Hi all,

I was countersinking 8 existing 12mm holes to 17mm top diameter on a 3mm thick cold finished steel plate, first with a 3 flute 90degree HSS countersink bit, then a 6 FL 82 degree HSS countersink. Only half of the holes came out to be alright, the other 4 all had some chatter and the holes are rough. My feedrate was 2.9, i tried both 500 RPM and 1000 RPM at the rate, all came out to be the same. what's funny is, all the holes that were finished smooth stayed smooth at different settings, those chattered always chattered. Could it be my holding is weak? the plate is located with roller pins then a small toggle. I was thinking adding another toggle.

Suggestions?

thanks

Daz