Cutters Double Metal Removal Rates

Field trials include finish-milling of aluminum engine blocks, motorcycle cylinder heads and transmission housings.

More manufacturers are evaluating aluminum castings, but they'll miss a key consideration if they don't factor in cutting tool advances. The product-development experience of one tool supplier dramatizes what could influence a decision: "In beta testing a new family of aluminum face-milling cutters, we were expecting productivity gains in the 20% range, but in all the field tests, two-, three- and four-fold improvements have been the norm," reports Mike Castner, vice president of marketing, Sandvik Coromant, Fair Lawn, N.J. Designed for high speed machining (HSM), the new tools are the first standard indexable-insert milling cutters rated for 40,000-rpm spindle speeds, says Castner. He says Sandvik Coromant is targeting automotive, airframe and pump and valve fabricators. The new tools use disposable polycrystalline diamond (PCD) inserts. Castner says most competitors attain only 20,000 rpm and use a more cumbersome reconditionable PCD insert design. Users can also choose carbide inserts if they don't need the enhanced wear resistance of PCDs. One rule of thumb justifies the added cost of PCD when milling aluminum containing 12% or more silicon. However, the choice of carbide or PCD depends on the alloy, the cut, the machine and the economics of the job, the company adds. PCDs typically cost eight to 10 times more per edge. One key to the new cutter's 40,000-rpm capability is the insert seating design. Mating inserts and seat pockets feature "security serrations" to secure and position the insert for very high cutting speeds, Castner adds. He estimates that at 40,000 rpm, the breaking loose of a one-ounce insert would pack a 700-pound wallop. The challenge is to minimize radial and axial runout and maximize tool-holding stability. "Repeatable radial runout is within 6 to12 microns, twice as accurate as anything else on the market -- and a level previously . . . unthinkable in indexable cutters." Such positional accuracy here creates the dynamic balance that makes such high spindle speeds possible, he adds. Gaining HSM benefits, however, requires considerations beyond cutting tools and indexable cutters. HSM leaves no margin for error. Tool and work holding become more important. One modular toolholding approach, for example, uses a tri-lobed coupling to develop several tons of clamping force and maintains 0.000080 inch repeatability radially and axially. With respect to part holding Castner emphasizes two cautions. "Rigid machines and setups are important to prevent movement and vibration, but take special care to avoid excessive clamping pressure that might distort the sensitive aluminum parts."

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