Large format printer -- Bed (physical not heated portion)

Hi guys, I have been trying to solve this one myself, but it's been 3 weeks without a solution.

I am building a 3d printer with a silicon pad 600mm^2. I was thinking I could get a machined metal plate but none of the local companies are even willing to give me a quote. The precision platestock has a reported tolerance of 1.5mm. I think I'd need less that .5mm, or less.. Anyone have experience with 2ft+ precision printing surfaces?

I guess you mean (600mm)^2

What about a big mirror? Too heavy?

"Cast aluminum tool & jig plate" (also called MIC 6) is very common at metal supply stores. My printer is a delta with 17 inch across max print length. I managed to find a remnant for half price.

Thinner Cast aluminum tool & jig plate (under 5/8" thick) typically has a guaranteed flatness of .015 inch (.381mm).

I'm not sure where you are, but here was the first link I found, with an average price, $82.65 for a 2" x 2" x 3/8 plate (+ shipping).

I found it difficult to print directly on the aluminum as nothing tends to stick to it, so I use a custom cut piece of mirror glass on top - because it's flatter than regular glass - and use Aqua Net Extra Super Hold hairspray, at 110C bed temp for ABS. This is a trouble free print surface - no messing about replacing tape. After about 10 prints just wash it with Windex, then alcohol, and respray a very light coat of Aquanet. But your first layer needs to be very even, and within .06mm or less of the desired height, so you'll need a good bed probe in order to print large items consistently.

I level the bed to within .02mm, and set the bed hight to .02mm of the desired height before printing each time.

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My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
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Get something nearly flat (e.g. 4mm aluminium sheet) and give it a 0.5mm thick coat of epoxy paint or automotive body filler. Put some kind of grinder head on your printer and send it to all points. Repeat if necessary.