Parameterized printable spring (Q68499)

It is a proof of concept of a kind of printable spring. Initially I designed it for using in my Prusa Mendel, because the z axis springs were very difficult to find for me. Then I generalized the idea and parameterized the spring. These springs conceptually consist of a strip of plastic that is bended. The outer cross section of the spring can be square, hexagonal or octogonal. The spring parameters are: * fid (fold inner diameter) * folds: Number of folds * th: strip thickness * Drill: Drill diameter * Diameter of the outer cross section Here there is a video of the proof of concept: http://www.youtube.com/watch?v=dbjiXFEyg_E