Getting Ready to Lap

[LubeckTech]

I am going to do some mold lapping and casting thru a nut to create a lapping bullet looks good. How do you keep the nut in place during the pour??
I will probably do some beagling first - what is the best material to use? Where does it go - each end middle or both??

[462]

If you want the mould to drop fat boolits, beagle it first. I use 3M aluminum foil tape, and your local hardware store should carry it. Placement of the tape requires a bit of experimenting, but I start by placing a piece on each side of each cavity, on one mould block. Once the mould drops the corrrect size boolits, you can decide if you want to lap it or leave it beagled. Lapping is permanent, beagling is reversible.

If you just want to clean up any rough edges from the cavity perimeters, than lapping is the way to go.

When lapping, I set the nut on top of the mould and pour carefully, making sure to keep the stream centered in the nut and to not let it run over the sides.

[dragonrider]

Remove the sprue plate and eyeball the nut over the cavity as central as you can then carefully position the mold and pour down the center untill you fill to the top of the nut. If you spill some over don't worry about just trim it off when cool, I use a coarse file does pretty well.

[onondaga]

I have done mold lapping a few times with success to bring bullet diameter up . The last one I did was a Lee C309-150-F to bring up bullet diameter to .313" from .309".

I cast the bullets with hex nuts. One I had to re-do as it was not centered well.

First I tried toothpaste and all this did was shine the mold cavity. Then I got serious. I made a polishing paste with dish washing liquid soap and extra fine pumice about 500 grit. . I used a small brush and applied the paste to the driving band areas on the hexed bullet and avoided the gas check area.

Close the mold gently and take 2 turns gently and don.t squeeze the handles like brakes. Next open the mold , remove the bullets and rinse the mold but don.t rinse your hexed bullets. Put the hexed bullets back in and close the mold. Now you can close the mold with slight pressure and rotate the hexed bullets. I gave about 20 turns. then washed everything.

I repeated that procedure 3 times and cast some bullets. They measured .311"

I repeated the procedure 3 more times and cast some bullets. They measured .313" This worked well for me and I successfully kept from ruining the gas check shank area diameter and gas checks fit fine. Some diameter increase was noted in the area of the bullet crimp groove and a little toward the nose but that was expected and harmless.

I bet valve lapping compound of 400-500 grit would also work fine but the soap was very easy to rinse out and clean between lapping procedures and casting trials. The liquid soap/pumice mix leaves no oil in the mold so casting trials were easy to complete and the molds were not contaminated with oil. An oil based abrasive paste would require a very thorough cleaning of molds before casting and I wished to avoid that, just rinse, warm the mold and cast.

Gary

[Doc Highwall]

Do not worry about having good fill out with the cast bullet the wrinkles hold the lapping compound and make the job go faster. Take your time on this as it only has to be done once and makes all the difference between a useless mould to one that will make the condom shooters jealous.

[Fredx10sen]

I have done mold lapping a few times with success to bring bullet diameter up . The last one I did was a Lee C309-150-F to bring up bullet diameter to .313" from .309".

I cast the bullets with hex nuts. One I had to re-do as it was not centered well.

First I tried toothpaste and all this did was shine the mold cavity. Then I got serious. I made a polishing paste with dish washing liquid soap and extra fine pumice about 500 grit. . I used a small brush and applied the paste to the driving band areas on the hexed bullet and avoided the gas check area.

Close the mold gently and take 2 turns gently and don.t squeeze the handles like brakes. Next open the mold , remove the bullets and rinse the mold but don.t rinse your hexed bullets. Put the hexed bullets back in and close the mold. Now you can close the mold with slight pressure and rotate the hexed bullets. I gave about 20 turns. then washed everything.

I repeated that procedure 3 times and cast some bullets. They measured .311"

I repeated the procedure 3 more times and cast some bullets. They measured .313" This worked well for me and I successfully kept from ruining the gas check shank area diameter and gas checks fit fine. Some diameter increase was noted in the area of the bullet crimp groove and a little toward the nose but that was expected and harmless.

I bet valve lapping compound of 400-500 grit would also work fine but the soap was very easy to rinse out and clean between lapping procedures and casting trials. The liquid soap/pumice mix leaves no oil in the mold so casting trials were easy to complete and the molds were not contaminated with oil. An oil based abrasive paste would require a very thorough cleaning of molds before casting and I wished to avoid that, just rinse, warm the mold and cast.

Gary

Thanks Gary for that information. I have two of those exact molds and one has always been hard to get good bullets from. So that one is getting lapped to open it up. :drinks:

[Armorer]

I have done mold lapping a few times with success to bring bullet diameter up . The last one I did was a Lee C309-150-F to bring up bullet diameter to .313" from .309".

I cast the bullets with hex nuts. One I had to re-do as it was not centered well.

First I tried toothpaste and all this did was shine the mold cavity. Then I got serious. I made a polishing paste with dish washing liquid soap and extra fine pumice about 500 grit. . I used a small brush and applied the paste to the driving band areas on the hexed bullet and avoided the gas check area.

Close the mold gently and take 2 turns gently and don.t squeeze the handles like brakes. Next open the mold , remove the bullets and rinse the mold but don.t rinse your hexed bullets. Put the hexed bullets back in and close the mold. Now you can close the mold with slight pressure and rotate the hexed bullets. I gave about 20 turns. then washed everything.

I repeated that procedure 3 times and cast some bullets. They measured .311"

I repeated the procedure 3 more times and cast some bullets. They measured .313" This worked well for me and I successfully kept from ruining the gas check shank area diameter and gas checks fit fine. Some diameter increase was noted in the area of the bullet crimp groove and a little toward the nose but that was expected and harmless.

I bet valve lapping compound of 400-500 grit would also work fine but the soap was very easy to rinse out and clean between lapping procedures and casting trials. The liquid soap/pumice mix leaves no oil in the mold so casting trials were easy to complete and the molds were not contaminated with oil. An oil based abrasive paste would require a very thorough cleaning of molds before casting and I wished to avoid that, just rinse, warm the mold and cast.

Gary

Very informative thank you! :goodpost:

[longbow]

I have successfully lapped out several moulds bringing diameters up by up to 0.003" using a procedure very similar to Gary's but with valve lapping compound.

Gary has given a very good procedure and I want to emphasize his comment on cleaning. If you do not clean the mould faces regularly the grit will get between them and open the mould some so lapping will produce oval cavities. You have to clean often to keep that from happening.

My procedure for multi cavity moulds is to cast and mike boolits selecting the smallest cavity to make lapping boolits (there is almost always a difference). I start lapping in the smallest cavity, clean and move to the next cavity until all are done. Cast new lapping boolits and check sizes again and repeat as necessary. If you are careful, you can even out cavity sizes as you enlarge the cavities and get all virtually identical.

You haven't mentioned mould material. Iron takes some work to remove more than 0.001" but aluminum goes fast. I have used an electric drill to turn lapping boolits in iron moulds where I was removing several thou but then finishing by hand. I would not be tempted to use an electric drill with an aluminum mould.

Here is a good article on lapping:

http://www.castpics.net/subsite2/MoldMods/MoldLapping1.pdf

This is from "Castpics" at the bottom of the page. Lots of good stuff there including Beagling info.

As 462 said, you should try Beagling first as it is lots less work and not permanent. It may solve your problem.

Longbow

[nanuk]

I read an article where you use steel plates

make an abrasive paste and spread it VERY thin on one plate
next roll a boolit between the plates which just coats the drive bands, which is usually where you want the diameter increase anyways.

then place the boolit back in the mould and do like Gary says.... and I would guess liquid detergent would make a great lubricant for the lapping process.

you can use this technique for lapping only the nose, by carefully embedding grit only in the nose.

[swheeler]

I read an article where you use steel plates

make an abrasive paste and spread it VERY thin on one plate
next roll a boolit between the plates which just coats the drive bands, which is usually where you want the diameter increase anyways.

then place the boolit back in the mould and do like Gary says.... and I would guess liquid detergent would make a great lubricant for the lapping process.

you can use this technique for lapping only the nose, by carefully embedding grit only in the nose.

I have used this same procedeure on several molds with very good results, Oldfeller had posted the whole process with pictures, it works great.

[Jailer]

If you have a morter and pestel you could grind up some egg shells and use them instead of pumice. Grind them to a fine powder and then mix them as Gary stated.

I can't remember now what brand it is, but there is a scowering powder (boraxo maybe) that the abrasive component is ground egg shells. Egg shells are gentle enough that I've heard it suggested on Car Talk for polishing fine scratches out of a car windshield. Should work real well for taking a small amount at a time when lapping a mold.

ETA: Of course this would only work on an aluminum mold.

[*Paladin*]

Good post! I just got a Lee 240gr RN mold in .429. I HOPED it would drop oversize like my other Lee molds. Unfortunately, this one is just about dead-on .429". So, I will be lapping this mold out to get hopefully closer to .431" or .432". Thanks for the info!

[excess650]

Aluminum cuts more quickly than iron, so be aware if its an iron mold.

I had used LBT bore lap that I got from Veral about 20 years back. It was a might coarse, so I mixed it with JB bore cleaner and tried it.

As of late I went to the Local NAPA auto parts and bought some grease based valve grinding compound. I seem to recall thinking it was way more coarse than necessary, but found that it wasn't the case. Like the LBT stuff, it could be cut with JB to make it less aggressive.