How flat is flat? (Water stones)

[Diomedes Industries]

Last night I was lapping my 800 stone last night to make it flat and parallel. I was using a set of calipers and was getting a little obsessed. How flat is flat enough? +/- 0.005?

How flat do you lap your stones?

TF

 

[KennyB]

Well, using a pair of calipers isn't the best idea. When flattening a stone it's pretty hard to put equal pressure on all corners, and so to have the stone vary in thickness on one corner or the other by .005" would be a hard thing to avoid. Measuring the actual flatness of the surface in that way just isn't possible; at least not in a very accurate way. Aside from all of this, let's say for example you have a big low spot in the center of your stone, your calipers will probably take readings from the high-spots surrounding it before ever hitting upon the low spot, and then if one corner of the stone is .005" thicker than the other, it just compounds things.

I don't know how flat glass is, but I would assume that the standard practice of rubbing a stone on sand-paper fixed on glass would probably give you a surface flatness of about +/- .001 - .002, because the paper itself will probably vary in thickness in terms of the distribution of abrasive, adhesive, and of course the paper itself.

DMT stones are flattened to within +/- .001 though, and I think that flattening on this surface should get you to +/- .001, which in my opinion is all one would really need. Most surface plates machinist's use are only accurate to +/- .0005, so .001 is nothing to scoff at. You can pretty much create a vacuum seal between two surfaces when you talk about that kind of flatness.

Finding an accurate way to actually determine that you've eliminated all the high and low spots and have flattened is the only real way to do it. That's why I would use the pencil-grid trick, or maybe a few light dashes of some magic marker once it's mostly flat since the slurry produced by flattening a stone tends to erase the pencil and can lead to a false reading of flatness. Marking a grid like so will leave the markings on the low spots, and once the markings rub away, then you know the surface is flat; at this point you could still get some wild variations with your calipers if you tried to measure it based on the thickness of the stone.

Realistically though, any flat surface like glass with some sandpaper on top will get the stone very flat. I've used laminate counter-tops as the backing underneath sandpaper, and they work very well. A good way that I've used to test if a surface is good for flattening, is to take a machinists' square and a small flash-light. Put the square on the surface, and the flash-light behind it. If you can see a crack of light shining in underneath the square, then it's probably not a good surface to use.

 

[Diomedes Industries]

I hear you on all of this - thanks for taking the time. I was using a machined piece of granite. It is flat within .005.

I will continue to use that and simply use my calipers to get a decent reading for making it parallel.

TF

 

[stitchawl]

Watching the old Japanese sharpeners at work in their studios, I'd say 'flat enough' was sort of like a tea cup saucer. These guys don't seem to worry about being perfectly flat. Every stone I see, and I see them often as I visit the shops when I'm in the area, has a shallow dish to it, perhaps as much as 1/2 an inch over a 12 inch stone. The guy who comes around the neighborhood on his bicycle sharpening all the housewives knives and cleavers has a stone that looks more like a shallow soup bowl!

Personally, I prefer flatter, but what do I know?


Stitchawl

 

[guyfalks]

I use a Starrett straight edge (.0004") to check for flatness. The various and sundry glass plates I have used to flatten stones all seem to as flat as far as I can tell using it. Another question to ask is just how flat doe a whetstone have to be; what would happen if it was off a bit? (My guess is nothing when dealing with knives. Precision tools and perhaps straight razors might be a different story.)

 

[Obsessed with Edges]

I think woodworkers need to be more concerned about absolute flatness, for sharpening plane blades, wood chisels & other things with precisely straight edges and sharp corners, where the full length of the blade's edge should be in flush contact with the stone. If the stone is dished, it'll make a chisel/plane blade's straight edge curved and/or round the corners off.

Most knife (cutlery) blades have enough curvature, they'll usually only make contact with a small portion near the central area of the stone. Unless you're going very shallow with the bevel, when other parts of the edge might scrub at the very edges of a dished stone, I don't think I'd be too worried about it.

 

[KennyB]

Well, I know more about keeping things really flat than I really do about water stones in general, but I think it's basically a matter or preference. A lot of people find that a stone that is dished might be too difficult to work with; I've found that if it's a coarse water-stone and it's already dished, then during the actual process it might get worse and have an adverse effect on the profile I was trying to accomplish. On the other hand I've heard people say that it's a good way to make your stones last longer by not flattening them frequently, but I think the better approach to that is to flatten early and flatten often, that way you knock out the high spots when they're still very minute and you don't remove that much material. If you wait until you have very large dishes, you will have to remove quite a lot of material to make it flat again.

I've also read from place to place that having a dish in the stone helps when grinding convex edges and that a lot of the stones are worn in like this to accommodate that, but I don't recall where I read it so maybe I just made it up. It has occurred to me in the in the past that a flat surface may not be the most ideal for every type of edge though; mostly when working with recurves. Thorn in my side, because I like to have things really flat, and just trying to profile the curve out of one winds up wearing a dish into my stone.

Then again maybe they're just so good they don't need it to be flat?

 

[pwet]

i flatten on dmt XXC and XC, the result is good enough for me. i keep my stones flat everytime. i lap them before every session, and the coarsest ones can be lapped during sharpening ... using the whole surface helps too, every 10 strokes or so i concentrate on both ends of the stone to even wear a bit.