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With 10mm on the rim on a 20mm deep bore, I would not try to treppan it. I think your going to need more "meat" to support the part and clamping forces while you cut. A part snatched from the chuck under this kind of operation can do some serious damage to you and the machine. Thats my opinion ....worth exactly what you paid for it..... Good luck!Dave
I'm with the others who have advised you against trepanning.Do you (or perhaps a nearby fellow machinist) have a mill with a rotary table? That's a very safe alternative for what you want - saves the slug and leaves a nicely finished hole that will require little or no finish machining.