For more information than you ever wanted to know about nuts and bolts check out Fastenal's 62 page Technical Reference Guide:
http://www.fastenal.com/content/docu...renceGuide.pdf
Edit: Oh yeah, they also have a neat poster you can get from your local store that has all sorts of fastener information (sizes, grades, head types, etc.). It's a nice quick reference to hang on the wall in your robot shop.