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Granite surface plate problem

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PekkaNF:
Unbelievable......

I have two surface plates, one is cheap chinese 200*400 mm and it is pretty accurate. I have faith in it, because it gives logical results.

Then I got one almost free, just perfect size, 630x400x100 mm, brand name (MAHR), but it has edges chipped, dents all over, one end rough and what is worst, it is not very straight!

I used brayer and rolled pretty thin layer of blue and armed with SIX straight edges of varying pedigree from 400-1000 mm length.

Idea was to check all sides and diagonals. I did 8 lines and union flag patterns with one straight edge a time, cleaned the edge and leveled the blue on surface plate between each.

Looks like the surface plate has low front, worn on the middle and rough spot on one end.

Pretty annoying really, because that was supposed to be my reference to check all other instruments.

I am pretty sure that only way out of misery is to buy a new or checked surface plate. Nobody here seems to have heard of surface plate reconditioning for a private person. It's only 75 kg or such I could transport it easily at the back of my car some 200 km. But nooooo.

Now the question: Chinese or (east) european? is there price break point that defines most likely good vs. most likely bad?

Pekka

Joules:
Look up a company that supplies and polishes head stones.  You may be able to have the surface ground and polished.  It won't be as good as a precision surface but better than what you have now.

PekkaNF:
I visited local rock mason shop and they had plenty of shiny rock. I had 0,01 mm/div. clock and straight edge with me. All had a lot of twist and waviness. Way too much. I think the polishing is is only visually fine.

I thought that they just had bad machines and went to other shop. They had a very nice saw and that produced pretty straight cut, but those plates were not near surface plates after they come out of polishing. Really weird. Maybe the rock they use is visually very interesting and proverbial to grind flat? Some of the prettiest granite you can feel the unevenes of the surface.

Pekka

Manxmodder:
As Pekka notes,most stone mason shops have grinding/polishing equipment that is suited to producinf a good aesthetic appearance. High precision flatness isn't of any concern to their purpose so long as the product looks good to the eye.....OZ.

ETA: Mahr is a reputable brand in metrology equipment,so the surface plate you have is likely worn.

BillTodd:
Have a search on youtube for oxtool's video of having his plates reconditioned. interesting and gives a good idea of how to go about fixing the plate.

Mahr is a top notch manufacturer so will be the best granite available and probably well worth reconditioning.

Bill

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