[Bill Kennedy]

Editor's note: This article originally appeared in the Winter 2009 issue of MICROmanufacturing magazine.

Machine tool builders offer a growing selection of machining centers that make parts with microscale features and to submicron accuracies. The machines come in a variety of sizes, from half-ton and smaller units with 4"×4"×4" work volumes to 5-ton and larger machines with work capacities of 20"×20"×20" and greater.

All the machine configurations are seen to be effective in making very small parts. The level at which that production is achieved, however, differs somewhat in relation to machine size. While the smaller machines typically are less costly and more energy-efficient than the larger units, the greater table capacity of the larger, more expensive machines enables them to create microfeatures on larger parts.

One of the key differences among the machines is the type of operations they perform and how they counter negative machining influences, such as vibration and heat. The more massive components of the bigger machines permit operations such as hard milling and rigid tapping, but require additional features to control machining forces. The small machines’ more compact constituents, as well as the minimal cutting forces generated by the small cutting tools they employ, serve to significantly reduce the need to quell vibration with sheer structural mass.

Critical mass?

Microlution Inc., Chicago, offers two small-scale micro machine tools. Its 3-axis, 363-S vertical machining center has a 2"×2"×2" work envelope, a 2-sq.-ft. footprint and weighs about 1,000 lbs.; the work volume of its new 4,600-lb., 3.4'×5.4' footprint, 5-axis 5100s VMC is 4"×4"×4".




Small machines, like the 900-lb. G-4 Utra from Atomectric (top), rely on their compact design and low cutting forces to prevent vibration. Large machines, such as Kern’s 17,600-lb. Nano VMC (above), rely on their massive bases. Photos courtesy Atometric and Kern Precision.

According to Andrew Honegger, vice president of Microlution, there is nothing inherently wrong with the traditional idea that greater mass improves the vibration characteristics of a machine tool. “Mass does a good job of eliminating vibration,” he said. The difference really is in the application. A larger machine tool may be able to handle larger tools and workpieces, but “the difference in our line of thinking is that we are designing machine tools that are specifically geared toward only the small tools and small parts. That is where the design constraints change a bit.” All Microlution machines are designed for cutters ¼" (6.35mm) in diameter and smaller.

Gary Zurek, president of Kern Precision Inc., Webster, Mass., agreed that the miniscule-part focus of the smaller machines may minimize the need for a massive machine base. “On the other hand,” he said, “I would argue that [with any machine] traveling at 1 to 2 Gs over 2" there will be vibration, and somehow these vibrations must be addressed.”


Although they may be less space- and energy-efficient than small machine tools, larger multiplatform machining centers have the torque to perform operations like rigid tapping and microdrilling, as illustrated in this 10mm-thick cutaway titanium part. The angled holes are tapped with a M3×0.5 thread, and taper from a maximum OD of 2.5mm to a section just 0.9mm long and 0.1mm in diameter near the part’s base. Photo courtesy Kern Precision.

As result, some of Kern’s machines for medium- to small-part machining are far from small. The company’s 17,600-lb. Pyramid Nano VMC has a 9.3'×10.5' footprint, has travels of 19.6"×19.6"×15.7" in the X, Y and Z axes, and can handle tools from 50µm to 18mm in diameter. The machine’s polymer-concrete-filled base and frame absorb up to 10 times more vibration than cast iron, improving tool life and imparting surface finishes finer than 0.05µm Ra, according to the company. The machine’s size also facilitates the use of hydrostatic drives, which also dampen vibration. Kern’s two smaller machines, the Micro and EVO, incorporate similar polymer-concrete bases but are targeted toward small and micro part production.

Conversely, builders of small machines say their equipment is designed to be proportional to the size of the parts they are making.

“We took a lot of effort to make our machines squat and stout, a structure that is as reasonably compact as possible,” said Thomas N. Lindem, president of Atometric Inc., Rockford, Ill. “We thought if we scaled it appropriately to the parts, it didn’t have to be an extremely heavy machine to be stiff.” Atometric’s G-4 Ultra micromachining centers have a work area of 4"×4"×4" (with an 8" stroke available), a 2.3-sq.-ft. footprint and weigh about 900 lbs.

Structural elements

Regarding the role structural materials play in vibration damping, Lindem said he and his father, Atometric founder Thomas J. Lindem, previously designed large machine tools at Ingersoll Milling Machine Co., where they utilized cast iron, welded steel, granite and synthetic-granite bases. The different materials, he said, “all have their advantages and disadvantages. I’m not advocating one over the other; they all make excellent systems, but for all of them you have to look at all their characteristics. Granite is a great natural dampener, but thick steel also has some very well known stiffness and frequencies, and we are using a heavy steel base.”

According to Lindem, a key consideration in controlling vibration and maintaining machining accuracy is keeping what he calls the “force loop” as small as possible. The force loop, he explained, consists of a circle or square drawn through the workpiece, fixture, table and machine base, around the structure of the axes and through the spindle and the tool. “The goal is to make those distances as short as possible,” he said. If the same part is machined in a larger force loop, any element of deflection will tend to increase vibration exponentially. That increase in vibration can be estimated by cubing the distances within the force loop. Lindem added that in many granite-base machines, the force loop is actually contained in the unit’s steel structure; in that case, a larger steel structure mounted on granite can develop more vibration than an all-steel structure that is smaller and more compact.

Mark Jackson, associate professor in the Department of Mechanical Engineering Technology at the Purdue University College of Technology, agreed that the small-machine concept makes sense with respect to controlling vibration. For example, he said there is a significant amount of work being done with Swiss-style lathes to machine medical parts, such as bone screws, from metal workpieces. “But they are still using existing techniques, which may not be appropriate for machining certain geometries,” he said. “There is a need for dedicated machine tools that not only consistently produce those features but don’t suffer from vibration or thermal changes that [lessen] the ability to consistently control the size of very small parts.”


The rotary axes of the G-4 Ultra micromachining center from Atometric are shown machining a 0.35"- cubed demonstration part from ½"-dia. bar stock.Photo courtesy Atometric.


The Microlution 5100s 5-axis micromachining center has a 3.4'×5.4' footprint and a work volume of 4"×4"×4". Photo courtesy Microlution.


A small V-22 VMC from Makino machined a mold for 5mm×5mm medical staples in hardened 420 stainless steel (50 to 52 HRC). Each staple required 0.10mm corner radii. Photo courtesy Makino.

When small parts are produced on large machines, he said, “chatter occurs because the machine tool is far too big; the size of the machine is disproportionate to the size of the components. You have a huge mass moving around, and the vibrations have to go somewhere. They are regenerated and actually replicated on the surface of the part.”

Scaling down machine tools may require an approach very different from simply downsizing the equipment, Jackson said. “We came up with a prototype tetrahedral machine tool that had three machined balls on the surface, connected via hollow columns to a fourth ball at the top of the vertex. The tetrapod, actually a truncated pyramid, is the most stable shape in nature. We fill those columns with oil, water or solids to tune the machine tool and damp out certain frequencies of vibration. All the vibrations attenuate towards the center of each circular ball.”

Not exciting

Another important step in controlling vibration, said Bill Howard, VMC product manager for Makino USA Inc., Mason, Ohio, is minimizing self-excitation between adjacent machine components. For example, although Makino’s Hyper 2J micromachining center is the company’s smallest VMC, the machine has an 8"×6"×6" work volume, a 6'×8' footprint and weighs 11,000 lbs. To minimize self-generated vibration, it is essential to ensure tight alignment and fit between the machine’s drive elements and its basic structure. “Two metal surfaces rubbing against each other, even if they are ground, are still going to have potential to vibrate or oscillate against each other,” Howard said.

To control that tendency, the ways of the Hyper 2J, the V-22 VMC (slightly larger and more production-oriented than the Hyper 2J) and other of the company’s machines feature guideways lined with Tercite, a self-lubricating, thermoplastic bearing material. According to Howard, the stiff, rigid material is hand-scraped to maximize accuracy and create pockets that evenly distribute lubrication between machine components. The lining also provides a cushion between the different elements of the machine tool.


A small V-22 VMC from Makino equipped with a 0.050mm-dia. drill machined 61 individual 0.002"-dia. microholes into a flat, 0.020"-thick, 303 stainless steel workpiece. Photo courtesy Makino.

The cutting process itself can excite vibration. Microlution’s Honegger pointed out that when machines use micro cutting tools, “typically the cutting forces are a lot smaller, so the input that is causing the vibration has a much lower magnitude.” In addition, small-diameter tools generally are applied at much higher spindle speeds than larger tools. The higher-frequency vibrations the small tools produce are easier to control because vibration-damping mechanisms are correlated to the frequencies, according to Honegger.

“The higher the frequency, the easier it is to damp, as long as you are careful about structural resonances. Mass is definitely an effective vibration-elimination mechanism, but based on the type of input that we have from small tools, you don’t need as much of it.”

It appears that both small and large machine tools for micromachining can effectively control vibration and other machining forces. What users have to decide, then, is what will the machine be used for, how flexible it needs to be and if the added flexibility is worth the added cost. µ
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About the author: Bill Kennedy is a contributing editor to MICROmanufacturing and Cutting Tool Engineering magazines. Telephone: (724) 537-6182; E-mail: billk@jwr.com.

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Contributors

Atometric Inc.
(815) 986-7352
www.atometric.com

Mark Jackson
Purdue University
jacksomj@purdue.edu

Kern Precision Inc.
(508) 943-7202
www.kernprecision.com

Makino Inc.
(513) 573-7200
www.makino.com

Microlution Inc.
(773) 282-6495
www.microlution-inc.com