Finish milling titanium

Traditional finish milling of titanium alloys has been accomplished via climb milling with cobalt and micrograin and submicrograin carbide cutters. The typical amount of workpiece material left for finishing was 0.100 “.

The cobalt cutters were run at cutting speeds up to 60 sfm. Chip loads were typically about 0.005 ipt but varied from 0.002 to 0.012 ipt. (If chip loads less than 0.002 ipt are used there is a danger of workhardening and tool overheating, which shortens tool life.) At these cutting parameters, tool life was about 90 minutes.

Traditional finish milling cuts on titanium alloys made with solid-carbide cutters and carbide inserts have cutting speeds from 100 to 120 sfm and chip loads of 0.005 ipt. The decision to apply cobalt or carbide cutters is influenced by tool availability and cost, the evenness of workpiece surfaces (intermittent cuts shortens the tool life for carbide cutters) and throughput requirements because carbide cutters remove metal about twice as fast as cobalt ones when using traditional cutting parameters.

The Boeing Co. found that leaving less material for finish milling—a maximum of 0.030 “—enabled milling at higher speeds. On finish cuts, this allows cutting speeds up to 600 sfm using carbide cutters. In addition, feed rates increased from 2 ipm to 40 ipm.

In the company’s progress toward higher speeds, it seems that the reason tools usually lasted more than 1 hour, which is good, is because a tool’s teeth are only engaged with the workpiece for a small amount of time. Most of the time the teeth are out of the metal and being washed with coolant.

To achieve 600 sfm and leave less material, we performed 5-axis milling on the roughing or intermediate cuts to leave a consistent amount of metal for finish cuts. This minimizes the intermittent cutter loads that shorten carbide tool life.

Tilting the cutter to lift its heel enables the coolant to more effectively reach where it is needed. Without the tilt, we generated too much heat because the bottom of the teeth always remained in contact with the workpiece, and we couldn’t machine faster than 400 sfm without sacrificing tool life.

One more step is often required. When side cutting, we rerun the final milling pass because the cutter is not rigid enough to meet our nominal dimensions. In theory, an end user could build in compensation for cutter deflection and eliminate the extra pass, but that’s a tough process to manage. If the operator pauses the machine for any reason during a compensated finish pass, the cutter tends to walk into the part beyond nominal dimensions, damaging the workpiece.