When Adam told me what he had in mind, I wondered if
he had forgotten to take his medication that morning. Basically, he wanted
to carve a signature from 0.0625" (1.59mm) thick MOP. "No big
deal", I thought. Except that, to preserve the character of the
signature, the entire cut would have to be made using a bit no larger
than 0.020" (0.51mm) dia. To make sure that complete fidelity was
achieved, Adam had decided to use an even smaller bit, one with 0.0156"
(0.4mm, 3/64") dia. I told him that the only tool that we had that MIGHT
be able to make the cuts that he had in mind was the 3/64" 3-flute
UltraBit (MM3I8-0156-006F). Although these tools are not designed
to handle materials that pack as readily as mother of pearl, the are
very tough and capable of surviving relatively abrupt changes in
material density. In any case, Adam decided to give it a try.
After a fair amount of experimentation, and remarkably few broken bits, he found that taking 0.005" (0.127mm) passes using a 60 KRPM spindle and a feed rate of 8 inches (203mm) per minute produced very clean cuts with virtually no surface damage. In his own words:Well, your .0156 in. dia with .060 in. loc* bit worked great....many depth cuts, but it got me there.
My method is to glue the blank down with hide glue (let it set overnight) and machine with many shallow cuts and no finish passes (to avoid lateral forces possibly breaking the bit). When it's done I just immerse it in a hot water bath and the tiny pieces release almost immediately. My pocket is loose enough I don't have any problem breaking them when inlaying.**
I was using an air turbine at somewhere in the
neighborhood of 60,000 RPM
and feeding at 8 inches per minute. This was done in .005in. depth
passes. It sounds like
maybe I could have gone a little deeper on my depth cuts, but you said
you were concerned about blowing the waste pearl out so I was playing it
safe...not having a lot of time to experiment. I was blowing about 90psi
air on the tool during machining.
Words of Wisdom
Cutting signatures can be very tricky due to the line width variations that result for the natural variations in stylus pressure during writing. Even when using a soft tipped marker (highly recommended), portions of the signature may be too thin to reliably machine (but then, I thought this project was too thin). If that is the case, a bit of post processing with a vector image processor (Adobe o Corel) can save an awful lot of pain and frustration.
Moral: If you call us up and ask our advice, it is probably safe to ignore it since we are apparently too conservative and hidebound to push the performance envelope with these tools.