First lapped mold today.

[wallenba]

I finished lapping my first mold today, a Lee 312-185r. I wanted a .315 for my Mosin M38. It took three casting/lapping sessions to get it oversize enough all around. I learned that you must avoid lapping the gas check base. They go on, but with difficulty. They drop about .319!! This makes me wonder about concentricity. Will that likely be a problem? Also, I wonder if I could have used a .321-.323 mold for an 8MM. Would the body of an 8MM be too large in dia.? The sizing die was easy, or I just got lucky and stopped right on the money. I used Wheeler lapping compound I found on Midway's site, and cast boolits with screw shanks in them.
Anything I missed that will make this easier if I have to do it again?

[longbow]

The biggest problem I have had lapping more than a thou or two is that there is a tendency for the cavity to open up larger at the seam than at 90 degrees to the seam as lapping compound and swarf wind up trapped between the mould blocks forcing them open a bit. Then less metal is removed from the wider dimension.

By lapping a bit, then opening and cleaning then lapping a bit more with no added lapping compound (just what is embedded in the lap) the oval cavity is minimized or eliminated. This seems to work very well with aluminum moulds since th ealuminum is so soft. It works with iron moulds but takes longer because the embedded grit doesn't cut fast.

If you were careful the boolit should be symmetrical but may be slightly oval. A thorough check with a micrometer or vernier should tell you.

A little oval shouldn't hurt anything anyway as Beagling produces oval boolits, but symmetrically oval, and they shoot fine.

I have found that casting through a nut (open sprue plate) sitting over the cavity then turning the nut with a box end wrench is easier for me with small (.30 cal) moulds. I use nuts or large screws for .44 and shotgun slugs.

I think the secret to success is to go slow, check often, clean up and make sure the lapping is even and go slower as you get close. Aluminum moulds open up quickly.

Longbow

[462]

Longbow,
Never gave any thought to the lapping compound, that accumulates between the blocks, being a cause of an out-of-round boolit. I'll be paying closer attention, in the future. Thank you.

Man, the knowledge available on this site is too incredible. I don't know if my brain can handle it all. I'll have to stop remembering other stuff so that there'll be enough room for more casting knowledge.

[wallenba]

Yes, mine are oval, but I figured that was part of the price of this effort. I would think that since there is a lot of Mosins out there with this problem, Lee Precision could make a mold and some sizers for that market. Still, if this doesn't work out I might give them a call to make me one. It would be under $200, but then that's more than I paid for both of my Mosin's put together! My casting addiction seems to have no limits lately. Next time I will definitly remove the gas check base from the casting/lapping spud, as that got enlarged too, and I am concerned with the g/c's ability to remain seated. I still have to reduce an expander plug for my RCBS die set, I have an 8mm one from an old Mauser die set I don't need anymore.