As a Technical Author I used loads of different tools for various reasons, mostly VERY expensive, but one of my favourites and most used ones is a freeby graphic program called Inkscape.
This is a vector type program, I.e. it draws "proper" lines. All you draw can be printed at any scale with superb results and it is very easy to use.
It's not really a 3D program like Sketchup, but it is still very useful, give it a go!
Bob