Cutting Tool Engineering
February 2010 / Volume 62 / Issue 2

Hold tight

By CTE Staff

MTU image 3.tif
Courtesy of Rego-Fix

The blisks MTU Aero Engines manufactures have stringent accuracy specifications, requiring the use of the Rego-Fix powRgrip toolholder system (inset), which includes a holder, collet and miniature press that generates 6 tons of force.

The growing and competitive blisk (BLade Integrated diSK) market requires continuous improvement of the manufacturing process and the development of more-efficient machining strategies. Blisks are found in heavy-duty compressor rotors in military and civil turbine aircraft engines and commercial compressors.

MTU Aero Engines GmbH, Munich, Germany, machines titanium alloy blisks and determined that conventional milling takes too long. To improve the process, the company developed toolpath programming to perform 5-axis circular stagger milling when roughing. With CSM, the cutting tool moves on a circular track through the canal between the part’s blades. Because of the changing width between blades, the diameter of the circular track is constantly adjusted, enabling surface finishes typically achieved when finishing to be imparted when roughing. “There are programs that can do circular stagger milling, but on a 3-axis basis and not on a 5-axis basis. You need the latter for air foils,” said Götz Lebküchner, materials and processing engineer at MTU.
Courtesy of MTU Aero Engines

A comparison of the toolpaths and cutting depths for conventional and circular stagger milling.

He noted that, compared to conventional rough milling’s feed rate of 40 to 70 m/min. and DOC of half the tool diameter, CSM enables feeds up to 150 m/min. and a DOC of twice the tool diameter. That achieves a metal-removal rate of 18 cm3/min. when applying a 10mm endmill, which can reduce cycle time by a third. “When cutting air, we accelerate to the maximum speed the machine can do,” Lebküchner said, adding that the faster the rotary axis accelerates, the better the results, and a machine needs linear motor drives to achieve that level of acceleration.

Initially, however, there was a barrier to implementing CSM. “If we were going to use this method to process the very thin-walled [2mm to 4mm] titanium blades, the tool had to be completely free of vibration,” Lebküchner said. “Otherwise, it is impossible to use the process safely.” That’s because vibration can cause chatter marks on the parts and chipping on the tools, which extend up to seven times the tool diameter out of the chuck—or 70mm for a 10mm endmill.

Circular stagger milling without vibration wasn’t possible with the shrink-fit toolholders MTU was using, and tool pullout was an issue, according to Lebküchner. As a result, the company tested various toolholders and found that the powRgrip hydraulic holders from Rego-Fix Tool Corp., Indianapolis, achieved the desired results. The holders provide a clamping force of 1,000 Nm and a TIR less than 3μm. “The powRgrip holders swallow the vibration of the tool,” Lebküchner said, adding that they also prevent tool pullout. “A holder covers more than 80 percent of your tool shaft, holding the force on the right space.”

In addition to different toolholders, MTU required different cutting tools for CSM. Unlike the standard carbide tools applied for conventional milling, the special carbide endmills for CSM are through-coolant ones with unique cutting edge geometries and preparations.

Compared to conventional blisk milling, CSM reduced processing time by 42 percent. In addition, tool life increased by up to 400 percent, reducing tool costs by 76 percent. During conventional milling, MTU consumed 18 tools when roughing, but applies only four with CSM. “There are four because they are different lengths, not because they wore out,” Lebküchner said.

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