is all granite created equally?

[jonnyfontaine]

is all polished granite absolutely flat? or are the slabs you find in woodworking shops that are dead flat to like .001 or so specially machined... i have a friend that is getting rid to a slab of polished granite from his house and said i could have it, but i don't have a good straight edge to check it out with, so i'm wondering if i can just assume that it is flat or not? thanks, jonny

 

[CarvedTones]

I don't know the answer, but I do know that depending on what the use is, some people have to get them machined from time to time. I have a piece of fake granite tile that I use for sharpening (I attach fine sandpaper from the auto parts store with spray adhesive) that is flat according to my best edge. Somewhere I have a marble slab I got from a kitchen place. It arrived with a cracked corner and they sent another, so I kept the first one (they did not want to pay freight to get it back) thinking I would use it in the shop, but it turned out not to be dead flat and it scratches easier than I expected (it is marble, not granite, so this may be irrelevant info). Those are nice in the kitchen, BTW. You put it in the freezer for a while before you use it and you get more time rolling dough before it starts getting tacky.

 

[McRabbet]

Generally speaking, if the surface reflects an image without distortion, it is pretty close to being flat. If you can look at a reflection of a straight object, like a windowsill or a door frame and it looks straight in the reflection, then the piece is probably pretty flat (distortions will show very obviously). If it is dished or bowed up, it will distort a reflected image.

 

[Mike Davis]

is all polished granite absolutely flat?

NO

or are the slabs you find in woodworking shops that are dead flat to like .001 or so specially machined...

YES

i have a friend that is getting rid to a slab of polished granite from his house and said i could have it, but i don't have a good straight edge to check it out with, so i'm wondering if i can just assume that it is flat

NO


or not?

Right

thanks, jonny

You can check it visually for accuracy to maybe 1/64 across several inches.

To get within .001" you need an Optical Flat.

Here is one of several manufacturers

http://www.lapmaster.com/accessories/flatness-instruments/default.html

 

[Gotcha6]

http://www.wttool.com/product-exec/product_id/17758
This is a link to a machinist's granite surface plate source. Price is relative to size & accuracy but they're a pretty good source on this sort of thing.

 

[Mike Davis]

50 millionths accuracy in a 12 x 18 for $50 is pretty darn good.

But, Jonny

the one question you didn't ask is

"Will a scrap piece of granite be good enough to sharpen chisels and flatten plane soles?"

Most likely, YES!

And free is always better than having to pay. :icon_thum

 

[Terry]

Jonny,
Back in my younger days I worked for the Rock of Ages in Barre Vermont in my Uncles Granite Shed. Depending on what the polished granite is used for it is generally flat, but not always. Most counter tops are flat with the exception of the bullnose on the front of the counter. I have some black granite in my shop that is polished and I will be glad to check it. But I called a high school friend tonight in South Ryegate Vt who owns Gandin Bros Granite and he confirmed what I just told you. Hope this helps. If you would like the number of my friend PM me and I will give it to you. Also you might want to use a laser level if you or a friend has one. If there is a break in the beam you have a rise or dent in the granite.
Terry:wsmile: