Planetary Gears

Firmly in the "useless" category is this a fun project to create an articulated toy. The goal is to draw people in and encourage them to turn the crank. After about a dozen visitors to my house have found it and played with it, I think it meets the goal rather well.

I mostly made it on my small CNC router, using Baltic Birch and Extruded Acrylic.

First, I made the gears from 12 mm (aka 1/2") Baltic Birch plywood. I cut them with a 1/8" compression spiral bit so as to minimize edge tearing for which plywood is notorious. Sanding the gear teeth and inside cuts was a bit of a challenge. I made some sanding sticks with the right profile and wrapped them with self-stick 100 grit sandpaper. Problem solved. I think they came out pretty nice.

Next I made the acrylic supports using 4.5 mm (aka 3/16") extruded acrylic from TAP Plastics. Cut with a single flute 1/8" upcut "O" spiral bit. The cut came out very nice. I also counter bored the holes to a depth of 0.3 mm to create pockets for the washers so they stand proud of the supports by about 1 mm and create a smooth bearing surface. That keeps the gears away from the acrylic and allows them to turn with minimal friction.

I used 1/4" bolts, washers and cap nuts for the base and two upper spur gears. The center gear and crank are held together with a 1/4" dowel. I had hoped that these could just be press-fit but that lasted about 1 minute so I very carefully glued them together. The bottom spur gear uses a dowel but I think I will replace it with a bolt and cap nut for visual consistency.

I used Sketchup to design this. After a bit of research, I decided to use a gear angle of 20 degrees. I chose 8" (er, 200 mm) for the outer diameter of the planetary gear and the spur sizes and number of teeth fell into place from that. It took a bit of experimenting to find the exact sizes to allow the gears to be loose enough to turn easily but still be reasonably tight. For CAM I used SketchuCAM. While it works, it presented a number of challenges via it's quirky behavior - much of which is due to the way Sketchup works. Also, SketchuCAM was derived from PhlatCAM which has it's own set of issues like splitting arcs into lines. I spent a fair amount of time getting the GCode right because of that. In particular, it was a bit of a fight to get it to emit arcs instead of lines. Maybe I'll do a blog on that issue. It's not impossible but takes a fair amount of effort and attention to get right. Had I been a little more adept at Fusion 360 which I got about halfway through the project, I would have used that.

I spent a fair amount of time experimenting to get the bearings right. If the wood gears rested against the acrylic supports there would be too much friction so I chose washers to keep them away from the supports. Unfortunately, 1/4" washers have a 5/16" hole so they would "droop" off center. To fix that, I counter bored the 1/4" holes with a 19.1 mm diameter, 0.3 mm deep pocket to hold the washers on center. Those numbers were derived via trial and error. I used thread lock to avoid over tightening the cap nuts so the gears can freely rotate and the cap nuts don't fall off.

In the lessons learned category, there are a number of things I'd change or try out. Per the above comments, I would use F360 rather than Sketchup. I'd like to try 3/8" or 9 mm BB plywood for a lighter look. 12 mm BB is just a little clunky looking. I would also use #8 machine screws. Not only to get a lighter look but also because after a large 15 drawer shop cabinet build and several other drawer projects, I have a large surplus of #8s. I'd also like to experiment with 5 spoke gears and thinner spokes. I really like the way the crank looks and wish I had done it for all the gears. I will probably add a wider and heavier base for it. The idea is to allow it to be turned without holding the base. Another change to the base would be to make the center element thicker, either by using a 2 mm (approx) thick spacer or a different thickness of wood so I can eliminate the washers in the base. In the process of designing and building, I learned a lot about tolerances and "allowances" for getting the various fits right. That may be the subject of another blog. I am always looking for ways to improve upon my designs and design skills.

Below, you can see the exploded views of the design.