I’ve made a few modifications to the RepRapPro Huxley extruder: Herringbone Gear Set, Idler lever/handle for easy filament changes, longer rods to set the spring compression and integrated ‘spacer’ blocks to support the extruder against the threaded rod.

Herringbone Gears

The gears that came with my RepRapPro Huxley extruder were a little warped and quite noisey in use. In my previous post I mentioned I’d printed a set of Parametric Herringbone Gear Set for Huxley extruder #30DoC.

The small gear is secured to the stepper motor shaft using a grub screw. The grub screw is held by a captive nut sitting in a small slot in the end of the small gear. After my first attempt at printing the small gear I noticed the slot intersected the shaft hole – it was too close to the hole. I made a small change to the OpenSCAD model for the gears to offset the position of the slot away from the shaft hole. The amended OpenSCAD model for the Herringbone Gear Set can be found here: http://www.thingiverse.com/download:88002

Modified M6 Block

I also decided to make a few changes to the ‘M6 Block’ that forms the main part of the extruder. The changes are labelled in the image below:

1. Thicker end section. This allows material to be removed at 3 without affecting the strength of the part
2. Added spacer blocks so that the part can rest on the threaded rod of the Huxley
3. Removed some material around the idler pivot to allow it to open wider. This, and the addition of a modified idler with handle, makes chaning filament and maintenace around the hobbed bolt easier

I sliced the new model in Skeinforge with Raft enabled. This automatically added support material under the overhanging parts.

Thicker Tongue Prongs

The Bowden tube end is secured to the M6 Block using a small ‘tongue’ part. I wanted the Bowden tube end to be held more tightly so modified the tongue part so that the ‘prongs’ which fit around the Bowden tube end were thicker.

Longer idler lever/handle

I changed the idler bearing block by extending one end to create a lever/handle. The holes for the M3 threaded rods were reamed out more to allow for more movement. The ends where the lever pivots were filed down to make them more rounded.

Preparing the parts for assembly

All of the printed parts needed some clean up and minor adjustments before they could be assembled:

• The Herringbone Gears needed a lot of cleaning up. The gear teeth were very rough and needed a lot of fiddly work with a file, sharp craft knife and sandpaper.
• The support material needed to be removed from the M6 block
• I used a drill bit, sharp craft knife and files to remove material around the area on the M6 block where the idler pivots.
• I used a craft knife and filing to reshape the end of the idler. I rounded the ends and removed material from the bridge spanning across the bearing supports. It was a process of removing a little material at a time, fitting the idler to the M6 block and checking how far the idler would open, removing the idler and removing a little more material. It was a compormise between how much the idler would open whilst not removing too much material and weakening the part.
• I replaced the M3 set screws that were used to set the idler spring compression with longer M3 threaded rods. This allows you to fully loosen the springs and bolts so the idler can be opened without the bolts coming free of the semi-captive nuts. Before this change every time I wanted to loosen the springs, enough to get into the area where the filament runs across the hobbed bolt, the nuts holding the M3 set screws would come loose and fall out of thier holes. These holes are very difficult to get to as access is blocked by the stepper motor.
• Wing nuts replace bolts on the M3 threaded rod. This makes setting the spring compression easier.

Assembling the Modified Extruder

The Herringbone Gears need to mesh together well. For this to happen the shafts of the stepper motor and the hobbed bolt need to be correctly spaced. The spacing for the gears is given in the ECHO string in the OpenSCAD file that models them – “Your gear mount axles should be 36mm from each other“.

I decided to print a small spacer jig to get the motor shaft and hobbed bolt at the correct distance. The simple block with two holes spaced 36mm apart is fitted before tightening the fixing screws for the stepper motor. It is then removed.

The gears are little tricky to fit as they will not slide past each other due to the herringbone gear pattern. To get them onto the shafts you have to hold them together with the gears messed as you push them onto the shafts. Once aligned the grub screw can be tightened on the small gear and the bolt added to secure the larger gear.

‘Tuning’ the gears

Even after spending a long time cleaning up the teeth of the herringbone gears they still did not run together as smoothly as I would have liked. There was certainly no backlash but they did bind at certain points. With them fitted to the shafts I had to spend a lot of time rotating them and then using files and a sharp craft knife to carefully clean each tooth of each gear to reduce the binding.

With everything assembled it was time to fit the new extruder to the printer. The extruder sits well on the threaded rod due to the added ‘spacers’.

Conclusions

The herringbone gears are certainly quieter than the previous gears. There seems to be very little, if any, backlash. The herringbone gear set are a challenge to print at a high enough quality that they will mesh togehter smoothly. Having to spend hours filing, scraping and sanding the gear teeth wasn’t a lot of fun as the angles of the teeth mean they are very difficult to get a file or knife into. If you’re going to print these you will need to make sure you are already capable of good quality prints.

The herringbone gears do make for a quieter extruder, especially during quick retractions – they don’t ‘clatter’ as the previous gear set did. Using the longer M3 threaded rods, wing nuts, adding a longer lever/handle to the idler and changing the shape of the parts where the idler pivots all make chaning the filament or getting access to the hobbed bolt a lot easier. I also added a blob of glue to each of the nuts holding the bottom of the threaded rods before putting them into the holes – they don’t fall out any more.

I haven’t really seen any improvement in the quality of the prints I get using the modified extruder but I haven’t yet printed anything too complicated with it.

Files, models and pictures

All the files and models for the modified RepRapPro Huxley extruder can be found on Thigiverse at: http://www.thingiverse.com/thing:24627

More photos of the extruder and each of the parts can be found on flickr: http://www.flickr.com/photos/acraigie/sets/72157630084972270/

The Huxley 3D printer is from RepRapPro