A spring-loaded bearing idler for 1.75mm filament. It should be compatible with the Makerbot mk7 and the Makerbot mk7 Drive Gear, but I designed it for a spur gear from SDP-SI: S10T05M020S0505. It has clearance on the bearing side so you can print a mirrored idler and use it in a dual extrusion setup.
The overhead lever design was inspired by http://www.thingiverse.com/thing:15718 . That design has no filament guide which is required with a spur gear. The idler bearing also seems to contact at slightly the wrong position when I printed it.
The body was insprired by http://www.thingiverse.com/thing:13727 which I quite like except for the fact that it protrudes on the idler side preventing use with dual extrusion. I also find that the force required can flex the lower part of the base because of the slight bounciness of the spur gear. The hinge on that design is also superior to mine but I did not see a good way to incorporate it in my design.
After about a year the pivot arm on my original print has flexed enough to require a larger spring. I've printed a replacement at 100% infill but not converted yet. I'd recommend at least 40% infill when printing this.
2x M3x18 flat head screws. These secure the base and the arm to the stepper.
1x M3x10 flat head screw to hold the bearing.
1x R2 (US size) or 623 (Metric) bearing
1x fairly strong spring.
Print one plate normally for single extrusion. Mirror and print again for dual extrusion.
Remove the builtin support from the curve that meets the stepper, from the idler where the bearing goes and from the hole where the bearing screw goes. You may need to run a 2mm drill through the filament guide or clean out the other holes, depending on your printer.
Tap the bottom side of the bearing screw for M3 and secure the bearing.
Mount the base to the stepper with the leftmost screw. Pinch a strong spring between the base and the arm while securing the right side screw. You can also try to insert the spring afterwards.