Spool spindle/center - universal

These adaptors will allow your spool to spin so much smoother.  I have made two models - the first will can be pressed into a spool center of 60 to 75mm with ease.  It has 20 spokes.  It requires quite a bit of finesse, but I did fit it into a 50mm spool center.  The second model has 15 spokes and is easier to press into a 50mm spool center.  This provides a snug fit that is easily transferred between spools.  

To make this work - the center is 8mm (it may need to be scaled slightly based on your printer settings) and will fit an 8mm axle (allowing you to mount your axle on common 8mm x 22mm x 7mm skateboard bearings).  Alternatively, if you wish to use a more frugal method, press 693 bearings (3x8x4mm) into the center and insert a 3mm rod.  

By providing a snug fit, this allows your spool to rotate smoothly with very little friction.  With the axle pressed into the universal center, you can quickly squeeze the center in and out of different spools to the mount on your custom spool holder (I will make and upload a spool holder next, although this can be easily inserted into most spool mounts.

I suggest that you print this with PETG as it is flexible and tough enough to be easily pressed into different spools and removed.  Alternatively, TPU allows for an even more flexible center.  I printed this with rapid PETG at 100% infill (since it was less than 2% more filament and saved time with printing).

update - I added a 10 spoke spool spindle/center that is even easier to compress to fit small spool centers