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Surface plates come in three grades: AA, A and B. The AA grade has the best flatness, while grades A and B have respectively 2 and 4 times less accuracy. Grade B is still much flatter than normal surfaces like tables, doors, windows etc. According to this table, a grade B 72"x144" surface has an overall flatness tolerance of 4400 microinch, which is better than 1/200".

I am looking for an intermediate form. Are there also 'grade C' surface plates? Those would be less accurate than grade B surface plates, but more accurate than standard surfaces. They would probably also be cheaper than grade B. Maybe this has a different name than surface plate?

Riemann
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  • Don't think so because at grade B the cost seems to mostly be material anyways. – DKNguyen Feb 08 '24 at 17:50
  • So where did you get that they come in 3 grades AA, A & B? Would C be number 4? But could you get a machine shop to make you one? – Solar Mike Feb 08 '24 at 19:30
  • @SolarMike The wiki page probably. In any case, I've never seen anything lower than Grade B for sale just like gage blocks. – DKNguyen Feb 08 '24 at 19:46
  • @SolarMike I saw that in the table that I linked in the question. – Riemann Feb 09 '24 at 12:30
  • https://shop.mitutoyo.nl/web/mitutoyo/en_NL/mitutoyo/1299830885531/Granite%20Surface%20Plate%20Grade%200%2C%20BS%20817/$catalogue/mitutoyoData/PR/517-918-0/index.xhtml and https://shop.mitutoyo.nl/web/mitutoyo/en_NL/mitutoyo/1299830885531/Granite%20Surface%20Plate%20Grade%201%2C%20BS%20817/$catalogue/mitutoyoData/PR/517-918-1/index.xhtml – Riemann Feb 09 '24 at 14:05
  • Here are two surface plates with the same dimensions. The more precise one is 9000€ and the less precise one is 8000€. However even the 8000€ one is for 0.576 cubic metres. That is more than 10 times the price of granite blocks as in the following link: https://www.stonecontact.com/p/granite-blocks-prices – Riemann Feb 09 '24 at 14:13

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