Starlight, Rapzunel Silver, Gold Happens, Vertigo Galaxy
Use a brim where necessary. First, print out the tester-tube.stl twice and see how well they screw together – Adjust slice settings as necessary.
Spin me right round.
Need to change the diameter of the screw? How about using different heat set inserts? The parts are setup with user parameters, so it's easy to change a value and automatically update the features in CAD. Tolerances use a "gap" to give clearance between mating surfaces – slight adjustments make big differences.