Surface Plate Advice | The Garage Journal

Call around to your local granite counter top guys. I had a pretty big piece left over from my granite counter top project and I kept it to make a surface plate. But then we moved and didn't have room for it. Out onto the street it went, never to be seen again. Dumb move.

I asked my granite guy and he said no one ever asks for the leftovers, it's just considered trash.

Good luck!

Granite countertops are far from precision ground. They're wavy as heck compared to an actual surface plate. Also, the stone itself is often pretty different. Surface plates use a fine grained granite that looks pretty blah as a countertop, though "black galaxy" is close to what's used for surface plates.

Sink cutouts are cheap, granite tiles are cheaper. If you're looking to do scary sharp, glass is flatter and smoother, so it will work much better. Actually, a thick piece of float glass is a much better all around surface plate than scrap from a granite shop.

A cheaper option than the big brands for a real surface plate is Grizzly. A 9x12 is around thirty bucks, with the 12x15 being closer to fifty. Granite surface plates are better for inspection and general use. Cast iron is preferred for lapping. Granite is more stable and wear resistant, which is nice for an inspection surface. Cast iron can be charged with lapping compound for making precision surfaces.

Granite plates are graded, B for general shop use, A for inspection and toolroom, and AA for lab work and for machinery like a CMM.

Generally, Grade/Flatness is a function of flatness deviation per inch or several inches, and repeatablity per inch. The size of the plate is directly proportionate to the difficulty to lap in and maintain the grade.

Grade B Toolroom-100 millionths
Grade A Inspection-50 millionths
Grade AA Laboratory-25 millionths

From Starrett: "The flatness tolerances for three standard grades are defined in the federal specification as determined by the following formula:"

Laboratory Grade AA = (40 + diagonal squared/25) x ." (unilateral)
Inspection Grade A = Laboratory Grade AA x 2
Tool Room Grade B = Laboratory Grade AA x 4.

If you follow the rule of thumb that your inspection device needs to be 10x the resolution of whatever you're inspecting needs to be, that will give you the range of what plate you want.

B is more than good enough for home shop use. If you can luck into an A or AA that's all the better. If you truly need the flatness of A or AA you'll want to have a company come in and lap it in. AA needs to be in a climate controlled room with very fussy instruments to keep its tolerance.

Starrett's pink granite is very nice, and I've heard great things about Standridge plates.

If you look at pretty much any machine shop auction, they are usually for sale and can be had for less than $100.

They are incredibly useful, I wouldn't be without mine. This one I got for free. Used to be part of a large CMM or some kind of inspection machine. Roughly 3' x 5' and ish pounds.

Granite countertops are far from precision ground. They're wavy as heck compared to an actual surface plate. Also, the stone itself is often pretty different. Surface plates use a fine grained granite that looks pretty blah as a countertop, though "black galaxy" is close to what's used for surface plates.

Sink cutouts are cheap, granite tiles are cheaper. If you're looking to do scary sharp, glass is flatter and smoother, so it will work much better. Actually, a thick piece of float glass is a much better all around surface plate than scrap from a granite shop.

A cheaper option than the big brands for a real surface plate is Grizzly. A 9x12 is around thirty bucks, with the 12x15 being closer to fifty.

I wouldn't have expected them to be precision ground but probably good enough for a typical residential shop.

Can you give some numbers to show the difference between the typical granite counter tops vs. low grade surface plate? How do they even measure it?

What are the OP's requirements for a surface plate?

I wouldn't have expected them to be precision ground but probably good enough for a typical residential shop.

Can you give some numbers to show the difference between the typical granite counter tops vs. low grade surface plate? How do they even measure it?

What are the OP's requirements for a surface plate?

Of course I can't give you numbers. That's the whole problem. Surface plates have a guarantee of flatness. Granite countertops do not. But from my experience, they're no flatter than a piece of MDF, and often far less flat, due to variations in consistency of the stone across the piece. The resultant "flat" surface is like a piece of wood sent through a thickness sander, with softer regions forming valleys, and harder parts forming ridges.

Is that sufficient for a residential shop? Perhaps. But if you have a Blanchard ground saw top, that's sure to be far flatter.

Of course I can't give you numbers. That's the whole problem. Surface plates have a guarantee of flatness. Granite countertops do not. But from my experience, they're no flatter than a piece of MDF, and often far less flat, due to variations in consistency of the stone across the piece. The resultant "flat" surface is like a piece of wood sent through a thickness sander, with softer regions forming valleys, and harder parts forming ridges.

Is that sufficient for a residential shop? Perhaps. But if you have a Blanchard ground saw top, that's sure to be far flatter.

I can, now:

FLATNESS TOLERANCE

The flatness deviation of a local area of 250 mm x 250 mm of the working surface shall not exceed :
1. 3.0 µm for plates of Grade 00
2. 3.5 µm for plates of Grade 0
3. 7.0 µm for plates of Grade 1
4. 15.0 µm for plates of Grade 2
5. 30.0 µm for plates of Grade 3

Of course, I have no idea how a residential (or commercial) shop would measure the above unless it was an actual SP with a cert.

And here are how the various grades are used:

Granite Surface Plates are manufactured in five grades. The typical application of these five grades of plates is as under :
1. GRADE 00 as per DIN : 876 : 84 is of laboratory grade. It is specified for precision measurement in gauge rooms and metrology laboratories for calibration masters.
2. GRADE 0 is of inspection grade. It is specified for general work in quality control.
3. GRADE 1 is of tool room grade. It is specified for production checking work through the shop.
4. GRADE 2 & 3 are intended for use in shops for general inspection. The choice of Grade 2 & 3 will depend on the application needs of the customer.

Got it from here: https://www.gmt.co.in/granite-surface-plates.html

So what are the OP's requirements?