Quite tight tolerance on the screw. When turning the bolt once outside the the rack, the thread gets better, so that it's much more smooth when turning in the screw the second time when placed in the rack. Also no resistance to overthightening, had to increase wall layers to make it 100% filled inside in order increase resistance to twist. Printed standing (screws on top) with supports. When removing the supports string used to break, solved by adding one more wall layer and make it 100% filled. But then still the end of the spring (the part snapping in the rack) breaks appart due to bad layer adhesion (shear effect). Tried with PLA and PETG