Flat and Cheap?

[nSomnius]

There didn't seem to be a best place to ask this, here is the best alternative. I'm working on a project that would benefit from a support of as near perfect flatness as possible, as inexpensively as possible. Material itself is actually not that important, it's a light weight low force application and it's best to think of it just as a table surface to place a light weight object on. The area would be, again, not critical, but about 7" x 7" ideally. If you have minimal design constraints and wanted a super flat registration surface for the lowest cost, what would you chose? Preference would be lighter over heavier, thinner over thicker if anything. I don't mean to sound outrageously vague, just hoping to help think outside the box.

[Geof]

Get the self-levelling floor topping compound that The Home Depot has. Make a frame out of wood and plywood, a good strong, stiff frame, maybe a torsion box style, with a rim around about 1" high and pour the topping compound mix into this.

[txcncman]

How flat is flat? MIC-6 aluminum plate?

[nSomnius]

Geof, were one to measure the flatness of such a surface, what do you think it'd be over that distance, discounting for meniscus at the edges? An optimal spec would be say +/- .oo3", in that realm?

@txcncman, that would be flat for this use.

[mcarvey]

Plate glass is fairly flat and may suit your needs if you don't need to drill into it. I did a quick search and there seems to be plenty of posts about glass flatness on various machining and telescope forums.

-Matt

[underthetire]

Go to home depot and buy a 8x8 tile of granite. Not too heavy, and is cheap and flat. You can drill in it with masonry bits if you needed to.

[Geof]

Actually I read your size too quickly and got 7 feet by 7 feet so my suggestion is not really applicable.