You can use this Hobbing Jig to get nice evenly spaced cuts when making a hobbed bolt.
I had trouble with some new ABS which had trouble feeding through my extruder. The ABS was a bit thinner (2.7mm) instead of the normal (2.85mm). It was also a lot softer than my imported stock. But because it was locally made, and about half the price of imported ABS, I really wanted it to work. So I set out to make a new hobbolt, which still works in my old extruder, but give better grip.
Then I saw this thing by Elk called "Transport screw workaround for wade extruder". (http://www.thingiverse.com/thing:9291) I tried it, and it works great! But I thought, to get a bit better precision, I need to make a jig. So my Hobbing Jig were born.
Download the OpenSCAD model and change the parameters as needed. You do not need to know OpenSCAD, to use it. It is very strait forward, but for those of you who really only wants a STL, I have included on with a disc diameter of 80mm, divided into 16 divisions for a M8 bolt. (PS: I only learned OpenSCAD today, and boy, am I impressed!)
- Print the thing.
- Put a M8 nut in the nut receptor.
- Tread your M8 bolt through from the bottom.
- Mark the position on the bolt where you need it to be hobbed.
- Put it in your drill press. (Do not over tighten as it will damage the threads)
- Fasten your Dremel-like tool equipped with a small cutting blade to the drill press table.
- Center the cutting blade on the center of the bolt.
- Align the cutting blade to the outer edge of the jig, by turning the drill chuck by hand.
- Switch the dremel on.
- Swing the drill press table towards the bolt.
- Make the cut and use the depth adjustment of the drill press to make the cut longer on the bolt.
- Align to next position and repeat until done.
The most difficult part is to fix your dremel-like tool to the drill press table, and to align the blad to the center of the bolt. The nice part of this is if you are doing a few bolts, this needs only to be done once.