The ArmDrive is a 1.75mm filament bowden extruder, designed to work with the NEMA 17 stepper motor. It mounts directly to any OpenBeam or equivalent sized extrusion. The MK7 and MK8 versions are each designed to work best for those particular drive gears, or gears with similar dimensions. The extruder is designed to be robust and reliable, while using as much easy to find hardware as possible. These files are the result of a surprisingly large amount of work, I hope you find they work as well for you as they do for me. Enjoy! Update - January 6th 2015 Revision 2 Slightly increased the diameter of the post holding the 608ZZ bearing to reduce play and allow more clearance around the outside of the bearing near the swivel shaft area. Increased the radius of the bearing cutout near the swivel shaft area by an additional .2mm to allow for more clearance for problem described above. Significantly strengthened the input filament alignment post. Added more support behind the spring retainer arm on the base-plate to reduce warping over time when under high spring tension. Update - October 24th 2015 Revision 3 is now available!The ArmDrive was designed to be printed at .2mm layer heights, any other setting may result in less accurate vertical layering.Suggested print settings: Layer height: .2 Perimiters: 4 Solid layers Top and Bottom: 4 Infill: .4ABS plastic is highly recommended. PLA will work in a pinch, but tends to warp over time, especially due to heat created by the NEMA 17 stepper motor.Drill bit sizes: (confirmed working) 15/64" for the upper and lower arm pivot holes 1/8" for all screw holes excluding the lower arm 7/64" for the two lower arm screw holesI don't have any metric drill bits for testing, but the hole diameters once drilled should be approximately: 6mm for the upper and lower arm pivot holes 3.2mm for all screw holes excluding the lower arm 3mm for the two lower arm screw holesUse caution when installing the lower arm on the swivel post! The post is narrow, and will break easily. Be sure to deburr the post and check for roundness. The post is quite strong once the screw is installed. If the post does break off, it is still completely usable as long as it is complete and located in place when assembled. The post merely acts as a standoff.One binder clip arm is often enough to provide more than adequate tension. There are four spring slots available for your super filament crushing desires, but beware, too much pressure can cause premature failure of the swingarm, and deformed filament.The alternate lower arm is designed to work with an M4 nut threaded directly on the bowden tube, whereas the standard lower arm uses the 4mm push to connect fitting. Take your pick!