I've been drawing now for 2 months, this is how far I am at the moment:
I model everything: Screws, bearings, gears, chains; then I know in the beginning how much do I need what. I take all the models from maedler, they don't look very good but they give me the right dimensions so I can fit them into my model.
I think I need two more months. Model is CAT 390 DL.