August 2012

OPTIMIZING ORTHOTIC MILLING

By Jordana Bieze Foster

Computer aided manufacturing is anything but a one-size-fits-all proposition. Customers have a range of variables to choose from, and making the right decision in most cases depends on knowing what type of orthotic material the machine will be asked to handle. At the Orthotics Technology Forum in Manchester, Robin Smith, applications manager for Delcam, outlined some key factors to consider:

1. Materials.  Different milling machines work better for some materials than others. Know the size and thickness of the material sheets you’ll be working with, as well as the hardness.

2. Working envelope. This is the space required to accommodate the material, and is a key consid­eration when deciding between a router and a machine tool. Routers, which typically have a large XY envelope but not a lot of depth, are good for soft materials like EVA (ethylene vinyl acetate) and low to medium volumes of polypropylene. Machine tools, with a more 3D working envelope, are designed for “heavy” engineering.

3. Fixture method. The means of holding the material down while it’s being cut depends, again, on the material. Believe it or not, double-sided adhesive tape works fine for low volumes of EVA. Vacuum tables work well for higher volumes of EVA, but the higher cutting forces required for polypropylene necessitate a fixture plate that is more secure.

4. Cutting tools. A rasp cutter is a good tool for cutting EVA but will “chew” polypropylene rather than cutting it cleanly. Sticky materials like polypropylene also can stick to the teeth of some cutting tools; using tools made of TiAIN (titanium aluminum nitride) is one way to help prevent this.

5. Ratser or offset cutting strategy. A ratser strategy is one in which the cutting tool passes back and forth along or across the material surface in parallel passes. An offset strategy, which follows patterns defined by the user, can help extend the life of the cutter and minimize sudden changes in cutting direction that will slow the feed rate.

6. Climb vs conventional cutting. Climb cutting, in which material is clawed away, works well for EVA. But conventional cutting, which pushes material away rather than pulling it, is more appropriate for polypropylene.

7. Waste extraction. Considerations include the size of the vacuum pipe, the availability of air flow to keep the cutting tool cool, and noise level.

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