John Alexander
posted this
23 February 2015
As anybody following this thread knows I received recommendations for lapping compound from 220 to very very fine for my aluminum mold and warnings about overdoing it. So I tried something in the middle -- 400 and went easy. This stuff is from the fire lapping kits of lapping compound that Merrill Martin passed out to interested shooters willing to try fire lapping and report the results back 20-25 years ago.
I tried one lap for about 40 turns, stopping and redoing the knurlng and reloading with lapping grit twice. This involved stopping after a few turns (two or three times) and cleaning the mold faces until the faces closed completely. Washed with CRC (clear) brake cleaner and little tube to get it to the right place under pressure swaged with clean q tip and pressure washed again. Bullets had some wrinkles that most would discard but gave reliable measurement on several bullets. I could see no diameter increase from the bullets I had cast earlier for a control either in line with the parting line or perpendicular to it.
Second try, this time with 220 grit. Same procedure, almost same result almost -- maybe .0001-2” diameter increase.
Bullets for laps have a hardness of 13-14 by LBT test method and relieved by lathe turning everywhere except the parallel nose which I would like to have larger and the nose and rear band for bearings to keep the lap straight. My results so far are radically different that what others have reported for aluminum molds as being easy to remove material. Maybe NOE uses a bullet proof aluminum alloy and heat treat? Could my gritty feeling lapping compound not be cutting. I will keep at it but so far I can't see that this aluminum mold is any easier to lap than the steel and iron ones I have lapped in the past.
Any and all suggestions appreciated.
John