From Cutting Tool Engineering Magazine May 2008
MICRO PRECISION PARTS MANUFACTURING LTD., Qualicum Beach, B.C., makes small parts for several applications, including advanced medical applications (see Productive Times on page 118 in this issue for more about the shop’s work). Micro Precision machines standard ferrous and nonferrous metals, as well as titanium and other exotic alloys, plastics and ceramics. Ceramics’ properties are ideal for medical applications, according to Steve Cotton, owner and president of Micro Precision. “Th e only problem is that they are very expensive to manufacture,” he said. “A part we can produce in titanium in 4 days takes 7 to 10 days in ceramic to achieve the same accuracy. Titanium and other materials are machined with carbide tools, but ceramics require diamond-impregnated grinding wheels. We tried a range of tools, some PCD, CBN and cubic zirconia, and they couldn’t touch ceramic. It just blew them up,” he said.
As an example of the challenges of dealing with ceramic parts, Cotton described machining a siliconnitride component for a neurological application. Ceramic was selected for its biocompatibility and nonconductive properties.
Micro Precision machined the ½"-long parts from solid blanks of silicon nitride. Th e parts featured complex 3-D contours with three contact surfaces, two of which specifi ed no tolerance. “Th ey had to be perfect,” Cotton said. Tolerances otherwise were ±0.0004".
The diamond-tipped tools Cotton applied included custom endmills as small as 0.016" in diameter, featuring grit as fi ne as 800 (25 microns) for fi nish passes. “If it’s a drill or an endmill,” he said, “you really need to be up about 150,000 rpm.” Th e small diameter tools had to be run at high rpm to produce suffi cient cutting speed.
Cotton machined the parts on a Haas OM-2A Offi ce Mill vertical machining center. Th e machine is relatively small with about a 5'×6' footprint and X-, Y- and Zaxis travels of 12", 10" and 12", respectively, but Cotton said it provides more than enough rigidity and accuracy for his work. To permit effective use of small tools, Haas recommended that Cotton acquire an airdriven supplementary spindle from NSK.
“We put the main spindle in the M19 hole position, which holds it still, then put in the structure.”
He noted that the stiffness of the part itself can be a concern. “Medical parts can be quite delicate,” Salmon said. “Although you could make the machine and fixturing ridiculously stiff, the part itself may not be. The part then would be the weakest link in the chain. You’re always doing that balancing act.” If the part is not stiff, grind less aggressively or modify the fixturing to better support the part.
Regarding the advanced grinding machines needed to effectively machine small ceramic parts, Salmon said he considers the industry in general to be “slow in incorporating what I consider to be mature technology.” He feels much of the inertia results from customer expectations regarding what a grinder should look like. End users want to see a grinder “that looks like a grinder,” he said. “What the industry associates with stiffness and rigidity is a machine that is made of cast iron and is the size of a ship.” A machine that is “the size of a dining room table, with superhigh stiffness” is thought to be too high-tech. “With all the granite-type bases, hydrostatic slideways, shear dampers, magnetically levitated bearings and configurations available today, you can build a machine that is extremely stiff and stable, and yet wouldn’t necessarily look like a conventional grinder,” he said. It’s hard to sell the latest technology to the traditional machine tool buyer who has been successful with the old method, he added.
Because many ceramic medical parts are extremely small, parts manufacturers need to match their machine tool to the parts, according to Andy Phillip, president of Microlution Inc., a designer and manufacturer of CNC milling machines for micromachining applications. Phillip said any machining application, no matter the part size, involves a number of key factors, including workpiece material, the size of the features to be produced and surface finish and tolerance requirements. “In a sense, the small scale of micromachining magnifies the results of changes in those factors,” he said. “If you change just one or two of those characteristics, you need to have a substantial understanding of what to do from there and how to address that application.”
The design of Microlution’s machines, for example, results from study of micromachining’s process mechanics, Phillip said. “There are specific design considerations, including placement of components to maximize rigidity, very high resolution feedback, stiction-free motion and ironless linear AC motors.”
Toolmakers are another source of ceramic machining application knowledge and recommendations for medical applications. For example, among the products supplied by Technodiamant USA Inc., Mt. Arlington, N.J., are diamond core drills. A diamond core drill consists of a hollow tube with a matrix of diamond grit and bonding material at the business end. The company produces the drills in diameters down to 0.0225", and the tools can hold hole tolerances of ±0.00025". Sales Manager David Slaperud said metal-bonded drills are most commonly recommended for machining ceramics, with the specific mix of grit size, grit concentration and binder determined by the intended application. “The more the customer tells us about the material they are drilling, the better idea we have of what recipe is going to work best,” he said.
The harder the material, the more friable the diamond grit should be. Friability describes a diamond grain’s tendency to fracture and generate new sharp edges while in the cut. A lessfriable diamond grain will not break off and expose fresh edges that would cleanly fracture a ceramic workpiece. For example, South African diamonds are known for being well shaped and rounded and are generally not preferable for some ceramic machining applications. Natural diamonds from other sources, as well as synthetic diamonds, may have different friability. Slaperud said Technodiamant’s smallest- diameter core drills usually feature 325 grit (about 50 microns), which can be used uncoated or coated with nickel or copper. The metal coating combines with the metal binder to hold the diamond grain more tightly in the matrix, which is advantageous when drilling hard materials.
The concentration of the diamond in the binder—measured in carats per cubic centimeter—is another consideration. “If you have too much diamond, then you may not have enough binder material to hold it,” Slaperud said. He added that more diamond is not necessarily better, because there may not be enough force to break the diamond particles into sharp cutting points.
Slaperud said grinding parameters are application specific, but when core drilling alumina ceramics with smaller drill sizes up to 0.050" in diameter, typical feed rates range from 0.2 to 0.5 ipm, running at 3,500 to 4,000 rpm.
Coolant flow is crucial in keeping the drill cool and clearing away grinding swarf. Technodiamant recommends up to 200 psi of through-tool coolant pressure. Without sufficient coolant, a drill will overheat. “If it heats up too much, then the binder will break down, the diamond will fall out or burn and your tool will fail,” Slaperud said, noting that a blunt drill can produce scrap.
About the Author: Bill Kennedy, based in
Latrobe, Pa., is contributing editor for Cutting
Tool Engineering. He has an extensive
background as a technical writer.