Introduction: Craft a Hobbed Bolt

Hobbed bolts, also known as knurled or extruder bolts, are the part of a 3D printer that pushes filament through the extruder, they are the physical connection from the rotary motion of the stepper motors to the linear motion of the filament. Needless to say that without one of these your 3D printer is but a hot dancing robot.

There are several tutorials on how to build one of these bolts online. Thingiverse is populated with objects that should help you build one but, if you are trying to, chances are you don't yet have a fully functional printer.

This instructible is here to help you build your own hobbed bolt using pretty much the same tools needed to build a printer, and provides one small improvement over other similar tutorials.

Step 1: Required Parts

First of gather these:

  • 8mm Threaded Rod.
  • 2 Skate Bearings.
  • 4x 8mm nuts.
  • A power drill or screwdriver.
  • A tap (thread maker tool) for 1/8" holes.
  • Something to cut the rod: A dremel tool or a hacksaw.
  • Something to clamp the bearings with: A vice or woodworking clamps.

I strongly recommend using all applicable protection equipment. At very least glasses...

Step 2: Cut the Rod and Prep the Bolt

Using a Dremel tool or a hacksaw cut the rod to length. Grind the edges so that you can thread nuts in from both ends, this will make it easier to install on the printer later on.

After cutting prepare the nut with a bearing on each end, as shown in the picture. Make sure the nuts are tight for they will have to withstand a lot of vibration later.

Step 3: Clamping

How you do this step depends on resources available. On my case I used a piece of scrap MDF I had around and a pair of woodworking clamps. If this is your case then mill a channel on the MDF surface with the dremel tool, place the bearings over the channel and clamp tightly.

Should you have a vice or an L-shaped extrusion and some zip-ties, these should work as well.

Step 4: Hobing the Bolt

This is where the magic happens... A similar process is described in several sites online, but there are two small details that make all the difference...

Install the 1/8" tap on the drill use it to make the hobbing.

When starting use the tap at 45º-60º to the bolt, this will force the bolt to rotate while scratching away the treads from it's surface. The constant rotation is important as it allows for a uniform circumference to be created.

After 30-40 seconds the threads from the bolt will start fading away, when this happens start moving the drill and tap closer to a right angle (90º) from the bolt. Go as close to right angle as possible without stopping the bolt from spinning.

Going back close to right angle is important so the final scratches align to the axis of the bolt. If the scratches are diagonal this will cause the filament to spin inside the extruder, instead of being pushed forward.

Step 5: Enjoy Your Completed Creation

Check that the scratches are deep enough, and close to alignment with the axis of the bolt. Happy printing!


Downunder35m (author)2015-06-02

With an 8mm bolt and your way of hobbing you will end up with a 6mm diameter or less.

Also having a round shape causes a lot of fine filament shavings as the outer parts of the hobbed area won't move exactly with the filament.
You can see this effect nicely checking the filament under a magnifying glass.

It is much better to perform the hobbing on a bolt with no thread at the end and to crate flat lines along the shaft, of course this requires using a lathe or drill press with milling vice for the job.
I tried a lot of different sizes for the hobbed bolt, same as variation like knurling in various patterns.
None of the grooved designs worked well enough, especially not when going down in diameter so much.
And unless you know how to change the stepping for your extruder you must create the diameter of the hobbed area to match you original system, otherwise you end up with bad print results.

If you do use this method of hobbing please use a drill press!
The grooves for the hobbing are not directional and very dirty on the edges again causing unwanted filament shavings that might block the extruder prematurely.
Here is a pic of my mine, in use now for 3 years straight and last cleanup was 2 rolls of filament back - not a single jam a chew in the time between....

Hobbed bolt_small.jpg
Kuuran (author)Downunder35m2015-06-02

I built a Prusa i3 and configured Marlin for it. There was no "original bolt", so this was not an issue. :-)

Granted that using a lathe should produce a much beter result, but having one around is not that common. At very least this bolt should allow you to print parts for your own lathe.

I've been using a bolt just like this one for about 6-7 months now. There was some shedding at the begginning, then I realized I was overtightening the filament compression coils on the extruder mount... Just loosened them a little and results were fine.

Downunder35m (author)Kuuran2015-06-02

Well, for a freshly build printer that needs the entire calibration anyway it won't matter too much :)
But if you want to stick to the rounded version:
Make sure the drill is in an fixed positions.
Best option would be to print a simple mount that allows no sideways movement but only up and down - like a drop saw.
Now insteaf of starting with the tapping start with a round. fine grinding tool to remove the thread (best to use a bolt with no thread in this area).
If the tap is 3mm use a 3 - 3.5mm grinding tool.
Once you got a nice and even groove replace the grinder with the tap.
Start with just enough pressure so the bolt spins freely while cutting and work your way in.
You will get a much smoother surface and can reduce the actual cutting action as you now only cut where you already have a groove.
The finnished piece should have well defined grooves with only very little damage due to missing the privious cut.