Summary

Photo added 7-18-20: Added photo of larger coupler mated to a Backmann knuckle coupler.

UPDATE 9-24-2019: I'm calling this done! Included are source files so you can customise the couplers or their shanks. 1:20 scale couplers, as pictured, were printed by scaling everything 160% and using 7mm bearing balls. I have also included examples of how I print them, with the body in two pieces to maintain outer detail and allow cleanup of the inner surfaces prior to final assembly. The STL files with thinner shanks are Kadee box compatible but (obviously) not self-centering.

UPDATE 8-18-2019: I have been able to get the detail level needed by printing with a .3mm nozzle in ABS. I suspect they'd be even better in PLA but I live in TX and the outside temps make me nervous. The pictured large couplers are 1:20 scale, small are 1:32 which I intend to use as 3/4 size narrow gauge couplers in F scale.

UPDATE: I've had some luck printing this after a minor rebuild of my Anet A8. Photos of the 1:20 scale version and 1:32 scale (unfinished) prints have been added. Suffice to say your printer needs to be well tuned to get these parts to print well.

This is my attempt at updating the beautiful coupler design provided above to make it have some semblance of functionality. Using a steel BB to lock the knuckle, a magnet should uncouple the design. I haven't printed this out yet, feel free to let us know how it works for you if you get to it first.

I have included the source file I used to verify fits with a pair of couplers mated. The format is from Designspark Mechanical.

https://youtu.be/mchGHJw_Mpw

https://youtu.be/EqSQ0ennc58