Making a Hobbed Bolt

A Hobbed Bolt is a part seen in about any geared extruder design. Its sharp teeth outright bite into the filament, allowing high transport forces.

Wade's original instructions

Note by Traumflug: he used an M3 tap for 3 mm filament. Later it turned out M4 or even M5 gives even more grip on the filament.

I used a hand drill and a file to cut the slot, then hobbed it with an M3 tap mounted in the drill, using bearings mounted in a vise to let the bolt spin - see the next videos. If you're not sure where to cut the pinchwheel slot, test assemble the extruder, making sure you leave room for the motor mount bolt heads under the 39 tooth gear, and mark the M8 bolt where it crosses the filament feed channel.

Filing the shaft

Note: The video shows the file being held stationary as the work turns under it. When filing on a lathe-like setup you should move the file back and forth to avoid clogging the teeth of the file, uneven tool wear, and heat buildup.

<videoflash type="youtube">2FU0v-3gloo</videoflash>

Using M3 tap

<videoflash type="youtube">Hl4Gf5R4YJg</videoflash>

Earlier Version

This is an earlier version with using threaded rod - it's harder to get the hobbing concentric when you cut it into the threads.

Other snapshots

Here's a collection of some tidbits.

Traumflug using a vice

This is a setup working without printed parts. The bearings are the ones used in the extruder later. The washer has flats to fit loosely into the vice and also holds back the tapping tool from running away. Adjust the nuts to the right to get the groove into the right place. A M5 tap was used here, which is said to be more resistant against clogging.

Useful helper parts

You can also use this http://www.thingiverse.com/thing:3780 bolt hobbing tool, or this http://www.thingiverse.com/thing:1802 one.

Where to place to hobbing

Using the smooth part of a shoulder bolt works better.

Eventually it should show up where the filament moves along, of course ...