Marble vs Granite surface plate

[Brento]

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]

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]

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]

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]

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]

Wish i could measure to see how it is.

 

[Dabbler]

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]

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]

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

 

[Ulma Doctor]

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