lapping a RB mold

[patkinson]

I recently purchased a lyman .437 RB single cavity mold.  However, balls are .432(pure lead).  Does anyone have a good method for lapping a steel mold?  I would to increase my mold to approx. .435.  Thanks   Phil

[WadePatton]

I'd cast a ball in that mould around a bit of metal to serve as a shaft, then apply lapping compound to that ball.  When the fit becomes sloppy, I'd try adding some "patching" with lapping compound OR simply cast a new ball in the enlarged mould. 

Repeat as necessary.

But I'm just yakking from the gray matter.  I'm SURE there are plenty here with more "mechanically correct" (with more experience) variations on how to make a hole bigger.  Listen to them first.  Always.   

[smylee grouch]

I have done just what Wade suggested to an aluminum mold and it worked, enlarged 2/1000, took all afternoon. Never tried doing it to a steel mold.

[heinz]

First, check your measuring equipment. That is a long way out for a Lyman. You can use a cast ball with a wood screw down through the sprue and rubbing compound or better valve lapping compound   Mount the wood screw with the head cut off in a drill press  Do NOT do this without a drill press.

[Robby]

Did you 'soot' the mold? That can easily reduce the diameter by a couple thousandth's. Never seen a Lyman that far off.
Robin

[Daryl]

I have a Lyman DC for sale- casts .400" X .400" in one cavity and the other is .400" X .394" - go figure! It's cheap - LOL.

I do prefer Lyman or RCBS moulds normally.

As to being under size or oversize, not a problem. My .570" mould casts .574" x .574" in my dead soft lead, which is perfect for most .58's.

[Maven]

Before I attempted to do something to a mould which can't be reversed, I'd try something else.  To wit, the following link on "beagling" a bullet mould does work, but will produce a slightly out of round elongated bullet or round ball:  http://castpics.net/dpl/index.php/the-library/moulds/23-modifying-molds/248-beagling-a-mold-temporary-mold-enlargement

You've nothing to lose by trying it, and it is reversible.



P.S.  Beagle, who came up with the idea, is the screen/user name of one of the original members of the Cast Boolits site.

[Daryl]

I have done something like that with 2 moulds, an RCBS & Lyman .458 bullet moulds that cast a few thou out of round bullets.  Using tinfoil and high-temp silicone (red) as an adhesive,  these moulds now cast round bullets.
I had to bring the moulds & lead up to temperature, then pour the lead into the hole to fill.  If using pressure, as in the dipper held against the mould's plate, the added pressure would cause a nasty flashing.  With the correct temps of the mould and lead, perfectly round bullets were case. I found this process quite simple.

If wanting to simply increase the size of the ball, tin-foil up to a couple thicknesses could be glued to one of the mould 1/2's and trimmed with a sharp knife, carefully, to the cavity perifery - this may magnification and care to prevent damaging the mould's cavity permanently.

[Maven]

Daryl, It works better if the foil or A/C duct tape is also wrapped around the sides of the mould as that helps keep it in place. 

[Steve-In]

An old timer in our club explained how to lap a mold.  He freshed out his barreloften and never bought a new mold.
You cast a ball with a nail in the mold.  Trim off any excess where the sprue would be.  You use lapping compound and chuck the nail in a drill press and spin fast enough that the lapping compound stays on the ball.  Take ONE side of the mold and rock it side to side, end to end and count how many times you do this.  Repeat on the other side.  After a couple of cycles clean and cast a ball to check progress.  Cast new laps as needed. 
It worked for him I have never had to try it.
I would start the drill out as slow as it goes.  Block surroundings with cardboard and wear cloths you can dispose of.   

[Daryl]

Daryl, It works better if the foil or A/C duct tape is also wrapped around the sides of the mould as that helps keep it in place. 

I simply glued on the tin foil \squares with the high temp red silicone 'glue' to the inside surfaces of the blocks.  They've stood the test of time - well, 20 years of it, anyway.