The portion of the book, "Cast Bullets for Beginners & Expert" by Joe Brennen Jr., was written by Ric Bowman.
There are several reasons to lap bullet moulds: new moulds with machining errors and burrs, corrosion and stains, equalizing volumes of multi-cavity moulds and casting problems. Other than for casting problems, most lapping has been done for cosmetic appearance and for match bullets, made as round as possible. I think everyone loves to make beautiful shiny bullets with zero runout, but it doesn’t make them shoot much better for most of us.
I have never been able to make moulds cast bullets larger than 0.002 inches greater than originally cut, and still keep them round, by lapping. If you need a bigger bullet, buy a mould that casts a bigger bullet as manufactured. However, lapping can make bullets drop from the cavity easier and remove baked on material from the surface of the cavity.
Lapping is not a “spur of the moment” decision, but takes some planning and forethought. The first thing you will need is ten of your best quality bullets cast and separated for each cavity you are working with. Wheel weight alloy is just fine except for polishing, and then linotype alloy is probably best, as bullets are larger, harder and more closely conform to the cavity. The three most useful lapping compounds are “Clover” brand automotive valve lapping paste in “fine” grit, Brownell’s 800-grit water based lapping paste and “JB” bore paste or “Flitz” polishing cream. (See picture)
To drive the lap, I use a small sheet metal screw that has a quarter inch head with a socket and “T” handle wrench. A small electric screw or nut driver may be used if you are careful and check your work often.
There are several reasons to lap bullet moulds: new moulds with machining errors and burrs, corrosion and stains, equalizing volumes of multi-cavity moulds and casting problems. Other than for casting problems, most lapping has been done for cosmetic appearance and for match bullets, made as round as possible. I think everyone loves to make beautiful shiny bullets with zero runout, but it doesn’t make them shoot much better for most of us.
I have never been able to make moulds cast bullets larger than 0.002 inches greater than originally cut, and still keep them round, by lapping. If you need a bigger bullet, buy a mould that casts a bigger bullet as manufactured. However, lapping can make bullets drop from the cavity easier and remove baked on material from the surface of the cavity.
Lapping is not a “spur of the moment” decision, but takes some planning and forethought. The first thing you will need is ten of your best quality bullets cast and separated for each cavity you are working with. Wheel weight alloy is just fine except for polishing, and then linotype alloy is probably best, as bullets are larger, harder and more closely conform to the cavity. The three most useful lapping compounds are “Clover” brand automotive valve lapping paste in “fine” grit, Brownell’s 800-grit water based lapping paste and “JB” bore paste or “Flitz” polishing cream. (See picture)
To drive the lap, I use a small sheet metal screw that has a quarter inch head with a socket and “T” handle wrench. A small electric screw or nut driver may be used if you are careful and check your work often.
Let’s talk about mass produced aluminum moulds first. One problem is that they are cut with the mould halves compressed tightly together. No one can hold the handles together as tightly as the fixture that was used in manufacture. This is the primary cause of out-of-round bullets with these moulds. In addition, the manufacturing process doesn’t always have the sharpest tooling and best chip removal process and that leads to burrs.
These burrs hold the bullet into the cavity. The other problem is that unless you are using a perfectly clean pot, ladle and alloy, every bit of dirt, slag and carbon seems to stick to the surface of the mould cavity. It not only makes ugly bullets, they are not in balance, and bullets stick to the mould making you hit the handle joint trying to get them to drop. While powdered graphite or mould preparation compound can coat the cavity, it doesn’t last all that long, besides occupying space that should be part of the bullet.
All aluminum moulds are soft and easily cut with lapping compounds. That is both their strength and their weakness. Very easy to get results but very easy to mess up and ruin also. When I get a new or a “new to me” aluminum mould, I clean it with hot water and automatic dishwater soap and a toothbrush. I scrub until clean and then hold it under the hot water tap, scrub until all the soap is gone, and about two minutes more. I dry it with a hair dryer I bought at a yard sale. Then prepare it for casting using what ever process the manufacturer suggest. When it starts casting well filled bullets separate ten good ones. Out comes the micrometer for measurements. Are they round? Now put them on an accurate scale, are they a consistent weight? Did they release from the cavity easily? Out comes the five-power loupe to look for flaws, burrs and problems. If the decision is made to lap here is how I proceed.
Remove the sprue cutting plate if possible, if not, you will have to work carefully not to let it get in the way and to make sure nothing get under it to mar the top of the mould. A bullet is placed in the cavity and a 1/16-inch drill bit is used to make a pilot hole as close to the center and as close to vertical as possible. I used to do this in a collet with a drill press, but found that doing it carefully by hand worked just as well. Do not let the bit come close to the bottom of the grooves or touch the mould in any way. It just needs to be deep enough to be the pilot for the screw. The screw needs to have a small shank and only needs about three threads deep to hold it tight. Place the bullet into its cavity and tightly close the mould. Start to screw the screw in the bullet with the socket. As it goes in it will expand the bullet. Now you will have to adjust the size of the pilot hole or shank size of the screw so that you do not deform the bullet. Increase the size of the hole until you can get the screw firmly into the bullet without expanding it too much. This is a “feel” process. You are not trying to hold the earth together, just enough to keep it straight in the bullet and rotate the lap.
Coat the lube grooves, driving bands and bore riding part of the nose with lapping compound. A volume of about half the size of a pea in plenty. Try to avoid the end of the nose. Place the bottom of the mould blocks on a flat surface such as a Formica workbench top, steel plate or square of plate glass. This will help align the mould blocks as the pins or guides will not be in full contact as they are closed. Insert the bullet into the cavity and hold it firmly together with the handles.
Now the “art” and not the “science” part of the project begins. The objective is to evenly squeeze the halves together, maintaining alignment, just tightly enough for the lapping compound to cut without jamming the bullet. I have used both a helper with an extra set of hands and small rubber bands evenly applied around the mould blocks. When turning the screw do not apply any more downward force than necessary, just enough to hold the socket on the screw. You are not trying to push the bullet down, just make it rotate. If you squeeze the handles too tightly, the bullet will seize in the cavity. You want it to have good contact to cut evenly, but not to keep turning the screw into the bullet. Ideally about 25 complete revolutions of the “T” handled socket driver will allow you to close the mould with about the grip of a firm hand shake, or the amount of pressure you use to hold the mould close when casting. If you are using the 800-grit compound, the coarsest I ever use on aluminum, you have increased the smallest diameter of the cavity about 0.001 inch.
Clean and dry everything just as at the beginning. Closely look at the cavity, as your goal is to have between a third and a half of the circumference of the cavity showing the different surface texture of having been worked. If it has not been touched, repeat the lapping process with another new bullet lap until you have about two thirds of the surfaced worked. When you reach that point, clean again and cast bullets until you start getting good ones and measure them to your specifications. If they are round enough for you and fall reasonably easy from the aluminum mould, it is time to polish. If not to your satisfaction, use another newly made lap. If you are trying to make the cavity larger in diameter, you must be careful. As the cavity gets bigger, you cannot keep the lap perfectly aligned with the mould and bullets will begin sticking or losing concentricity. If they meet your dimensional needs, it is time to polish.
Polishing is to remove burrs and improve the surface finish of the cavity. Make a new lap and coat it with your polishing compound. JB is commonly used because most shooters already have it for bore maintenance, but others prefer Flitz because it is faster acting. The process is the same as using coarser grit but it takes about 100 revolutions to get an acceptable finish. This is where a small battery powered screwdriver comes in handy because you can check your work after five seconds of rotation. You do not have to make a new lap for polishing, but you must insure that the face of mould is clean so that the halves meet correctly. Laps made from linotype last longer and may be used several times. However if you have increased the diameter of the cavity very much, you will need a lap made from its new bigger size. Again, clean and dry your mould and test for bullet quality. (A quick note on bronze moulds. All modern bronze moulds are made by custom and semi-custom makers. The nature of the material and the quality of manufacture are so good that only once have I ever had to work on a bronze mould. That was to remove one small mark on the center flat of a flat nosed bullet. Once you have used one of these jewels, you will know why so they are so popular.)
Ferrous metals are the traditional materials for moulds and have one great advantage; they are hard, tough and will take more abuse than softer metals. However, they have one large fault, they rust, at times in as little as six hours! To work, they have to be oil free and that leaves them vulnerable to oxidation. I feel the leading causes of failure of the bullet to release from the cavity are burrs and “staining” or shallow oxidation of the surface of the cavity. These are easily removed with lapping. When iron based moulds rust they are both pitted and the surface of the cavity is raised by the creation of ferrous oxide that occupies a larger volume than the underlying material. Even if we remove the bump caused by rusting, a pit will form when the rust falls away from the surface metal. This pit tends to lock the newly cast bullet into the mould and makes bullet removal difficult and lapping can only ease the problem and not remove it.
Let’s look at an iron mould that we are trying to save. Prepare it for casting and make the ten of the best bullets possible. While it is cooling, prepare your first lap. This is where the Clover fine grit comes into play. It is about 340 to 400 grit and it will imbed into your lap. Using your first lap, rotate it about 50 revolutions. Clean the mould in petroleum based solvent like isopropyl alcohol. Closely examine the cavity through your magnifier. If not all of the red colored rust is gone use another lap of the Clover compound. If only light staining is left, you can now proceed to using an 800-grit compound and a new lap. Fifty revolutions will normally even out the cavity surface. Then the polishing compound, another new lap, should get you to a workable finish. To get the best possible finish, you will have to cast a couple of new bullets to use for the final polishing laps.
How many revolutions, or how long do you rotate the lap? This is in the “art” and not in the “science” category, unless you want to make the calculations yourself, as it is to complex for me. Here are the factors: The larger the diameter of the lap (bullet) the faster the outside surface is going at a set number of revolutions. A 45-caliber lap cuts faster than a 30-caliber lap.
The coarser the grit the faster it cuts. The more pressure closing the mould haves, the faster the cutting takes place. The harder or larger the lap, the faster the cutting takes place. An excess of compound does NOT make the lap cut faster. The lap will only imbed so much, and the rest is thrown out by centrifugal force.
The first mould 30-caliber iron mould I ever lapped I used medium grit Clover valve grinding compound and a quarter inch drill motor. I squeezed as hard as possible without running the screw deeper into the wheel weight alloy lap. The motor ran exactly 30 seconds. What a mess when the next bullet was cast from that mould! The nose was 0.0015 larger on one side and the bottom driving band was 0.003 larger on the opposite side. The mould block had to be hit with a mallet to release the bullet. Too fast of rotational speed, too coarse of grit, and lack of control with the drill motor set up some bad vibrations making the lap wobble in the cavity. Since then, I have learned that lapping with a “T” handle driver or low speed battery screwdriver is less error prone.
All aluminum moulds are soft and easily cut with lapping compounds. That is both their strength and their weakness. Very easy to get results but very easy to mess up and ruin also. When I get a new or a “new to me” aluminum mould, I clean it with hot water and automatic dishwater soap and a toothbrush. I scrub until clean and then hold it under the hot water tap, scrub until all the soap is gone, and about two minutes more. I dry it with a hair dryer I bought at a yard sale. Then prepare it for casting using what ever process the manufacturer suggest. When it starts casting well filled bullets separate ten good ones. Out comes the micrometer for measurements. Are they round? Now put them on an accurate scale, are they a consistent weight? Did they release from the cavity easily? Out comes the five-power loupe to look for flaws, burrs and problems. If the decision is made to lap here is how I proceed.
Remove the sprue cutting plate if possible, if not, you will have to work carefully not to let it get in the way and to make sure nothing get under it to mar the top of the mould. A bullet is placed in the cavity and a 1/16-inch drill bit is used to make a pilot hole as close to the center and as close to vertical as possible. I used to do this in a collet with a drill press, but found that doing it carefully by hand worked just as well. Do not let the bit come close to the bottom of the grooves or touch the mould in any way. It just needs to be deep enough to be the pilot for the screw. The screw needs to have a small shank and only needs about three threads deep to hold it tight. Place the bullet into its cavity and tightly close the mould. Start to screw the screw in the bullet with the socket. As it goes in it will expand the bullet. Now you will have to adjust the size of the pilot hole or shank size of the screw so that you do not deform the bullet. Increase the size of the hole until you can get the screw firmly into the bullet without expanding it too much. This is a “feel” process. You are not trying to hold the earth together, just enough to keep it straight in the bullet and rotate the lap.
Coat the lube grooves, driving bands and bore riding part of the nose with lapping compound. A volume of about half the size of a pea in plenty. Try to avoid the end of the nose. Place the bottom of the mould blocks on a flat surface such as a Formica workbench top, steel plate or square of plate glass. This will help align the mould blocks as the pins or guides will not be in full contact as they are closed. Insert the bullet into the cavity and hold it firmly together with the handles.
Now the “art” and not the “science” part of the project begins. The objective is to evenly squeeze the halves together, maintaining alignment, just tightly enough for the lapping compound to cut without jamming the bullet. I have used both a helper with an extra set of hands and small rubber bands evenly applied around the mould blocks. When turning the screw do not apply any more downward force than necessary, just enough to hold the socket on the screw. You are not trying to push the bullet down, just make it rotate. If you squeeze the handles too tightly, the bullet will seize in the cavity. You want it to have good contact to cut evenly, but not to keep turning the screw into the bullet. Ideally about 25 complete revolutions of the “T” handled socket driver will allow you to close the mould with about the grip of a firm hand shake, or the amount of pressure you use to hold the mould close when casting. If you are using the 800-grit compound, the coarsest I ever use on aluminum, you have increased the smallest diameter of the cavity about 0.001 inch.
Clean and dry everything just as at the beginning. Closely look at the cavity, as your goal is to have between a third and a half of the circumference of the cavity showing the different surface texture of having been worked. If it has not been touched, repeat the lapping process with another new bullet lap until you have about two thirds of the surfaced worked. When you reach that point, clean again and cast bullets until you start getting good ones and measure them to your specifications. If they are round enough for you and fall reasonably easy from the aluminum mould, it is time to polish. If not to your satisfaction, use another newly made lap. If you are trying to make the cavity larger in diameter, you must be careful. As the cavity gets bigger, you cannot keep the lap perfectly aligned with the mould and bullets will begin sticking or losing concentricity. If they meet your dimensional needs, it is time to polish.
Polishing is to remove burrs and improve the surface finish of the cavity. Make a new lap and coat it with your polishing compound. JB is commonly used because most shooters already have it for bore maintenance, but others prefer Flitz because it is faster acting. The process is the same as using coarser grit but it takes about 100 revolutions to get an acceptable finish. This is where a small battery powered screwdriver comes in handy because you can check your work after five seconds of rotation. You do not have to make a new lap for polishing, but you must insure that the face of mould is clean so that the halves meet correctly. Laps made from linotype last longer and may be used several times. However if you have increased the diameter of the cavity very much, you will need a lap made from its new bigger size. Again, clean and dry your mould and test for bullet quality. (A quick note on bronze moulds. All modern bronze moulds are made by custom and semi-custom makers. The nature of the material and the quality of manufacture are so good that only once have I ever had to work on a bronze mould. That was to remove one small mark on the center flat of a flat nosed bullet. Once you have used one of these jewels, you will know why so they are so popular.)
Ferrous metals are the traditional materials for moulds and have one great advantage; they are hard, tough and will take more abuse than softer metals. However, they have one large fault, they rust, at times in as little as six hours! To work, they have to be oil free and that leaves them vulnerable to oxidation. I feel the leading causes of failure of the bullet to release from the cavity are burrs and “staining” or shallow oxidation of the surface of the cavity. These are easily removed with lapping. When iron based moulds rust they are both pitted and the surface of the cavity is raised by the creation of ferrous oxide that occupies a larger volume than the underlying material. Even if we remove the bump caused by rusting, a pit will form when the rust falls away from the surface metal. This pit tends to lock the newly cast bullet into the mould and makes bullet removal difficult and lapping can only ease the problem and not remove it.
Let’s look at an iron mould that we are trying to save. Prepare it for casting and make the ten of the best bullets possible. While it is cooling, prepare your first lap. This is where the Clover fine grit comes into play. It is about 340 to 400 grit and it will imbed into your lap. Using your first lap, rotate it about 50 revolutions. Clean the mould in petroleum based solvent like isopropyl alcohol. Closely examine the cavity through your magnifier. If not all of the red colored rust is gone use another lap of the Clover compound. If only light staining is left, you can now proceed to using an 800-grit compound and a new lap. Fifty revolutions will normally even out the cavity surface. Then the polishing compound, another new lap, should get you to a workable finish. To get the best possible finish, you will have to cast a couple of new bullets to use for the final polishing laps.
How many revolutions, or how long do you rotate the lap? This is in the “art” and not in the “science” category, unless you want to make the calculations yourself, as it is to complex for me. Here are the factors: The larger the diameter of the lap (bullet) the faster the outside surface is going at a set number of revolutions. A 45-caliber lap cuts faster than a 30-caliber lap.
The coarser the grit the faster it cuts. The more pressure closing the mould haves, the faster the cutting takes place. The harder or larger the lap, the faster the cutting takes place. An excess of compound does NOT make the lap cut faster. The lap will only imbed so much, and the rest is thrown out by centrifugal force.
The first mould 30-caliber iron mould I ever lapped I used medium grit Clover valve grinding compound and a quarter inch drill motor. I squeezed as hard as possible without running the screw deeper into the wheel weight alloy lap. The motor ran exactly 30 seconds. What a mess when the next bullet was cast from that mould! The nose was 0.0015 larger on one side and the bottom driving band was 0.003 larger on the opposite side. The mould block had to be hit with a mallet to release the bullet. Too fast of rotational speed, too coarse of grit, and lack of control with the drill motor set up some bad vibrations making the lap wobble in the cavity. Since then, I have learned that lapping with a “T” handle driver or low speed battery screwdriver is less error prone.
Picture 2 shows a used wheel weight lap charged with JB compound that has been turned 100 revolutions in a Hensley and Gibbs #119 30-caliber mould to clean out 50 plus years of crud, rust and surface staining. The scrap lead bullet cast from the lapped mould shows acceptable surface finish and falls easily from the cavity.
While I only use single cavity moulds, or one cavity from a two-cavity mould, to make match bullets, others don’t like to do that. Moulds cut with cherries do not always have identical cavities due to
While I only use single cavity moulds, or one cavity from a two-cavity mould, to make match bullets, others don’t like to do that. Moulds cut with cherries do not always have identical cavities due to
the manufacturer not insuring that there are no chips in the cutting fluid and moulds faces meet perfectly at the final cutting. Once you develop some skill at lapping, you can equalize cavities. This is done by alternating laps made from front and back cavities. It is a lot of work, but if careful you can get cavities to cast bullets within 0.1% of average weight over a sample of 10 bullets from each cavity.
If you have the ability to cast decent bullets, you have the ability to lap moulds. Think about what you are trying to accomplish and take your time. Remember it is a lot easier to take more metal out than trying to put some back.
If you have the ability to cast decent bullets, you have the ability to lap moulds. Think about what you are trying to accomplish and take your time. Remember it is a lot easier to take more metal out than trying to put some back.