To follow on from previous designs, if we target less extreme gear ratios we have more room to move with our choice of gears. In particular, it would be most beneficial to have fused sun gears in both stages. This would allow for the drive force to be distributed over a larger contact surface, assist in initial post print processing and improve efficiency.
A fused sun gear introduces a constraint that the sun gear teeth is a multiple of the corresponding planet gear teeth, so that it is in effect a mirror of the planet gears and the two stages turn in unison with the planets in a 1:1 ratio. Even with this constraint, some absurd gear ratios are still possible (up to 616:1 within the range of gears specified in this customizer) but around 50:1 to 100:1 seems a good target based on existing designs.
Incorporating 3-layered approach from https://www.thingiverse.com/thing:3210223 and https://www.thingiverse.com/Gear_Down_For_What/about to make a practical gearbox. Eventually I'll probably make this gearbox compatible with the others, but for now I'm still playing with the arrangement of gears.
Where this series differs is the departure from the usual teeth relationship of R=S+2P. We are allowing some flexibility in our ring gear tooth count by dropping teeth and stretching the ring gear (easier to stretch than compress and maintain clearance). This is what allows more choice of gear combinations.
This model is a bit less forgiving in terms of input values. Use this spreadsheet to choose appropriate values.
In summary, what this design brings to the table are:
- Customizable for many gear ratios and dimensions
- Single piece sun gear, increasing efficiency, strength and distributing load
- Potentially more available gear ratios by violating R=S+2P constraint
- Print-in-place fully assembled. Potentially we could have multiple degrees of freedom of a robot arm printed in one pre-assembled unit.
(One last thing: The scad is a dog's breakfast, I know. I've been chopping and changing so much to experiment with different ideas. It probably has bugs too. I'll tidy that up, eventually, when I settle on a final design, which might be never. Welcome to the home of unfinished projects.)
Check slicer using "Layer View" (Cura) to make sure exactly one layer gap is present between the two outer ring gears - otherwise they will print as one piece. The gap is slightly larger than 0.2mm by default which should work with a 0.2mm layer height. This may also depend on your slicer.
While you're there, check the gear teeth for clearance - sometimes the gears are touching. Often the slicer will fix this problem anyway or make it worse. You won't really know until you see the results of slicing.
To print solid, leave infill at 0 and add more to wall thickness. It will print quicker with nice concentric lines.
Separate the layers of the outside ring gears before turning. There should be a one layer gap after slicing (check in preview). Using a knife or sharpened spatula, crack them apart so that they can turn relative to each other.