While still a tiny fraction of the aerospace composites market, the use of metal-matrix composites (MMCs) and ceramic-matrix composites (CMCs) in engine parts has grown substantially over the last few years because they can run at higher speeds and handle more heat.
But as Aaron Howcroft, global product manager—composites & PCD for Sandvik Coromant (Fair Lawn, NJ) explained, “they are very difficult to machine to the tight tolerances needed inside the engine, to the point where you’ll use one diamond tool to machine one small feature versus the 40 you could machine in traditional alloys.”
The solution to this, said Jeff Wallace, general manager of the Aerospace Center of Excellence at DMG Mori (Hoffman Estates, IL), is the company’s patented ultrasonic technology. DMG Mori adds a piezoelectric head to a standard toolholder interface such that the cutting tool both rotates normally and oscillates in Z during the cut.
Specifically, the machine feeds power through an induction coil and the resulting material deformation of the piezo resistor in the head causes the oscillation. The frequency of the vibration is on the order of 50,000 Hz and is highly tunable to the application. The Z-axis movement is 2–4 µm, depending on the model, creating a small jackhammer action. The separate head isolates the vibration from the machine spindle so as not to damage its bearings.
Originally developed to cut hard, brittle gemstones, Wallace says DMG Mori has invested “thousands of hours developing cutting parameters and tuning our ultrasonic technology to produce CMC parts, having done several large projects with one of the major aircraft engine builders. We use diamond impregnated tools and usually specialized coolant. The system cuts CMCs very well, even adapting to the variations we see from different suppliers.”
It turns out the ultrasonic technology works well on the full range of composites and eliminates the need for coolant on everything but CMCs. “All the CFRPs we’ve tested can be cut dry,” said Wallace, “and in half those cases the customers were trying to cut wet. We find that the ultrasonic action sets up more of a shearing action in which we cut the fiber in CFRP rather than rip it, which we’ve confirmed microscopically. It cuts so cleanly that it surprises everyone. About three years ago we also started to apply ultrasonic technology to cutting superalloys and found it doubles to quadruples tool life using standard, off-the-shelf tools.”
The only downside has been a relatively high cost of entry. But the system is available on a wide variety of three- and five-axis machines and Wallace says one target for 2018 is lowering costs to make the technology easier to justify.