When in Doubt, Build a Mendel

Okay, I don't know why I'm doing it.

I've been having fun with my Makerbot. I've had occasional interesting projects, and occasional uninteresting projects. I've been a little frustrated by the maximum size the Makerbot can handle, but not that frustrated. I also tend to be annoyed at the people who use machine tools only to build handy tools for machining.

But for some reason, when I saw Spacexula's Mendel Production Set, I thought, "Hey, that'll be fun, let's try printing parts to build another 3d printer!"

Now, to be fair, I was interested in how he'd packed several parts onto each print. When all my Makerbot prints have been 45 minute waits for a single part, it was pretty cool for the first print to give me seven or eight decent parts. From there, I just kept going, slowly filling a shoebox with the completed parts. Then I ordered the various hardware, and... well, now, I've got the mechanism for a Mendel almost done and I'm kind of curious what I'll do with it.


It took about two weeks of printing to get almost all the parts done, with several prints on weekends and (if I was lucky) one or two complete prints during the week. I had remarkably good luck; maybe 60% of the prints worked fine the first time. I had some other problems with prints curling and jamming the print head. Some of the larger parts such as the lower carriage went really badly, with too much plastic at screwholes jamming the printer. I even resorted to using a failed print on the carriage when I couldn't complete a full print.

I assembled the Mendel this weekend after making a big parts order to McMaster and the bearings here. Both shipped immediately and arrived quickly. I still am waiting on the stepper motors (which I'm getting from Alltronics - I'll let ya know how that goes), and once those are running, I'll see how my mechanism works.

Helpful hints for building a Mendel:

• The Nyloc locknuts are a bear to work with, hard to put on and a pain for disassembly. I ended up falling back to regular M4 nuts for connections that didn't look like they were liable to shake loose. If I do have problems, I'll either add a second nut or a Nyloc nut.
  
• The ground steel rod is hardened, and my hacksaw wouldn't scratch it. I ended up using a Dremel moto-tool with a cutoff wheel to cut the hardened outside layer of the bar, then used the hacksaw to cut the bar.
  
• I ended up needing to drill out all the screw holes to get the screws in. When screw holes need to line up in parts, I used the machinist trick of drilling out the top piece separately, clamping all parts together, then using the top piece as a jig to keep the holes aligned.
  

It's also been interesting to compare the Mendel to the Makerbot. If I had to choose Makerbot or Mendel, I'd go for the Makerbot - assembly was quicker, it feels more robust, and various design points just feel better. The plastic bushings and guide rods for the X and Y axis require much less tuning, and the magnetized build platform makes it easy to print, then pull the platform out to scrape the print off the build platform.

The Mendel wasn't that hard to put together, but the printed parts sometimes annoyed me because they weren't as precise as machined or laser-cut parts. The Makerbot went together much better, and feels more robust, while I keep wondering which printed parts will fail first. For places where accuracy matters (such as on the guides for the X axis assembly), a little bit of debris or extra raft material can throw off rod spacing, and makes me think I'll have problems with alignment and friction.

Well, THAT didn't work!

Definitely no luck with the homebrew heated platform. The 10 ohm nichrome wire pulled about an amp of current (not surprising for a 12 volt power source), but took 10-20 minutes to heart up. I poked a wire into the silicone between the layers of glass and dragged out the middle of the wire, and soldered a new wire on. By connecting the new midpoint to one terminal and the original two wires to the other terminal, that meant I had two wires in the platform that were half as long, and it drew (not surprisingly) 4 amps... and started heating the plastruder circuits too much. (It heated up really fast, though!) I then just connected up one side for 2 amps of power... and the glass quietly cracked.

Next step: two amps of current seemed ok for the plastruder, so if I remake the build platform (and try glass again), I'll try stretching two 10 ohm / 16" nichrome wires inside. The platform can definitely use the extra nichrome to spread out the heat source and hopefully keep the glass safe.

Heated Build Platform

After seeing some of the great results folks have had with heated build platforms (such as Hydraraptor's groups of pieces, I wanted to try one too. I'm mostly interested in cutting the warping of long parts, avoiding printing rafts, and experimenting with the nichrome wire.

Here's a photo of my build platform; I based it on Hive76's design that they're selling through Makerbot. I far as I could tell, they use a couple pieces of glass, nichrome wire, and silicone sealant to make their build platforms.

I tried the same, though I sealed all the wires between the two layers of glass with tons of silicone. I used the same 10 ohm piece of nichrome, soldered the ends to longer pigtails of copper wire. I then cut two pieces of 4" square glass with a glass cutter and straightedge. I used dots of silicone and carefully placed weights to get the wire in place. I also added a spare thermistor and wires for measuring temperature, then covered everything in silicone, added a spare piece of copper wire at the far side of the platform to keep the pieces of glass parallel (because of the soldered connections on the near side), then smushed the two pieces of glass together.

I also used a bit of Radio Shack pre-drilled circuit board for the circuitry to connect the thermistor to the spare A6 analog port on the extruder controller.

I found 16 inches / 10 ohms of nichrome wire isn't enough to get sufficient heat; it took around 15 minutes to heat the glass to 55 degrees Celsius, and while the ABS would bond to the glass at hotspots above the wires, the plastic wouldn't stick to the glass elsewhere, even if I made sure the glass was clean and free of grease.

Looks like I'll stick with my 1/2" plexiglass build platform I currently use, though I might take another pass at the heated build platform in a few weeks.

Also this weekend: my original extruder nozzle/heater died with a broken wire, so I made a new one from spare parts. The original one put in a fair number of hours. I also found that ReplicatorG version 13 (for the heated platform support) also is much quieter about moving the stepper motors - that was a nice surprise!