This clock is a marble track. The second hand lifts the marbles along the outer edge of the clock and releases them at the apex. From there, the marbles will roll down a short path to the base of the clock, where it will come to rest and wait for the second hand to pick it up.

Printing: 

There are five pieces total: 3 clock hands, the clock face, and the base. Every piece except the clock face can be printed face up without support. The clock face requires some support beneath the cavity for the clock mechanism. Additionally, the clock face may be too large to fit inside a buildplate flat. To solve this problem, I split the model into two pieces to fit on my CR-10 Mini. With some more postprocessing and better tuned in settings the pieces should be seamless when glued together.  

I have also included a 14.5D marble to print if you have no marbles or fitting substitutes at home. To print the marble, I would suggest printing in halves and gluing them together. The easy way to slice the marble into halves would be to drag the sphere halfway through the ground plane, duplicate it, and slice that. 

Assembly: 

Insert the clock mechanism in the cavity in the back of the clock face with the rotating parts stuck through the hole. Secure it in place with the bolt included with the clock mechanism (or glue it if you don't have the bolt). Attach the hour hand and minute hand (in that order), making sure that the pieces fit snuggly and are locked in place. Finally, install the battery and mount the assembly onto the base. 

The clock mechanism that I used. (It's a multi-pack but there are single items and some cheaper options.) → https://www.amazon.com/Movement-Mechanism-Pasters-Different-Replacement/dp/B08SJDZHLT/ref=sr_1_9?dchild=1&keywords=clock%2Bmechanism%2Bwithout%2Bhands&qid=1623109728&sr=8-9&th=1