I recently published my designs for swappable extruder heads for the hypercube on thingiverse and GitHub. While the bowden design is a relatively simple progression from the original hypercube design by Tech2C, the direct drive extruder is a little different and I'd like to share some of the background on how I got to where I did.

mmmm, cable braid...

What was the aim?

The problem I was faced with was flexible filaments. The bowden extruder design is great for almost everything, but I found it really hard to reliably print flexible filaments due to the long bowden tube and unconstrained filament path. A direct drive extruder seemed the obvious choice, but there was a suprisingly small number of direct designs available for the hypercube mounting system on thingiverse. The closest I could find was the design by WF3D, based on a modified Prusa Mk3 head.

This design is simple and reuses a well tested design, but to use it, it requires altering the frame of the hypercube to make space for the stepper motor. Simple if you've got the standard z-axis setup, but tricky in my case (a subject for another blog post).

Why is it so hard?

In trying to come up with a design that did work, I realised what everyone else had been struggling with, which is space. The volume available for the print head on the hypercube is pretty small, especially if you don't want to reduce the build space.

A closeup of the idler mechanism

Specifically, the large components are the stepper motor, the idler mechanism (whatever provides the pressure to push the filament up against the drive gear) and the blower fan. For the blower fan, I borrowed inspiration directly from the Prusa design and mounted it low on the front, which makes the ducting design a little easier. The hard part is the idler mechanism and the stepper which occupy the same part of the toolhead, and cannot (not easily at least), be moved very far from each other. In particular, I found that the mounting points for the stepper motor would always come into conflict with the mechanism for the idler.

Source: https://www.thingiverse.com/thing:2543095 [cropped]

One of the really clever parts of the original Prusa design is that one of the mounting bolts for the stepper motor is reused as the axle for the idler mechanism. My design turns the stepper axis 90° and then keep the idler arm as small as possible, and as close to the stepper as possible. The hardest part of this was then working out how to tension the idler, without getting in the way of the filament path. There wasn't space for two tensioning screws (as per the Prusa design), so the only solution left was to have an off-centre tensioning bolt mounted just to the side. Combined with a very low profile stepper motor allows the head to fit just within the volume allowed for the hypercube print heads.

Off-centre tensioning bolt for the idler arm

Have you got a hypercube printer? Try the extruder head yourself and let me know how it works for you, either through the comments on github or on thingiverse.

Github: https://github.com/alanmcruickshank/hypercube-extruders

Thingiverse: http://www.thingiverse.com/thing:3761366