QCAD.org Forum

One type of curves which cannot easily be generated from the existing primitives in QCad is Cycloid curves (e.g isochronous curves) and the related Involute curves (used for, among other things, constructing gears). I know I could start learning script or import a coordinates from a spreadsheet but I think it would make a compelling case to include this as a primitive(s).

QCAD uses the DXF/DWG file format for storing drawings. Unfortunately, this means that entity types (primitives) that are not supported in that file format such as these types of curves are not really an option. Perhaps, spline entities could be used to represent such curves.

There is a paper, here http://web.mit.edu/harishm/www/papers/involuteEWC.pdf which describes the use of splines for approximating an involute curve in CAD systems. I think the approximation would be pretty close for the range of curve that is used in a gear for example, not sure about an extended length curve. The gear wheel generator script I wrote, in the part library, does not use that but just calculates points on a polyline (I've intended to work on it, but..)