Today I learned that
fiberglass is really tough stuff.
I was using my 18v Dewalt drill as a 1" hole saw. The hole saw has an approx 1/4 centering bit, that enters the material first.
Surprisingly, after I started slowly, then ramped up to high speed, the centering bit got part way into the
fiberglass and then suddenly seized up, causing the drill to torque violently out of my grip. The force was great enough to shatter the hardened steel of the bit.
Upon reflection, it appears that the bit is too good at biting into the material. A slower approach, slicing less deeply into the material yields better (less violent) results. I accomplished this by using the drill on a quite slow speed.
The hole saw blade didn't seem to mind higher speeds.