When I broke my leg I was given a plaster cast. After a couple of months this was removed at the hospital using a rotatory saw. This sliced through the cast with ease, but I noticed that when it touched my skin, it didn’t hurt at all. How is this possible?
They probably used a form of cut off disc. These don’t have teeth like a wood cutting blade and use abrasives like diamond. The abrasives are usually fine enough that it won’t hurt your skin unless you press it in with some force. Think of it as a very thin, fast moving sanding disc.
Chances are that it was actually an oscillating saw even if it had a round blade. Those just swing back and forth a little bit. That makes them good at cutting hard things and bad at cutting soft things.
It doesn’t actually rotate, it just vibrates back and forth, it ‘cuts’ through the cast because its hard and sturdy, when it hits your skin it does no damage as your skin actually moves with it due to its flexibility.