Dr. Lawlor's Code, Robots, & Things

December 9, 2012

Printrbot Assembly: 2012 Howto

Filed under: 3D Printing, Printrbot — Dr. Lawlor @ 8:34 am

Last month I built up a Printrbot Plus kit, which was a lot of fun and seems to be printing well.  But both the hardware and software for this do-it-yourself 3D printing technology is still pretty immature, so here’s my guide to the current rev of that particular hardware.

The definitive place to start is Tim Stark’s official assembly instructions, a solid photo guide.  It leaves out a few problems with the current kits, though, including:

  1. At step 55, my Y motor stepper’s shaft was too short to reach the tiny setscrew.  I drilled and tapped the plastic pulley for a new lower #6-32 set screw, which still just barely reaches the motor shaft.
    Printrbot Y setscrew
  2. At step 68, while attaching the heated bed to the carriage, you need to insert *something* at least 2-3mm high underneath, or you’ll find the carriage hits the deck before the printhead even gets close to the bed.  I put an aluminum sheet under the bed to even out the heat distribution, which is pretty nonuniform by default.  My under-bed sheet is a little over 2mm thick, cut in a “U” shape to clear the thermistor.  I used the long M3 screws instead of drilling out the PCB for #6-32, but either would work.
    Printrbot bed
  3. At step 104, the 5/16″ black hex head bolt needs the whole stack listed at step 136.  If you wait any longer, you won’t be able to use a hex key to keep the bolt from rotating as you tighten the nylon nut on top.
  4. At step 142, my X carriage belt pulley was just a little too tall to clear the zip ties holding the Z linear bearings.  I sanded and filed the pulley down, and mounted it *very* close to the stepper surface.
    Printrbot X pulley
  5. Many people, including me, got badly crooked gears for the extruder at step 160.  I couldn’t even fit the hex head of the hobbed bolt inside, and cracked the brittle castable material while trying to chisel some clearance.  It’s a chicken-and-egg problem if this is your only printer, but if you can get something working you can print new extruder gears.

Optional improvements:

  1. I dunked all the wood parts in urethane, which makes them look better, keeps screws from backing out, and reduces dimensional changes when humidity varies.
  2. I wanted to protect the extruder and bed wiring, since flexing back and forth repeatedly across sharp zipties will eventually cause wiring faults.  So I spiral-wound grip tape around all the exposed wiring, and ran a little steel wire out from the extruder to guide the extruder wiring into a gentle curve in *front* of the carriage. The sharp corner and stretched wires of step 168 make me wince.
  3. My Y axis belt stretched after installation, resulting in blobby prints.  I tensioned it using a rubber band hooked on a binder clip, pulling sideways to take up belt tension.  Keep it far enough back that it won’t get sucked into the stepper even when the bed is fully forward.

Lots of folks seem to get discouraged when things don’t work straight out of the box, but this is brand-new technology: you’ll need some tools, talent, patience, and creativity.  But the Printrbot is a solid kit, and with a few tweaks makes a reliable printer!


Coating a Printrbot 3D printer in Polyurethane

Filed under: 3D Printing, Printrbot — Tags: , , — Dr. Lawlor @ 3:35 am

One whole generation of do-it-yourself 3D printers (from 2009’s Makerbot Cupcake to 2012’s Printrbot Plus) are made from laser-cut wood, held together with small screws and bolts.  There are good things about this: wood is cheap, light, stiff, and environmentally friendly, and laser cutting is fast and precise.  But wood is fairly weak and splinter-prone, wood structures warp with humidity changes, and the bolts holding everything together tend to vibrate loose when printing.

You can improve each of these drawbacks by dunking your wooden printer assemblies into polyurethane.  Dunking lets the polyurethane soak into the fibers, which strengthens them somewhat.  It reduces the rate moisture can diffuse in, reducing warping with humidity.  And polyurethane soaked onto the threads of the screws keeps them from vibrating loose, but is still removable for servicing.

Last month I built up a Printrbot Plus from a kit, and tried coating the wood.  I dunked each of the major assemblies (the base, bridge, extruder, and printbed) after adding screws and nuts, but before adding any electronics or linear bearings.  Here are the parts ready to go in: assemblies are on the right; loose parts on the left.


Here they are after coating in polyurethane.  I had to brush the poly onto the top deck, since it’s too big to fit in the 1 gallon can, but everything else got dunked on both ends, and then brushed into the middle.  This process was pretty messy, so I wore rubber gloves.


This “quick drying” poly still took about a day to stop being tacky and smelly; the solvent stench is so strong that I wore an activated carbon respirator while dunking and painting.  Total poly consumption was tiny, 50 mL or less, although you need a much bigger container to allow dunking. There doesn’t seem to be any substantial dimensional change, and everything assembled fine.  The poly brought out some beautiful chatoyancy glinting within the wood, an unexpected benefit, and added a nice warm glow to the wood.

This printer has been run hard for several dozen prints and a few cumulative days of continuous printing since then, and haven’t had a screw back out yet!

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