You know that there are computer aided design programs that can do engineering drawings like the one below but they are too difficult to learn without going to night school. Never mind, there is a good chance that there will be a helping hacker/maker to answer questions as you engage in hands-on self-learning.
OK. So you covered that at school and you would now like to use a 3D modelling package but they are all too expensive and hard to learn. Help is at hand with Google's FREE SketchUp program. This is very easy to learn and with a suitable "plug-in" can produce files for the latest open source 3D printers. See the image below and our PRODUCTION page.