I am just finishing a Printrbot Simple kit. Fortunately I found a file online in a format that will work for a gear to test the printer. Otherwise it would be quite awhile before I made any useful parts. I am finding 3D CAD to be much more difficult than I thought it would be. I learned 2D CAD quite easily. Not so 3D!
One big problem is that all my computer input devices are 2D. So inputing a model in 3D requires much more keyboard entry of numbers, rather than drawing with a mouse, trackball or pen tablet. So I am trying to adjust to the new paradigm.