This Hollow Clock is a remix of the Hollow clock 2 - silent and smooth by Shinsaku Hiura "shiura" and licensed under the same Creative Commons - Attribution - Non-Commercial - Share Alike license CC BY-NC-SA 4.0.

The assembly instructions and the BOM (parts list) are here.

The STL files are on thingiverse.

I was having problems with the clock keeping time, mainly due to two reasons:

1. The gears sometimes skipped
2. The Arduino itself does not keep particularly good time

To solve these problems I made the following changes:

1. Changed the gearing to use involute gears
2. Added an axel to the reduction gear
3. Added a housing to the main clock face
4. Used a mortice and tenon joint to connect the clock face to the base
5. Recessed the gears to reduce friction
6. Used a Raspberry Pi Pico instead of an Arduino - this has a build in real time clock and so keeps much better time

Items 3 and 4 stop the clock face from "jumping" and skipping a tooth.

Additionally I changed the design of the base and cover, and the new cover can be printed without supports.