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/index.html
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.
For complete instructions got to http://www.3dprinterclocks.com/page23.html
I used ABS for all the parts, and you will need 1 of each STL part except for the following:-
Counter Front/Back - 2
Counter reel 2 - 2
Lost motion - 2
Lost motion -Drive Pin - 2
You will need some extra components to complete the build:-
1 piece Ø 2 mm steel rod 80 mm long
2 piece Ø 2 mm steel rod 100 mm long
2 Springs as used in a retractable pen
You may also need a:-
Ø2 mm drill for the 3 holes in the Mainframe for securing then ends of the Ø2mm shafts.
A Ø2.1 mm drill for drilling out the bores in the parts that are a running fit on those shafts.
Acetone to solvent bond some of the parts together.
The 4 components that make up the frame are held together with Tabs and Slots, these need to be a tight fit but not that tight that you find it impossible to dismantle them again once test fitted. I would suggest therefore that you adjust the tightness with a file until the fit seems reasonable but not excessive.