Corona 3 Typewriter Improved Replacement Spool Nut

The ribbon spools on a Corona 3 typewriter are somewhat unique. They will not operate without the spool nuts which both retain the spool, and select the direction of ribbon travel. Both spools are actively rotated clockwise during typing (including the spacebar), but because they are fed from the front (left spool) and rear (right spool), this clockwise rotation causes both spools to try to pull on the ribbon. Ribbon reversal is manual, you must loosen the nut on one spool, while the other one must be tight. Both loose, no movement. Both tight, something may break as they both try to wind.

These frequently are lost on these ~100 year old machines. You can't just replace them with a matching nut (if you can find one), as the originals are quite sophisticated, actually, they have a built-in retaining clip mechanism that uses a spring force to keep pressure on the threaded rod, keeping the vibration and rotation from unscrewing the loose nut (the one that is tight will be retained by the friction from retaining the spool).

For fun, I wondered how replacement nuts would look 3D printed, and what material would be best. Two designs are uploaded here, the "rigid" one is my initial design, which you can print in any rigid plastic and it should work, I tried PLA, PETG, and Nylon, and all worked fine. My silk silver PLA looks quite striking, actually, and closest to the original look. The Nylon felt great and would be of course the most durable. But, how to solve the spring retention problem?

It finally hit me that using FLEX material, and printing the thread a bit small, would solve the problem! Screwing it on, the material expands to fit the threaded rod, but that generates a retaining force (like the original spring mechanism) that keeps the nut firmly on the rod despite any vibration or rotation at all times, not just when not snug against the spool. I actually think this is superior to the original mechanism. Printed in black NinjaFlex, on a smooth build plate, the top ends up glossy and shiny and looks fantastic. After all, this machine is painted in glossy black (stunning) paint, and there is already plenty of gleaming accent chrome, I think these glossy black NinjaFlex nuts look fantastic. And they function properly. The second STL "FLEX" is the one with the thread reduced for this option.