This is a derivative of MiseryBot's excellent spider coupling http://www.thingiverse.com/thing:7678

I basically modified few values to have it fit the 8mm diameter of my trapezoidal spindles - the original thing was adjusted to fit the common M8 threaded rods whose diameter is slightly smaller than 8mm. Additionally I corrected the M3 nut trap sizes since they were little too small to match ISO 4032 nuts.

Finally I used the insert-positive to quickly make a simple, but great working mold for use with awesome SUGRU!

This is the best performing coupler I used so far. Note that the spindles need to be supported via extra bearings for radial guidance as well as some kind of fixing collar on top of one bearing for axial support.

I recommend printing the couplers with at least 50% infill for durable results.

Instructions for molding Sugru:

• Grease all parts of the molds gently (silicone grease works fine but vegetable oil should do the job, too)
• Apply some Sugru of your favourite colour; a 5g pack will do for two insets
• Use 10-20mm M3 screws an nuts to tighten the top and bottom part of the mold; some Sugru might sqeeze out a little
• Wait for about 24 hours to allow the Sugru to cure a little; it doesn't do so very well inside the mold, because it's pretty much sealed in this state
• Cut off top and bottom mold parts using a sharp and preferably thin knife; avoid tilting the parts too much, it would deform the still partly uncured inner Sugru
• Wait for 24 hours again. The Sugru should be entirely cured now and can be easily pushed out of the mold