Nineteen minutes into the second period and the University of Ottawa’s women’s hockey team, the Gee-Gees, trails by one. Out of nowhere, the Montreal Carabins’ left wing bruiser Helga Lefèvre smashes Gee-Gees centre Holly McDuff into the boards, fracturing her cheekbone and leaving a ragged gash above Holly’s hairline. Blood turns the blue line red, but the crowd roars as the intrepid junior shrugs off the pain; she pulls a third period hat-trick and leads the team to victory.
At the hospital, the doctor repairs Holly’s cheek with a tiny metal plate, then uses staples to reattach the flap of skin to her forehead. Because he’s worried about an infection, several blood tests are needed. With Holly safely in recovery, her boyfriend uses his smart phone to snap a photo for Facebook. “Gnarly scar, Holly,” he says. “It looks like a maple leaf.”
Bone plates, surgical staples, blood testing equipment, even smart phones and their ever more capable cameras—none would be possible without miniature cutting tools. Yet as Cory Cetkovic points out, a host of things must go right before cutting tools smaller than a human hair can be used successfully, including the right toolholder and the right feeds and speeds.
Cetkovic is an application engineer for BIG Kaiser Precision Tooling Inc., Hoffman Estates, IL, and product manager for the company’s Sphinx brand of cutting tools. He explains that micromachining produces that realm of parts and part features measuring less than 0.5 mm (0.020 in.), with feature sizes smaller than 100 microns (0.004 in.) becoming more common every day.
Cetkovic will tell you microcutters are not simply smaller versions of that 12 mm (0.472 in.) endmill you loaded into the machining centre this morning. As cutters become smaller, web thickness relative to tool diameter increases, making chip evacuation vs. cutter strength a delicate balance. Edge sharpness, too, becomes more difficult to achieve as cutters shrink in size—the slight edge prep used on a roughing end mill may be larger than the entire flute depth on a microtool. This makes rubbing a real concern with Lilliputian cutters, since depth of cut during machining is often a fraction of the tool’s edge radius. Lastly, Cetkovic says to ditch the coating. Its benefits are marginal, and may even be detrimental on tool diameters smaller than 0.2 mm (0.0078 in.).
Built-up edge, rapid tool wear, unexpected tool breakage—all the problems seen in macro milling are magnified exponentially as tools become smaller. However, there’s a straightforward way to alleviate many of these microcutter challenges, Cetkovic says: crank up the speed. “It’s very common that customers are using micro-sized endmills without the appropriate surface footage.”
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