Category Archives: Experiment

Experiment Extruder Transmission

Experimental Off-Carriage Extruder Motor

There was a recent thing on Thingiverse called the Vert-X-Belt-Truder by Johann which describes a way of moving the extruder motor off the carriage. The benefits are a lighter, and potentially faster, carriage. The Bowden cable is another way of achieving this goal. A comment by Martin Price (martinprice2004) caught my eye:

…keep the x axis as current prusa, but turn the extruder 90 degree around the z axis. Drive the extruder using a motor mounted at the end of the axis with a square or hexagon bar mounted horizontally. The extruder drive gear moves along the shaft when the x axis moves, but can still be driven by the motor at any point.

I had already done something similar for moving the X-axis motor off the carriage and so I quickly threw together a proof of concept to thrash out how a design would work.

Sketchup Design

The results are shown in the following video (please excuse the varying focus, the many “er”s, and the fact the audio is muffled between 2:10 and 3:35 – I didn’t realise I had my finger on the microphone – but I don’t say much of value here anyhow, only that it seems to work and the drive gear moves through the drive shaft. Also that the test “print” is running at 200mm/s (travel:250mm/s))

I utilised the Compact Geared Extruder by Matt Roberts (Thingiverse) because it’s small, modular, and as it is designed in OpenScad it’s easy to hack with. I went through a couple of iterations to come up with a bracket that would hold the drive gear in place as the extruder moved along the square drive shaft. As I put the parts together I realised the same effect could be achieved using herringbone gears, so long as the drive gear is held close to the driven. The coupling is a rubber roller from an old Xerox laser printer, as is the square rod. The NEMA 11 motor is simply zip-tied to the X-axis motor end, and the idler is simply a bearing zip-tied to the X-axis Idler end.


A few things I learned as I put it together which might be worth considering if anyone recreates this.

  • Positioning of the motor and idler dictate where the drive gear will be placed, and so I would align these precisely with the X-axis rods and place the drive gear with a bracket. Then I would make position of the extruder flexible (using slots for the bolts for example) so it can be accurately pushed back in line with the drive gear, giving a good fit.
  • The bracket holding the drive gear would be made more robust – reducing lateral movement and possibly adding bearings or bushings either side of the drive gear to support the rod.
  • The bracket should have a way of removing and inserting the square rod, otherwise it’ll be a pain to disassemble the whole thing. Of course if bearing or bushings are used then it might be easier to design it so the rod slides out along the X-axis when disassembling.
  • As mentioned above, using a helical gear such as a herringbone might remove the need for a bracket at all. I guess this would depend on the amount of flex along the driveshaft, and whether the drive gear keeps engaged during the back and forth.

If I get chance to fix the hobbed bolt of the compact extruder then I will try to get this setup printing for real. Here’s a couple more photos and videos just of the running.

[gallery¬†include=”942,946,944,943,941″]

 

Experiment Transmission

Experimental Z Axis Gear Train

I was thinking about alternative ways of driving the second Z axis rod. Using two motors seems a bit redundant, and timing belts have to be the right size and can be tricky to source. This isn’t much of a problem for most people wanting a 3D printer – but if you’re experimenting with the technology then having something more ‘reprapable’ has it’s advantages.

One way would be to use a gear train between the rods. This seemed like an interesting line to pursue, as it gave me an excuse to learn how to design gears, and what’s the point of having a printer if you can’t play a little. So, firing up Openscad and the excellent “Parametric Involute Bevel and Spur Gears” script from Greg Frost, I worked out how to build a set of gears to connect the rods on my test rig. I also put together a basic frame to hold them in place using Tinkercad (which coincidentally tested how big a part my Prusa can actually print).

Gear train mounted on top

Having the gears in this curve was not actually planned to be honest (my calculations were a little off), but it did show a way in which access to extruder could be possible if they were mounted on the top of the machine – assuming some sort of guard around the gear mechanism.

Testing showed immediately a clear problem with this approach – backlash. The poor tolerances between the gear teeth accumulates along the train resulting in a noticable delay between the start of the driver gear and the start of the final driven gear when the direction of movement is changed. This is nicely shown in the video.

There’s a few things to note:

  • The problem only occurs when changing direction which, on the Z axis, does not happen for the duration of a print except¬†at the very start after the axis has gone to home.
  • This could potentially be mitigated by going to home and then priming the gears by moving in the opposite direction for a very small distance. I imagine this would be difficult to do with precise.
  • Another idea is to move the driving motor to the middle gear. This would mean that each end would turn at approximately the same time, but there would be still some play between the driving and driven gears.
  • Increasing the tolerances of the gears would also reduce the backlash. However this depends on the quality of prints, and also how high a quality can be hoped for in printed plastic parts.

The backlash problem seems horribly obvious with hindsight. Perhaps I could have worked it out from a bit more thinking, or from a bit of googling or asking on the forums, but by being able to quickly build a prototype I got to learn about making gears and think more about the problem than I would from just reading about it. And maybe posting this will help others in some way. The joys of 3D printing!