Contract manufacturers that perform precision micromachining play a useful role developing prototypes of biomedical parts smaller than 2 in³. Micromachining can replace or complement traditional EDM, forming, stamping, and injection molding for plastics, aluminum, brass, titanium, stainless steel, ceramics, and glass. For example, prototypes traditionally micro-injection-molded can be machined instead, thereby eliminating the need for expensive molds. Micromachining is also suitable for micro medical parts requiring intricate 3D freeform geometry.

Micromachined parts are typically manufactured on ultra-precision machine tools that can take up 50 ft² of factory floor and cost over $300,000. But many traditional milling machines do not do a good job of machining small parts because they were not designed for that purpose. Larger machine tools are expensive to operate. They gulp cutting fluid and burn energy while moving their huge internal components. Furthermore, traditional machines are complex and require experienced, skilled technicians and support staff.

These issues make for an inflexible process that requires high-volume production for profitability. Traditional machine-tool manufacturers have responded by incrementally building-in more precision and performance to their products. But this only makes equipment larger, more complex, and more expensive.

A better way

A better way to make low-volumes of small parts comes from Microlution Inc., and its proprietary machine tool technology called “micro-mesoscale machine tools” (mMT). The company grew out of about eight years of machine-tool-design research at the University of Illinois at Urbana-Champaign. The research was focused around the idea that small parts should be manufactured on small machines intended for microscale manufacturing. Thus, the mMT milling machines are 1/10 the size, 25% to 70% the cost, and less complex than traditional machine tools.

Althought mMTs are smaller than conventional machines, the new designs are more than just scaled-down versions. Micro scale cutting is much different than that done on a macro scale. University researchers used a variety of modeling techniques including finite-element analysis to understand specifics such as tool dynamics and cutting forces.

Researchers found that micro milling removes a chip from each flute of the cutting tool that is about the same size as the flute's cutting-edge radius (2 to 5 µm). In contrast, macro cutting removes chips that are much bigger than the edge radius. Researchers determined that when the size of the chip (chip load) falls below a threshold known as the minimum chip thickness (a function of the cutting edge radii), the tool simply rubs or ploughs through the surface instead of cutting it. High precision only comes when the chip load remains above the minimum chip thickness. For micro parts, this requires a machine tool with specialized performance characteristics.

A walk around the machine

The 310-S milling machine is a high precision, 3-axis CNC with 2-µm positioning accuracy throughout 63 mm of travel in X, Y, and Z directions. High-resolution Heidenhain optical linear encoders mount to precision-ground granite sub-structures to provide excellent positional accuracy. The encoders have short grating periods, which reduces the magnitude of difficult-to-correct short-period errors.

The machine also features excellent contouring performance, critical for making biomedical parts. The capability stems from the encoders, along with high specific-force linear motors, a combination that allows what is called high-bandwidth tuning. This involves high servo-loop frequencies and rapid encoder updates that make the CNC quick and responsive.

The 310-S also uses ironless ac linear motors with zero cogging-torque. This reduces disturbance forces while cutting and increases machining performance. The motors provide up to 5 gs acceleration, which, along with spindle speeds of 50,000 to 200,000 rpm, make for smooth part finishes. With a 600-square-mm footprint, the machines easily fit in offices and labs. The mMTs use 20 A at 120 Vac and 6 cfm of compressed air at 100 psi.

A special ball-and-V kinematic mounting system for the workpiece and spindle lets users remove and replace both with sub-micron repeatability. For instance, users can interrupt machining, remove the workpiece, inspect it, and then replace it — all without reregistering the part. In cases where a machine is handling multiple projects, several users can easily go from one setup to the next without losing part registration. The mounting system also lets users replace the spindle with a laser distance-sensor to take measurements such as surface profiles or roughnesses.

What gives the mounting system such accuracy is a workpiece pallet with four precision ground balls that fit into two full and two split Vs. The assembly is held in place by four rare-earth magnets. The spindle base plate has a similar arrangement. The mounting provides a total of six points of contact and thereby constrains six degrees of freedom for an exact fix. Sub-micron repeatability comes from the mirror surfaces on the ground balls.

In addition to providing micromilling machines, Microlution prototypes parts and performs contract manufacturing with its own micro machines. This helps prove-out the machines, as well as increase the knowledge base.

Part volumes depend on how many parts fit on a pallet. Up to 100/pallet are possible. A planned automatic tool and pallet changer will support mid to high-volume runs. Also in the works is a 5-axis milling machine and a microturning machine.

HOW MICROMACHINING IMPROVES ON TRADITIONAL METHODS

Traditional Manufacturing	Micromachining pluses for prototyping and low-volume production
Injection molding	Micromachining directly produces parts traditionally micro-injection molded. The new process avoids purchasing intricate molds for only a few parts, and saves upwards of $40,000 on intricate molds, while providing a faster and higher-quality alternative.
Sterolithography (SLA)	Micromachining uses a broader range of materials than are possible with SLA, and generates parts that are 5 to 10 time more accurate than those from SLA.
Traditional macro machining	Micromachining provides better accuracy and small-part handling than available on traditional CNC machine tools. Its small footprint and simple utility requirements make factory installation easier than traditional CNCs.
Stamping and forming	It directly machines many parts that traditionally require stamping or forming, or both.
Electrical discharge machining (EDM)	Micromachining has flexibility to work on a wide range of materials. EDM only works with metals.
Laser-based machining	The process provides higher productivity and higher-quality parts as compared to laser-based machining.

A prototype nozzle

Bioscience technology firm iCyt in Champaign, Ill., (www.i-cyt.com) which focuses on devices to measure and handle cells, needed help developing a prototype nozzle with a 0.070-mm orifice that had to align with the internal geometry of the nozzle. Traditional manufacturing methods just did not cut it. The company turned to Microlution, which used a 310-S micro-milling machine to make the parts to a tolerance of +/- 0.001 µm.