Lapping Aluminum Mold

Hi John

Here is a old post from the CB-L Yahoo list By Steve Hurst I saved, maybe it will give you some more idea's on lapping.

Steve used to contribute lots of articles for the Cast bullet

Lapping moulds I enjoyed the recent thread on lapping. I have lapped a few moulds here are some more ideas.

It is quite common to find the nose diameter to small on mass produced molds. If the nose diameter is too large, the bullet will not fit, it comes right back to the manufacturer with a nasty letter in the box. Too small and it will fit and shoot and even though it shoots poorly this is OK with lots of folks. To avoid this, most commercial mould makers plan to error on the small side.

Believe me, even custom mould makers do not get it right the first time. I have returned more custom moulds than mass produced moulds, when I pay $$$ I expect diameters as specified. Lapping a commercial mould will get you a mould to cast correct diameter bullets with your alloy for your rifle or pistol, often for a lot less money than a custom mould.

Many cast bullet shooters “bump” bullets in a lube sizer to expand the nose diameter to fit the bore. I find this a tedious procedure that also limits the hardness of the alloy you can use. I prefer to modify the mould so the bullets are the correct size with whatever alloy I want to use. Here is how I do it.

Make a dozen well filled out bullets of hard alloy like typemetal for each cavity. I have tried soft alloy bullets for lapping and they do not work well. The soft alloys are less effective; they cast a smaller bullet and wear very quickly. You need a well filled out bullet, so make sure you heat the mould blocks to full operating temperature. This means you may have heat the blocks or cast 50 or more bullets. What the heck, you can shoot the leftovers.

The best tool for honing is a small “T"-handle tap wrench and taps. These are available in a small set with common screw size taps and the correct drills for a few bucks. Do not be cheap; the tap wrench is a superior tool. I know; I have tried other methods of turning the lap. The tap handle makes it much easier to control the lap and avoid off center pressure that can cause an egg shaped cavity. Pick a tap appropriate to the size of the bullet, with the outside thread diameter smaller than 25% of the bullet diameter.

I read the recommendation in the other post about using Allen wrenches. Having the handle out on one side is exactly what you do not want. Been there, tried that with a screw and vice grips. The handle on one side makes it easy to get off-center pressure. If you want to go cheap, get a brass wood screw, use small round file to cut the head slot half-round. Solder in a small piece of brass rod. This will get you a “T” handle that is easy to use.

You need to drill a hole in the base of each lap bullet without damaging it. Do NOT do this in the mould blocks; the chance of damaging the blocks is too great. A lathe and collets will do a perfect job. However, without a lathe, put two hard wood blocks in a vise or “C” clamp, Drill a hole slightly smaller than the bullet diameter between the blocks, use the blocks in a
vise or “C” clamp to hold the bullet. Discard the first bullet clamped in the blocks since it may distort as the wood forms to the bullet's shape. Check the second bullet with your calipers or micrometer to make sure the pressure used does not distort it. I do not have a lathe; I use the wood block method.

Drill a hole in the base of the bullet with the correct drill, use tapping lube or WD-40 on the drill to prevent the drill from sticking. If you get the hole off center, throw that bullet away and try again. Now you know why you needed a dozen bullets. A drill press is nice, but there is a cute little drill alignment tool that makes the job easier. This is just a folded piece of metal that slips in next to the drill. Freehand works, but you may need the whole dozen bullets. Drill the hole as deep as you can, you need at least 5 times the diameter of the tap for a good grip.

Turn the tap into the hole, the bullet diameter should not increase when you turn in the tap or screw. Taps are better because there is a place for the metal to go. This is usually not a problem with typemetal, but it can be with softer alloys. If you expand the base of the lap, you will get a tapered bullet. Check with your micrometer.

For iron moulds I use Clover brand 220 grit carbide in a grease base. For aluminum moulds, a 320 grit carbide or aluminum oxide works better. Aluminum is softer and cuts faster and the finer abrasive slows the cutting and gives better control. Aluminum oxide leaves a smoother cavity in aluminum, but I cannot see any difference in the bullets. I sometimes use a smaller grit abrasive, 320 on iron and 400 on aluminum. I want to go slow, so I have time to do it right. YMMV choose a larger grit if you want. Grits over 400 mesh cut very slowly.

Examine the lap, if there is a visible seam at the part line or whiskers from the vent lines, you will need to trim this off with a sharp blade. Apply a small dab of abrasive to the lap and smear it over the lap. Do not use too much, the amount of abrasive needed is very small.

Hold the blocks in set of correct mould handles, preferably made by the maker of the blocks. It should not be necessary to say this, but I know people that try to use one set of handles for blocks from various manufacturers. If you lap a mould in incorrect handles, you can permanently damage the mould. The blocks should be loose enough so that they can freely align on the pins.

Insert the lap into the cavity and close the blocks. It will help to align the part line on the lap 90 degrees to part line on the blocks. Turn the lap while slowly closing the blocks; adjust the closing pressure on the handles to get slight resistance to turning the lap. As long as there is resistance on the lap, it will continue to cut. Turn clockwise several revolutions and then reverse your direction for the same number of turns. Try not to put any sidewise pressure on the tap handle, it is best to work with the handle vertical. Open the blocks and remove the lap. Remove all material from between the faces of the blocks so that they can completely close. It is normal to find lead shavings from the lap along with compound forced out between the blocks. Reinsert the lap and repeat, you may want to add a tiny bit of compound. When the lap turns easily, with the faces of the blocks clean and firm pressure on the handles, stop. If doing a multiple cavity mould, repeat in the other cavities using a fresh lap in each.

After the lap turns easily, clean the cavities, and check for signs of progress. On iron moulds, “white” spots inside the cavity indicate that you have removed metal. Un-lapped portions will remain blue. Iron moulds have a blued surface that indicates where the lap is cutting. Aluminum blocks do not, so you should blacken the cavity in aluminum moulds with a Magic Marker to get the same effect. If you see an un-lapped section on one side, put the lap back in and favor that side with gentle pressure.

If the un-lapped area in the backs of the cavity, away from the mold face, you probably have not kept the faces of the mould clean. Another cause is failure to keep firm closing pressure on the handles. Continue to turn the lap until firm handle pressure does not give resistance to rotation. You might have to favor the backs of the cavities. Stop when you have completely removed the colored surface.

Generally, a “white” cavity in an iron mould will cast at least 0.0005” larger. You can usually use another lap and perhaps a third in iron moulds before the cavity gets too loose on the lap. In aluminum, you will get about 0.001 to 0.0015"-diameter increase with one lap and a “white” cavity. You will probably need to stop and cast a new set of bullets after one lap in an aluminum mould. If you use a loose lap, you can end up with an egg shaped cavity. Stop and make a new set of lapping bullets.

Between laps, or after you cast a new set of lap bullets, clean the mould and use Magic Marker to blacken the mould cavities. On iron moulds you can use Magic Marker, or one of the chemical bluing solutions to color the cavities. This will let you keep track of where the next lap is cutting.

You can lap one band or the nose by applying compound to only that portion of the lap. Do not use much, as it will tend to spread out as you turn the lap. When lapping one band, change the lap and clean the cavity if the abrasive spreads out. Use a new lap with less abrasive. You may find it better to apply some grease to the rest of the bullet to make it turn easier if you are only trying to lap a small part of the bullet.

When I first lapped one section of a mould, I cut off the other part of the lap. This turned out to be a bad idea; it was difficult to control the short lap. I could not keep it centered and the cut was out of round. It was much better using the whole lap. The portions of the lap that were not cutting helped to center the lap. I was able to correct the mistake I made with the short lap.

I have not tried to even out the cavities in a multiple cavity moulds, all my moulds have had well matched cavities. The idea that you can make multiple cavities similar by using a bullet from the larger cavity to lap the smaller one sounds OK, but I have not tried it. After I get the size right, I try a fresh lap gently in each cavity in multiple cavity moulds. If a cavity feels tighter than the others do, I carefully turn the lap a few times in it and re-measure with a fresh lap. Using this procedure, I have not been able to measure any differences in the bullets cast in different cavities after lapping.

I have lapped over 10 moulds to increase their diameter, including several four-cavity moulds. I have lapped three aluminum moulds. I have increased the gas check diameter on Lyman moulds to get a tight fit in the gas check. I have increased the nose diameter on bore ride bullets so the bullet nose matched the rifle bore diameter. The largest increase was 0.003” the bullets were perfectly round as far as my micrometer could measure. I have not ruined a mould, all the lapped moulds work better.

This is a slow process, it took an afternoon to do a 0.0015 increase in a two-cavity iron mould, two laps, stop and cast new laps, lap again. Doing a 0.001 increase in two four-cavity moulds took most of the day. If you are in a hurry, you can ruin a mould. Using a drill to turn the lap is a recipe for disaster. This is a hand job where feel is important. Go slow, stop and measure, look and learn and you will be OK.  

Good luck-----J