Lapping oil stones

[John C Cox]

Perhaps I have been a bit naive about buying things, but I sort of assumed that new sharpening stones would come in pretty flat. Imagine my surprise when basically every Arkansas stone I've gotten has been wavy or fat in the center. Sure, I get that we're not buying reference standards, but still... Not even close.

I suppose it is sort of like chisels and planes, flattening sharpening stones may be another one of those rites of passage, but I need another hobby like I need a hole in the head.

Is there any way to flatten oil stones that doesn't involve spending yet another pile of money and hours and hours of time? Maybe it's just the ones I bought, but still....

I sure hope it gets easier once they're flattened that first time.

So... I started going through my Arkansas stones, and so far, I'm o-fer on them showing up flat. (Granted, I have a Dan's on the way, and I hear they're good.).

The Norton soft had a smile. Concave on one side and convex on the other.
Both Natural Whetstone softs came in weird and wavy and low on opposite corners. (one was sold as a hard... but it was softer than the soft.).
The Smiths was wavy and low in the corners.
The Norton Hard translucent is thin on the ends and fat in the middle.

The Smiths was the smallest, and also the easiest to flatten, as in objectively easy. I was done in less than 20-minutes. I didn't measure the density (it's glued onto a plastic base) but I would guess low.

The Norton soft was the next easiest, but easy here is relative. I probably burned 3-hours on it. It is also the lowest density Ark I've measured so far at 1.97. Still, it's almost 0.030" thinner than when I got it.

The NW soft stones are where the problems kicked in. They're both wavy and fat in the middle. I've probably got 5-hours in them experimenting with methods, and I've only got one side of one pretty flat to the last half-inch of the corners (low), but all the other sides are still sort of wavy.

The Norton Hard/translucent... I think I'm going to punt on this one and send it out to get it lapped. Yikes, it's both really hard and fat in the middle on both sides. It would probably be ok if it wasn't that far out, but it's almost 0.030" thicker in the middle.

If this is typical of the breed, it's got to be one of the reasons people esteem water stones so highly. A hard stone, like an Arkansas, coming in out of whack makes sharpening take even longer.

[Richard Coers]

If you are just working on the bevel of a chisel or plane blade, why does it have to be flat from end to end? The bevel is short and will ride over the crown with no issue to the blade. When it hollows out from side to side, then you have an issue.

[Jimmy Harris]

I haven't bought an Arkansas stone in 30 years, so I can't say if that's common now.

But how are you lapping them? 20 minutes to take down a few thousands of an inch seems excessive. It usually takes me about 2-3 minutes to flatten a stone. I've even done it on concrete sidewalk before, and it didn't take anywhere near that long. You might be using too fine of an abrasive to flatten your stones.

[Jim Koepke]

Hope to hear about your Dan's stone soon. All of mine from Dan's have been flat. Other brands have varied.

On my softer stones it seems working blades on the high areas have helped to smooth them out. Putting stone to stone also seems to work. One has to be judicious about working down the high areas and not just going back and forth over the surface, making two really bad stones in the process.

jtk

[Rafael Herrera]

Lee Valley sells coarse lapping grit for less than $10. That and a float glass pane is the fastest way to lap a stone.

This Washita took about 15 minutes to lap.

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[Kevin Adams]

That’s a nice stone, Raf, is it 2x8’ish? How’s it working? Can’t wait to see the box you make for it!

Kevin

[scott lipscomb]

I've used Silicon Carbide Grit with some sort of thin oil (probably mineral oil mixed with wd40) on a granite countertop off cut. I've done my washitas and India stones in a few minutes. They are aggressive after that treatment, but settle down fast enough.

edit: mine were slightly dished-I suspect trying to flatten a convex surface could require a different technique than just rubbing it on a flat surface...

[Tom M King]

Decades ago I flattened the coarse stones using Carborundum (Silicon Carbide) grit left over from when we ground telescope mirrors as teenagers. I'd use an old window pane on a cinder block outside, just rinse the swarf off with a hose, and throw the window pane away. I don't remember the grade of the grit but it was one of the coarse ones. The Hard and Black never needed any flattening. We always used water to make the slurry for grinding the glass mirrors. It worked just fine for the stones.

For waterstones today I use replacement Atoma diamond sheets, 140 and 400 grit, on a granite surface plate. My oilstones have not seen enough use to need to be flattened since mainly switching to water stones.

[Rafael Herrera]

That’s a nice stone, Raf, is it 2x8’ish? How’s it working? Can’t wait to see the box you make for it!

Kevin

It's 7.5 x 2 x 1 in. It looks like it's a well selected stone. It took me a while to find it in the shop. I finally found it in my "handyman" toolbox, I had forgotten I placed it there. It doesn't get used often, but it has a good bite (probably from the lapping I did.) I did an edge refresh on a chisel and it feels really good, uniform grit, no hard spot.

It came in a box, walnut, with the name of the previous owner carved on the lid. It's nice enough, so no new box for this one.

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[Dave Zellers]

Is there any way to flatten oil stones that doesn't involve spending yet another pile of money and hours and hours of time? Maybe it's just the ones I bought, but still....

If your new stones are not flat, don't spend any more time or money on them. Sent them back, wait for your stone from Dan's, and if that lives up to expectations, then spend your money on flat stones from Dan's. I have 3 stones from Dan's (soft, hard, and black) and they all were, are, totally flat.

[les winter]

In my experience, stones from Dan's come flat. They also offer to flatten out of flat stones for a reasonable price.

[Kevin Adams]

Looks like the previous owner used a chunk of walnut right from the log. I quite like it. And that really is a nice stone. Looks like an AI chisel, too, which are my go-tos at the bench.

Hope all is well, take care.
Kevin

[John C Cox]

The Dan's hard came in today. It's big and it's flat. Definitely no lapping needed on that one.

[Monte Milanuk]

Lee Valley sells coarse lapping grit for less than $10. That and a float glass pane is the fastest way to lap a stone.

This Washita took about 15 minutes to lap.

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Question for you (and the others that have mentioned they used silicone carbide lapping): how do you not get those large, coarse particles embedded in the surface of the stone?

[Rafael Herrera]

Question for you (and the others that have mentioned they used silicone carbide lapping): how do you not get those large, coarse particles embedded in the surface of the stone?

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.

[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