3d printer updates 3: Almost there (for real)!

When I last left off, the mechanics of the 3d printer were near enough complete, with the wiring closely following suit. This progress and the progress that will follow all occurred in only a few days between the RPI Formula Hybrid season and final exams (hence the abrupt halt in work and weak photo documentation).

Now that the (substantial amount) of wiring was in a test-ready state, I began the process of tweaking Marlin (my open source controller firmware of choice) and debugging stepper motor connections. For a long time, I kept getting very strange "stuttering" effects from the steppers. I found the culprit to be a combination of swapped wires, poor limit settings in Marlin, and uncalibrated current control potentiometers on the stepper drivers . Once the x and y axes were moving smoothly (and scaled to correctly replicate mm inputs), I switched attention to the z.

The new NEMA 23 stepper motor was working great on the z, right up until the moment when it wasn't. Every time that the motor passed a certain point during upward travel, the speed would bog down, an unpleasant noise would be emitted, and usually the leadscrew clamp would slip. Initially, I though this was another case of misaligned guide rails or screw marring, but it never occurred on the downstroke, and worked fine at every other place on the rail. Many tests later, I finally noticed an edge of the aluminum rear panel that had warped in far enough to catch the gantry when it slid up from below. The motor had literally been trying (and failing) to tear the machine apart! With no immediate fix available, I simply shimmed the z-axis rails a fraction of an inch inwards by adding a washer to all connection points. This solved the problem, and shouldn't have any effect on accuracy due to the 3-point leveling build plate.

After fixing another relatively minor mechanical problem (incorrect build plate spacing), I was finally ready to tackle the extruder: one of the few parts on the entire machine I just purchased instead of designed. The extruder was a MB8 knockoff with no frills, bought more than a year ago for the initial 3d printer project. Initial wiring went well, with the thermistor responding properly and the hotend heating relatively quickly. A small amount of smoke was produced at full temperature, but I attribute this mostly to months worth contaminants and grease that had been collecting on the surface. It gradually stopped smoking after being on for a while.

Unfortunately, after loading it up with filament, things started going wrong. The first few inches of filament would readily squeeze out the end of the nozzle, and then slow to a crawl. Meanwhile, the internal driving gear would grind away at the seized filament, jamming the extruder and necessitating a tedious teardown of the assembly. About 10 time-consuming disassemblies and reassemblies later, I determined the cause of the problem to be unacceptable heating of the brass feeder tube leading up to the hot end. Before the filament had even reached the nozzle of the hot end, it would have already become sufficiently soft and malleable (technical term: gooey) to buckle over itself and stick to the walls of the tube.

I quickly came to understand that I needed a dramatic temperature transition between the feeding tube and hot end to allow the process to run as intended. There was nothing I could do about the conduction up through the brass tube, but I had hope that I could at least introduce some forced convection on the midsection of the tube. The ideal design would have circular concentric fins running up the tube with a fan blowing over them. With the night getting late on my last day to work on the project, I went for a hail mary option.While I ran over to the machine shop to turn a crude aluminum heatsink, my friend biked to the local (not yet shut down!) Radioshack to grab some Arctic Sliver heat paste. The result was a marginally better hot end:

With only a single (wide) parting tool left unbroken in the shop, and no time to grind a new one, I was only able to get two fins out of my stock. Despite the improvements, it still was not enough to get the extruder functional before the deadline. With no time to order a new one before flying across the country to California, the printer has sat at RPI all summer. While I'm sad to have to wait so long to finally see it run, I'm at least glad that many of my design elements that I was most concerned about worked. 

With the printer on my mind's backburner all summer (as I've been busy having too much fun interning at Tesla Motors), my eventual plan was to do a full extruder swap with whatever the internet claims is the best design. The downside, of course, is that this also requires re-waterjetting a new mount plate and possibly more parts to ensure functionality. So imagine my surprise when browsing Amazon revealed this. I love it when pictures in my head turn out to be real things! While it is not compatible with my extruder design, I'm excited enough about the design that I intend to buy one and modify it to accept the new hotend. If that too fails, I may have to plan on the original full replacement.

In the near future I'll try to write an update about the continuing progress of MechEtroller, a build log of an exciting year-long Formula Hybrid project, and possibly something relating to this: