When I installed the new fine-tooth filament drive gear (wheel, whatever) in the M2, I ran some numbers that suggested replacing the fixed-position screw with a (more-or-less-)constant-force spring. Some recent discussions on the M2 forum suggest, at least to me, that the drive gear is, indeed, less forgiving of filament diameter variations, drive housing wear, and suchlike than the chunkier old gear.
Having recently bought an assortment of longer M4 screws, I finally got around to installing an appropriate spring from the Big Box o’ Springs and another washer to capture it:
Before doing anything, I measured the gap between the filament drive body (on the left) and the lever arm (on the right) holding the idler bearing: 21 mil = 0.53 mm.
I don’t have a number for the spring constant; it’s rather stiff.
After installing the spring, I cranked the screw to restore the same gap as before, which should mean the spring is exerting roughly the same force on the arm as the fixed-position screw.
The general idea: the spring allows the flexible arm to move as the filament diameter changes, while maintaining roughly the same pressure on the drive gear, thus producing nearly the same depth-of-engagement grooves in the filament. Maintaining “the same pressure” requires the motion to be relatively small compared to the spring preload distance, which seems reasonable with ±0.1 mm diameter variations and maybe 5 mm of preload.
The new filament drive gear hasn’t ever stripped out (after that initial finger fumble), so this will be more of a test to verify that the spring doesn’t make the situation worse.