The concept of hard milling and/or HSM is still a relatively new concept. The technology needed to facilitate these new methods has only been designed in the last six to seven years. In examining the market,

it is clear that tools manufactured ten and fifteen years ago for the conventional machining will not work in unison with the newer technology. Cutting geometry needs to be tailored for high-speed and feed applications where greater cutting forces and more heat are common. The advancement of machine tool and cutting tool technology has always gone hand-in-hand. The current advent of new machine tool technology has made it even more important for new operators to use tooling designed for these machines.

Three important characteristics every tool for HSM should have are accuracy, rigidity and longevity. In the die/mold industry, a synonym for accuracy is radius tolerance. The tolerance of any ball mill is critical if a moldmaker wants to avoid hours of benching time. Imagine machining a complex cavity mold, such as a bottle-bottom mold, using a 1/4-inch ball end mill that really measures 0.248 inch over the diameter. The amount of rework required to bench the cavity could very easily be the difference between a profit and a loss.

Other critical tolerances include the shank diameter. Shrink fit tooling, which is ideal for HSM applications, requires tools with h6 tolerance shanks. Not only must the shank maintain an h6 tolerance, but also true roundness as well. Roundness and diameter tolerance are two separate quality standards, and if they are not met, it can lead to the tool becoming permanently lodged inside the shrink fit holder. The bottom line is that the additional cost of buying an accurate tool is nothing compared to labor costs of added benching time and/or scrapped tooling.

The key feature of the cutting tool that dictates rigidity is the core diameter. The machining requirements set forth by the principles of HSM, place heavy emphasis on the tool and its ability to withstand axial and radial forces. Conventional end mills incorporate a standard core diameter thickness of approximately 50 percent of the tool’s diameter. The core diameter is directly proportionate with the gullet depth of the flutes.

This design is good for standard milling operations in carbon steels, but it is not rigid enough for high-speed applications in die steels such as D2 and H13. End mills for the die/mold applications should use a shallower gullet to sustain a thicker core diameter. HSM methods incorporate depths-of-cut that do not exceed any more than 10 percent of the tool’s diameter. As a result, far less chip room is needed in comparison to conventional machining methods; hence, gullet depth can be sacrificed for a thicker core diameter.