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PhilDAS-
kitchen top offcut of granite
I wondered about that, but you not only have no idea how flat it is, its is also much thinner than a surface plate so it will sag under its own weight, and any instrument load you put on it. A 900mm×600mm surface plate is 100mm thick, which makes it (100/30)3=37 times stiffer than a 30mm thick worktop.
gbj_tester
What is the cheapest form of a "surface plate" or similar very flat surface for machining, engineering, etc sort of projects (ideally fairly big - maybe 24" x 36"?). I'd probably be happy with some localised damage or deformation, and material isn't super important (interested in any input on relative benefits of steel/stone/phenolic board? - not sure what other options there are?).
I am finding small used surface plates on ebay around £100 but wondering if I'd find something cheaper with a different search?