screw extrusion lib for polyhedron

thingiverse

When it comes to threads, most people's approach is to use a for()-loop with many segments, each consisting of hull(){shape(n); shape(n+1)} or a set of simple polyhedrons. This takes a long time to render and has its restrictions due to the early passing of work to openSCAD. As an alternative to this, I have written the function screw_extrude_v(), which returns the required parameters for polyhedron(). Besides the standard-parameters for such functionality, it receives a vector with the points of a polygon that makes the shape of the thread and calculates and returns both vectors and a roughly calculated convexity required for polyhedron() for further processing. The primary target for modifications is the first parameter "points", while the second parameter "faces", which describes how to turn the points into a surface, is rarely something to change, though changes should be possible here too, for example to slice an object and combine the rest with another object in the same format. The most simple form to use it is this: v = screw_extrude_v(P, r, p, d, sr, er, fn, rf); polyhedron(v[0], v[1], convexity = v[2]); for a thread with constant diameter or v = screw_extrude_v(P, r1, r2, p, d, sr, er, fn, rf); polyhedron(v[0], v[1], convexity = v[2]); for a thread with growing/shrinking diameter, like for US-garden tubes or tightening threads on ball-joints. Another notable feature is the sr (starting ramp) and er (ending ramp) parameters, which, giving an angle, make a soft start and end of the thread without the need to cut its end off with difference(), which still doesn't give a proper easily gripping thread-ending.