- At least x4 m3 30 One spring for the IRobertI Arm. You can reuse the strong spring of the um2+/um3 but I think is way too much force for what is necessary. Remember this isn't a brute force feeder, bondtech gears do push the filament very precisely. So just pushing hard with the spring doesn't help feeding with quality. - Some x2 M3 10-12-14mm depends if you use it on um3 (you can reuse his small m3) or on a um2+