You didn't mention the diameter of the disk you were cutting out; this would affect the time it takes. My recent experiences with trepanning have been cutting a 13-1/2" dia. disk from 3/8" thick plate, using 120 grit. This took about two hours, mainly because my motor was under powered. A 2-1/2" dia. hole through a 2-1/4" thick Pyrex mirror took an hour and a half, also with 120 grit. Things to check--cut some teeth or v-shaped notches in the edge of the metal so grit can get in. A notch about every inch along the edge should be enough. If the sheet metal is thin, it might be moving the coarse abrasive out of the way instead of trapping it under the edge of the metal. A thicker sheet metal or finer abrasive might help. 1/16" thick is not too thick; it just makes a wider groove. Apply as much pressure as the motor will take and move the cutter in and out in the groove to let more abrasive/water mix flow in. Good luck.