Has anyone built a DIY large print bed ?
I’m thinking of building a 500 x 500 x 500 printer, and wondered if anyone has found a “this everyday item makes a fantastic flat bed” ?
Yes, but I used cast aluminum plate. I guess that’s an everyday item for me because I am a hobby machinist, but might not be otherwise. 
If you are going to print only in PLA, some suggest that a heated bed isn’t necessary. Heated makes flat harder, because wrought aluminum has internal stresses that often cause it to warp when heated. The flattest everyday thing you can probably find is glass, and there’s at least been suggestions that mirrors have closer tolerances. But I don’t think I’d want to glue a heat pad to a glass bed.
I wouldn’t start your design with the bed, though. What kind of motion system do you have in mind?
I would probably only use it for PLA, so a mirror maybe a good start.
Planning on cartesian, as I have 2 already I could use as a reference.
Sure, but there are lots of ways to do Cartesian kinematics.
That’s quite large for a bed flinger. Bed mass will increase as the square of the size of course. At that size, most go for moving the head in X and Y and the bed in Z. Larger than that the bed is often completely stationary and the X/Y motion platform raises in Z. You can stack X and Y and get Cartesian kinematics without using a gantry and flinging the bed of course…
I would expect some sag in the mirror at that size. I’ve never tried an unsupported mirror. Are you planning mesh bed unflat compensation?
I’ll probably build an extrusion base for the mirror to sit on, then mesh level with an induction sensor or BL Touch.
Good point regarding the design - I havent got very far with the practical side of things- it’s all early days. A moving bed sounds like the way to go.
I guess if I had a stepper on each corner, they could also be used to level the bed.
Note that making the extrusion itself flat is itself difficult, and “potato chip” warping is common and hard to see. Turns out that flatness is the core of all precision. But I’m considering the fact that your bed might sag at that size.
The four-stepper approach is specifically for dealing with flexible beds that can warp that way. The Voron is an example. I call it going out of your way to spend extra to make things worse. 
It would be cheaper to just do it right and buy a 500mm x 500mm piece of cast aluminum plate and use a single stepper to drive it up and down. If you use a kinematic mount, you never have to worry about it being either out of flat or out of tram. (“tram” is what people usually mean by “level” — it means all axes of movement being at right angles to each other. “Level” came to be substituted for “tram” I think because on lathes, extremely precise levels (“master precision levels”) are the most convenient way to set up tram.)
At that size, you might consider a larger nozzle and high-flow hotend if you want prints done in a reasonable amount of time.
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Great stuff - I’ll consider my options 
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