Mechanical counter

A mechanical, stackable decimal counter. The rightmost digit is driven by the user turning a knob, each subsequent digits turns as the previous one turns from 9 to 0. 

Assembly:

There are pictures to help with the assembly.

1. Take the right endcap (the thick part with the window) and insert the knob into the cutout, so that the bevel gear shows through the window. 
2. Take the knob holder piece (the very thin cover part) and insert it into the cutout. Make sure its oriented correctly (one side has a cutout that closes around the knob shaft) and that the sliders on its side are aligned with the slots in the endcap. Push it in to hold the knob in place.
3. Put the carry_in gear on the axle in the endcap so thats its bevel gear meshes with the knobs bevel gear.
4. Put the carry_in spacer on the axle. If you now turn the knob (and hold the spacer down) the gear should turn.
5. Take the first digit wheel housing and put the digit wheel in. Make sure the full gear on the digit wheel is facing “down” and the one-tooth gear on top is visible.
6. Slide the digit wheel housing into the slots on the endcap. Make sure the sliders, pins and holes are aligned properly, then push it onto the endcap all the way. If you now turn the knob, the digit wheel should turn as well (Theres nothing yet that holds the digit wheel in, so you may need to hold it in with a finger). If no more digits need to be added, continue at step 10.
7. Take the two sides of the carry gear housing and the carry gear. Insert the carry gear into one of them, then put the sides together. Align the pins and holes and push them closed. The carry gear should rotate freely on the axle.
8. Now slide the carry gear housing onto the back of the digit wheel housing. Make sure you orient it correctly, the side with the holes should face the right endcap. Align the holes, pins and sliders, then push it in all the way. The one tooth on the digit wheel should turn the carry gear.
9. Repeat from step 6 for as many digits as you want. The next digit wheel housing slides onto the previous carry gear housing. Make sure the holes on the digit wheel housing align with the pins on the previous digit wheel housing. The holes on the carry gear housing are only there for the right endcap, so you dont need to worry about them anymore.
10. If all the digits are assembled, the left endcap can be put on. Slide it onto the back of the last digit wheel housing, make sure its aligned, then push it down.
11. The backplate pieces (backplate and endcap spacers) are used to hold it all together. First slide in the backplates, then the endcap spacers. Pay attention to their orientation, see the pictures for that. Note: If you only want to use one digit, the endcap spacers are not needed.

Number of the parts that need to be printed:

Both endcaps: 1 each

Both endcap spacers: 1 each if number of digits ≥ 2, 0 for single digit. Note: They are distinct parts

Backplate: Number of digits minus 1, or 1 for single digit

Carry in gear, knob, knob holder, carry in gear spacer: 1 each

Both carry gear housing parts and carry gear: Number of digits minus 1, or 0 for single digit

Digit wheel and digit wheel hosuing: One each per digit

Print instructions:

All parts should print fine with .2mm layer height and default settings.

Heres what ive used:

Both endcaps: 3 perimeters, 20% infill

Both endcap spacers and backplate: 3 perimeters, outer perimeters first, brim.

Carry in gear: 100% infill, seam position random, brim.

Knob: 3 perimeters, brim, and using a height modifier 100% infill on the bevel gear end.

Knob holder: 100% infill, brim.

Carry in gear spacer: Brim (inner and outer), random seam .25mm layer height.

Both carry gear housing parts: 3 perimeters, brim, 30% infill.

Carry gear: 5 perimeters, 50% infill, brim

Digit wheel: Thick bridges, brim, and using height modifiers 100% infill on the gears (0-3mm and 18-22mm) and .15mm layer height, 5 perimeters and 25% infill for the digits part of the barrel (7-14mm). Seam painted on.

Digit wheel hosuing: Brim, 25% infill.

The templates for the gears used are from STLGears.com | The Free Gear Designer.