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2011 11 16 CNC notes
I spent about 4 hours in the shop today working on making holograms. My initial attempts were somewhat successful (trying to print the flower3 pattern), but it was extremely difficult to get the correct depth for scratching. The range between being too light to see and so heavy that the point jumps and creates a rough scratch is so narrow that I couldn't consistently find it, and the variations in depth across the plastic were enough to make good results pretty much unattainable.
I decided to try another cutting tool, and ended up trying a #3 center drill in a milling collet (instead of the compass point in a drill chuck). Hand-tests suggested that this might work well, and might even be able to make nice scratches on the polycarbonate I picked up from the artist supply store last week, which the compass point only made rough scratches on. I tried the center drill both stationary and spinning at low speed (~300 RPM). Stationary worked quite well: the scratches were consistent and extremely smooth, but much, much wider than I was hoping. Spinning the bit didn't work at all. Instead of making a smooth scratch, it made a series of closely-spaced cut marks as it traveled, and these provided enough variation in the surface to ruin the optical effect.
I also tried a special engraving tool which Ilan Moyer let me borrow. It had a 30 degree tip which was essentially half a cone (split along its axis), with a very slightly flattened tip. I tried this in the mill collet at around 2600 RPM, but it just created a very rough white scratch which didn't reflect at all. Ilan also has a vinyl cutting tool which I will try tomorrow.