To provide the high-performance results demanded in today’s moldmaking, many indexable-insert ballnose endmills use PVD-coated, precision-ground, micrograin- carbide inserts. Experience shows that such inserts perform best with sharp edges,
which represents a departure from the conventional wisdom that extra-strong edges are required to stand up to hardened materials at very high feedrates.
The micrograin-carbide inserts used for high-speed finishing of molds are designed with a free-cutting insert geometry that allows the high cutting speeds and high feedrates needed to obtain ultra-fine surface finishes at acceptable productivity rates. The ultra-sharp edges complement the free-cutting insert geometry by effectively penetrating the hardened work material at the small axial and ultra-small radial depths of cut required to minimize stepovers (cusp heights) and achieve the desired finish quality.
A current application of this tooling shows the results that can be obtained using micrograin-carbide inserts with an ultra-sharp edge. To finish-mill a mold made from D-12 die steel hardened to 55 Rockwell C, a ballnose endmill was assembled from a modular endmilling system using a precision 90-degree ballnose insert in a very tough PVD-coated milling grade with medium-duty geometry. Run at a 0.020 inch axial doc, 0.010 inch stepover (radial doc) and 260 ipm feed, the endmill produces a 15-microinch surface finish with 90-minute tool life.
PCBN-tipped inserts are also available as an alternative to the PVD-coated micrograin carbides for high-speed finishing of molds. The real value of PCBN for high-speed finishing of hardened materials lies in the future, when requirements for still higher cutting data can be expected. In the meantime, the PVD-coated micrograins with ultra-sharp edges offer proven capabilities to meet today’s requirements as well as or better than PCBN, and at significantly lower cost.