Lapping Molds

[insanelupus]

Two questions.

Should a person use a brass screw to use with a T handle when lapping a mold to prevent marring the mould itself?

Once you lap the mould and gain .001-.002" if you need to go a little further, what is the problem with recasting a bullet in the new diameter and lap it again?

[NSP64]

Not a thing is wrong with what you suggest.

[runfiverun]

seems easier to beagle after lapping.
at least worth a try.

[slughammer]

The hard part is keeping the cavity round. Best advice for lapping is to embed the grit by rolling the lapping boolit between 2 steel plates, then wash off the excess with a bowl of soapy water and a tooth brush. Clean the loose particles out from between the mold blocks often. Anything that keeps the blocks from closing will make the cavity larger at the parting line than it is across the blocks.

Not my idea, I seem to remember these tips coming from Oldfeller. I have done it and they do improve the process.

[longbow]

slughammer is right on about oval cavities!

I have lapped more than one mould and do find it difficult to remove more than 0.001" to 0.002" without producing and oval cavity. Typically I have used valve lapping compound which is already mixed with oil. I always clean up the mould blocks between lapping runs but have not gone the extra mile of rolling the lapping bullets between steel plates then cleaning them up to produce a "burr" of sorts. I also have read here that this will ensure a round cavity if you are careful.

There are some articles about lapping here: http://www.castpics.net/ in the "moulds" section.

Also, being somewhat lazy I use and electric drill turning slowly at least to start. Then I may finish by hand turning the lap. I usually accept a slight ovaling of the cavity then Beagle to reach final diameter which works just fine.

If you are looking to remove much more than 0.002" or 0.003" and keep a round cavity you will have to be very, very careful.

Another point that occurs to me - I have only lapped iron blocks. Aluminum or brass will respond much faster to enlargement so if you are planning on lapping a Lee mould you may be best to use very fine lapping compound and work very carefully and slowly.

Longbow

[insanelupus]

Thanks guys. I plan to lap Lyman/Ideal moulds as those are what I have. I'll cast in Lyman 2 and hope they are of correct diameter. Seems many of those moulds are a touch undersize and after I cast, if they need a bigger diameter I'll lap them. I don't anticipate needing more than .002" or so, most of the time, but was just thinking ahead. Hadn't thought of the steel plate idea, but it's worth a shot!

[leftiye]

A suggestion for lapping. Don't use a screw. Use a Torx bit driven into a hole in the base of the boolit. I've used various screws driven into holes bored in the bases of boolits (in a collet on a lathe) and never gotten a coaxially straight one yet! I.E. they don't screw in straight. Plus, if they're (screws) not tight and with a good bite, they unscrew when you try to reverse directions when lapping.

[mtgrs737]

I use a small two flute tap like the ones that are used to tap scope mounting holes. I think the one I use is a 5-40 and I have little or no problem with it backing out when I reverse the drill.