Recutting RB Mould slightly larger...

[Collector]

I'm addicted to nonstandard sized bores.  Th easy thing (which I've done) is to just order what I want from Jeff Tanner.  I  prefer Lyman moulds and there may or may not actually be a vintage Lyman in the size I want.  This is for a rifled bore of .480 caliber, for which I'd like a .475 RB mould. 

Given a choice of recutting a .465 (+010) or a .470 (+.005) Lyman RB mould, which one would be preferable?  And, after recutting, would reheat treating it be required, or at least advisable?   

As always, thanks in advance for your time and replies.

[rich pierce]

I don't think any heat treating is done.  The molds contain molten lead.  That would temper any steel that ever had any hardness.  Making the cherry is the hard part.  The sprue hole will be bigger after re-cutting.

[Macon Due]

Collector
No more than you are talking... .005", just Lap it out to that size. Cast a lead ball in it then drill a small pilot hole down thru the sprue. Turn a metal screw into the hole and cut off the screw head.Coat your 'lead lap' with valve grinding compound and chuck it in your electric drill.  Remove your mold blocks from the handles and clamp them around the 'lap' turn the drill on slowly and gently squeeze and you can feel it start to lap. You will feel when it is no longer cutting. At this point clean the mold well and cast another few balls and measure. Generally I had to cast about 2-4 new 'laps' before I had it lapped out enough. I have done bullets as well [Lyman 500g. RN]. I reckon a drill press would work as well but might be a bit harder to control.....
Macon

[KNeilson]

I have now lapped a few molds Ive made to improve the cavitys shape and surface. I think .005 is possible but it will take you a while. The first lap I made I did in a similar manner to what has been suggested, but the screw wobbled and loosened too much for me. However that also  could have been the super soft lead I was using. Lately Ive casted my laps from wheel weights, seems to be a more durable material yet still will hold your abrasive. First I hold the mold in a drill press vice and then with the chuck lower a modified  nail (filed the head to "T" shape) into the cavity before pouring the material. I seem to be able to get the laps spindle centered better this way. The lap will require a little work to remove the excess lead (sprue etc). I started by using a eggbeater drill and progressed to hold the lap in the drillpress and clamping the mold onto it by its handles.  I have a selection of grits for my tumbler and have been using a fairly fine size (320) so far. Cant really say how well a coarser grit will speed the process up. But the finer grits Ive used have done a thou or two quite easily....  Id be interested in hearing how it works out for you.. regards.          Kerry

[Collector]

Macon Due,

Very interesting approach... I like the idea! (because it sounds like I could do it myself)  

Any inherent problems with maintaining dimensional uniformity, in a round cavity, using this approach?

[Macon Due]

Collector
I had none, the molds were just fine.I was totally satisfied. I just bought a 2 tube pack of valve grinding compound [1 coarse/1 fine] If I remember right I just used the coarse.

Macon