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Granite surface plate, $100

Toronto, ON

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The mere fact that it is sitting on a cabinet says to me that it's junk. The three point mounting is prolly lost. With a stone that size and bigger, it must sit on the 3 points it was originally lapped on. Otherwise it would need recalibration and prolly resurfacing relapping which totally blows the price point.

I'd love to find a 36x24 on its 3 mounting points in that price range. But every single one I've found so far has just been placed on a surface or an edge brace. None have been mounted properly.

From what I understand, the smallest stone that can be used without the 3 mounting points is 12x18x3.
 
Susquatch said:
The mere fact that it is sitting on a cabinet says to me that it's junk. The three point mounting is prolly lost. With a stone that size and bigger, it must sit on the 3 points it was originally lapped on. Otherwise it would need recalibration and prolly resurfacing relapping which totally blows the price point.

I'd love to find a 36x24 on its 3 mounting points in that price range. But every single one I've found so far has just been placed on a surface or an edge brace. None have been mounted properly.

From what I understand, the smallest stone that can be used without the 3 mounting points is 12x18x3.
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If only this was longer I could cut this into a few strips and use the two ends for my router rails
 
Good deals on some nice plates. I wouldn't worry about the Airy points on something as small as these. That is more applicable to much larger plates at least 3x6 and larger as the weight goes up and it CAN (not that it necessarily actually will) start to affect stability and accuracy if it's not supported exactly how it was lapped and certified. I'd say any plate under that size would be pretty stable in any orientation, or any mounting points. Or course there are many different grades, and types of granite being used for stones, some more stable than others so that's not a blanket rule.

Not saying that to stir a debate, but I don't want a passerby to thumb their nose at these because they think they're worthless because of it. They very well may be, for a variety of other reasons, but it's very, very unlikely that a small plate like this would be affected by a non factory 3point mounting from sitting on a bench. IMO temperature swings in a typical home shop are a much bigger problem to maintaining accuracy and calibration.....

But $100 for a 24x18 plate is good deal, and a nice size for a home shop for doing layout, and other metrology uses without taking up too much space. Unless of course you're splitting microns, or running a 3rd party metrology lab in the shed......:D
 
That's a hundred bucks less than the cheap 12x18 Busy Bee cost me delivered, mind you the delivery was more than the plate.
I guess that's the price for living in the middle of nowhere!
 
JustaDB said:
Can you elaborate on this, pls?
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I got to wondering about this a while back when I discovered that most stones for sale are not supported properly.

Remember this golden rule of machining - everything moves when subjected to a force. Yes, EVERYTHING - even solid Granite!

Now, think about this. If a stone is lapped flat on 3 points at the manufacturer, the edges droop down from the weight of the granite. Pure physics. If the user then mounts it in an edge frame, it droops in the center - the exact opposite of how it was made. That's MAXIMUM deflection from design.

According to my math, the maximum deflection due to its own weight for a 2ft stone 3" thick is about 0.0005. You can probably double that for a reverse flexture (lapped on 3 points vs mounted on edges). A thou is not a lot. But the flatness specification for a two foot B grade block is approximately 3 tenths. So mounting it incorrectly triples the error from its certification. That's for B grade. Quite obviously it is much much worse for the tighter grades. Also consider that for most applications you are using references across a length. A reverse slope from one edge to the other magnifies the effect (just plain old geometry).

For reference, granite surface plates are graded based on their flatness tolerance, which determines their precision and intended use. The common grades are:

Grade AA (Laboratory Grade): The most accurate grade. Used in controlled environments like metrology labs for ultra-precision measurements. Flatness tolerance is the tightest.

Grade A (Inspection Grade): Suitable for quality control and general inspection work. Flatness tolerance is twice that of Grade AA.

Grade B (Shop Grade): Used in workshops and production areas for less critical applications. Flatness tolerance is four times greater than that of Grade AA.

I know that the foundation of all metrology is a granite surface plate. Any metrology lab knows that too. But I reasoned that maybe a small stone wasn't as susceptible as a big one. So I did the math. And it turns out that this is true. But not nearly as much as I had suspected. The smallest you can safely go without a meaningful impact (greater than the cert specs) is 12x18. Any bigger than that and the natural deflection of the stone due to its own weight exceeds its quality certification. HOWEVER - If the stone was originally lapped on a flat surface or on its edges, you can safely go bigger. Perhaps even as big as 24 x 18. I didn't do the math for this - I wouldn't even know how to go about it. The important question is, how was it made?

You can go to any of the stone makers websites and find lots of info about how to properly mount a stone. They make a really big deal about using their 3 point system, and they all tell you why.

If you don't care about the accuracy of a granite surface plate, then why buy a granite surface plate? Just buy a piece of granite countertop! In the end, you have to ask yourself why you want a stone in the first place. If it's just to do some simple height measurements, the stone quality prolly doesn't matter. If you want to calibrate your metrology equipment, or do some precision comparison measurements, it probably does.

For me, it boils down to this. If I just want to do good measurements, my mill bed is good enough (and was for many years). I don't need a stone at all. It will just consume valuable space in my shop. If I want to do precision measurements and calibrate my metrology, I need a good surface plate and it needs to be reliable. You need specialized equipment to verify the reliability of a stone - which I don't have. So the only way I can get close to being reliable is to use a certified stone the way it was designed and lapped to be used.

I also remember this from my days in industry - the shop machinists and the guys in the quality lab NEVER got along. It was always like the Hatfields and the McCoys.

Obviously, your mileage may vary. Just pick your poison.
 
A lot of good information there, Susquatch.

Another little consideration is the professional stone re-lapper, recertification people don’t like to work on smaller stones. From what I’ve been told a 2’x3’ plate is about the smallest. Nor sure I understand why.

And speaking of re-lapping, recerting, who does this in Canada? Specifically Manitoba? Had planned when needed just to take my plate down to Minneapolis, but shunning the US now.
 
Lucky7 said:
And speaking of re-lapping, recerting, who does this in Canada? Specifically Manitoba?
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@TorontoBuilder looked into this a while back. As I recall, he found a guy in Michigan who regularly came to Canada with his equipment. We were going to pull together a group session, but the idea fizzled due to cost and logistics. I bet the situation is much worse in Manitoba.

My opinion (today) is that I would prefer to buy a used Grade A Stone knowing I don't need that precision but that it would give me a buffer that would keep it within the grade B tolerance for my use without needing to resort to a recert specialist. I am looking for a Grade A 2ftx3ft stone. There are some out there but they are always mounted incorrectly. I have no interest in a Grade F stone... ;)
 
Lucky7 said:
From what I’ve been told a 2’x3’ plate is about the smallest. Nor sure I understand why.
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I'm sorry. I should have commented on this but didn't. I think it's just plain economics. Nothing more, nothing less. Who wants to pay a recert outfit more than they can buy a new stone for?
 
Susquatch said:
@TorontoBuilder looked into this a while back. As I recall, he found a guy in Michigan who regularly came to Canada with his equipment. We were going to pull together a group session, but the idea fizzled due to cost and logistics. I bet the situation is much worse in Manitoba.

My opinion (today) is that I would prefer to buy a used Grade A Stone knowing I don't need that precision but that it would give me a buffer that would keep it within the grade B tolerance for my use without needing to resort to a recert specialist. I am looking for a Grade A 2ftx3ft stone. There are some out there but they are always mounted incorrectly. I have no interest in a Grade F stone... ;)
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So if I understand correctly surface plates should be checked for accuracy on a regular basis but hobbyists just about never do because they don't have the means and probably don't need such a great level of accuracy anyway. In the hobbyist spirit of DIY you would think we would have figured out how to do it ourselves. I think it requires an auto collimator and a Repeat-O-Meter. These are very expensive, perhaps over priced tools. I know of only one hobbyist near me who has such tools and I'm sure he doesn't have the time to measure all his friends surface plates. There are videos on YT that show how to make your own Repeat-O-Meter. Project #1618 on my to do list! Any one out there checking the accuracy of their surface plates or contemplating making a Repeat-O-Meter????
 
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? Perhaps, also somewhat/maybe more to the point, how are you going to check it, if the plate was what you were going to use to check measuring tooling that you have? This, I think leads to, just what kind/amount of accuracy do you need/think you need?
If you buy a new stone, just how is it put in a box/crate to maintain accuracy? This perhaps also brings up can they be “tweaked” slightly with how they are mounted?
Questions, questions, questions, ah, si, more tequila!
One finger typed while @Canadium posted.
 
Canadium said:
So if I understand correctly surface plates should be checked for accuracy on a regular basis but hobbyists just about never do because they don't have the means and probably don't need such a great level of accuracy anyway. In the hobbyist spirit of DIY you would think we would have figured out how to do it ourselves. I think it requires an auto collimator and a Repeat-O-Meter. These are very expensive, perhaps over priced tools. I know of only one hobbyist near me who has such tools and I'm sure he doesn't have the time to measure all his friends surface plates. There are videos on YT that show how to make your own Repeat-O-Meter. Project #1618 on my to do list! Any one out there checking the accuracy of their surface plates or contemplating making a Repeat-O-Meter????
Click to expand...
a flatness gauge is on my todo list.
 
Susquatch makes very fair points, though in my experience only labs or QA/QC departments go to such lengths.
For my own personal use I have a 12" x 18" Grade B and I'm very happy with it.

If I were shopping for a larger Grade A stone, I would have a definite need for the precision, so not a chance I would go to the Used market. Too much risk.
 
Here are a couple of QA Service Providers in Ontario.


Alex Tole M.Eng - NTS Ops Mgr.
Atole@sci-labs
Kitchener, Ontario

​

Canada - AESI Acumen Engineered Solutions International Inc​

5575 North Service Rd., Ste. 401​

Burlington, Ontario, L7L 6M1, Canada
Phone: 905.875.2075 or 888.849.7237

Contact them to get current services and fees
 
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