cast iron lap

tdelewis · Dec 25, 2019

I have never used a cast iron lap and wondered how it works. I would think that the diamond powder would cut into the lap as well as the polishing the graver. How do they work so they create the polish without cutting into the cast iron?

mitch · Dec 25, 2019

that's not how laps work. the hard, sharp abrasive material embeds itself into the softer substrate (be it iron, lead, whatever), where it's held fast so it can abrade the harder material being lapped. it's kinda like you're creating a sort of sandpaper, with the grit stuck to one material so it can sand/grind what it's being rubbed against.

yeah, i know, it sounds weird. i thought the same thing when i first learned about lapping 40 yrs ago...

Archie Woodworth · Dec 25, 2019

I have used a brass plate that I "charge" with diamond powder. Just as Mitch said, the diamond dust is embedded into the brass. Then use it to put the final polish on the graver edge. It only takes just a few strokes to polish. Works great and the sooo much cheaper than an iron or ceramic disk.

Sam · Dec 27, 2019

The crackly surface of the cast iron (see microscopic view) carries the diamond particles. Yes it's very soft, so you have to be gentle with it. You're only polishing so a lot of downward pressure isn't needed. Just sweep the graver across the rotating lap and it'll polish like a piece of glass.

tdelewis · Dec 27, 2019

I checked out GRS. They have a video on how to prepare the cast iron lap. I will purchase on in the near future.

cast iron lap

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