# Marble vs Granite surface plate



## Brento (May 23, 2021)

So this weekend i bought a granite lectern and i plan to use the granite as my surface plate and make a cover for it to maybe do flat top sanding on it.  I have heard for a hobbyist shop a granite countertop would suffice. I am curious what everyone would think about a marble top? I may be able to get a marble top as well from the seller if he can find it.


----------



## rabler (May 23, 2021)

Marble is going to wear much faster than granite, and therefore need more frequent recalibration.  It also chips easier, and is more prone to being etched by anything acidic.


----------



## SLK001 (May 24, 2021)

Marble isn't as stable as granite.  I only know of cast iron and granite being used as surface plates.  Also, some granites are more stable than others.


----------



## Brento (May 24, 2021)

So if i got marble i should maybe use that as a flat top sander as long i have a cover over it to help protect it?


----------



## Dabbler (May 24, 2021)

Two of my friends have tried counter tops for GSP, and they both have been disappointed, especially as offshore SP in granite are are around 40$ CDN - probably cheaper in the US.

Both have found that they are beautifully smooth, with local undulations as much as 3-5 thou.  This is an artifact of how they are polished.


----------



## Brento (May 24, 2021)

Wish i could measure to see how it is.


----------



## Dabbler (May 24, 2021)

Brento said:


> Wish i could measure to see how it is.




You can use your mill - you place the plate on it using your tramming gear.  You indicate each of the corners so that they are as close to the same measurement as you can get.  Then just travel back and forth over the whole surface noting the high and low spots and marking them with a sharpie.  You can take some 400 wet and dry and wet sand the high spots, to get a more usable surface.  

That's if your granit is free - otherwise, it is easier/better to just buy an offshore plate.  My 2 offshore plates are easily within a tenth over the entire surface.  My mitutoyo plate is much more accurate.


----------



## T Bredehoft (May 24, 2021)

I was gifted with a sink cut out of "granite", white, so probably mostly ceramic filler of some sort. Using a Federal indicator on a standard base, Lentghwise I found less than .0005.  Crosswise, if I held the base in the middle and swept from edge to edge I found about .001, high on each side, with .0000 in the middle. As a fella I ised to work with would often say "Good enough for the girl I go with."  Its abouit 2 feet long and 18" wide.  I don't work for NASA, so when I need a surface plate, this will be it.


----------



## Brento (May 24, 2021)

The granite was 10$. @T Bredehoft thats about the same size as mine like 1.5" thick.


----------



## Ulma Doctor (May 24, 2021)

thick glass(>3/8") is, generally speaking, flat enough for some folks need for accuracy
glass platens are often used as backing for sandpaper when lapping small parts flat

i tested countertop granite against a surface plate, and it was nowhere near as accurate as a surface plate
but run what you brung, if you have no need for extreme accuracy


----------



## T Bredehoft (May 25, 2021)

While gainfully employed I had access to an (approximately) four by  five foot black granite surface plate. Tool room quality. It was flat, no question, my Kitchen Counter Granite will do for my projects. Absolute accuracy is great, but it's cost is not returned in my projects, no revenue from them, just pleasure of work.


----------



## SLK001 (May 25, 2021)

If really interested in flatness, get three pieces of free granite (all close to the same size) and some lapping compound and then lap the three plates to the flatness you desire.  Gingerly discusses the lapping process in one of his _Build Your Own Metal Working Shop from Scrap _books.


----------



## RJSakowski (May 25, 2021)

Countertops are made to be smooth and shiny.  Flatness is not a spec.  I found this out when I tried to extract a flatness spec from three major countertop manufacturers.  The best that I could get was 1/8" over a 5' x  9' surface.  A surface plate is made to be flat.  Without that specification, it isn't much use as a surface plate.  This doesn't preclude its use as a setup surface.

Another source of granite surfaces is monument makers.  Inevitably they will mess up an inscription.  A neighbor made a number of benches for the house and yard from reject tombstones.


----------



## Brento (May 25, 2021)

SLK001 said:


> If really interested in flatness, get three pieces of free granite (all close to the same size) and some lapping compound and then lap the three plates to the flatness you desire.  Gingerly discusses the lapping process in one of his _Build Your Own Metal Working Shop from Scrap _books.


If i could get two more i would possible do that.


----------



## Brento (May 29, 2021)

Just to revive this thread a little today i am picking up 2 more Granite pieces. Orange/pink/black. One 12x20 and other 25x5 both 1 inch thick. I am thinking about lapping them in. What compound do i need? How much? Any pre work i need to do before i start?


----------



## Dabbler (May 29, 2021)

Any abrasive powder will do in the lower grits, say, 1000, but the finishing work is always done in Diamond, (I don't know why).


----------



## Brento (May 29, 2021)

The orange ones are the new ones.


----------



## Dabbler (May 29, 2021)

for the 3 plate method to work, each plate has to be the samd size and shape.  That long one could be cut into 3 for practice.  It isn't a no-brainer, it takes a little practice to do a good job.


----------



## Brento (May 29, 2021)

So they have to be all same size cant be a little different in size?  I know its no brainer to do i just dont know what compound to use.


----------



## Dabbler (May 29, 2021)

Places that sell telescope grinding materials like  Eftonsience is a good source for the right grits.  

You grind the similar to making a telescope, except you don't rotate the plates, but you do walk around the work, making the strokes radial.   Usually the lower grits are carborundum, and the higher grits are diamond.  

Here's one link on what you want to do, but there are lots of others.


----------



## SLK001 (May 29, 2021)

The "3 pieces of equal size" requirement is to prevent dishing of your plates (or, one concave and one convex).  A "dished" set will _look _good, because all areas are ground, but in reality are unusable.  Grind Plate A and Plate B in a circular pattern for 100-200 circles,  then grind Plate A and Plate C in the same pattern, then grind Plate B and Plate C the same way.  Since you probably don't have a Repeat-O-Meter, so grind your surfaces until the scratches are all over the plates.  Proceed to the next finer grit and repeat the same grinding.  At the end, you'll have three usable "surface plate lite" that can have quite good flatness specs.

McMaster has the grinding powder (use silicon carbide or diamond powder).


----------



## Brento (May 29, 2021)

Ok so it has to be the same size


----------



## brave_ulysses (May 29, 2021)

GotGrit.com
					






					gotgrit.com


----------



## Dabbler (May 30, 2021)

@Brento I didn't realize that 'got grit' was still in business!  When I was building telescopes (aeons ago)  we got our supplies from Telescope Supply, now long defunct.


----------



## Brento (May 30, 2021)

I went to got grit really quick but all they had was 5 pound containers. Does anyone make a kit for the material. I am also waiting as i need to still find 2 more plates of the same size. I was hoping i could use the big orange one with the big black on and get another to do the lapping. Its gonna be hard to fond the same sizes.


----------



## Dabbler (May 30, 2021)

one of 2 [Edit] _More like 5_ of these will suffice for a 12 X 16" surface plate using the 3 plate method.   [Edit]  _No you are better off with buying 5 packages of Got Grit abrasive...
_





						Amazon.com: 7pcs 20 gram Diamond Lapping Paste Polishing Compound For Fine to Final Polishing Grits 0.25 to 5.0 microns : Industrial & Scientific
					

Buy 7pcs 20 gram Diamond Lapping Paste Polishing Compound For Fine to Final Polishing Grits 0.25 to 5.0 microns: Finishing Compounds - Amazon.com ✓ FREE DELIVERY possible on eligible purchases



					www.amazon.com
				




*I think I need to state again:*  by the time you've purchased the materials for lapping you could have bought 2 offshore surface plates:









						SURFACE PLATE 12IN. X 18IN. X 3IN.
					

Precision Surface Plate - Black Granite.  Harder than steel and does not nick or burr.




					www.busybeetools.com
				




P.S.  Surface plates are way cheaper in the US than Canada.


----------



## Brento (May 30, 2021)

Im gonna have to check my black one to see the state it is in and then i guess break down and buy a smaller one i dont need a big one but it would just be nice.


----------



## SLK001 (May 30, 2021)

Go to almost any auto parts store and get some valve grinding compound.  That should suffice.  You don't need a super polished surface, just a flat one.


----------



## Brento (May 30, 2021)

Yea still need to get 2 more plates though


----------

