Vibra-Free’s bridge-structure casting provides a stable platform to accurately produce small, intricate components and features even though the machine’s table travel is 24"°—16".
Compumachine introduced the Vibra- Free machine in 1998. Originally intended for hard milling and cutting graphite, users soon found it worked well for other applications. “During the time that we marketed this machine as a mold and die machine, we also found that a lot of people bought the machine because they had very small parts to cut,” Chen said. “We came across those types of applications almost by accident.”
The machine tool builder now offers three models, two of which are appropriate for micromachining. Prices range from $135,000 to $220,000. The JS model is the heaviest, but its top spindle speed is 16,000 rpm—generally too low to rotate tiny tools fast enough for effective cutting. The VF model, however, reaches 55,000 rpm, and the HD model has a 32,000-rpm spindle, which is useful for mold and die making, Chen said. “People put a lot of detail on molds and dies, and a lot of detail means a lot of small endmills are needed to reach small corners, for example.”
When applying microtools, spindle runout becomes a bigger issue than when macromachining, where a runout under 0.0005" is typically OK. In the micromachining world, spindle runout needs to be 0.00004" (1μm) or better for every tool. To control runout, Chen said a closed-loop vector drive is needed to keep spindle orientation, or tool angle, consistently the same from tool to tool. Such a drive is typical for lower-rpm machines but not for the ones with spindle speeds higher than 30,000 or 40,000 rpm, according to Chen.
Chen added that Compumachine and a customer have developed proprietary routines to control runout. “It’s kind of an add-on module to the machine,” he said.
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| This 440C stainless steel part, produced on Microlution's Model 363-S 3-axis micromilling machine, features an array of 0.060"-tall posts that taper from a 30° cone to a 0.02"-dia. cylindrical midsection and a 0.008"-dia. tip. Photos courtesy Microlution. |
Pushing the spindle runout envelope is the Hyper 2J micromachining center from Makino Inc., Mason, Ohio. The direct cutting tool-to-spindle chucking interface reduces total dynamic tool shank runout to less than 0.5μm, said John Bradford, Makino’s micromachining R&D manager, who works at the company’s micromachining and microfabrication laboratory in Lacey, Wash. He added that the machine offers positional accuracy of ±0.3μm and repeatability of 0.2μm “anywhere in the entire stroke.”
With its ultraprecise capabilities and construction characteristics, the Hyper 2J is typically used in development centers rather than general production facilities. “I liken it to a high-precision gage or master specifically geared toward R&D laboratories,” Bradford said.
Similar to the macroworld, minimizing vibration is critical when micro-machining. The materials used to build a micromachining center aid in that effort. For example, the Hyper 2J is built on a 10"-thick granite base, designed to isolate thermal transfer from the facility foundation, as well as to dampen thermal vibration and to provide a stable platform throughout the machine’s life by avoiding any sagging or twisting of the casting unit, according to Bradford. “It is Makino’s responsibility to provide the same level of machine performance and accuracy characteristics even over a 10-, 15- to 20-year life span of the machine tool,” he said.
Ensuring that two parallel components, such as two slideways for an X-axis slide, are accurately aligned and encounter the least amount of resistance as the axis moves through its stroke is also important from a machine-life perspective. “You’re basically creating a true accurate axis of movement without any friction or resistance from one side to the other,” Bradford said. “I like to compare Makino’s build philosophy to that for a Moore jig grinder or a Moore jig borer.” (See "Machining optics" sidebar.)
Machining optics
PART TOLERANCE AND SURFACE finish requirements for the optics industry usually exceed those for metallic components, even microscale parts. Form accuracy of 0.000002" and a surface fi nish of 1nm Ra is typical for the optics industry, according to Yazid Tohme, R&D manager for Moore Nanotechnology Systems LLC. The Keene, N.H., company, a division of Moore Tool Co., Bridgeport, Conn., which builds jig borers and jig grinders, builds ultraprecise 2- to 5-axis turning, milling and hybrid machine tools for the optics industry.
The optics are not microsized but can have microfeatures that require the use of microtools to produce. Workpieces include the nickel plating of steel molds and germanium, silicon and zinc-sulfi de lenses. Many are free-form optics that have never been produced before. “Our applications department spends a lot of time trying to fi gure out how to make such parts,” Tohme said.
Most of the machining is done with natural or synthetic diamond cutting tools; rarely are carbide tools applied.
The Moore Nanotechnology machines have granite beds and feature hydrostatic bearings and guideways. The spindles generally have air bearings, with the heavier spindles being hydrostatic.
“The machine components have a long life because everything in them is noncontact,” Tohme said. “For example, we use linear motors so when you move the machine nothing wears other than the diamond tools.”
He noted that the lathe spindle speeds are up to 10,000 rpm, and milling spindles are available up to 120,000 rpm, but too much runout can be an issue. “If you want to push the accuracy of the spindle, the one with less runout will only go up to about 60,000 rpm,” Tohme said.
As in most types of machining, controlling heat is critical to achieving the desired results. One method is showering an enclosed machine with temperaturecontrolled air to maintain a temperature within ±0.1° C. “What really kills us is not the temperature,” Tohme said. “It’s the temperature fl uctuation.”
In addition, vibration control is required. “We set the machine’s granite in a frame, and between the frame and granite are isolators,” Tohme explained. “The isolators can be passive, like air bags, or much more complex active vibration isolators.”
For micromachining applications, Tohme noted that as parts tend to get smaller and smaller, the machines tend to get larger to accommodate additional tools, such as cameras to aid in tool setups and touch probes for measuring surfaces. “They make it larger to make the machine more productive,” he said.
The machines from Moore Nanotechnology start at around $250,000 to $300,000 and go up to $800,000 to $1 million, depending on the options, accessories and number of axes.
—A. Richter |
Dampening with oil
Liquid materials also can be employed to dampen vibration when micromaching. The hydrostatic drives and guide ways in the Pyramid Nano machine from Kern Micro- und Feinwerktechnik GmbH & Co. KG, Eschenlohe, Germany, incorporate a defined and controlled micro oil film that eliminates metal-tometal contact and dampens vibration, according to Burkhard K. Rother, recently retired managing director of the machine tool builder.
“There is no friction, no heat generation and practically no wear,” he said. “And we make use of that oil to cool the bed of the machine and the axis by having the oil temperature permanently controlled to an accuracy of ±0.25° C.”
The Pyramid Nano has a positioning accuracy of 0.3μm, can impart surface finishes as fine or finer than 0.05μm Ra and has a workpiece accuracy of less than 1μm. “We mill a test part on each machine before it leaves the factory and all the measurements taken on the workpiece have to be within ±1μm,” Rother said.
Another Kern machine for production micromachining applications is the Kern Evo, which has a 0.5μm positioning accuracy and can impart surface finishes equal to or better than 0.1μm Ra. It comes with an ATC for up to 95 tools. “It’s primarily meant to work 24- 7,” Rother said.
For prototyping and R&D, there’s the Kern Micro, which has a 1μm positioning accuracy, a 2μm workpiece accuracy and can impart surface finishes as fine or finer than 0.2μm Ra. The Micro is the smallest of Kern’s three models and is the lowest cost machine among them.
Although the Pyramid Nano is Kern’s biggest machine, with a 500mm°—500mm°—400mm travel, Rother said that it provides the highest accuracy, primarily because of the hydrostatic drives and guidings, which require a machine to be a certain size to use the technology and cannot be integrated into bench-type machines. A machine tool builder can integrate precise linear drives into a small machine, but Rother noted that linear drives work with energy-intensive magnets. “When you have a linear motor, you need a lot of energy in your axes,” he said. “This energy heats up your entire machine so you need additional energy to get rid of that energy because temperature is a tremendous enemy when talking about accuracy, especially temperature fluctuation.”
Microlution’s Phillip concurred that high-precision micromachining operations must be run in a temperature-controlled environment. “You can’t just have the machine sitting in a building with the bay door open and hold tolerance,” he said. “Consistency in terms of the operating environment and the stable operation of the machine are very important, and that’s something we and our customers pay a lot of attention to.”
Compumachine’s Chen noted that for high-speed machining, Vibra-Machine’s spindle is controlled to ±1° C. “In order to control tolerance, you have to control the environment,” he said.
To control the temperature on Haas’ Office Mills, Hayes said the machines have electronic thermal compensation to adjust for ballscrew growth and shrinkage through a series of parameters and algorithms. “A lot of people ask us if they can get linear scales, chillers and whatnot,” he said. “We say we don’t have it and you don’t need it because of how the machine is designed.”
With machine tools becoming so precise and overall micromachining systems being able to accurately produce tinier and tinier parts and features, how much further can micromachining centers go?
“The grain size of the workpiece material itself starts to become the limiting factor of the actual part feature or part that you’re producing,” said Makino’s Bradford. Once a part or feature gets smaller than that grain size—for example, 0.2μm or 0.3μm—then a technology is needed to add base materials molecule by molecule to build up a finished part or feature, similar in nature to a stereolithography process, as opposed to tearing it down, such as via traditional milling or even EDMing. Even in buildup processes, which also include MEMS (microelectromechanical systems), a high-accuracy micromachining center is required.
“Generally, in those processes, you have to produce a hard master copy first before you can begin to replicate in large quantities the parts that would be copied from that master,” Bradford said. “That master still needs to be produced on a high-level machine tool, using a more traditional CNC machining approach.”
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Available options: Machine tool builders expand micro offerings
