Lapping a Lyman Mold

[Ben]

I bought a Lyman 358156 , 2 cav. mold, ( used ). Unfortunately on the seams of the bullet , it miked .355". Move 90 degrees with the mike and it mikes .3580". The bullets were bad out of round.

No way that I'd ever be happy with numbers like that. I decided today to lap the mold. I'd lapped Lee Aluminum molds with good success before. Lapping an aluminum mold is fairly quick. Lapping a steel mold like this Lyman is slow.

It took a little over an hour to cast bullets ( usually 4 of them at a time to use as laps ), drill the appropriate hole in the center of the base of the bullet. Thread the threaded screw shank into the base of the bullet about 1/4 ". Coat the bullet with abrasive compound, spin in the mold with a drill on slow speed , stop....clean, repeat, clean, repeat, clean, repeat.

It seemed that about every 20 minutes , I was gaining .001" on the next series of 4 bullets that I'd cast for the new laps.

About an hour later, I had accomplished my goad of a nice round 358156 gas check bullet that sizes out to a perfectly round .3580" any point you measure on the bullet. I'm a happy camper.

By the way, my gas check shank now is also a better fit with Hornady .35 cal. gas checks.

Here are some photos, if you've got a mold like mine with the same problems, it can be corrected with a bit of time and sweat.

http://img.photobucket.com/albums/v494/haysb/005-54.jpg

http://img.photobucket.com/albums/v494/haysb/001-86.jpg

http://img.photobucket.com/albums/v494/haysb/004-69.jpg

http://img.photobucket.com/albums/v494/haysb/003-77.jpg

http://img.photobucket.com/albums/v494/haysb/002-84.jpg

http://img.photobucket.com/albums/v494/haysb/012-27.jpg

http://img.photobucket.com/albums/v494/haysb/007-49.jpg

http://img.photobucket.com/albums/v494/haysb/011-32.jpg

http://img.photobucket.com/albums/v494/haysb/013-30.jpg

[bhn22]

I've always had a miserable time getting the hole centered. I finally ended up pouring my laps through a nut, set on top of the mold. Then I'd remove it, coat the cavity with compound, and use a 1/4 in. air ratchet to turn the laps slowly. Rubber bands around the blocks keep the tension on the blocks consistent.

Just another way to do it.

[Ben]

bhn22

I feel certain that if 10 people tackled this task, that each might do something slightly different in method vs. what you've seen above.

Ben

[Mal Paso]

Good Job! I used 320 grit diamond and a full day to fix a 4 cavity. Mine was round so I only did the drive bands. I used the mold centerline to scribe an x to center the screw.

Great casting mold once you fix the size.

[Idaho Mule]

Thanks Ben, very informative. I have been envisioning doing the same thing to a particular 429421, maybe i will get it done sooner now. JW

[bosterr]

Hey Ben, what grit did you use?

[Ben]

I had this mold on my bench for months.

Today, I said....." I'll either fix it today,
or ' deep six it ' today." One of the two
is about to happen.

I just sized about 100 of them a few minutes ago........Dead nuts on .3580" now.

[longbow]

Ben:

Good going! It is satisfying to "fix" a mould and have it produce nice round boolits of the correct size for your needs.

I have lapped a few myself. I mostly use the screw but then I have a lathe so can get holes centered easily.

A couple of tips for those that don't know but are planning to try lapping a mould:

- if you use the "cast through the nut" method mentioned by bhn22 watch that you don lap the top of the mould under the nut. I've used card board to space it up so after casting there is clearance under the nut.
- What Ben says bears repeating "...stop....clean, repeat, clean, repeat, clean, repeat..." Clean the mould faces OFTEN! What happens if you don't clean OFTEN is that grit gets between the mould blocks and holds them apart and if you do not clean regularly you will wind up with an oval cavity.

Lapping is not hard to do but there are a few things to watch.

I usually cast the lapping boolits, drill, install screw, oil the cavity and lapping boolit then spin slowly with an electric drill closing the mould until the lapping boolit fits the smallest cavity and spins freely. Then I apply lapping compound and run the same boolit through each cavity, wiping mould faces each time and sometimes just because.

After one circuit I will clean the mould completely take another of the same size boolit, coat it with lapping compound and repeat. Once I have the lapping boolits of the first size spinning freely in each cavity it is time to cast new lapping boolits to move up in size.

Clean, lap, clean lap, clean, lap... for each cavity. Once a circuit is done, cast and check size. Repeat lapping and cleaning cycles as required using the new set of larger boolits. Taking out a thou or tow is usually pretty easy but taking much more than a couple thou is significantly more work and harder to get good results in my experience anyway.

As Ben says, I am sure there are lots of ways to lap a mould but the above is what has worked for me.

Longbow

[462]

Nice job, Ben.

I used Ben's method to enlarge a Lyman 429421, taking a few hours over the course of two days. More time was taken up in cleaning the mould, casting new lapping boolits, running them through a sizer die, and measuring, then actually working on the die. And, yes, doing it to an aluminum mould is much faster.

I tried the pour-through-a-nut method once, but the nuts did leave their marks on the mould. Thanks, Longbow, for the cool spacer tip.

Anymore, I use Beagle's method, and let the sizer die make them round.

[Ben]

462

I'm also leary about using the " pour thru the nut " technique also.
I've always had real good luck with the technique described and illustrated in post # 1. This technique leaves no marks on the top halves of the mold. Think I'll stay with it.

Ben

[dragonrider]

I use the nut method, never thought about the cardboard, very excellent idea.

[Ben]

Hey Ben, what grit did you use?

Clovers Brand Coarse. It did not give the grit size on the container - - only described as coarse ( see photo above ) , I'd guess about 180 or so.

Ben

[Green Frog]

I've only had to lap a couple of Lyman moulds to clean them of slight surface rust/roughness, not to true them up or change bullet diameter. I never had to use anything but 400 grit start to finish, and IIRC, only a couple of bullets per chamber. They really dropped bullets well after! :mrgreen:

Froggie

[Ben]

An up-date :

I was looking through my mold collection and found a Lyman 358477 with a tag tied to it that said....." Cast .3565 " X 357 ".

So, it got the same identical lapping treatment.

The bullets are now sizing well to a nice round .3580" also.

The lapping system works, on a steel mold like a Lyman it will take a good bit of time, ( this 477 took over an hour to do ) but the results are well worth it.

Ben

[w5pv]

Looks like you fellows did a jam up job and non of the pictures resembled a goats rear sewed up with grape vine.