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[Metrology] Flatness Of Granite Counter Top

CPCaballero

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#1
Does anyone know how flat the typical granite counter top is and its suitability for use as a surface plate? We will have a few cutouts from a kitchen remodel job and I'm looking for uses for them.
 

Subwayrocket

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#2
i'd be interested to know . Have a buddy that owns a granite business. They let me pick thru dumpsters of beautiful cut offs ...some large . Nice for making water features with ...never thought of a surface plate though .
 

Bob Korves

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#3
Unknown quantity. Will vary from one to the next. Surface plates need to be held to very tight tolerances and counter tops do not, so they are not. If you want a surface plate, import ones are being sold at really cheap prices. Look for a free shipping deal, and pull the plug. You will likely get a good one, though quality control of Chinese surface plates, like their other stuff, is often somewhat lacking. Granite counter tops would likely make a great lapping plate for non critical work, as will plate glass.
 

PaPa_Jack

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#4
I have wondered the same thing. How many of the members on this forum actually have any need to be that accurate? I can't recall seeing or reading anything about any project that required that level of accuracy. Some of the gunsmithing people might benefit from .0001 accuracy, but I am not sure it even matters that much there.
 

T Bredehoft

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#6
If cost is minimal, get three and lap them all together. eventually they will all be flat, but you need three. No guarantees about how long they'd stay good, either.
 

Ulma Doctor

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#7
:sherlock::nail biter:

ok ladies and gentleman
i have a 6x6 piece of counter top grade rock that seems to be polished up pretty good at my shop,
i'd be willing to give it a rub on my recently recalibrated plate and see the result.
something tells me beforehand that the counter top will blue up pretty good, but then i have been wrong before!:grin:
pictures to come!!!
 

4gsr

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#9
Debur the corners and try again!:grin: Just kidding..:cool 2:

Be interested how many impressions it takes to get about 70% coverage from scraping.
 

4gsr

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#10
I wonder how much stress relieving the rock sees when it is sawn up into smaller square pieces as you have shown?
 

Tony Wells

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#12
Since it does show contact only at the corners, I'd suspect it does have something to do with being unconstrained now that it is sawed out. If this is a sacrificial piece, grind back the corners a bit and see if the contact spot grows, or disappears. I'm sure it could be lapped flat, but as thin as it is I wouldn't count on it being stable.

I watch the installers put in Italian marble on the nurse stations at the hospital, and it comes in pretty good size slabs, all pretty much cut to print. Where the piece has to be longer than they can practically handle, they shim and play with it until it is very close, and epoxy it all up, then polish to blend. It takes a long time, but the seams are invisible. There is some talent involved. You cannot even see a flaw by eye from a low angle. They have it down really well. But marble isn't as hard as granite. I wonder about the abrasive they use. I'll have to ask or get some next time we do a remodel.
 

Bob Korves

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#13
here is the test:
calibrated surface plate to Grade A, cannode blue ink , test @72*F
https://plus.google.com/u/0/collection/E8Tpv

before
(snipped pics)
after:

end result: FAIL

the granite counter top is not flat, touching in only 4 corners and hollow in the middle >.001"

your results may vary,
use only as directed, yadda yadda yadda :p
One good test is worth a thousand expert opinions, and perhaps a million hobby-machinist opinions... Thanks, Mike!
 

Uglydog

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#14
Will make a really classy bench top! Easy to wipe down.
Much easier to maintain than the diamond plate to which I default, although rounds stuff seldom roll off the diamond plate.

Daryl
MN
 

PaPa_Jack

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#15
I seem to recall reading that some colors of granite are more stable than others. Black granite is supposed to be the most stable. I realize that most of us are striving for the holy grail of accuracy but I hope new people, like me, won't get hung up on that. I worked with troubled teens for a while and many of them got hung up on meeting some unattainable goals and it made them feel like a lesser person because of it. The title of this forum is HOBBY-Machinist. I get a little frustrated when I can't get something just right, but I always fall back on the fact that this is supposed to be fun. If I thought that a perfect granite plate was a necessity, I would be severely disappointed. It is a goal, but not a requirement. That money could buy a lot of stock to play with and practice.
 
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4gsr

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#16
PaPa Jack,

I couldn't agree any more.

Some of us will strive for the perfect flat rock we can find. Just in our blood. And I've used the concrete floor for a surface plate, too! Not so much for flatness but for a setup we had to do to check concentricity on a 30 foot seal assembly that had to run within .010" T.I.R. end to end and .003" T.I.R. between connections. Each seal assembly had around 12 connections in the 30 foot of length. The real fun was stabbing the seal assembly into the seal bore without it hanging up! We did that vertically. I got way off subject here. Sorry, Ken
 

kd4gij

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Active Member
#17
I have a piece 30x30x1 1/2 thick. It is plenty flat enough for the layout I need to do as a hobbyist. My neighbor that was doing granite work at the time even rounded the corners and polished the edges before he gave it to me.