# Granite plate



## tarmo120 (Jan 27, 2013)

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?


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## mitsue (Jan 27, 2013)

if you have a good straight edge check out you local granite countertop supplier. lot of pieces thrown out there I bet.


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## tarmo120 (Jan 27, 2013)

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.


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## mitsue (Jan 27, 2013)

tarmo120 said:


> 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


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## Big_John (Feb 6, 2013)

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.


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## itsme_Bernie (Feb 6, 2013)

Wow John!  

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


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


Bernie


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## DMS (Feb 6, 2013)

I don't know how they do it with granite, but you can create your own references in cast iron using the 3 plate method

http://www.google.com/url?sa=t&rct=...9YCACA&usg=AFQjCNHBSfn5wK5jDVmON_ZMRKTwxxFtuA


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## mitsue (Feb 6, 2013)

Big_John said:


> 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.


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## Big_John (Feb 7, 2013)

mitsue said:


> 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.


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## Ray C (Feb 7, 2013)

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.





Big_John said:


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


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## fastback (Feb 7, 2013)

I think you guys already know this but before WWII for the most part all that was used was steel or CI for surface plates.  Since there was a shortage of iron the industry turned to granite.  After the war this continued since granite has great wear resistance and never rusts.  I am fortunate I have both.  The one I actively use is the granite one, its around 12 x 18 and is a B grade.  I plate I think that that one is 18 X 24.  At Least I think it is.  I have built covers for both of my plates and keep them covered unless I am using them.  


Big John, I for one would like to know how they check the surface plate for flatness.  Thanks for the offer.


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## Big_John (Feb 7, 2013)

Ray C said:


> 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.
> 
> ...



I'd like to keep any discussions on forums like this technical in nature. 

An 18 x 24"  Grade B plate can be bought for $154 from MSC. That's an  import though, much softer than a good pink Starrett, but very  serviceable in a home shop where it's use is limited. Used plates can  sometimes be bought for much less, although sometimes they may have  excessive wear. Again, it depends on the accuracy you need. A worn  surface plate is better than no surface plate. 

Coincidentally,  my wife used to work for one of the largest granite counter top  distributors in the US. She got a good laugh out this. She understands  the difference.


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## Big_John (Feb 7, 2013)

fastback said:


> I think you guy already know this but before WWII for the most part all that was used was steel or CI for surface plates.  Since there was a shortage of iron the industry turned to granite.  After the war this continued since granite has great wear resistance and never rusts.  I am fortunate I have both.  The one I actively use is the granite one, its around 12 x 18 and is a B grade.  I plate I think that that one is 18 X 24.  At Least I think it is.  I have built covers for both of my plates and keep them covered unless I am using them.
> 
> 
> Big John, I for one would like to know how they check the surface plate for flatness.  Thanks for the offer.





http://www.qualitydigest.com/aug03/articles/03_article.shtml 

Here's a good article on plates and gives some history.

As I explained, plates are checked for flatness and repeatability. 

Repeatability is checked with a "Repeat-o-meter" (I didn't make up these names). Basically, it's a sled about a foot long that flexes in the center. An indicator measures the deflection of the end of the sled as it is drawn over the plate. You can do the same test with a gage block, a height gage and a good dial indicator. The block is placed at different spots around the plate and a measurement is made. This will tell you the repeatability of the plate. The Repeat-o-meter is just a lot faster and makes more repeatable measurements. 

Here's a picture I.... ummm... borrowed off the web.




The one I use has a better Federal gage head and electronic readout.

Flatness is another measurement. There are a few ways to do this. Laser, electronic levels, autocollimator and a straightedge system called "planakator" is used. I've personally used all those methods. Most common though are autocollimators and planakator. I use both, but for most field work, the planakator system works best for me. 

Here's another borrowed image:




Basically it's a granite straight edge that is leveled to the top of the plate and an indicator is run between the plate and the straight edge. A plane is established and a grid of check points laid out. Unfortunately, many of the "fly by night" calibration outfits don't do this important test.

Now if the plate doesn't pass these tests and isn't too far gone, it can be repaired by lapping. A cast iron lap (sometimes an old cast iron surface plate) is charged with diamond or carbide grit and the table is lapped back into tolerance.

Here's a video of one of my guys working up a sweat.






If this really appeals to you, here's where you can buy the stuff. http://www.tru-stone.com/pages/price_lists/price_list_popup.asp?id=8 

Most likely, not in reach of the guy in his home shop, but that's what you need to really do it right.


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## darkzero (Feb 8, 2013)

Big_John said:


> An 18 x 24" Grade B plate can be bought for $154 from MSC. That's an import though....



In addition, an import 18 x 24" Grade B plate can be purchased from Enco for $46. I got my import 18 x 24" Grade A plate from Enco for $37 shipped which now costs $114. Shars also offers black granite plates for similar/great prices. For the average home user, I doubt most would even notice a significant difference, if at all, between the ones mentioned. Of course not always true but price does not have to dictate how useful or good something performs. If it works well for you then that's all that matters. There will always be someone who has a different opinion.


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## fastback (Feb 8, 2013)

I also saw that granite plat on sale at Enco.  It is black granite and is 3 inches thick.  Also come with some kind of certificate.  Oh, it is foreign made.  Sounds like a decent deal.


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