How coarse of abrasive to lap out the drive bands?

[RU shooter]

I have a rcbs DC mould I'm considering with much anxiety to open up the drive bands about 3 thou. I've polished up a few alum. Lee moulds but never actually removed metal before . What grit abraisive should I be looking at to attemp this ? I have some garnet abraisive at work it's 80HPA not sure what that equates to in normal lapping grit it's like very fine play sand . Too coarse? Also very worried about getting the cavities oval how to I prevent this .

thanks , nervous Tim

[btroj]

Think of it this way- how are you gonna put it back if you take off too much?

[RU shooter]

Think of it this way- how are you gonna put it back if you take off too much?
Exactly Sir. That's why I'm asking for guidance and advice . Im not to proud to admit what I don't know or have experience with . Is there any places or people that offer this kind of service of lapping out a mould ? I'd gladly pay rather than ruin a nice mould .

[DougGuy]

80 grit is VERY coarse. It would put scratches in the mold surface that no amount of polishing would take off until you gained about .015" on the size of the mold. If you want to go slow, then go fine. Many of the better auto parts stores sell valve grinding compound in coarse and fine, see if you can get some of the fine as this will cut rather slowly. Well, the smaller and finer the edge you have, the faster this fine compound will cut, so go slow at first and check often to see where and how much metal is being removed.

For polishing or finishing, Meguiar's makes auto finish abrasives in quite an array of grits for fine polishing, swirl remover, etc, and these work well on metal. Their #4 is fairly coarse and will cut quickly, #7 is fine and will polish nicely, #9 is very fine and will have the surface of whatever you use it on, reflecting your face if you polish it long enough.

The way to lap something, is to make a custom size or shape piece of aluminum or brass in a lathe (or good drill press) and use this in a drill press along with fine compound, close the mold halves on it with light pressure and turn it fairly slow maybe a dozen turns then clean it all out and measure the mold.

If you want to send the mold this way, send me a PM, I do have a decent lathe and all of the compounds I mentioned.

[RobS]

I am not familiar with the High Performance Alluvial Garnet Abrasives line however I would start with something in the 200 grit lapping compound area and then work toward 320-400 grit to finish. Go slow and check often. Wheeler makes lapping compounds as does LocTite Clover

[captaint]

You can contact Eric at Hollowpoint Mold Service. He opens up mold bands for a fee, of course. It ain't cheap, bet he does a terrific job. With a 2 cav mold, 2 bands each boolit it shouldn't be out of line, cost wise.

[Char-Gar]

I have not lapped out a mold, but if I were going to do so I would use Clover 320 grit paste to start and when within .0005 of where I wanted to be, go to Clover 600 grit paste. I would cast a bullet with a screw in the base to turn it and file some grooves in the bullet to hold the emery paste.

I sure would not use 80 grit anything.

[tomme boy]

I have used valve lapping compound from the auto parts store. Comes in a small tube and is a paste. Not sure what grit it is. I have done only LEE moulds.

[texassako]

I have lapped several molds for various reasons including enlarging drive bands because I seem to accumulate other peoples abused molds. I use the automotive valve lapping compound for steel molds like your RCBS. I clamp the blocks together to keep grit from getting between the block surfaces and causing oval cavities. I lap by hand and count revolutions so I can keep all cavities close to the same. Keep in mind that it is going to cut everywhere your lap touches including the nose, and you can alter your bullet to lap what you want as long as you still have enough supported to not wobble when turning. I think Eric only does base bands now.

[ph4570]

I start with 320 and finish with 600.

[Char-Gar]

Brownells sell small containers of lapping compounds in three grades. The coarsest is 600 grit, followed by 800 and 1,000 grit.

When I lap the forcing cones of revolvers, I use Clover 320, clean and finish with Clover 600.

[Bent Ramrod]

I also use Clover 320. Three thousandths is a lot of metal removal, though. Go slowly, and re-break in the mould and cast some test boolits in between lapping sessions. The test boolits will be your new laps, if you need to continue.

Keep the abrasive paste and lead fragments out of the gap between the blocks and by all means avoid any off center condition in the turning of the lap. I put the lap in a collet in the lathe and drill and tap it to keep everything centered. The lap should turn pretty slowly, maybe 100 rpm max, and a minute per session is all I do before a test casting. Generally, I have to grip the block handles rather loosely at first, but once the lap is spinning freely in the cavity, I squeeze as tight as possible for the minute. Those over center handles that clamp might be handy for this but I either give it a death grip on the handles or use a vise sometimes. Even so, the more metal I try to remove, the more likely the cavity will become oval.

[country gent]

Lapping is a slow proocess that can be done easily. start with casting several good bullets from each cavity keeping them seperate so you know which are which. drill a small hole for a stem in the base of each bullet for a tem or screw, I perfer a small stem of 1/8" - 3/16" dia. With a bullet from number one cavity coat with 320-400 grit lapping compound and carfully insert into #1 cavity closing blocks around it. with a tapping tee handle turn back and forth 3-4 times and rotate 1/4 turn till 2 full rotations are made. move to second cavity and repeat. With a bullet cast from #1 cavity again coat and insert in #2 cavity work as before the repat in #1 cavity. check sizes. Go to 600 grit or so and with bullet from #2 cavity start in #1 cavity and work to rotations repeat as before. Cast a few bullets to check actual size and proceed as needed. I have used tooth paste, flitZ, or simichrome paste to finalk finish in this manner also.

[beagle]

I have done this on several moulds. Got to have a DP or lathe. Go down to Ace Hardware and buy several 1/4 bolts with a long shank. Cut off threaded portion. Turn head to the band dimensions you want. Use this mandrel, very precisely set, to open the bands. The steel screw is harder than the iron RCBS uses in their moulds and will take it out much slower than valve grinding compound which tends to round band edges./beagle

[longbow]

Let's back up the bus just a bit.

You say you want to open up the drive bands... not the nose? Very difficult to do lapping unless you have a lathe to turn lapping slugs. I guess you could very carefully cut the nose off then lap but boolits are awkward to hold without damaging.

Now if the nose doesn't matter then it makes life easier. However, speaking from experience, lapping out more than a couple thou from an iron mould is work and it is difficult to keep cavities round.

Some good info here:

http://castpics.net/dpl/index.php/the-library/moulds/23-modifying-molds

Yes, 80 grit is way too coarse! I use valve lapping compound which if I read the can right runs about 425 grit. Pretty fine but not polishing grade and it left my NOE mould with a noticeable mat finish compared to how it arrived but I needed 0.316" boolts not the 0.314" the mould was made to cast so I took a chance and it worked well. If you can get a finer grit to finish it will do a nicer job.

Anyway, I have lapped several aluminum and iron mould with valve lapping compound. Taking out a thou or two is not too hard but the cavities do tend to go oval when going much beyond 0.002" if you are not careful. Clean, lap, clean, check, lap, clean, check, lap, until you reach the size you want. Once lapping slugs get loose it is time to cast more with the "new" cavities. Note all the cleaning. If you don't clean often and carefully grit gets between the faces and so holds the mould halves apart a bit. My experience anyway.

So, yes it can be done but it is work and you do risk ruining a mould if you aren't careful.

Have you thought about Beagling? It is about the simplest mould modification you can do and 0.003" is doable with Beagling:

http://castpics.net/dpl/index.php/the-library/moulds/23-modifying-molds/308-beagling

Think about what you want/need then go about it informed and prepared. Lots of good info on this site.

Longbow

[RU shooter]

Thank you all for your advice and sharing your knowledge, I shall look for the correct grit abrasives you all have mentioned and give it a try when I have the time.

Thanks ,Tim

[detox]

Practice on one cavity only. You can screw up a good mold quickly when learning or when you think you know everything?

Oval is caused by the mold not closing all the way when lapping. Be sure mold is shut tight with no compound in between flat mold surfaces to prevent closing. DO NOT lap gas check area. I use Wheeler 220 grit compound and light Remington oil to help cut better. Polishing is not needed after opening with 220 grit.

[.22-10-45]

I have use Clover 320 & finished up with 600..attempting to make a 49gr. Lyman 225415 round and increase dia. .002" Well, driving bands were now round & dia. increased..However, I realized the grease grooves were still egg shaped..shot a bit better..but starting out with proper dimensions is best way to go.

[bhn22]

Here's my secret: Wheelers Bore Lapping kit from Midway. It includes 220 grit, 320 grit, and 600 grit lapping compounds, as well as steel plates for it's intended function of rolling the compound into bullets for fire lapping. One kit could conceivably last a lifetime. Moulds aren't as hard as rifle barrels, and I certainly wouldn't start out any coarser than 220 grit. You could put a really light coating of grease on the bands you don't intend to lap, as well as the nose areas. Beagle is correct of course, you do risk rounding off edges that you don't intend to.

[Ben]

I use this exclusively.
Works well for me :


http://img.photobucket.com/albums/v494/haysb/PICT0001-22.jpg


http://img.photobucket.com/albums/v494/haysb/PICT0002-16.jpg


Here is the coated bullet after it has been spun in the mold cavities :


http://img.photobucket.com/albums/v494/haysb/PICT0003-17.jpg

[JonB_in_Glencoe]

I have a rcbs DC mould I'm considering with much anxiety to open up the drive bands about 3 thou. I've polished up a few alum. Lee moulds but never actually removed metal before . What grit abraisive should I be looking at to attemp this ? I have some garnet abraisive at work it's 80HPA not sure what that equates to in normal lapping grit it's like very fine play sand . Too coarse? Also very worried about getting the cavities oval how to I prevent this .

thanks , nervous Tim
if you haven't read this sticky yet...read it, you will be glad you did.
http://castboolits.gunloads.com/showthread.php?257858-Expanding-a-boolit-mould-the-quot-WEET-quot-method-(In-parts)