Lapping a Lyman mold

[Todd S]

I have been trying to lap a steel mold and I'm having a heck of a time. I have done four 6 cavity Lee molds with no problems. The steel mold though, is another story. It takes sometimes 6 or 7 hard whacks with a wood handle on the hinge to drop the boolits. This is a second hand mold that required some attention when I recieved it. There was a slight amount of surface rust on the inside. I scraped it with a wooden chopstick and removed it. When the boolits drop, they look pretty good. Now for the problem. I am just trying to polish the cavities. I was trying to use both car polish and comet. The boolit would get wedged in the mold cavity and would not move. When I pulled the boolit out, it had little wings on it from the cavity edge. One of them actually had the screw drive all the way through the boolit and touch the bottom of the cavity. It just left a very small dot. How do you guys get the mold lapped without the boolit seizing?

[bhn22]

Leave the sprue plate open, and set a couple of (hardware) nuts on top of the cavities. Pour your lead through the holes in the nuts and make sure you have lead remaining in the nuts themselves when you finish pouring. Then use a ratchet & socket on the nuts to turn the laps. I wouldn't use car polish, but would look at perhaps mixing Comet or perhaps Bon Ami with a little bit of oil to make a paste or slurry instead.

[dubber123]

Get the lap spinning with the mold open a bit, and then close the mold around it. It will chew the first lap up pretty quickly, but will take the highest of edges off first. Once the mold closes, open it and clean off the inevitable flakes of lead between the blocks, then continue to lap. I only spin them for 10-15 seconds when they need this, then pour another lapping boolit. Some molds are drilled so far off center you will never fix them enough to drop a boolit freely. Good luck.

[leadman]

I use the lapping compounds from a kit I purchased from Midway to lap steel molds. # different grits of compound. The Comet will work if made into a paste with water but it is slow, which sometimes is a good thing.
Coontrol of the handles is very important when doing this. I hold the handles with a finger in between the handles so I can open the mold if need be, sort of lay the handles in your hand palm up.

[runfiverun]

it sounds more like the cavity's are not aligned.
look closely at the edges of both halves on both ends of the cavitys, if they are mis aligned you can jiggle them back and forth by adjusting the alignment pins.
the front pin will move the right cavity forward and the back pin will shift it rearward.
once you have the halves in perfect alignment then you can polish the cavitys out with some toothpaste and comet, just a whitening toothpaste, or a little with some baking soda.
flitz polish is also good for doing just a polish.

the above advice on lightly holding the handles then squeezing them together once the lap is turning freely is what i do also.
i also make the lap from a pretty hard boolit and roll it in the lapping compound to lightly embed it in the places i want it.

[BABore]

Are you drilling a big enough pilot hole in the boolit before you running your screw in? Measure how much the screw is expanding the boolit. Likely part of the problem. The nut method works better.

[Bent Ramrod]

The abrasives you are using might work OK on aluminum but are not effective on steel, especially steel that might have rust pits or burrs. Get some Clover 320 paste. Use a drill that has the kind of trigger that adjusts the speed; you need to turn the lap slowly until the excess metal and abrasive ooze out between the blocks to be cleaned away. I clean between blocks and close the mould slowly as the lap spins the same way as Dubber, but I take off the sprue plate and every other attachment that I can. The lap spins slowly until it is spinning with the blocks closed as tight as I can hold the handles, then I pull the trigger all the way to full speed. (The high speed setting on my Black and Decker is maybe 150 RPM.)

With real lapping compound, you shouldn't have to spin the lap more than 30 seconds or so at top speed unless the mould is really gouged and pitted up. Properly done, this won't change dimensions but will get rid of edge burrs and small pits quite nicely. You can always stop, remove the lap, wipe out the cavities and inspect the extent you are polishing. The end of a toothpick-full of Clover can be put on the lap and spread around, the lap inserted into the cavity, and lapping resumed, if necessary.

I usually do a lapping run like the above, wash the mould, reassemble it and cast a few boolits to gauge the effect. The lapping will expose clean metal, which will have to have more or less of a break in before you get good production, but a few good ones can be selected from a bunch of ten or so to look at, and it is easy to determine whether they fall out easily or not. If the effect I seek hasn't happened, I do another run with the old lap, or, if I really want a change in dimension, I use one of the freshly cast boolits for the next lap.

[detox]

Rust will make ugly boolits. I removed rust from one of my Lyman moulds using JB Bore paste, 45 cal bronze brush and cordless drill. I coated .357 cavities with JB paste then spun brush inside cavity until rust was gone. There is some pitting in mould, but hardly noticable on boolits.

I wonder if Cryogenically treating a steel mould would prevent it from warping?