Ive been seeing a lot of posts on lapping polishing a mould and what to use so thought Id make a quick post.
Lapping polishing compounds are by nature abrasive and vary as to use and grit. Lapping is a precision finish and can produce sizes to exacting size finish roundness. In practice a soft material in our case lead is impregnated with the compound and becomes a cutting tool. Lead or brass ill cut hardened steel in this manner. In tis earliest uses copper blades with sand cut stone blocks.
the various compounds are
Lapping compounds
Silicon carbide, usually a green color. This is for carbides, steels hardened and soft, Grit sizes run from 80-1000 grit and bought by the grit size. Breaks down to stay sharp and can provide some very fine smooth finishes. It comes dry water based or oil based.
Aluminum Oxide, usually a yellow color. this works best in softer materials aluminum brasses coppers and soft steels. Grit sizes run 80-1000 grit and again bought by grit size. Breaks down slower and is a harder abrasive. it also produces very fine finishes. Comes dry oil based and water based also.
Valve grinding compounds are usually a form of silicon carbide compound oil based and running around 80 grit for coarse and 120 grit for fine. fast metal removal and decent finishes can be obtained but amount of stock removal makes it use in moulds limited.
Diamond lapping compounds. color designates concentration of the diamond. These are used for very hard materials carbide glass and fine polished finishes yellow is the more popular all around here. It takes very little diamond to do a area. with a nylon brush in a dremil it will make mirror finishes.
Simichrome and Flitz are metal polishes that when used as lapping compounds will provide nice finishes with little metal removal
Tooth Pastes are a very light polish that work again for fine finishes and little metal removal.
Comet barkeepers friend and scouring powders I class with the valve grinding compounds as being on the coarse side for this work.
One I have been using to hand polish parts with ( a rag or towel and a dab on it) is nufinnish automotive polish. I suspect this will be similar to the simichrome or flitz maybe even a little finer.
Automotive rubbing compounds tend to be water based. They run in the 1000-1500 grit range and normally are used to remove light scratches from Paints and Clear coats. They work here for fine finishes and little stock removal.
In use impregnate the lap by spreading a light coat of compound on a steel plate and lightly roll lapp between it and another steel plate this impregnates the compound into the surface of the lapp.
Remember in holes .0005 on a side stock removed equals .001 on the dia.
In addition to the above compounds there is a way to use the grinding ash that builds up around your bench grinders also
In a container of light oil gently sprinkle the grinder residue on the top of the oil
At 5 mins gently pour off the oil whats in the bottom is the biggest coarsest grit.
At 15 mins again pour off the oil into a container this is slightly finer.
At 30 mins pour off
at 1 hour.
at 2 hours. You can continue this for the day and end up with your own lapping compounds from something normally thrown away. The Older Machinists Handbooks have this procedure in them to follow. It does work. I have found you et more of the finer grits from the surface grinder than the bench or pedestal grinders.