Lesson: Never order something and not test it, especially if its from a questionable source.

I had ordered my heated bed, an ALU-MK3, from Amazon through a generic, overseas vendor. I knew the risks involved in purchasing items from lesser known manufacturers, but I was on a budget and even the $30 I saved wasn’t worth it.

The bed remained in its box until I got around to installing it recently. That was very silly of me. I should have inspected and tested it well over a month ago. Functionally, it’s as described. It heats up just fine and is a nice solid piece of aluminum. It’s unfortunately warped, however, and pretty badly, too.

There’s dishing near the top left corner and cupping in random places. It’s blatantly visible, too. But because of my procrastination, I feel the fault is mine on this one. Since I couldn’t return or exchange it, I just ordered a new one from reprap.me.

This new bed appears much better. It uses a SMD type thermistor (which was graciously soldered on for me upon request!) and uses the original wiring configuration as outlined on the reprap wiki, found here. This time, I did the straight edge test as soon as I took it out of the box and it’s as flat as flat can be – I couldn’t even see light gaps, let alone fit a piece of paper underneath the straight edge. Fantastic! The folks at reprap.me also packaged the bed up inception-style, nested several layers in. It was well protected and arrived in good condition. I’ve read some chatter on the reprap forums about their products – some good, some bad, but from my limited interactions with them so far, they’re excellent and have great customer service (especially soldering that SMD thermistor – that would have been a nightmare for me). Doing something small like this for your customers speaks volumes for the quality of service, in my mind, so kudos to them!

I subbed out the standard M3x14mm hex socket cap screws for M3x16mm button head hex socket cap screws. I wanted them to be a bit longer as the standard 14mm length won’t leave much thread left after adding a layer of cork and using (1/4″ aluminum) spacers to prop the bed up high enough to clear the cork. I added a drop of medium strength loctite to each one since I’m going with solid mounting. My frame has most of the M3 holes pre-tapped, as is the case with the bed mount, so while I technically don’t need the M3 nuts, I’ll probably swap out those for even longer screws down the road to give me a bit of extra thread underneath to tack those on for added security. For now, the loctite should do the trick.

I had originally purchased the 2mm laser cut cork insulator from reprap.me as well, but after some thought, I put it aside as a spare and went back to the design I was going to use with my original bed based on the myriad of complaints surrounding the MK3 Alu heat bed’s heat up times (thicker is better, I hope – but also adds more weight…which increases momentum…which makes it harder to stop – which could induce jerk and therefore ringing – there’s a LOT to think about when building these things…). I picked up some 1/4″ thick, 12×12″ cork tiles from Michaels (a hobbies / arts / crafts store in Canada – not sure if they’re global!), used the 2mm cork insulator as a template and traced it out onto the best looking tile. Cork is really tough to cut (I wish I had a hot wire or laser cutter for this), even with a brand new knife – even after trimming the one tile, my wrists killed. You can’t just cut it straight by dragging your knife along the cork, you sort of have to “saw” it first – think of it like making perforated lines in a sheet of paper. Once you get the perforations in, you can make 1 – 2 final passes with a regular dragging motion which should complete the job. From there, you can use a sharp hobby knife to clean up any ragged edges.

Some people like to punch holes in the corners where the mounting screws would go, which I had originally planned on doing. I had a sharp awl, which I used to try and make said holes but they are so close to the edge, the cork ended up breaking up, so I just nipped it and the remaining corners off. Besides, I want the cork to float a tiny bit above the bed carriage / mount, and I DON’T want to tighten the bed down directly on top of the cork (I feel this would cause leveling issues since cork compresses and each corner could be potentially different).

I took the cork and glued it to the underside of the bed, lining up the nipped corners and the cutout for the wiring, using red permatex silicone. I wiggled the cork back and forth to evenly distribute the silicone – you don’t want lumps here, if possible (it won’t affect the levelness of the bed, but it can be unsightly and cause the cork to want to droop, not to mention it creates air gaps which has the potential to lower your insulation effectiveness). I do have ultra copper, but I would worry about possible shorting if a path on the coils is exposed due to say, wear / rubbing. Red is fine since it’s good up to 343C – I highly doubt I’d ever be pushing the bed that high, even if it were capable of it.

I took that setup and sandwiched it in between two bamboo cutting boards, with a 10lb weight on top for a couple of hours, just to make sure the cork was held tight against the bottom of the bed during initial curing. After, I flipped it back over onto the cork and let it dry overnight. I don’t think that cork will be coming off anytime soon – it feels extremely well stuck and looks very tight and even / flat against the bottom. (which could be bad if I ever need to service the bed – but in that case, what, really, would there be to service underneath there – maybe repair some trace paths?).

Soldering the leads required a larger tip to be used with my iron, a higher temperature and a bit more patience, but was relatively straight forward. Since I’m going with 12V, the ground bridges 2 and 3 (for this particular bed, the other one I bought – the same model essentially – had 12V+ being bridged – it’s quite confusing, but just follow the guide on the bed itself and be SURE to take pictures of the guide BEFORE you attach any insulation because you probably won’t be getting it off anytime soon if you glue / secure it like I did!), and the 12V+ is on #1. I should have had the + wire pointed away from the SMD, as illustrated on the reprap site, but stupid me, I was afraid of touching the SMD so I soldered it facing towards (so the rubber / protected part of the wire is facing away…and slightly on the heated bed – hopefully that doesn’t melt the wire!) instead. I took some kapton tape and placed it over the exposed solder joints for a bit of protection (yeah, it isn’t the prettiest / neatest job, I know). I zip tied the wires together and then formed a strain relief loop and attached the bundle to the bed frame underneath for added protection. Finally, I ran the 4 wires through a piece of sleeving and connected it to the control board.

Once the printer is complete (so close!), I’ll run the PID autotune against it, for about 8 cycles, set to a target temp of about 120C (just so I don’t have to do this again when I do finally start printing with more exotic materials). The board I’m using has a direct entry in it’s GUI (basically a GUI slapped on top of Marlin’s Configuration.h, I think) for the hot end and bed PID values, so I’ll just jot them down when I run the autotune and plug them in after. I’m still trying to grasp PID, PWM, etc. From the little I know, however, I believe when you run the autotune against a specific target temperature, it just gives you the best PID #’s that will get you to the target temp in the most efficient manor – but you can still push the bed or hot end higher or lower than the original temp used in the PID tuning. The # of cycles allows the autotune program average out the PID values (I think…again, not very educated in that area…).

I forgot to mention that I have already wired up the end stops – sorry, I couldn’t help it – I was on a role! There’s not much to talk about with those, anyway – they’re just an average mechanical end stop. I can tell you that I had to extend my Y end stop a bit because with the spacers and the thickness of cork, the micro switch didn’t touch the bed any longer. Ideally, one would sub out the standard switch for one with an extended arm, but I had success just taking a piece of shrink tube and a bit of scrap from snipped zip ties, and creating an arm extension. As the saying goes: “It ain’t pretty – but it works!”.

 

Phew. My updates are long. I’m wordy. But this blog, while a good way to reach out to others was originally designed as a journal to offload my thoughts – and I have a LOT of those kicking around inside of my noodle. Love me or hate me, I yam what I yam.

Cheers,

– E