Building a mechanical counter
This decimal counter works like most mechanical counters do, such as this tape counter from an old reel-to-reel tape deck. Before everything went digital, counters like these were used in a lot of devices including cassette decks, car odometers to utility meters.
When not being advanced by the two teeth, two of the four teeth on the back of the small eight-tooth gear push against the disk shaped part of the wheel on the right. Only when the notch in the disk comes by can one of the four full width teeth turn through it. This locks the gear's position when it's not being advanced.
I prototyped a lever/yoke mechanism to ratchet a wheel right or left by one increment when a lever (top to bottom in the photo) is tilted in either direction. This mechanism turned out to be more tricky than carry gears!
Cutting it outHaving prototyped the carry and advance mechanisms, I was confident everything would work, so I made 1:1 printouts from my SketchUp CAD model using my BigPrint program.
It took 40 minutes to cut all the gears and rotors. CNC could cut faster, but if you factor in CNC setup time, the bandsaw still wins.
I could have used a bandsaw to do this, but then I would have had to cut a slot in the side of the frame and then glue it back together.
Increment/decrement mechanismTwo levers provide detents (click stops) for the digit wheel and the lever center. Solid hardwood is ideal for these parts, though I could have made these from plywood as well. Here's the templates glued onto the wood.
With one face cut, it was easier to transfer the dimensions for the thickness cuts onto what was left than to glue the templates onto the non-flat surface.
Adding numbersAttaching the gears and carry notches to the sides of the rotors. Getting the alignment right involves a few tricks (more about that in the plans and the video).
To make sure the numbers will end up in the right place, I assembled the counter, then increment the right-most wheel by one click at a time and mark the center of where the digit must go. The position immediately after the one that also increments the second rotor is "0". I mark this, along with the next position as "1", just to make sure I don't get the direction mixed up.
I printed out all the numbers in mirror image (a printable version is included in the plans), then place them ink side against the rotor, and rub the back of the printout with a pencil. This transfers some of the ink onto the rotor.
The right-most rotor is locked to the shaft with a setscrew. A knob attached to the end of the shaft can be used to spin the least significant digit (rightmost rotor) rapidly by hand. This speeds up turning the counter back to zero after counting.
Ron Walters' 1/2 scale
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