Making a Hobbed Bolt

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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.

Making a hobbed bolt is often the first part of building a Threaded Pulley as part of a Cold End.

Contents

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 more grip on the filament and a smaller tendency to clogging.

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.

Using M3 tap

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.

M8 1.jpg

Other snapshots

Here's a collection of some tidbits.

Traumflug using a vice

M8 3.jpeg

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 the hobbing

  • Many people cut the hobbing into the unthreaded smooth shaft of a bolt.
  • Some people cut the hobbing into the threaded part of the bolt.
  • A few people cut the hobbing into the head of a cap-head bolt. (see Yegah).

Using the smooth part of a shoulder bolt works better. Better than what? better than a normal non-shoulder bolt? better than using the threaded part?

M8 2.jpg

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

M8 2inside.jpg

Anthony Aragues' investigations

Anthony Aragues at suddendevelopment.com came up with a different way to make the drive bolt using a Dremel to cut slots: http://suddendevelopment.com/?p=68.

Cloudmaker, after trying this out, noted: "I just tried this, took 5 minutes and grips incredibly, works best on the threaded part of the bolt, produces nice spikes which enter the filament :-)" and added, some longer tests later: "It works great in the original Wade's extruder, but I didn't manage to get it to work reliably in Greg Frost's variation of the Wade. A (yet to be designed and printed) rig to rotate the bolt in evenly spaced steps (12 maybe?) and to assure linear movement of the Dremel, thus giving very even spikes and even thrust to the filament, would be best for consistent even nozzle output."

An easy way to produce the teethed bolt can be found here on Thingiverse.

Closeupteeth.JPG

Myndale has done a lot of testing of this design across four different extruders and reports the following:

  • The M8 nuts on the main drive bolt must be very tight. The teeth are cut from the original thread, so the bolt tries to unscrew itself out of the extruder during operation. Alternatively try using Herringbone gears which automatically self-centre.
  • The grip on the filament can be very high, so make sure the hot end is mounted securely or you can damage your extruder.
  • Works terrific with PLA but ABS tends to be problematic. ABS is both softer and more slippery, so the idler tension has to be just right. Too loose and it slips, too tight and the sharp teeth shred the filament.


This close-up photo shows the variation inside a slightly-modified Wade's extruder, the rows of teeth embed themselves almost entirely in the filament resulting in extremely tight grip.
If you don't have access to a dremel, it is possible to cut the teeth with a normal metal saw, cutting the bolt at an angle.

Method using an indexer

There is some uncertainty about the optimal number of cuts on the bolt's circumference. Lanthan has got an early failure (less than 10 hours printing) with 16 cuts. Better results might be expected with 12 - 15 cuts (to be experimentally determined - also, the quality of the bolt does probably matter)

Lanthans failed bolt.
Basic manual indexer for a 3 - axis CNC. Better precision, calibrated cuts. See also Thing_10110.

Indexed Groove method

This method is a combination of the above methods. After cutting the shaft, instead of using a tap, use a dremel to cut slots perpendicular to the groove. Try cutting them as close as possible.

This has been working great for Alertik for 20+ hours of PLA printing so far.

MakingHobbedBolt-IndexedGroove-Step1.JPG
MakingHobbedBolt-IndexedGroove-Step2.JPG
HobbedBolt-IndexedGroove.JPG