Inserts for difficult materials

Superalloys are hard; some grades of titanium are machined at 330 Brinell hardness. With conventional alloys, cutting zone temperatures greater than 2,000[degrees]F soften molecular bonds and create a flow zone for chips. In contrast, the heat resistance that makes HRSAs so desirable keeps them hard throughout the machining cycle.

HRSAs also tend to work-harden as they are cut, notching cutting inserts to premature failure. The difficulty cutting HRSAs is compounded where unpeeled stock is covered with abrasive, knife-edged scale that wears cutting edges down even more quickly.

Given their machining difficulty, superalloys are cut slowly. For example, Inconel 718 is milled for brake keys with Sandvik GC2040 grade carbide inserts at 200 sfm. Turning speed for the same alloy with Sandvik 7020 CBN inserts in an outside turning/facing application is 260 sfm. By comparison, uncoated carbide inserts typically cut tool steels at 400 to 800 sfm. Feeds for HRSAs are generally comparable to those used when machining tool steels.

The choice of cutting inserts to machine HRSAs depends on the material and the workpiece. Carbide inserts with positive rake geometries will cut thin-walled HRSA stock effectively. However, thick-walled parts may require ceramic inserts with negative cutting edge geometry to create a more productive plowing action. While dry machining is preferred in most difficult materials to maintain uniform edge temperatures, titanium requires coolant even at very low speeds.

by Wayne Mason