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Precision Ground Toolroom Stones

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rpseguin

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Bob,
I know this thread is old.
What have your results been like with surface grinding flatstones?
I think Professional Instruments uses an air bearing grinder to grind theirs.



The cheap Shars wheel is now doing a fine job. It was only off about .001" radially when new and had a few striations in the face. I dressed those out pretty easily. The wheel has been if beautiful balance from the beginning. Others who have purchased them have been a lot less happy with what they received. It took about half an hour to dress out the .001" runout and most of the other imperfections. Grinding the first two stones pretty much cleaned up the rest, and has been getting better with use. The wheel needs to be used by plunge cutting the entire wheel face as much as possible to help the final truing of the wheel diameter and face. For getting the stones really flat it helps a lot to use a small step over on the final couple light passes.

A Norton wheel would be a big help out of the gate. It will likely be truer and better in every way. It is just the cost of it... There are additional issues that can come up from the wheel adapter and the grinder spindle that can and will cause problems no matter how good the wheel is.
 

Janderso

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Since you woke up this thread, I’ll add my $.02
I was fortunate to be invited to Bobs shop to grind my Norton stones.
We spent a few hours grinding them in as Bob is a persnickety devil.
I use my stones all the time.
They are an essential accessory to have in your shop.
Now that I have a surface grinder, I plan on grinding some smaller stones.
 

Rex Walters

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Bob was also kind enough to help me make a pair.

I do find them extremely useful for removing burrs, though I’d like a much smaller pair for handscraping. Mine are 6” stones so they can sometimes bridge over and won’t completely remove a burr in the middle of the span while I’m still roughing (when the burrs are most pronounced, but before the surface is very flat).

It may be an imaginary improvement, but I make it a habit to rub all surfaces of the stones against each other to ensure that I don’t wear a curve into the faces.

That is, with two stones A and B, and four larger faces, A, A’, B, and B’, I rub the faces against each other until I hear/feel all four combinations running smoothly: A on B, A on B’, A’ on B, A’ on B’.

The three plate principal should ensure they remain truly flat this way.

Again, it may be my imagination, but it does sometimes feel like I’ve removed a bit of curvature from one of the faces I’ve just used. I think always rubbing, say, the same two orange faces together would likely be a bad practice and pretty much guarantee you would eventually create a spherical surface (like grinding a mirror for a telescope).
 

Janderso

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I do rub mine together but re-watching the video, Robin is meticulous about it.
 

Bob Korves

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It may be an imaginary improvement, but I make it a habit to rub all surfaces of the stones against each other to ensure that I don’t wear a curve into the faces.
Flat is probably not as important as we might think, Rex. The flat stones do a fine job on cylindrical surfaces and Morse tapers at taking off burs picked up in service. That is the antithesis of flat. "Flat" when talking about flat stones is about how the stones are ground flat so there is a large surface area with no raised points, but with lots of low spots with sharp corners that cut anything that can get into the openings. As long as there is enough flat surface to support the part being rubbed, there will be no damage to nice and flat shiny surfaces. And whenever the stones are not cutting, a nice flat and shiny surface will not be damaged, only defects that stick up above the surface, which get removed. A couple thou over the 6" length of a stone does essentially nothing bad, even though I would rather have them flat, to get a better feel of how many and where the defects are.
 

Rex Walters

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A couple thou over the 6" length of a stone does essentially nothing bad.
I’m not sure. You’re essentially saying making the stones smooth is more important than making them flat.

I’m unclear weather diamond grinding the stones does something to the embedded grit, or if the “magic” of the stones is just due to the much lower pressure per unit area when the stones are truly flat.

I think abrasion requires a minimum pressure per unit area. The larger the area, the more pressure required to abrade. Keeping the stones flat ensures the area is as large as possible and prevents abrasion except at burrs or other very small-area defects protruding above the surface.

A cylinder or taper effectively presents a line (or very narrow rectangle) to the stone. If the stone was also bellied, you’d essentially be stoning a point or very small area (like a burr). I think it’s important that the stone be quite flat when stoning a cylinder or taper (or anything for that matter) else you risk abrading the surface.

I’m pretty sure that if I really bear down on a cylinder while stoning that I can scratch the surface even with a precision ground stone (because the area of the line presented is still pretty small). When stoning a large flat surface, I probably can’t apply enough pressure by hand to cause abrasion.

I’m more curious about how they work the more I use them.
 

Bob Korves

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I’m not sure. You’re essentially saying making the stones smooth is more important than making them flat.

I’m unclear weather diamond grinding the stones does something to the embedded grit, or if the “magic” of the stones is just due to the much lower pressure per unit area when the stones are truly flat.

I think abrasion requires a minimum pressure per unit area. The larger the area, the more pressure required to abrade. Keeping the stones flat ensures the area is as large as possible and prevents abrasion except at burrs or other very small-area defects protruding above the surface.

A cylinder or taper effectively presents a line (or very narrow rectangle) to the stone. If the stone was also bellied, you’d essentially be stoning a point or very small area (like a burr). I think it’s important that the stone be quite flat when stoning a cylinder or taper (or anything for that matter) else you risk abrading the surface.

I’m pretty sure that if I really bear down on a cylinder while stoning that I can scratch the surface even with a precision ground stone (because the area of the line presented is still pretty small). When stoning a large flat surface, I probably can’t apply enough pressure by hand to cause abrasion.

I’m more curious about how they work the more I use them.
Rex, I know that my 1946 B&S surface grinder does not produce flat surfaces. It has about .0015" of curvature over the 18" length of the mag chuck, and about .0005 over the 6" width. Many to most hobby shop surface grinders and many commercial machines will have the same issue or worse. It has been scraped in at some point during it's previous life. We have ground about 15 precision flat stones so far on my SG. They all work just fine. I can rub the stones together and then rub one on a steel surface, hardened or not, no lubricant, with my entire body weight bearing on the stone, and zero galling occurs. Just shiny metal, less the high spots. It is a simple matter of adequate surface area in contact with the work and the way that the 'flat' surface spreads the load while the recesses cut anything sticking up. All the stones we have made on my SG are close enough to "perfect" to work like they should. They also work very well for de-burring cylindrical and Morse taper tooling, but I would definitely not not put my full weight pressure into those operations, not enough surface area in contact. The precision stones definitely do a far better job on the curved surfaces than ordinary stones can do, the undamaged surfaces remain unmodified, and the original geometry stays intact. Robin Renzetti shows stoning curved surfaces with his precision flat stones in one of his videos.

"I’m unclear weather diamond grinding the stones does something to the embedded grit, or if the “magic” of the stones is just due to the much lower pressure per unit area when the stones are truly flat."
It is the latter. And they just need to be "very" flat...
 

Rex Walters

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I can rub the stones together and then rub one on a steel surface, hardened or not, no lubricant, with my entire body weight bearing on the stone, and zero galling occurs.
Yup, though I’ve been afraid to apply much pressure when stoning cylindrical or tapered things.

We agree that it’s the flatness that makes them work, and appear to now be arguing over just how flat they need to be (which is so unlike me — laugh).

I’m pretty sure the requirement to constantly rub (lap) them together makes them flatter than right off the grinder. I believe the constant rubbing isn’t just to eliminate foreign pieces of metal protruding above the surface, it’s also to abrade the stones themselves and keep them flat.

Your grinder produces roughly the same amount of curvature to every face of every stone you’ve made, no? Putting the stones face to face then doubles that error. Right off the grinder, I remember mine felt a little crunchy for the first few rubs, then quickly settled in to that slick feeling of good contact throughout.

I rub all four large faces together until they all feel slick and smooth. A with B, A’ with B, A with B’, A’ with B’. The only geometry that would allow this to happen is a quite flat plane, no?

I’m pretty sure they wouldn’t work with a full thou and a half of curvature, but it doesn’t really matter. The stones we made on your grinder definitely do the job, and I’m going to continue rubbing them together constantly even if it’s just a good luck charm! :)
 

Bob Korves

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I’m pretty sure they wouldn’t work with a full thou and a half of curvature, but it doesn’t really matter.
The .0015" is over the length of the 18" chuck, (the chuck actually measured .0012" out of flat, don't want to oversell... ;) ), so that would be .0004 over the 6" length of the stone, not much. I have experienced the same roughness when rubbing two freshly ground stones together, every time. I think it is swarf that gets into the lower recesses while grinding the stones during the last pass, rather than from abrading the solid stone quickly parallel. And yes, it does go away completely with a few seconds of rubbing them together.
 

Bob Korves

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The stones are flat and co-planar enough that when you hold one at a very slight angle to a light source, like a fluorescent lamp, you can quite easily see a good reflection of the light in the stone. It works even better to have strong light from behind you illuminating an object, to get a good view of the reflection on the stone surface. I have no idea how to quantify how perfectly angular a reflection we are seeing, or how to measure it with simple apparatus, but that might be a good way to determine actual flatness if it could be done to a good enough result to be meaningful.
 
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