For a couple of years now I have been pining for a 3D printer. I told myself all I needed was one good reason to justify the 1k hit to the wallet. I told myself it would be a great thing to do with my kids but the reality is they are more impatient than I am. Two months ago I was on a Google hangout with a co-worker of mine who I've known for many years. We were talking and the topic was microcontrollers (totally not work-related but it was our Friday Arduino Club happy hour hangout) and specifically the lack of a solution that fell somewhere between breadboard and custom case.
I thought I was the only one who had this complaint but he shared the same sentiment. I told him I had an idea based on the Mac Pro's triangle skeleton inside a cylindrical case. My thought was I could create a top and bottom and connect it to three rods and then snap in my MCU and any perf board or wire harness directly to the rods and encase the whole thing inside a soda can with top and bottom removed.
I said all I need is a 3D printer and it would be a piece of cake. He moved to the side, pointed behind him and said "Like this one." An hour of questions ensued. Matt told me his printer was a kit which he put together and it cost about half as much as a typical printer. The printer was a member of an open-source 3D printer family called Reprap whose tagline is that it is humanity's first self-replicating machine. Yeah, you heard that right. Certain types of repraps can print 60% of the parts to create a new reprap. The model was a Prusa i3v with an 8" heat bed he got from Makerfarm.com and he was very happy with his experience. Matt urged me to buy it as a kit and assemble it myself so I would understand what each of the parts do and how to tune them.
He also told me about the instruction manual. The steps include a picture of all the parts you need followed by a link to a YouTube video link. You click on the link and watch a nice fellow named Colin assemble the section. Matt said the best part of the whole experience was that if you ever get stuck you fill out the question form on Makerfarm and Colin emails you the answer in a New York minute. I was sold and bought the 12" that night. Then I began to watch the video in anticipation of a UPS shipment 10 days later.
Several emails from Colin (#superFreakinAwesomeGuy) later, I was up and running and super happy. Altogether it took about a week although some people can do it in four hours but I have kids.
I do have to say it was quite fun to put it together. I started printing and everything was going well for two weeks. Then I started a long print.
It was a Friday night and the print did not complete; I did not want to leave it running so I shut it down with only 30 minutes remaining. We came home around midnight and I was a little tipsy but I thought I would finish up the print. I started heating the bed and after a few minutes I saw a trail of smoke come from the controller area, then it disappeared. I quickly pulled the plug and looked for signs of danger. In the pictures above you can see the print bed in red. This is a flat plate covered with glass which heats the surface to help the material stick and prevents it from warping as it cools.
This is basically a copper trace that weaves back and forth with only about 1 Ohm of resistance and it can draw up to 20 Amps of current. To protect the Arduino they added an externally-mounted relay which only draws a few milliamps to energize the coil. The problem is that the work side of this relay has those green Molex screw terminals to which you need to jam in 12AWG wire, and the other end of the wires that connect to the bed are always moving. Most guys put a zip tie to prevent it from moving in the screw terminal but for me TL,DR (Too Late, Didn't Read). Sure enough, I could see signs that this screw terminal had melted.
I sent this picture to Colin and he said:
"I have seen this before. If the screw terminals loosen up over time, it creates extra heat which leads to sparking, etc. The best way to resolve this is to unsolder the green screw terminal and solder the wires directly to the circuit board; this will prevent future issues."
Keep in mind the printer and relay design are part of the reprap Prusa i3v spec, so this is no fault of Colin's. Had I done more research I would have learned that this is a fairly common problem. I wasn't a big fan of soldering 12AWG cable to the tiny through-hole pads, so I looked for a better solution. In addition I was not completely comfortable with how the fat power wires fit around the lugs on the power supply. Although the instructions call for a particular type of speaker cable from Radio Shack (http:// www.radioshack.com/auvio-25-ft-12awg-speaker-cable/2780462.html) which is labeled 12AWG, it is much thinner than 12AWG. I measured conductor diameter at 2.7mm and with the insulation it comes to 5.4mm which is closer to 9AWG than it is to 12AWG.
My plan was to replace the screw terminals with M3 bolts and find ring terminals big enough to hold 12AWG or even 10AWG but still be small enough to not touch each other on the relay assembly. I also would sandwich the ring terminal on the new post between two M3 bolts. Also I would replace the fat speaker wire with silicon insulated 12 gauge wire from my favorite supplier Sparkfun.
Below is my modified relay assembly with the new posts (lugs) without the connecting wires. I will walk you through each of the three posts starting from the bottom. As you can tell, the original screw terminals had four ports; however the neutral was just shorted together by the trace at the bottom so I knew that I only needed one post and I could just put the two ring terminals on the single post.
The middle post was where all the work was. This is the terminal that connects to the positive side of the heat bed. Since it was too close to the top lug, I needed to drill a new hole and move it forward so that each lug had plenty of room and didn't short. Now I needed to figure out how to get a conductor back to the pad that is connected to the trace that goes to the relay.
I ran a strip of 5mm copper tape [https://www.sparkfun.com/products/10561] from the pad to the bottom of the post under the nut. Then I laid as much uninsulated copper wire as I could around the post and back to the through-hole pad in the original hole. Then I loaded it with solder which should give me more than enough of a conducting area. For the top lug, which is the power in, I just had to drill the hole in the pad bigger; actually I drilled it and then tapped it with an M3 tap I had lying around.
The next challenge was to find a ring terminal (crimp type) that would support 10-12AWG but have a small enough head to not touch its neighbor on the adjacent post. If you go to your local hardware store, crimp connectors for 10-12AWG are always yellow which is what we need, however they are typically sold with a stud size #8 (.416mm D) but we need something smaller: a #6 (3.5mm D). A quick Amazon search for 5UGP1 will yield what we are looking for.
The label on the package rates the insulation at 221 F (105 C) which is pretty good considering that for PLA (which I used) the bed temp is 60 C and the silicon wire is rated to 200 C. I measure the max temperature on this lug at 38 C (100 F). Below is a picture of the ring terminal connected to my silicon wire. Also in the background you can see the crimp tool I used as well as another uninsulated #6 ring terminal I bought, just in case. I had my doubts about crimping over soldering but if done properly it is impossible to pull out.
According to YouTube you should not tin your wire before you crimp it. Who knew!
Since I had a bunch of these ring terminals I also put them on the leads that connected to the power supply and they just barely fit. This was a connection I did not feel good about before because if you put thick stranded wire on a screw-down terminal some of the strands tend to splay out and can cause a potential short or even worse -- a fire, if they hit the neighboring lugs. The image below shows the new set of wires connected to the power supply. Notice the perfect fit inside the lug dividers.
You might notice the two leads with the red shrink tubing. In addition to the 12AWG silicon-insulated wires, the connection also contains a 22AWG lead that goes straight to the head cooling fan. Wiring the cooling fan directly to the power supply is considered best practice.
I would strongly urge anyone with a 3D printer to at least invest in a crimping tool and swap out the power connectors. Since these connections are only an inch above the base of the printer and hard to inspect, I sleep much better at night knowing this connection is good.
Below you can see the modifications made to the relay assembly. The nuts on top of ring terminal prevent the wires from rotating at all, but even if they did, the conductor will not make contact with the other posts.
I enjoyed my trek into the territory of 3D printing immensely, but as with many other hobbies, side trips arise that are equally enjoyable. I spent many nights lying in bed and planning how to modify this relay bed to provide a safer connection and I am quite pleased with the outcome.