Manufacturing tiny medical components can be tricky. Parts can have features that measure less than 100 micron — about that of a human hair. At such fine scales, the physics of materials are different than they are in the macro realm. For example, a material's ability to hold an edge can become a limiting factor when tool sizes are smaller than the parent-metal grain structure where small parts are prone to production problems such as heat distortion, which can quickly chew up a micron tolerance-range.
Fortunately, contract manufacturers that do micromachining can help develop concepts for making small parts into manufacturable designs. A few examples come from companies that use tools so small they can be difficult to see.
Machining with Swiss-type tools
The key to micromachining is getting a handle on what is actually going on according to J.W. Childs, a spokesman for American Micro Products Inc., Batavia, Ohio, (american-micro.com). His company targets biomedical, aerospace, and diesel-fuel injection industries, with a focus on miniature parts with unusual designs. Part runs range from a few for prototyping to production of millions of pieces in materials including stainless steel, titanium, Teflon, and Inconel.
“Knowledgeable contract manufacturers develop a formal process, which is critical for dead-nuts parts and bullet-proof projects,” says Childs. “For example, we bring together key personnel to discuss how parts will be manufactured, while the part is still being quoted. Our pre-production talks cover form, fit, and function, with the level of detail going down to exactly how the chip will be generated.”
Measurements are also critical says Childs. “Companies can't make parts they can't measure. Micromachining requires gages such as laser or special precision micrometers that have excellent repeatability and reliability.” Gages for parts with tolerances of ±1µ, for example, must measure to 1¼10µ.
“Customers have given us existing designs to reproduce and asked us to figure out how to make a new part. We've even manufactured off of proverbial napkin sketches,” says Childs. “One part now in production, for example, replaces the stirrup bone in the inner ear.” The parts are 0.10-mm in diameter.
“We use Swiss-type machine tools, which are suited to precision jobs because the cutting is done right at the revolving guide-bushing,” says Childs. “Unlike conventional machines, Swiss-type machines only expose workpiece sections being machined, so parts are supported at all times. This almost eliminates deflection, which, of course, can cause inaccuracies. Some machines here have five axes and most are robotically fed.”
The firm's focus on unusual designs makes it necessary to produce tooling in-house. “Our toolmaker designs and makes tools for special features and job requirements,” says Childs. “Tools are carbide or high-speed steel, with some as small as a couple of thousandths of an inch in diameter.”
Small machines for tiny parts
“Some small medical components must be monolithic, or carved out of solid blocks, to help guarantee part geometry,” says Tom McDunn, director of advanced manufacturing at EIGERlab, Rockford, Ill., (EIGERlab.org). The facility is a combination research lab and commercialization center funded by the U.S. government to foster technology use in industry. As such, the organization hosts several contract manufacturers tasked with commercializing designs for customers ranging from individual doctors to large corporations such as Medtronic Inc., Eli Lilly, Abbot Laboratories.
“These parts typically require ‘right-sizing’ the machine tool, ” says McDunn. “Issues of mass and thermal growth usually defeat making small, super-precise parts with large, conventional machines. A typical machine tool running small parts has a machine-to-workpiece-volume ratio of about 1,000,000:1. But with miniature machines, it's closer to 10,000:1. Right-sizing eliminates thermal-growth problems,” says McDunn.
Equipment at EIGERlab includes 3, 4, and 5-axis horizontal and vertical benchtop micromachine tools based on technology proven at the University of Illinois-Urbana Champaign and Northwestern University. According to McDunn, the facility's 3-axis machine tool's specifications include 100-nm resolution, 1-micron positioning accuracy, and a 40×40×60-mm working envelope. Machine spindles operate between 20,000 and 200,000 rpm, with newer variable speed machines running up to 500,000 rpm. The tools include end mills and drills ranging from 0.005 down to 0.0001-in. diameter, and below.
“One application that requires monolithic parts involves metal mixing-blades for a lab-on-a-chip for the bio-med industry,” says McDunn. “The customer had tried stereolithography to build blades with a method similar to one used in making semiconductors. But optical buildup proved exceedingly expensive. It also presented some of the same problems found with laser cutting, such as imparting too much taper on shapes.”
To get around these pitfalls, one company at the facility machined tiny blades, or propellers, out of nickel blocks. “Machining makes it easier to hold blade tolerances and gives a nice tolerance between blade tips and the cavity the propellers operate in,” says McDunn. “We also machine square cuts for fluidic channels in the body of the laminate. Photoetching tends to round-off edges.”
To complete the propellers, the company machines a hole in their center so they can spin on a post. The customer also had difficulty getting a shaft and a hub working so that the little mixer spins. McDunn says this is easier with micromachining because it gives a better fit than photo processes.
Small machine tools also come in handy for another job involving tiny titanium turbine impellers with wall thickness of about 0.006 in. They are used in a blood-booster pump for infants. The pump is shaped like a pill, about 2 × 3 3/8 in. diameter. The device contains a motor that spins impellers to pull blood through the device.
The company is also working with a physician on a design for hypodermic needles that get inserted into liver tumors and transmit radio waves. The company mills holes ranging from 50 to 250µ in the end of stainless steel or titanium needles at various angles to pass fluid and radiation.
“We mill the holes because they must have a sharp edge and milling gives us good control over hole-depth and form,” says Scott Erickson, project manager for Alion Science and Technology, the EIGERlab contract manufacturer. “It's always difficult getting a drill started on a round surface, let alone one only 800 µ in diameter. And it would be hard to EDM the blind holes.” The company machines lots of five or ten at a time, which lets the physician experiment for an ideal hole pattern.
Holes in hard stuff
Putting holes in exceedingly hard materials, however, might require EDM says Steve Heisel, an owner of Aurora Micro Machine Inc., Buffalo, Minn., (auroramicromachine.com). The company specializes in making round and shaped holes in catheters, needles, and pacemaker parts, among other medical devices. Production runs span from one piece to 37,000 parts per week. Many of the holes are under 0.003-in. in diameter.
“EDM doesn't care how hard a material is,” says Heisel. “Drills can't put holes in diamonds, for example. But EDM works with hard materials because it creates heat that melts the material. In addition to diamonds, EDM can put holes in sintered carbide, silicone wafers, and certain ceramics.”
Heisel says EDM is a cost-effective way to put holes in small parts because it eliminates deburring. “Holes made with a drill bit always have a burr because the drill pushes metal aside. And it's almost impossible to remove burrs from tiny parts so it is quite expensive. Also, resorting to chemical etching to remove burrs becomes another expense because it adds a secondary operation,” he explains.
The company uses Sarix EDM equipment says Heisel because the machine is extremely precise compared to similar equipment and it makes smaller and finer features. “The equipment uses a solid, round, rod electrode made from carbide or brass. Clamping the workpiece on the machine table grounds the part. Electricity jumps between the electrode and workpiece when the machine brings them close together, which creates a spark and the heat to melt the material. On round holes, we typically spin the electrode for a nice, round, true hole,” he says.
Machines must keep feeding this electrode because the EDM process removes material from the electrode along with putting a hole in the part. “For instance, we typically buy 12-in. long carbide electrodes,” says Heisel. “For parts with runs of 37,000 per week, we get about 3,000 holes per electrode.”