Lapping oil stones

[John C Cox]

I'm sure other people had better results with SiC powder, but it tended to leave a belly in the stone unless you were pretty careful. I think it might be great for a dished stone, but you can easily take it the opposite direction.

It also left my stones with a "pebbled" surface.

[Monte Milanuk]

It also left my stones with a "pebbled" surface.

Any guesses on how it was causing that?

[Monte Milanuk]

The powder is very hard, but also very brittle, it breaks down pretty quickly and turns into a slurry. You have to add fresh powder to continue grinding. The resulting mixture of SiC dust and water is washed when done. If there's any residue embedded in the pores, it's not noticeable.

So you're saying the SiC particles fracture and break down, getting finer and finer as you continue lapping?

I figured both the stone and the plate would be washed clean, just wasn't sure if some of the lapping compound would get ground into the surface of the stone.

[Mel Fulks]

Lapping can damage your stones, just be careful.

[Rafael Herrera]

So you're saying the SiC particles fracture and break down, getting finer and finer as you continue lapping?

I figured both the stone and the plate would be washed clean, just wasn't sure if some of the lapping compound would get ground into the surface of the stone.

Yes, the particles pulverise. The pictures I posted above were taken after rinsing the stone on warm soapy water.

[Rafael Herrera]

It also left my stones with a "pebbled" surface.

Any guesses on how it was causing that?

If you're lapping waterstones, some of which have a softer matrix, the coarser grit will leave a roughened surface. I'm assuming that's what you mean by "pebbled". It happened once when I was lapping a waterstone, I was using 90x powder. I used finer grit sizes, I don't remember, maybe 280x, to smooht the stone out. I don't use waterstones anymore.

I've lapped Washitas, India (fine, medium, coarse), crystolon (coarse, medium, fine), Hindostan, and Queer Creek with the coarse 90x powder. I've not felt the need to go above that, I have powder in finer grits but it doesn't make a lot of sense to polish stones that are supposed to be abrasive. They will eventually settle up if the lapping left them a little rough.

Perhaps with slates, a coticule, and a Charnely Forest I've used 180X powder or finer, I'm not sure. They don't get a lot of use, so they're still flat.

It's pretty obvious if the surface is too rough, like on my waterstone, but for the stones I listed above they were pretty good after the 90x grit.

[Rafael Herrera]

I'm sure other people had better results with SiC powder, but it tended to leave a belly in the stone unless you were pretty careful. I think it might be great for a dished stone, but you can easily take it the opposite direction.

Since almost all the stones I've lapped are vintage stones, all of them were dished. Like in the pictures I posted. I stopped when I got mostly a clear face, the center was still a bit dark, but I didn't spend more time getting rid of that. You can direct the pressure and limit the powder to under a particular area of the stone, I've done that. I've also seen people on YT dump a table spoon worth of powder on the plate. You want a uniform layer of abrasive between the stone and the plate, having a mound of grit is a bad practice. A small amount, like half a tea spoon is good enough. Figure of 8s movement, turning the stone around also help keep the abrassion uniform.

[John C Cox]

The "pebbled" surface is the best way I can describe it. It's very different from the flat/shiny surface you get with fixed abrasive. The action of SiC powder is sharp grit rolling around between two surfaces. It's not stuck down, so it sort of finds its place. I agree that if I had an old dished stone, I'd probably try that, as it is pretty aggressive, and it's going to eat the part you want.

I think suppliers erring on the side of a slight belly sort of makes sense, as the wear tends to dish the stones... So that's maybe a few more years before somebody complains with "Hey, your stone dished way too fast...". The Norton Translucent sort of went overboard, as it had a pronounced belly.

I ended up using sandpaper on my surface plate. It is fairly expensive ($1.00/sheet) but I've got my Norton Translucent lapped flat to P320 on both sides except for the last half inch on 2-corners. For reference, I took off 0.040" of thickness to get it there.

In the end, after trying everything else, dry lapping using quality sandpaper gave me the best and fastest results. As in zero to done in ~2 hrs. Wet sanding at the coarse grits seemed to lock up the surface of the Translucent Ark fairly quickly, where dry sanding ate the stone pretty aggressively. Perhaps wet sanding or shifting to diamond plates would more make sense above P220, as the paper simply loads up too fast above that.

Anyway, thanks for the feedback and the advice. I think given the pain of doing this, I'll probably try to maintain them flat so I don't have to go crazy and punt again.

[Jim Koepke]

You can direct the pressure and limit the powder to under a particular area of the stone, I've done that. I've also seen people on YT dump a table spoon worth of powder on the plate. You want a uniform layer of abrasive between the stone and the plate, having a mound of grit is a bad practice. A small amount, like half a tea spoon is good enough.

Rafael, you made three important statements (underlined) in your post on which many people may not have figured.

Many may think lapping is just going back and forth over an abrasive material.

Sometimes one has to work just a small area at a time.

Like a plane sole, if it is humped in the middle, work it on a thin strip of abrasive so only the bump is abraded. It might need to be worked sideways in some cases.

jtk