wow, that's pretty cool.I am a recently retired tool maker so I do understand all aspects of machining parts. Interesting. so you basically rely on the press fits and if you need to "hold" the collar you use a collar clamp . I just bought from McMaster Carr some precision ground heat treated metric shafting, 4mm and 5 mm . and I will see how this works out. I believe you are correct and this is actually no big deal as long as we can procure metric "precision " heat treated shafting . Thank you. I will let you know how I make out .