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Granite surface plate, $100

Bandit said:
Hmm, I guess this raises the question, if the plate was not mounted in a 3 point setup, is it going to take a “set” over time?, and will it “recover” given time?
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My understanding is that they can take a set - over time - lots of time. And that they do not recover in the normal sense - at least not predictably. When they are made, they are aged before lapping. The other related problem is locating the original support locations.

tesko said:
For my own personal use I have a 12" x 18" Grade B and I'm very happy with it.
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Again, my understanding is that 12x18 stones are not set on three points and that they will hold B grade accuracy just fine sitting on a reasonable support system. I think this size of stone is perfect for most hobbiest's needs.
 
Susquatch said:
My understanding is that they can take a set - over time - lots of time. And that they do not recover in the normal sense - at least not predictably. When they are made, they are aged before lapping. The other related problem is locating the original support locations.



Again, my understanding is that 12x18 stones are not set on three points and that they will hold B grade accuracy just fine sitting on a reasonable support system. I think this size of stone is perfect for most hobbiest's needs.
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Large granite surface plates are typically marked with their Bessel points, like in the picture.

You should be able to see where the Bessel points were by the slight wear, tint or matte finish left behind from sitting on them when it was being lapped. Usually these supports are placed around 22% from the edge of the plate.

However, where it gets troublesome is if a sag has set in from many years of not having sat on these original Bessel points.

I'm sure it can be fixed, but now your looking at special measuring tools, knowledge, labor and patience, all necessary for a successful restoration.

So yeah, I see your point.
 

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Bessel points.....I mistakenly called them Airy points earlier, those are different (2vs 3 points)....I stand corrected. I DO remember there there is a formula to calculate them but not what it actually is. That part of the brain is pretty dusty from college....I never was good at remembering formulas anyway..
 
Dan Dubeau said:
Bessel points.....I mistakenly called them Airy points earlier, those are different (2vs 3 points)....I stand corrected. I DO remember there there is a formula to calculate them but not what it actually is. That part of the brain is pretty dusty from college....I never was good at remembering formulas anyway..
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IIRC I posted a PDF of the standards for granite surface plates with the calculation somewhere. Or maybe I just shared it with @Susquatch

just checked, I didn't even share it with the old hairy dude.... the 25mb pf is too large to upload

But googling ASME-B89.3 should allow people to find the file
 
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Dan Dubeau said:
Bessel points.....I mistakenly called them Airy points earlier, those are different (2vs 3 points)....I stand corrected. I DO remember there there is a formula to calculate them but not what it actually is. That part of the brain is pretty dusty from college....I never was good at remembering formulas anyway..
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No issues, it's basically the same math. The points where the bend equals the sag but for 3 instead of 2 or 4 points. The problem with recalculating is the potential to reverse them and the interim set. I called one of the makers (not Mitu or Starrett) and chatted about all that. He basically said he couldn't recommend doing it. He also said anybody who didn't mount their stone that way probably didn't maintain it properly either.

I don't remember formulas either Dan. Never could. It's not the way my brain works. I always had to develop formulas from first principles during exams. Didn't take much longer than if I remembered them because I automatically knew how to use them. Served me well over time. But Profs all hated it. Made me a poor teacher in later years too cuz most students are memory based learners. I don't think it hurt you either!
 

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Asking $41,000, down from $49.

(An aside, how could you list this, and not list the grade?? It’s like listing a forklift, but not the capacity)


If your friend doesn’t mind something a little smaller, and a lot cheaper, there’s this 5’ x 10’ Starrett B grade for $8000.

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When we were having plates relapped and checked, they would lap plates as large as 18x24, upside down on the larger ones. Then flip them back over, check them, then return them to their usual location. Mind you most of these were thicker, like my 12x18 that’s 4” thick.
 
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