Professional commercial 3D-CAD software is horrifyingly expensive. There are some free alternatives out there, and to get some idea of their quality and usability I started trying out one of them. I used BRL-CAD, a free open source application, originally developed by the U.S. Army Ballistic Research Laboratory (BRL) and still used by the U.S. military to model weapons and systems.
BRL-CAD is nothing like any other 3D-CAD software I ever worked with. The user-interface, the way models are structured, the way solids are created, the way the model is saved in a database, it is all very unconventional. However, BRL-CAD seems to be very stable, compact, extensive and powerful, but because it is extremely user-unfriendly I do not consider it yet to be a reasonable alternative to commercial 3D-CAD systems for machine development. Nevertheless I made a few models and one of them I converted into an .STL file (also using a BRL-CAD application) and had it 3D-printed by Shapeways.