I have spent a LOT of time trying to solve the thread tension problem and have finally come-up with a solution that provides smooth, fully adjustable thread tension.
Basically forget about the spring.
I use a threaded rod, two UHMW plastic washers (very low friction) on each side of the spool, and two opposing rare-earth magnets instead of the spring, A single nut at the end is used for tension adjustment (by hand). The lack of a spring means that there is nothing physically touching the washers resulting in unobstructed smooth movement of the thread spool. The force of the opposing magnets allows for infinite adjustment of the tension. You can actually tighten it hard enough to prevent spool movement without the opposing magnets touching. The key here is that the magnets have to be opposing, not attracting - like some other designs I have seen.
This system works flawlessly and is the best I have found to-date.
I got the magnets from Lee Valley: [
www.leevalley.com]
I made the UHMW washers myself out of a 1/4 inch UHMW sheet.
I will try to take pictures tonight and post them to illustrate further.
