This is an attempt to combine various aspects of Adrian's and Wade's extruder drivers. Specifically, It keeps Adrian's extruder clamping system, uses a prime number of teeth for good wear characteristics but uses Wade's hobbed bolt to do the driving.
As the name suggests, the design uses Herringbone gears. This ensures that backlash is substantially reduced, which should result in the extruder requiring slightly less reverse, but more importantly run quieter. The design of the drive gear is somewhat experimental. As with other gears, this uses a grub screw to secure the gear to the motor shaft. However, on this design the hole goes through the root of one of the teeth. This takes up less space, and also lets you adjust the position of the gear without taking the motor off. However, it is quite intricate. Many people have managed it, but your printer needs to be running well to do it.
Manufacture of the bolt is the same as Wades, but you'll need an M8x70 instead. I find it best to assemble the whole extruder, before marking where the hobbing needs to go on the bolt. A stack up of tolerances during build and assembly will probably mean that if we measure it off the CAD model it'll be wrong, or atleast not optimal..
Also, on my pictures the layout is not optimal. Ideally I think it should be Gear, 3X Washers, Spacer, 2X Washer, Bearing, Gap, Bearing, Washer, 2X m8 Nuts. It might be an idea to increase the size of the spacer, and do away with some washers, but I personally like that it allows some adjustment in either direction.
The final two nuts do not need to be done up tightly, doing so would mean the bearings become a little rough. The herringbone gears should keep everything aligned by themselves and the bolt takes all the torque, so the nuts only need to be lightly tightened.
When assembling this extruder, it is somewhat critical the gears are perfectly aligned. Herringbone gears try to self-center, if the alignment is incorrect they'll probably get damaged. I find the best way is to assemble everything, not tightening the grub screw on the drive gear, give the whole thing a couple of turns during which the gears should align, and then tighten the grub.
On my photos you'll notice I've used m4 nuts to tighten the idler. Ideally, you should use wing nuts instead as these allow you to adjust it by hand.
13/12/10 - Updated Idler to beef up the filament guide
18/12/10 - No design changes, but the STLs have been simplified to ease slicing.
06/01/11 - Updated drive gear holder to reduce the holes for the bearings to give better constraint. You may now need to press fit them in, or give them so encouragement with a hammer. Fortunately the bearing supports are open and therefore can flex a little to hold the bearings tightly. Also I've uploaded a new idler bracket without the filament guide, I don't think its actually needed with a hobbed bolt. It seems to be printing as well without it anyway.
02/02/11 Updated the driven gear so that the herringbone gears now contact in the recommended "stable" manner. If you already have this extruder and want this, you only need to print the driven gear, and then just flip the drive gear. Also the idler bracket has been updated for better placement of the holes. I've uploaded updated IGS & STEP files for those who use them.
15/02/11 I've pulled some of araspitfires updates into the main release as they are much better:D. Also thanks to bobbens, all the STLs should now be orientated correctly for skeinforge users.