New waterstones and a question

[Mike Henderson]

Well, not really. If you rub the two stones together so the bumps on one slide around and the bumps on the other slide around, pretty soon all the bumps on both are worn off...and both stones are flat. This is the recommendation of Rob Cosman of Lie-Nielsen (see any of his videos). He recommended to me on the phone that I keep two 1000 grit stones, rub them together to flatten both, and then use them both to flatten the finer stones. It works fine, every time. The diamond stone might cut faster, though.

Again, the trick in the two-stones approach is that BOTH stones have to be wearing away at once, along an intersection plane that's already more or less flat. If you had a lopsided diamond stone (which would not wear down), and tried to true a softer stone with it, THAT would be bad news.

I'm afraid that rubbing two stones together will NOT give you a flat surface. It will give you two surfaces that fit together. For example, one surface may be concave and the other convex and they'll fit together very well.

People who grind their own telescope mirrors know this. They have a glass tool and a mirror blank and by rubbing them together with grinding compound between them, they create a spherical concave mirror (which is converted to parabolic in the final steps).

Mike

[Mike Henderson]

Thanks Mike. What is the appropriate grit of sandpaper for flattening a 1000/4000 stone? Do you use some sort of adhesive to attach it to a sheet of glass or say an unused piece of marble or granite?

When I used sandpaper for flattening water stones, I wasn't very particular - I used what I had handy. That said, I wouldn't use too fine a grit because it takes too long and I wouldn't use too rough a grit because it leaves big scratches in the stones. Maybe 100 to 150 grit. Maybe someone else can give an opinion.

Mike

[Brian Hale]

The flattening stone and tray was part of the group price and that was cheaper than just the 2 stones by themselves so returning it isn't an option. However the back of it might be used as an extra course stone?

As a side note, precision granet inspection plates for the machining trade are made by rubbing a combination of 3 plates against each other with a lapping compound between them. I'd think that using just 2 stones would create concave surfaces but perhaps using the 3 stone method would work provided they all were of equal abrasiveness. Maybe.

Brian

[Rob Blaustein]

I'm afraid that rubbing two stones together will NOT give you a flat surface. It will give you two surfaces that fit together. For example, one surface may be concave and the other convex and they'll fit together very well.

People who grind their own telescope mirrors know this. They have a glass tool and a mirror blank and by rubbing them together with grinding compound between them, they create a spherical concave mirror (which is converted to parabolic in the final steps).

Mike

If you rub along several axes shouldn't that help? I can see where rubbing two stones together along only one axis can give rise to what you describe, but would that also occur if you switched directions often?

[Mike Henderson]

If you rub along several axes shouldn't that help? I can see where rubbing two stones together along only one axis can give rise to what you describe, but would that also occur if you switched directions often?

Nope - if all you use is two surfaces, all you get is two surfaces that fit together - not two flat surfaces. As Brian mentioned, it is possible to get a flat surface by using three surfaces, but I don't know the sequence or other requirements.

But I can guarantee that two surfaces will only give you two conforming surfaces, not two flat surfaces.

Mike

[Ken Bryant]

Your two 1000 grit stones, after wearing through similar sharpening, will be worn in similar ways -- i.e., both convex or both concave. Empirically, if you rub them in a more or less circular fashion, all the high spots are hit, and they come out quite flat. I will agree, though, that if you had a convex stone and a concave stone, and carefully (and perversely!) rubbed them together along the axis, you'd only make the problem worse.

Again, see Rob Cosman's videos. This wasn't my creation, but his.

[Rob Blaustein]

Maybe the answer is that the two surfaces will be flat enough--not good enough for optics, but maybe good enough for sharpening something small like a chisel where a very small deviation from flat would be ok.

[Mike Henderson]

Not to beat this to death, but let me give you an example. First, why does your working stone wear more in the middle-center than around the outside? It's because no matter how you try, you use the middle center more than the outside.

Now, suppose you take two stones that aren't concave the same amount. For purposes of illustration, let's assume that one of them is actually flat, while the other has a concave in the middle-center, and you begin rubbing them together. No matter how you try, you will abrade the outside of the flat stone more than the center because the outside will be making contact while the center is not. Eventually, the stones will make full contact but the flat stone will now be convex while the originally concave stone will continue to be concave. Using only two stones it is simply not possible to guarantee that the two stones are flat - in fact, it's more likely that they will not be flat.

If you use these stones to flatten other stones, you will transfer the shape of one stone to the other. So if you use the convex stone on a concave working stone you will absolutely not flatten it - it will continue to be concave. And if you use the concave stone on another concave stone, the one that is less concave will become convex.

You can use two stones you rub together, but don't try to fool yourself into thinking they are flat.

Mike

[Rob Blaustein]

For purposes of illustration, let's assume that one of them is actually flat, while the other has a concave in the middle-center, and you begin rubbing them together. No matter how you try, you will abrade the outside of the flat stone more than the center because the outside will be making contact while the center is not.

Mike--I envisioned a different sort of movement that I think would not lead to what you are proposing. What if you started rubbing your concave stone against the flat stone but orient them, at least initially, 90 deg to each other, as in the attached photo. Moving the concave one (the upper one) side to side will remove material uniformly from the flat one, and also remove the material from the upper one too, and evenly if that one is also allowed to move. Does that make sense?
waterstones.jpg

[Alan Turner]

Brian,
We use 180 grit wet/dry black SC paper, on a granite sink cut out, and it is pretty quick for our Nortons at PFW. We leave all three in water all the time. The 220 is not a great stone as it dishes pretty quick, and is difficult to re-flatten.

[Mike Henderson]

Mike--I envisioned a different sort of movement that I think would not lead to what you are proposing. What if you started rubbing your concave stone against the flat stone but orient them, at least initially, 90 deg to each other, as in the attached photo. Moving the concave one (the upper one) side to side will remove material uniformly from the flat one, and also remove the material from the upper one too, and evenly if that one is also allowed to move. Does that make sense?
waterstones.jpg

Rob, the point I was trying to make is that you cannot gurantee flatness with only two stones. I cannot see any technique which will allow you to say with confidence that the surfaces are flat.

If what you are doing works for you - if you have used it and tested the surfaces after using it and they are always flat - I would say continue to use it.

Mike

[Roy Wall]

Thanks Mike. What is the appropriate grit of sandpaper for flattening a 1000/4000 stone? Do you use some sort of adhesive to attach it to a sheet of glass or say an unused piece of marble or granite?

Rob - I do what LN taught me......

I bought a 12x12 granite plate from Woodcraft.....good to .00005 flat or something crazy.... wet a sheet of 120 sandpaper and stick it to the granite...then run each stone over to flatten.

[Ken Bryant]

works for me. works for Rob Cosman.

[Dave Burnard]

If you rub two surfaces together, like abrasive stones or glass plates and abrasive, and orient them randomly (mathematically speaking) - what you get are spherical sections. So you might get two flat surfaces since that's a special case of spherical sections but you're much more likely to get some form of concave/convex pair. This is how you do the initial grinding when making a telescope mirror or glass lens

However if you do the same thing with three surfaces and also randomly switch which two you are rubbing together - you can only end up with a planar surface. That's how optical flats (or telescope diagonal mirrors) used to be made.

So you can do it with three (similar) stones, and I've heard of some folks doing this. You just need a bigger stone pond

[Monte Milanuk]

This topic is of interest to me as I just ordered my first waterstone (Norton combination 1000/4000 grit) and figured I'd try the drywall screen or coarse sanding belt as shown in one of Frank Klausz's videos.

On the two stones vs. three stones for flattening each other... I've seen Rob Crosman's video on hand planes where he flattens the stones against each other (two stones), but I also understand the concept of three independent surfaces normally being required to guarantee that any one is perfectly flat. The question that comes to mind is if this changes materially by rubbing in a circular or figure-eight pattern such as some people appear to use when flattening stones, vs. a more linear forward and back motion?