Robust yarn spinner with simple parts/hardware and somewhat adjustable tension.

Design uses 2x608 bearings setup back-to-back and clamped together, providing a compact, sturdy base. The spindle is 100mm tall, which was designed to fit 2.5oz yarn balls.

Parts:

• 1 M8x30mm socket-head-cap screw (SHCS)
• 2 608 skateboard bearings (look for "speed" bearings, that are loose and spin freely, like you'd use for a fidget spinner. Precision bearings have thicker grease and won't spin well for this application)
• 2 M8 standard washers, not fender washers. They must not touch the outer shell of the bearings to spin freely, just the inner journal (metal ring in the center)
• 1 M8 Nylon lock nut
• 3 M3x10mm socket-head-cap-screws (SHCS)

Assembly:

• Insert the M8 screw into the coupler and push/turn until it's fully inserted. The head should be fully in and below the top of the coupler.
• Insert 608 bearings into the base, one in the bottom and one in the top. The top bearing should be flush with the top surface, and the bottom inserted all the way inside.
• Add M8 washer to M8 screw and insert through base bearings. Turn over and add second M8 washer to screw, then M8 lock nut. The nut will get tight very quickly but needs to be screwed on until it is clamping the washer against the bearing. You don't need to clamp really tightly, just so it's slightly snug and the coupler doesn't wobble.
• The coupler should spin freely at this point. If it doesn't, loosen the lock nut just barely, and check again. If the coupler spins freely but is loose and wobbly, tight the lock nut barely.
• Insert M3 screws through top of spindle holes until they slightly protrude through the bottom, line up with holes on top of coupler and screw into place. The coupler holes are slightly undersized so the threads will cut their way in and hold securely.
• The screw heads should be flush or slightly below the top surface of the spindle when fully inserted, but be careful not to overtighten as it may strip the plastic threads.

I included the STEP CAD file in case it's helpful or for easier remixes. There's a M8x30 shoulder bolt with M6 threads and associated lock nut in the model, but it's not necessary for smooth behavior. I had them on hand so I designed around it, but regular M8 hardware will work as well.