Advanced lightweight materials such as honeycomb structures, composites, fiber optic, carbon fiber and foam are gaining favor in aerospace manufacturing and other markets due to their superior physical properties that increase energy efficiency in aircraft and rockets. Material properties include high strength, stiffness, low density, and fatigue and corrosion resistance. As a result, aerospace OEMs and their suppliers are challenged with determining how to optimally cut these advanced materials.
Traditionally, they have been milled using a more conventional cylindrical carbide end mill featuring a sharp edge on the bottom to cut the material and a series of teeth to rip it away above the cut. However, this method can result in tears, burrs and jagged edges. There is also a limit in the shapes it can make, while the cutting tool puts significant stress on the workpiece and workholding due to high machining forces.
Ultrasonic knife cutting is an ideal solution for efficient cutting of these materials. Less cutting pressure is required because the blade is moving via sound waves converted into micro oscillation of the knife. A generator creates an ultrasonic frequency, which a converter transforms into a linear movement. By transmitting a mechanical amplitude, the knife oscillates about 0.04 mm. It is fast, accurate, reduces dust, eases workholding, improves tool life and enhances quality.
To implement this solution, a suitable five-axis machine tool with a programmable spindle axis, behaving as a sixth axis, is required. A fork-head style milling machine (with the machine head having two rotary axes) is a good option. Here, the rotary and tilt axes are located in the machine head and the part sits on a table. The rotational axis is typically limited to slightly more than 360 degrees of movement, while the tilting axis might be 110 degrees in both directions.
If an axis limitation is encountered during a machining operation, the cutting head must retract, rewind and reapproach the part. This, as well as controlling the knife orientation, puts a lot of pressure on the programmer to understand and make decisions with toolpath planning.
Conventional five-axis machining centers can often be used in this application by implementing an ultrasonic device and having a programmable spindle orientation. Creating the required six-axis NC programs has been a challenge in the past. Traditional CAM processes are accustomed to using a cylindrical tool, relying on the post-processor to generate an NC file. Programmers use a manual methodology to control the knife orientation on the sixth axis, then use separate NC verification software to confirm the results.
Now CAM software, such as hyperMILL from Open Mind Technologies, has a dedicated knife-cutting strategy for toolpath planning. This capability offers a productive, accurate, safe and smooth cutting solution that prevents hazardous rotations and retractions. The hyperMILL VIRTUAL Machining Optimizer process is used to control the orientation of the spindle as the sixth axis, aligning the cutting blade to the workpiece and providing collision checking against a typically asymmetric spindle system while enabling optimal machining within limits for demanding machines with limited rotary axes.
In virtual machining center environments, individual part programs can be linked with smooth, safe positioning points that allow the cutter to remain close to the workpiece. This provides a high level of assurance and time savings versus moving to a home or safety position between jobs.
Smooth connections are better for the machine tool, as they eliminate fast moves with hard stops and sudden directional changes. An optimized virtual machining process also anticipates axis limitations and potentially pre-position or “pre-wind” rotary tables to allow completion of the entire cut in one motion or invoke a retract and safe rewind position to continue within a valid axis range and avoid collisions.
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