Lapping Mould cavities

[FISH4BUGS]

I purchased five 8 and 10 cavity H&G moulds that had been neglected and in a storage unit for who knows how long. They had rusted a bit and with a TON (and I mean a TON) of TLC, Naval Jelly, steel wool, sharpend wood dowels, and oil, they have been cleaned up rather nicely. I would estimate that I have at least 8 hours of TLC work in each one so far. Worth every second I might add.
Some of the cavities have some minor corrosion in them. My understanding is that you need to lap the cavities to bring back the smooth cavities like the original condition.
Do I use one cast bullet per cavity (taken from and used in that cavity only) or can I use one bullet for all eight/ten? I have purchased some valve grinding compound and will use it judiciously.
Are there some detailed description of the process? I know I need to put a screw in the bullet after casting, put some valve grinding compound on it and gently turn it a few turns.
Other than that, I have no idea how to do this. I am not the most mechanically inclined human on the planet so any and all help is appreciated.
Please be as detailed as you can be.
I want to bring these moulds back into service. They are truly works of the machinist's art. A friend of mine that runs CNC machines and does aerospace and defense work to incredibly tight tolerances (but has been doing machine work for 35 years) marveled at the quality of workmanship and skill it took to create these on Bridgeport lathes.
Your assistance is appreciated.

[toallmy]

I have lapped some molds , but I am definitely not a professional . I cast several boolits for this project with the mold , then drill a small hole in the base ( not while in the mold in case it slips ) screw a small screw that I can turn with a 1/4 in hand held driver in to the base . Don't use to aggressive of lapping compound , you just want to smooth out the cavities not make them bigger . I don't really concern myself with witch cavity the cast boolits come out of . But cleaning the mold face wile doing this is pretty important , you will feel the grinding when you slowly close the mold halves together . Go slowly with a hand driven screw driver and mild compound you can clean up a cavity pretty easily . Just remember you can't put it back so don't take out to much .

[toallmy]

I forgot to mention change the lapping slug often , you can use the same slug in multiple cavities before changing it though .

[country gent]

The traditional method for lapping is simple but requires attention.
Cast good bullets from each cavity and Id them as to cavity they came from. You want one from each cavity to start with.
You need to put a stem in the bullets to hold on to it. A screw or allen head bolt works, I use a piece of 1/8" square key stock. I usually make the 1 1/2" long

Drill base as deep as possible but don't break thru, use a drill size the same dia as the corners of the keystock.

This square stem allows you to use a t handle tap driver to turn the bullet easily. And allows for an idea of rotational movement

Epoxy or super glue this into bullets and let cure. This makes your stem to turn the bullet and hold it On multi cavity moulds this is the tricky part as removing .0005 on a side is .001 on dia. To just clean up the cavities and repolish them a fine compound is better, I find valve grinding compounds to be on the coarse side. Flitz or toothpaste may do what you want, this will become more apparent shortly.

Mount the lap in the tap handle and impregnate with compound #1 cavity lap and start #1 cavity
Carefulley close the lap in the blocks under light to medium tension using a vise. Rotate lapp in a Tapping motion back and forth 3/8 turn or so 3-4 times and rotate 1/2 turn repeat the 3/8 turn back and forth thru 2-3 full rotations. Repeat with this lap thru the cavities. Clean mould and discard lap.
Mount lap #2 and start with cavity 2 repeating above thru all the cavities.
do this with all the laps and check for finish and size. If acceptable then your done if not cast some new laps and start with lap#1 and cavity 2 working thru all as above.
By rotating the laps thru the cavities you mould should finish close to .0005 size from cavity to cavity.
I prefer to use finer compounds and work a little longer being careful, its easier to remove more metal than to put it back.

[Bent Ramrod]

I’ve never lapped a multicavity mould, all cavities, but I have done a fair number of single cavities.

In most cases, it was just to smooth the surface imperfections, but in a few, it was the enlargement of some part or all of the cavity.

I fire up the pot, heat up the mould and cast a bunch of boolits as usual. Sometimes this exercise would convince me that the surface imperfections didn’t interfere with mould function and were cosmetic only, so the lapping plans were tabled for that mould. Otherwise, I would select a couple of the least bad castings to make the laps.

I’ve never had any luck casting a boolit around a screw or nail or whatever. I put the castings in a collet chuck in the lathe, drill a hole in the base and tap it with a coarse thread tap. I run the tap in and leave it in there for the extension. I roll the boolit in Clover 320 grit and chuck the tap in a variable speed electric drill, set to as low a speed as I can get. 30 RPM is plenty.

I put the mould (minus sprue plate) in its handles, put the boolit in the mould, hold the closed handles lightly in my left hand with the drill in the right hand, and pull the trigger. The tap usually goes in another partial turn and then the boolit starts turning. I hold the handles just tight enough not to stop the turning and let it run for maybe 10 revolutions, then I open the mould, set the drill down and wipe the faces and cavity with a cloth. The excess abrasive and a few lead splinters pry the faces apart, and if left there, will give you an oval boolit pretty quickly.

If the few revolutions have taken care of a visible problem, I stop there, clean the mould and cast a few boolits to check. If I can see that the problem is still there, the lap goes back in and again I hold the handles in left hand and trigger the drill in my right. I squeeze a little harder this time, and run the drill for maybe half a minute. Generally, this time I just wipe down the mould faces, reinsert the lap and run it for a minute, squeezing the handles down tightly. By this time, the lap should spin freely with the blocks tightly together. After another minute, I wipe the faces, inspect, and may go on for another minute, in extreme cases.

By the way, even with this method of centering the lap extension, I always feel a little runout as the lap spins, and try to hold the mould so it follows this wobbling rather than resisting it. Casting around screws, or drilling in a drill press for the lap attachment would probably cause even more wobble, so I don’t see how a rigid setup for the mould would help.

At this point, I figure the original lap is done with. The extra castings were just insurance in case I mangled the first one. The mould gets cleaned and a casting session follows. I’ve only done iron moulds, so a break-in period is needed before I can evaluate any real improvements. But if the cavity is now shiny without dings, rust spots or tool marks (or whatever the original issue was), I figure it’s done. If further enlargement is needed, I again pick a few of the best castings and repeat.

Any serious enlargement (0.002” or more) will take several of these lapping and casting sessions and probably will make the cavity out of round, but a slight amount of this generally doesn’t matter that I can see. Often the cavities are out of round to begin with, and, if undersized or damaged, are useless anyway.

I have used the collet to turn away areas on the boolit that I don’t want enlarged, such as when I want an undersized nose to ride on the lands but don’t want the driving bands enlarged. I turned a grossly-disproportioned 311291 to a perfect fit in my Dad’s Arisaka 7.7 by this method. Undersize gas check shanks can also be enlarged this way, by turning off the driving bands and most of the sides of the nose, leaving the end to guide the lap in the cavity.

For a multicavity mould of all the same boolits, I would probably select the biggest diameter casting, lap that cavity, and then use the same lap for progressively smaller diameters. If it’s just to remove surface rust and minor pits, a half-minute and then a full minute of slow spinning, as described above, should do the job.

[Grmps]

http://www.castpics.net/subsite2/Mol...umentation.pdf

I've had to lap several Lyman 4-cavity molds (new molds, looked like they had only been used once or twice and sold for pennies on the dollar) to get all the cavities the same size (some were 5 thou or between the largest and smallest )

Cast several molds full of the best boolits you can out of the hardest alloy you have.

mark the boolits with the mold cavity they came out of so you know which need the most attention

Choose the largest/best boolits and drill a 1/4 inch deep (depending on boolit size) hole just large enough to accept a small bolt/screw (you want the bolt/screw to be snug but you don't want to expand the cavity) (the leementing PDF shows pouring the cavity through a nut)

I use a fine valve grinding compound (courser than metal polish) and a fine metal polish "Fitz"

Some like to use a hacksaw and cut a couple grooves in the sides of the boolit to hold the compound, otherwise, you need to clean the mold face often so it will close completey around the boolit when the compound squishes out.

This is a long slow process (I spin mine slowly with a drill keeping it perfectly lined up.

Start with the worst cavities and the courser compound. Spin 15 to 60 seconds with your chosen method on the worst cavities (Keep track of what/how you do it)

Clean the mold, cast the best boolits you can then compare them visually and weight wise for progress over the amount of time polished with which compound. This will give you an Idea of how long with what compound and your method will remove how much material from the cavity.

Start slow, learn what your method and different compounds do and go from there.

Like previously stated, YOU CAN"T PUT MATERIAL BACK ON

There is much satisfaction when you get all the cavities clean and dropping the same weight.

[jsizemore]

I've found the valve grinding compound to be too aggressive. Lapping, polishing compound and toothpaste at a slow rpm or by hand is best. I use one of these after drilling a 1/8" or smaller hole in the base of the boolit:

https://www.dremel.com/en_US/product...es/401-mandrel

It's the one that's used with the acorn shaped polishing felt.

[Grmps]

▲▲▲ I like that, can't go too deep ▲▲▲

[FISH4BUGS]

Many thanks to all that responded. After reading all the responses and looking at the issue, I think i will try the Dremel Tool with the 401 Mandrel. I understand the options but I think i'll start there.
I have the Dremel but will order one of the 401's. I googled them and shipping will cost more than the mandrel!
First and foremost I will test out the moulds to make sure that they are not OK as is. If the bullets stick or they have surface imperfections, I will go the Dremel route to clean them up.
If that does not work then I will try some of the other options.
Many thanks to all for responding, and for the level of detail. I understand much better now!
I'll let you know the results. It may be a while, for I need to make a real living in all this hobby fun!
It will be great to see these beauties back in service!

[Petrol & Powder]

I don't think jsizemore meant for the Dremel 401 shank to be used with a high speed Dremel tool. It's just a convenient shank to use as a lap stem for a slower speed tool.

And if you need that 401 shank, Lowes generally sells all of the Dremel accessories, I'd check there first. Might save you the shipping cost.

I'll also second the opinions that valve grinding compound will likely be too aggressive/coarse.

Good Luck !

[FISH4BUGS]

Interesting.
I called Home Depot and they only sell complete accessory kits for something like $20.
I will call Lowe's when I get back into the office. Thanks for the tip.
My Dremel has a very slow speed, but CAN get right up there in RPM's. I thought that might work at the slowest speed.
Maybe the "doing it by hand" thing IS a better idea.
Glad you pointed that out BEFORE I started doing this!

[jsizemore]

I'm sorry. I should have given a better explanation. I chuck the mandrel in my battery drill which can go as low as 30 rpm. While turning the boolit in the mold with lapping, polishing compound or toothpaste, I hold the handles in my hand my left hand and the drill in my right without my arms, wrists or hands braced against anything. That way the mold and drill can float. Any misalignment is mitigated by allowing both to float. Slower speed and smaller grit is always better. If the mold and drill are wobbling too much, use another boolit/lap. Slow and easy. Same technique as neck turning brass. I clean the compounds after each turning with q-tips and denatured alchohol.

[Grmps]

Dremel Tool 401 Mandrel

Price: $2.69 Free shipping for Prime members when buying this Add-on Item.

https://www.amazon.com/Dremel-401-Ma...el+401+Mandrel

HomeDepot Dremel
1/8 in. Shank Rotary Tool Accessory Mandrel for Polishing Bits
4.0 out of 5
(1) Write a Review
Questions & Answers (1)
For use with Dremel rotary tools (sold seperately)
Supports polishing bits such as the 414, 422 and 429 Dremel bits
Screw a polishing wheel or tip onto the mandrel's threaded head
$2.69 /each

[JonB_in_Glencoe]

I use a 'sheetrock' screw, with the Head cut off for a boolit mandrel. I spin it with a battery drill.

I use automotive "Rubbing" compound (the red stuff, it has Rouge in it)

I prefer to use boolits cast from a cold mold, the wrinkles hold the rubbing compound.

I use one boolit per cavity, after each cavity, I clean the mold faces before spinning a boolit in the next cavity.

This may be obvious, but I time the spinning...so the time is the same for each cavity.

[Walter Laich]

I have a slightly different way to cast the bullets

I take a 3/4" nut and center it over the cavity AFTER moving the sprue plate out of the way

this gives me the bullet tied to a nut that can be turned by a wrench or a ratchet.

be careful getting the valve compound on the bottom of the nut as it will make circles on the tops of the cavities. I can push these sideways as I turn them in the cavities to put pressure when I think it needs it (seam area).

only done this on aluminum molds--does a great job and haven't opened any up too much

[dkf]

I cast many bullets to lapp with. I put the bullet back in the cool mold, mark and drill/tap the bullet base for a screw. Think I used an 8-32 pan head phillips last time. I use an actual tap drill and a tap for cutting the threads in the bullet.(as straight as possible) Obviously you do not want to drill too deep and drill through the bullet and hit the mold. You only need about 3/16"-1/4" of threads for it to hold good without stripping. Then I put the lapping compound on the bullet and spin it with the cordless drill.(proper bit to fit the screw) I time how long I lapp each cavity to try and keep it consistent. I have the handle on the mold to keep the halves closed and to adjust pressure. McMaster sells small 2oz. vials of lapping compound in various grits, I have them in 180-600 grit. I'd start with 320 for a short time per cavity and see how much it cuts. Iron molds tend to want to lapp out of round in my experience. You just want to clean up the rust so stay with fine lapping compound. You can slightly expand the lapp and help compound retention with some skive marks in the bullet from a sharp cutter/knife. Iron goes a lot slower than aluminum so you have a little bit of time to figure it out before you start screwing up a cavity.

[Cosmic_Charlie]

You know, I'll bet you could tweak a mold this way to drop bullets that were perfect for a particular rifle without the need to size. And also to get all the cavities evened up.

[country gent]

That's what the process was originally for, many many years ago manufacturering wasn't near as accurate. Barrels varied .002-.004 in dimensions, moulds varied also. So you either bought moulds till you got one to fit, sized a to large down, or lapped a mould out to here you needed it. The process was used by mould manufactuerers as a means to finish the moulds. Benefits of a PROPERLY lapped mould are bullets dead to size, rounder truer bullets, bullets that drop easier, and better finishes. Modern machining technology has made this much easier to accomplish from the machine.

Still when working with some of the old firearms bores are on the larger size, throats may be "big" and leades may be eroded. A mould lapped to fit here can make a big difference. Trapdoors are a good example as some need a .463-.464 bullet to seal the grooves and fully engrave. Its a tool for use when things arnt going to work otherwise. Its a process that has been used for 100s of years to accurately finish flat surfaces, fit parts together, Holes, and cavites in moulds. not a fast process but very accurate.

[FISH4BUGS]

I put some of the valve grinding compound between my finders and rubbed them together. VERY gritty. I think one poster had it right - TOO AGGRESSIVE!
I'm going to get some finer stuff and see how it goes.

[country gent]

To just polish and clean up try a little tooth paste, nu finish ( I have been using it to final polish brass copper aluminum and steel with good results), Flitz, Simichrome, J*B bore paste. For what youwant you don't really want any real metal removal just to polish the surfaces nicely. all of the above will do that