Granite plate

tarmo120

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Locally I don't have any suppliers who have a granite plate for machinists.
But I do have many tombstone and worksurface suppliers.

What I wish to know, is do I need to get a special polished and ground plate or does a regular polished tombstone or worksurface do the job?
 
if you have a good straight edge check out you local granite countertop supplier. lot of pieces thrown out there I bet.
 
Ok, I will do that. Thickness would be, the thicked the better. I'm guessing 1 inch and thicker would be fine.
And the contour size maybe 20''x25''?
Or is that too big?
I also googled and found out that the best is black granite.
 
Ok, I will do that. Thickness would be, the thicked the better. I'm guessing 1 inch and thicker would be fine.
And the contour size maybe 20''x25''?
Or is that too big?
I also googled and found out that the best is black granite.

I have a 12" x 18" x3" B grade. 1" is probably ok for home use. Set it on three rubber feet, two set it from one end and one centered and set in on the other end. that should help keep it flat. (there are specs for foot locations depending on plate size in the machinery handbook)
Darcy
 
I'm late to the discussion, as usual....

It depends on what you need to measure and how accurate your measurements need to be.

A cast off piece of countertop is just that... a countertop. It's not a precision tool like a surface plate.

Surface plates are checked for overall flatness and repeatability. Overall flatness is simple, that is how much the plate is concave or convex. Grade B for a 18 x 24 (typical bench top size) has a flatness tolerance of .0003". Grade A is half that and AA half again.

Repeatability is slightly different. That means any spot on the plate can measure within a certain spec. Using the same size plate in Grade B, you would be able to measure at any spot within .00011". Grade A is .00006" and Grade AA is .000035".

BTW, plates do wear with use. Shops with a good calibration program have their plates checked on a regular basis. BTW, that's one of the things I do for a living. If someone really wants to know how it's done, I'll be glad to share.

Better plates have higher amounts of quartz visible in the granite. The pink Starrett plates are very hard, as is the Rock of Ages plates. Plates that are jet black are cheaper and a lot softer.

For home shop use, you can buy a cheap Grade B plate from someone like MSC. Keep it clean when you are using it and covered when you aren't and it will serve you well.
 
Wow John!

I'd love to hear how it's done!


Any way to replicate the method, to less accuracy, with home tools?


Bernie
 
I'm late to the discussion, as usual....

It depends on what you need to measure and how accurate your measurements need to be.

A cast off piece of countertop is just that... a countertop. It's not a precision tool like a surface plate.

I suggest you put a staight edge on a piece of granite before you make blind statements like that. Most home shops don't require a B grade plate. It makes a cheap alternative to spending a few hundred dollars for a flat surface. I'm betting you never have checked a piece of granite with a good straight edge. No it not perfect but better than nothing and affordable for home use. If you need better than .0003/12" then buy a surface plate.
 
I suggest you put a staight edge on a piece of granite before you make blind statements like that. Most home shops don't require a B grade plate. It makes a cheap alternative to spending a few hundred dollars for a flat surface. I'm betting you never have checked a piece of granite with a good straight edge. No it not perfect but better than nothing and affordable for home use. If you need better than .0003/12" then buy a surface plate.

I suggest you ask yourself why you need to reply to my post in that manner.
 
All,

Within the written word, different interpretations of statements often arise... I don't get the sense of any major dispute here but let's not head down the wrong track and let's stick to technical discussion.

That said, a piece of counter top or a precision plate are likely made of the same substance but the manufacturing process and quality controls for each are very different. That much, I think we can all agree on. In a home shop environment, we improvise according to our budgets and needs. For the sake of those who aren't familiar with things like this, lets just get the message across that sometimes, you can get away using substituted tools with good results but, you can't rely on it in all cases...

Peace...

Ray

EDIT: I want to point-out that counter-top material is usually very thin. A and B grade slabs are thick for a reason and it's only partially what you think... It's to control the rate of expansion and contraction under ever-so-slight temperature variation conditions. The point being, there's a difference between dedicated tools and substitues and it's important to at least understand these reasons so when we take shortcuts, we're aware of the possible pitfalls... In all likelihood, these kinds of factors won't come into play in a home shop environment.



I suggest you ask yourself why you need to reply to my post in that manner.
 
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