I had always wondered how the mechanical counters worked, and it turned out to be nothing like the way I thought it would be, but something simpler and more elegant. Having discovered this I worked out this design that could be 3D printed. Research on the Internet several designs that kept all the gearing internal to the number reels, but the one that is more appropriate to help with the understanding of the workings was the one shown on the left. This has 3 number reels that allow you to count to 999 and uses a lost motion gear pair in between each reel, that transfers the motion to the next reel after 1 complete revolution of the lower digit reel. 

This design is based on the model developed by Matthias Wandel, you can see it here http://woodgears.ca/counter/in... I have changed some of the features to make it suitable for 3D printing, and simplified the Actuator and made it a simple push operation.

You can get the full instructions from here http://www.3dprinterclocks.com...