Marble vs Granite surface plate

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.
 
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.
 
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.
 
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.
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If i could get two more i would possible do that.
 
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?
 
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).
 
The orange ones are the new ones.
 

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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.
 
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.
 
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.
 
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