Saw this today, definitely something to keep an eye on, if the hobbed inserts are going to be hard to come by.

V-Struder

Yes, it looks like a promising design. Belts are readily available and not expensive (even from RS!) - e.g. [uk.rs-online.com]. I may have a go at printable design for the Ormerod. It looks like the groove in the pulley used to push the filament is lined with something in that link - maybe an O ring?

Maybe a hobbed bolt can be made reasonably easily with a Dremmel and printed jig (e.g. [www.thingiverse.com]), though as I have never tried it I have no idea whether the practice is as easy as it appears to be in theory.

Dave

Here is my take on this:

I think that this concept can be realized with a much simpler design. It is all theories but may be worth a try. The magenta cylinder is a spring applying pressure to the lever that in its turn keeps the belt under tension.

Possible downsides to this whole design is that the filament is bent at the same radius as the large wheel. That may break the filament over time, when the machine is not in use. Maybe it can be overcome with a bigger wheel, the one in these sketches has a diameter of 50 mm. A bigger wheel also gives more grip surface on the filament. It is also possible to have the filament go 180 degrees round the wheel for even more grip, but then again, bending the filament to radius that narrow may cause problems.

In this design it is the belt that drives the filament and the big wheel is the support, the design in the first post it is the other way around. So a o-ring in the groove of the big wheel should not be needed.