The hobbed bolt part 1

It would seem that the most common extruder for filament is Greg's Wade Extruder, which uses a hobbed bolt to drive the filament.  This is an M8 x 50 (or 60) bolt with a groove in the smooth part of the bolt shaft. In this groove is cut "teeth" using an M6 tap (sometimes larger, sometimes smaller).  The bolt is driven by a stepper motor via a set of gears.  The filament is pressed into the groove by the idler block via a bearing.

The exact position of the groove is not easily obtained. You can buy hobbed bolts on ebay for USD10, and I have seen grooves at distances of 21, 24, 26 and more mm from the bottom of the head of the bolt.

One improvement made by Eckertech on their Eckstruder is to reverse the hobbed bolt orientation, as the groove can run into the thread on a 50mm length bolt.  In their version the groove is much closer to the head.

Inspection of the eckstruder STL file indicates that the groove should be 13mm below the head, and this LOOKS right on ebay photos of eckstruder hobbed bolts.

Tomorrow I will attempt to make a hobbed bolt by clamping the M8 bolt in bearings in my small bench vise, and hobbing it with the tap in the electric drill.  I will first initiate a groove using the small angle grinder.  I will post photos.