I recently had the opportunity to compare three different filament drive bolts for Wade-style extruders with each other because one of our extruders began to slip occasionally at 60°C air-temperature in the heated chamber while another worked absolutely reliably under the same conditions.
To verify that my oldest Hobbed Bolt (bottom) made by Wolfgang (reprap-fab.org) isn’t just worn out after over 2500 hours of operation I ordered a replacement (top) from Lazlo at arcol.hu:
The Hyena v1.0 is manufactured very precise and has quite sharp edges – although no knife-like sharp teeth. The comparison in operation, however, unexpectedly showed no better grip than before.
Thereupon I took a closer look at the bolt of the functioning extruder (also from reprap-fab.org – somewhat more recent) and saw a significant difference from the others: the (tap) teeth are cut less deeply so that there is virtually no circumferential groove in which the filament gets pressed.
This marginal difference seems to be important – here are two pieces of ABS from the extruder in comparison (top: Hyena Hobbed Bar; bottom: working Hobbed Bolt):
The outer teeth circumference of the working bolt is about 17% larger than the others (25.56mm vs. 20.10 mm) and therefore bites with more distance and particularly deeper into the filament by an (almost) straight tooth flank.
In the long term we will need a bigger drive bolt radius to distribute the extruder’s force onto a longer section of the filament in order to deal with the conditions in a heated chamber (softer filament, more resistance by a longer melting zone in the Hotend). This will probably be the application of two “Mini Hyenas” that we ordered via Lazlo’s IndieGoGo campaign. Possibly along with another “big thing” on which Jonas and I are currently working…
[Update]
I didn’t emphasize enough above that these were actually three different drive bolts that i compared (#1 Old hobbed bolt by Wolfgang, #2 Hyena v1, #3 more recent hobbed bolt by Wolfgang). The older #1 hobbed bolt performed equally well (or: not so well) as the #2 Hyena v1 – as epected due to their similar geometry. Both are grooved so that the inner section of each tooth flank is positioned closer to the center of rotation than the outer sections. That’s why they move at different angular velocities – which is a bad idea, it burns a lot of work just damaging the filament instead of pushing it downwards.
The second hobbed bolt (#3) has teeth flanks that are almost parallel, not grooved – all points on the tooth flank are moving at the same velocity resulting in cleanly cut filament, a LOT more torque and thus less slipping. Also the spacing of the teeth is wider and the teeth themselfes are longer (cutting deeper into the filament). As Wolfgang told me he is aware of his improvements and intentionally changed the bolt design due to the the named reasons, that he also already observed.
I had the chance to also test the pair of Lazlo’s Mini Hyenas (v1) lately that unfortunately but understandabley showed an equally bad performance. Don’t get me wrong – these are fantastically machined, beautiful pieces of hardware. I appreciate that a lot. Their geometry is just counterproductive for their intended use as a filament drive gear.
Tagged: Filament, Hobbed Bolt, RepRap
