Tim Hatch

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Printrbot is alive 15 Aug, 2012

Earlier this year, I got wind of the Printrbot 3-d printer kit. After the Kickstarter was funded, but (well before) it shipped, Brook released the source files under an open source license, so I found someone who could print me a set. A common problem in hobby 3-d printers is the chicken-and-egg problem: most reprap printers incorporate printed parts, so how do you get started?

Anyway, after very slowly assembling the v1 parts, there are obviously a few places that the design could be improved. Answering question I had myself before building (based on building a “Pro” sized 8 × 8 with plastic parts, rather than lasercut):

  • 8mm rod is sufficient for the vertical (Z axis): this depends mainly on the weight of the extruder and its associated (X) carriage. I’m using a NEMA-17 motor on a Greg’s Wade’s extruder.
  • The lower frame and its threaded rod with nuts is really easy to put together. Sadly the lower frame doesn’t have a very big footprint, and I have to keep the acceleration that moves the plywood bed (Y axis) turned down or the machine can flip itself.
  • Using different pitch belts works out fine. None of the belts needs to be continuous. However, one of the toothed pulleys needs to be put on backwards (with the setscrew on the side away from the motor) which means you need comparatively long shafts.
  • The extruder holder doesn’t seem to fit standard extruders; I made a substitute out of aluminum angle and lots of spacers.
  • The Z couplers work out okay — I initially had some old steppers (1984 date code) and they would lock up, but newer steppers work out fine without additional bearings, this axis isn’t stressed. Wires are likely to get caught in the screws however.
  • The allowance for the lower motors is enough for average size motors, but not some large ones that I had on-hand.

  • The homing switches are necessary, despite the fact there weren’t any v1 printed parts to hold them. The X one (on the upper left) will get a fair bit of stress because it moves up and down. With the output from Slic3r it needs to home all axes before each print and the X axis again only after it finishes (this could be substituted with a G0 X0 with the same effect), so I need to hold the wire carefully to make sure it makes contact.

Initially I tried a press-fit plastic Y pulley, but it would get soft after a lot of use and slip. With Eric’s help last weekend I replaced it with an aluminum one, and it’s much more accurate now. The only Y problems that remain are from the way the belt pulls it, and the fact the belt is off center. The X axis is using a printed pulley with setscrew, and I haven’t had any trouble from it. The X idler is a little tricky to get right, I ended up mixing a 5/16” bolt I could find locally with a mix of metric and SAE washers.

The space provided for tensioning belts is basically useless; you need to remove an idler, zip-tie to tension, then reassemble.

After completing my build, I started to consider what I’d want in a printer if I started from scratch. The main points, Y axis riding on stationary rails and a belt path like Mendel has (which requires an extra idler), more airflow around the Y motor, something more rigid to hold the top of the Z axis, and printed parts that allow for easier-still customization, all seem to be design goals of Nophead’s Mendel90. For people starting out, I’d suggest that rather than a Printrbot (if you can get someone to make you the necessary parts).