Not to offer a solution, but this topic reminds me of a time when I wanted a hole cut in a Sinar board (about 3mm aluminum). I had a machinist friend who was also a supervisor in a large plant's machine shop for many years. He also had a GRAND shop in his basement. He put the board in his lathe and starting cutting, but stopped just before breakout, then with his pinky poked the piece out. I asked how he knew when to stop cutting. He said he could tell by the cutting sound. I asked what if the material was a different material. He said it would have a different sound. We're talking experience here.