Summary: NEW VERSION!!! Now with braille numbers and easy to feel direction indicator! ALL gears have been revised. The changes make assembly easier with better fit and smoother operation. This Math manipulative is designed to be playful from the first touch. It lies flat on desk top, so it will also work well with overhead projection systems. Behind the mechanical fun there are several simple ratio exercises. The eight tooth "Idler" gear is supplied to add reverse rotation without affecting the ratios. Each interchangeable gear has built-in graphics to indicate the number of teeth as well as rotation position. The addition of braille numbers will help those with impaired vision. Some gear ratio formulas can be found here: http://en.wikipedia.org/wiki/Gear_ratio. Rubber bands provide tension to keep the wheels engaged no matter what the combination. FYI, the "T" on the 16 tooth gear stands for "teeth". INSTRUCTIONS: This thing is designed to print on ALL machines, I'm sure it will have good results on the tiniest of machines as well. All parts fit within 105 mm x 105 mm build area. No support or raft is needed. There should be VERY little clean-up. Assembly is simple, just look at the photos provided. The rubber bands are just 3/4" or 1" office supplies. One part that may get a little tricky is the Idler gear. That part is really two parts in one. In my test, it just needed to be broken loose and spun around a few times to free it up. The axle post is now a snap-fit into the center hole in the base unit so that it can be added or taken out as needed. Be sure to clean out the center holes in the gears after printing so that it is not too difficult to get them on and off of the pins. Have fun learning!