This is a technical gift to my nephew: plywood planetary gears. I had the opportunity to order some water jet cut parts for a work project, and after seeing the results I tried it in a hobby project also.

CAD model

I drew the gears in QCad (design file), which is a great open-source program for simple 2-dimensional CAD-drawing.

The biggest challenge was fitting the round teeth around the gear perimeter:

1. Choose teeth width w and tolerance x to use between meshing teeth.
2. Choose number of teeth n.
3. Calculate gear diameter d = w * n / π and teeth angle α = 360° / n.
4. Draw a circle with diameter d and then on it's circumfence, smaller circle with diameter w + x.
5. Starting from the intersection of the two circles, draw a third circle with diameter w - x.
6. Cut the small circles in half and remove unnecessary halves.
7. You should now have a basic form for the teeth, one concave and one protruding curve.
8. Use the rotate tool to make one rotated copy of the basic form, rotating around the middle point of the big circle.
9. Adjust the endpoint of the form with snapping so that it aligns perfectly with the copy.
10. Remove the copy (because it has not been adjusted for alignment) and make n new copies.
11. Remove the big circle. Your gear is ready.

I used x = 0.5 mm for the gap between the gears. In retrospect this was a bit too large, leaving too much play in the gears.

Plywood

I ordered my parts locally from Tampereen Vesileikkaus Oy, for a total of 60 €. I guess the starting costs raised the price somewhat, I might have got it cheaper from some laser cutting place on the web.

The material is 6.5 mm film faced plywood. The film caused a bit of a problem with the water cutting because small details chipped away. However, I got replacement parts for free and they somehow managed to avoid the chipping.

Painting

I painted some ducks with templates made by my wife, to decorate the gears. Paints are water-based acrylic and there is a bonding primer underneath, both applied with a brush.

I quite much liked the end result. I've heard that the yellow ones are used more as toy pancakes than gears, though :)

Next time I might try something a bit more creative.