One of the reasons to own a 3D printer is to repair or replace parts that have worn or are broken. After pulling apart our self propelled lawn mower (Craftsman Briggs & Stratton 700 series)to find out why it stopped self propelling. I find out the plastic gears are ground down to nothing. The gear is one unit which happens to be the wheel. After looking up the price I decided to give my Airwolf 3d printer a try. After printing a few test parts to confirm the gears were correct I printed a pair out of ABS. The lawn mower worked for about a month then worn down. So having a 3D printer I printed another set in a day out of Nylon and lets just say I will never have to make another set for a while.