I keep chipping and/or breaking carbide drill bits while drilling receivers for scope mounts. I am using Brownells Do-Drill, slow feed rates, and a very rigid (mill vise) set-up. It seems that they always chip or break during penetration of the hard inside skin of the receiver.

Any recommendations from the crew? This is getting very expensive!

Hotshot

Have you tried using non-carbide bits? If you're not drilling too fast, and you're using enough oil and you're still breaking the bits, I would try using a bit that isn't as hard.

I use regular high speed steel drill bits, and of 25 Mausers, the only one I couldn't get through was a '93 Mauser that the rear bridge had a hard spot inside. After I made three bits dull without progress, I heated the whole rear bridge up until red and let it cool. Then I was able to finish. But the K98, VZ24, and Turk Mausers have all been soft to drill.

I'd try changiong bits halfway. Cut through the outside case hardening with the carbide bit then switch over to a HSS bit. It should just run through the steel then.

They cut through the steel ok, but as hotshot has noticed they often hang up on the spall created when they break through the other side, and that hang up caused the cutting edge of one side to snap off. The old hardness VS toughness thing again.

Thanks all. I'll give the method of changing over to the HSS bit after breaking through with a carbide bit. If the the HSS stalls penetrating the skin on the backside, I'll switch back to the carbide. It appears that some of the O3A3's are heat treated to high Rockwell values on the outside.