This model illustrates an interesting planetary drive variation that produces a very low gear ratio.

Click the video link to see it in action:

Instructions

The video shows how it works and includes some construction tips.

You'll probably want to make the Tube cutting tool as well:

https://www.printables.com/model/192596-tube-cutting-tool

I started designing this in metric units, but was only able to find brass tubing in imperial sizes, so I simply used both measurement systems. If you live in a metric part of the world, you should be able to adapt to locally available materials by drilling out the bearing holes as necessary

Category: Engineering
 

Update Available 
 

https://www.printables.com/model/997656-high-reduction-ratio-planetary-drive-revisited