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Does anyone know where to get a 300 mm (11.8 inch) diameter round by 2 or 3 mm thick aluminum plate for the RMAX bed?

I am going to laminate a thin piece of polyetherimide to it and the boro glass build plate doesn't transfer the heat to the PEI well enough. From an earlier post (http://forum.seemecnc.com/viewtopic.php?f=54&t=2298) PLA and ABS bond very well to and easily release from polyetherimide so that will be my build surface but it needs to be heated for it to work well.

I can't answer for something of the thickness you specifically mentioned, but this post has a great resource for some 5/16" cast rounds: http://forum.seemecnc.com/viewtopic.php ... num#p14316

I cut one down and jumped to 24V and things are awesome. About 11 min from room temp to 105 degrees and it is *flat*.

Also, I'd try to get cast aluminum over mill sheet/plate as it is much more thermally stable.

This guy has a round 300 mm x 1 mm thick glass filled Polyetherimide sheet and a 300 mm round x 2 mm thick aluminum plate to go with it (you might need to have google translate the page from German to English):

http://www.mtplus.de/3.html

Anyway, I'm guessing a 2 mm thick aluminum plate will more than likely develop slight warpage when I try to clip it directly to my ever so slightly warped Onyx heated bed.

Any ideas on how straighten out the Onyx bed so the assembly will be flat when I clip the 2 mm aluminum sheet w/ PEI to it?

Also, wondering if maybe there is a heated bed replacement for the Onyx that will give me a flatter surface?

I would agree with your hunch about the 2mm plate warping. Your flatness concerns are exactly why I went with the 5/16" thick cast plate. In short, no, you won't be able to straighten/flatten the Onyx. Even if it was perfectly flat at room temperature, it's an edge-constrained thermally expanding disc. It can't expand radially due to the screws, so it can only bulge up or down.

I took the approach that the Onyx was only a heater, not a work surface. In that respect it does a fine job. If you want flat, you'll most likely have to find a surface that you can add heat to, not a heater that can be a work surface.

Thanks Edward, I'll search the forums for converting to 24 volt power supply. Can you tell me what you used to do your conversion and will it all fit inside the RMAX?

No problem. I hope I didn't sound discouraging toward your idea, it's just that I *can* tell you what works really well, and was easy to install, too. I'm sure you can apply any surface material without a problem to an aluminum plate setup like mine.

Basically I sandwiched the Onyx between a wood insulator and the aluminum plate, both of which were slightly larger in diameter than the Onyx. I drilled through holes in both the aluminum and wood that aligned with the Onyx's mounting holes and another set of 6 at a diameter slightly larger than the edge of the Onyx. I used this second set of holes to come from the bottom with bolts, through the wood and into the aluminum, in order to sufficiently hold the Onyx to the bottom of the aluminum plate. I applied some high-temperature silicone thermal compound to the top of the Onyx (between it and the aluminum) to facilitate heat transfer. This whole assembly is then mounted on small spacers on the MAX in the normal Onyx holes.

It's really not complicated, just a sandwich.

For 24V there is a good thread around here that goes into more details than I can or want to here. You'll need a SSR (Solid-State Relay) and a sufficiently large 24V power supply. You need to caution that the resistance of the Onyx is ~1.5 Ohms (you should measure yours to be sure), so I=24/1.5=16 amps which is quite a powerful PSU at 24V (that would be just less than 400W). I found one that is 12 amps and was able to adjust down the output (there is a pot) to ~19V in order to keep things within spec, so I suppose I shouldn't say I run at 24V. I think you could also software limit the current by limiting the maximum PWM duty cycle for the bed PID loop, but I haven't tried it and don't think I need to or will.

I have been documenting all of my mods and won't make any promises as to when, but since I've been personally a bit out-of-service these past few days, and likely a few more, I might attempt to get a mod-log together and post it in the build forum some time this weekend. At that point you'll see a bit more clearly what I am describing.

Thanks Edward, look forward to seeing your build log.

Ran across this site yesterday...

http://www.sandsmachine.com/alumweb.htm

That's exactly what I referenced above when linking to the other thread. Good find!



Also, that's CHEAP for MIC-6 plate.