Here is the hobbed bolt I have been using for the last 10+ months with better results than anything else (check also the complete feeder at http://www.thingiverse.com/thing:73082). Time to share!
Though it looks like "old school", it is extremely reliable and could prove useful to makers that do not want to buy expensive machined bolts, or that build their own filament feeders or extruders.
As for me it compares favorably to MK6 and ultimaker drive bolts. Grinding and slipping is something I never experienced in the last 6 months but once with a brittle wood filament.
A few great features (imho):
- it has a bigger diameter than M8 threads, hence it provides a better grip
- square and large teeth replace spiky grinding small ones (hey, I do not want a file but a driving bolt)
- central groove always leave material on the filament, which prevents grinding and crushing
- plastic does not accumulate in the grooves as with some other hobbed bolts (including machined ones... that was a nice surprise), so no slipping occurs
- it's easy to make (just need a drill and a dremel with steel cutting wheels plus a few clamps)
- it's obviously ridiculously cheap
I described the process in my short hobbed bolt review here:
- not very good looking
- hard to make it consistent, though the thing is impressively tolerant :)
- compatibility with stock filament feeders / extruders
This thing is part of the filament feeder I made and used for the last 6+ months with excellent results (I will post it one day, now that I am at revision 5!). I printed PLA at an amazing 350+mm/s with it on my ultimaker at my fablab (with witnesses).
Would it help to design yet another hobbed bolt tool? Actually, I am a bit skeptical because the cutting wheel usually wears out so quickly, that the varying diameter ruins the purpose of the tool in the first place anyway. The angle and teeth shape still would not be consistent imho.
Check, e.g. http://www.thingiverse.com/search?q=hobbed+bolt+tool ?