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.

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...

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.

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.

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.

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