V2 - Gear housing, improved meshing, reduced size

With V2, I finally got around to modeling some housing to keep the cross-spherical gear and monopole gears meshed together. Finding a balance between the gears meshing smoothly but not having so much play as they worked themselves out of mesh was tricky but I think the tolerances in this version are a good compromise. Everything in this version should be press-fit, but a tight one - I had to use work gloves to push some of the dowels in. Through use (or persistent assembly/disassembly) the dowels can work loose. Feel free to glue everything to be safe.

When assembling, it can be finicky to align the gear teeth before clamping everything together. The easiest way I have found is to align the ‘eye’ of the cross-spherical gear with the ‘eye’ of one monopole gear, then revolve the gears in mesh until another cross-spherical ‘eye’ aligns with the other monopole ‘eye’ and press the whole thing closed. The magic of ABENICS will sort itself out from there.

Gear Housing

The housing is printed in two halves and held together with dowel pins (the same ones that hold together the halves of the cross-spherical gear). Each half features a print-in-place bearing to mount the monopole axles to. The bearing tolerance is tight - again, to avoid too much play - so once the housing has printed, make sure to break the seal on the bearing and spin it a few times. The bearing will work itself smoother with use.

Improved Meshing

I revisited my OnShape FeatureScript code and found a way to double the resolution of the monopole gear hobbing process, as well as provide some relief and achieve a true mesh between gear surfaces. This, coupled with the reduction in size of the overall setup, has resulted in monopole gears that are indiscernibly smooth when printed. 

Reduced Size

I have scaled all models down. The pitch diameter of the cross-spherical gear is now 35mm. The resulting model fits nicely in your hand and is quite an addicting fidget toy!

V1 - Original

 V1 is much larger than V2 and does not truly mesh. 

Attempt at modelling the cross-spherical and monopole gears from the ABENICS paper listed here: https://ieeexplore.ieee.org/document/9415699.

I created this model a few years ago, using it as an opportunity to get more familiar with OnShape + FeatureScript, and I was quite impressed.

The cross-spherical gear was easy enough to model with OnShape but the monopole was much trickier, and I ended up having to write custom FeatureScript to achieve the hobbing process desired. The monopole was created by incrementally rotating it around a regular spherical gear and performing a ‘boolean’ cut in OnShape at every step. I think it is the process most people have taken to model such a gear so far.  There were some limitations to this process; for example I could only squeeze out about 100 increments of rotating & cutting per revolution of the monopole before OnShape crashes, so the teeth faces notably chattered. This number of increments is reduced even further when trying to add a bit of offset with the boolean cuts. As a result, the monopole has no relief when meshing with the spherical gear. It works acceptably in practice as a proof of concept but obviously this is not a true meshing of gears.

I don't doubt I could have achieve marginally better results by using SolidWorks or another platform that would allow me to get more cutting increments out of the hobbing process but OnShape was certainly comparable, especially for the low low price of free.

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