February 2009 / Volume 61 / Issue 2|
Finish milling titanium
By Edward F. Rossman, Ph.D.
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.
High-speed milling of titanium alloys is all about cutter life, and heat is a tool’s enemy. Applying sharp cutters helps keep heat down.
In addition, take bottom cuts in pockets to net dimensions during roughing and avoid having to tilt the cutter during finishing because the pocket floor is already finished. Program the finishing pass to be about 0.001 " above the pocket floors and eliminate the need to bottom cut. Another way to save time would be to write a separate program for the final spring pass, removing less than 0.002 ", and run the tool at 1,000 to 1,200 sfm. The cutter should handle this productivity-enhancing speed.
The biggest road block to achieving higher speeds when finish milling titanium has been convincing NC programmers to program for these higher speeds. Another major inhibitor was the reluctance of older machinists to run at the higher levels.
One of our milling tool suppliers successfully high-speed finish milled titanium alloys with M-42 cobalt cutters. All our tests were with carbide for finish milling. We did some fresh testing of M-42 at high speeds on finish cuts, removing 0.025 " to 0.030 " of material. We had success at about 400 sfm with 0.005 ipt to confirm the supplier’s results.
To me this result is ironic. The supplier heard about high-speed finish milling of titanium alloys, but did not think about switching to solid-carbide tools. We experts who developed high-speed cutting with solid-carbide tools presumed that cobalt would not work at high speeds when removing less material (0.030 ") and never bothered to test cobalt cutters at high speeds. TiAlN-coated tools were tested. Sometimes we are not as smart as we think we are.
Carbide is the primary cutting material for high-efficiency finish cuts at 600 to 800 sfm, but we found cobalt cutters can be applied for finishing at speeds as high as 400 sfm—compared to the traditional 60 sfm—provided the radial DOC does not exceed 0.030 ". This allows most of the teeth to be cooled while not in the cut. CTEAbout the Author: The late Edward F. Rossman, Ph.D., was an associate technical fellow in manufacturing R&D with Boeing Integrated Defense Systems, Seattle. Rossman’s Shop Operations column is adapted from information in his book, “Creating and Maintaining a World-Class Machine Shop: A Guide to General and Titanium Machine Shop Practices,” published by Industrial Press Inc., New York. The publisher can be reached by calling (212) 889-6330 or visiting www.industrialpress.com.
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