CVJ (Constant Velocity Joint) Fidget Toy

This Birfield-type Constant Velocity Joint is very addictive to fiddle with.
Although it requires two hands to operate it turns pleasantly smoothly, while it will transmit a surprising large amount of torque over a large angle, up to 45° for the single CVJ version, twice that for the double CVJ coupler.
I've added some knurling on the shafts to add grippiness for easier handling.

Interestingly, I've used exactly these type of CVJ's in my RC-car that I designed and built some years ago as it's shown in my banner image, so I know they're very pragmatic, as well.

Tolerances for this 'toy' are pretty tight and for smooth operation it's essential to use balls of the right material and dimension.
I've added an image of the BB balls I use for all my CVJ and bearing needs.

After printing the shafts need to be glued into their respective parts (see images), and it is crucial to take notice of the orientation of the inner race on its (correct) shaft, it will not work when flipped !

Assembly is easy, but a bit fiddly (see the attached image for a schematic guide) :
1. after glueing, glide the cage over the inner race on its shaft, notice: the cage has no orientation, it works both ways
2. insert the combination into the housing part. Attention: tight fit, the holes in the cage should glide over the narrow edges ot the housing
3. maximize the angle between the shafts and by rotating the different parts align the grooves of the housing and inner race with the holes in the cage
4. insert the first ball and by rotating 60 degrees to the next groove incrementally keep adding balls
5. the last ball will go in pretty tightly, but with a bit of force it should snap into place

Please note:
- because of the relative roughness of 3D printed parts dust and grit will accumulate in the grooves and ways after a few days of turning, which will make the joint choppy and jerky: this will need to be cleaned ! 
- when dis-assembling, removing the balls is tricky for the first of the six balls, see attached images for some tips and tricks for that, the remaining balls will easily detach
- after a thorough cleaning (wiping down with a rag or cloth), and possibly replacing worn (matte) balls, re-assemble and apply some small drops of grease
- Fortunatey, the cleaning needs to be done only once ! After that it will run for a long, long time

Printing :
- as of now PLA is the preferred material due to its hardness, toughness and printing quality, other materials are to be tested
- as the shafts are are long and thin and printed vertically I advise to use a brim
- for all other, rotating parts it is essential to not have a noticeable seam, I use the 'Random' Seam position, I never tried a Scarf joint type seam, might work
- the remaining settings are standard

For anyone interested in some background information see:
http://archive.is/xVEk

Note: as usual, any stereo images are of the cross-eyed type.
Link: https://en.wikipedia.org/wiki/Stereoscopy