Page 1 of 1

lapping sizing dies

Posted: Tue Feb 22, 2011 7:02 am
by brettb7575
Could you please describe your method of lapping sizing dies to get a tapered entrance. Also do you lap forcing cones if the entrance is undersized. Thanks alot

Re: lapping sizing dies

Posted: Tue Feb 22, 2011 4:20 pm
by mtngun
Before I had the tools to make my own dies, I would modify store-bought dies by lapping them.

I'd start with a die at least 0.001" undersize, preferably 0.002" undersize. Even though I tried to lap only the entrance, it was impossible to avoid lapping the rest of the die at least a little bit. Plus, storebought dies are usually out-of-round and have burrs, anyway, so they benefit from light lapping.

Make a bit to hold a piece of emory cloth in a drill chuck. Just hacksaw down the middle of a piece of say, 1/4" round rod, far enough to hold a piece of emory. Now wrap the emory around the bit and have at it with the drill. A 1/4" die grinder might work even better. Try to stay at, and just inside, the entrance of the die.

The emory will quickly dull, so you'll either have to keep replacing it, or else freshen it up with valve lapping compound.

The process is very slow, typically taking 30 - 60 minutes.

Alternatively, you might be able to use a tapered carbide burr, with everything securely held in a drill press. But carbide burrs are expensive, and the finish tends to chatter, so it would still need to be cleaned up by lapping.

Re: lapping sizing dies

Posted: Tue Feb 22, 2011 4:26 pm
by mtngun
Regarding forcing cones, I've yet to run across one that is too small. More often they are too big.

Brownells offers a kit to recut forcing cones. That's what I would use if the need arose.

If you are referring to the groove diameter being small where the barrel screws into the frame, due to an interference fit of the threads, I've yet to run into that, either. Firelapping would probably take care of it -- but it would enlarge the throats, too -- so if you didn't want to enlarge the throats, it might be better to hand lap the barrel.

Re: lapping sizing dies

Posted: Wed Mar 07, 2012 2:22 am
by kaytod
On the chance that you do have a revolver with a "choked" barrel, one that is undersize in the frame thread area. You can fire lap it by applying the lapping compound at the rifling end of the forcing cone with a cotton swab. This will lab only the barrel portion and not the cylinder throats. I suggest using the compound sparingly and cleaning often so you can observe your progress.

Hope this helps.