Multiple extrusions/sweep question for gear generation

[wentam]

I'm attempting to port my gear generator from openscad that can produce arbitrary helix patterns, like this:

Getting the general gear profile generation working was simple, and splines are awesome:

But I'm having a hard time finding a good solution to set up the helix generation.

I would expect to be able twistExtrude multiple times (p=p.wire().twistExtrude(5,20).wires(">Z").twistExtrude(5,-20)), but there's something I don't understand because this just errors out. How would I extrude the top face of an object?

I had a little bit of success with sweep and an auxSpine

That's very satisfying, but trying to define a herringbone profile with a polyline as the auxSpine just does strange things

Source is here: https://github.com/wentam/Compound-planetary-gearbox-generator/blob/db55c7ff6a5a3625f614d7179e7d08a9f393230f/test.py

I'd any appreciate ideas on how to best solve this!

[fedorkotov]

The method you are missing is toPending(..). But it seems that there is a bug in twistExtrude(..).

This works

result = \
    cq.Workplane('XY')\
      .rect(5,5)\
      .twistExtrude(5,20)\
      .wires(">Z")\
      .toPending()\
      .extrude(5)

But this does not

result = \
    cq.Workplane('XY')\
      .rect(5,5)\
      .twistExtrude(5,20)\
      .wires(">Z")\
      .toPending()\
      .twistExtrude(5,-20)      
      # same with .twistExtrude(-5,-20)

Both variants (with distance 5 and -5) extrude backwards and give this shape

[fedorkotov]

As a workaround you can probably use mirror(..)

result = \
    cq.Workplane('XY')\
      .polygon(6, 5.0)\
      .twistExtrude(5,60)\
      .mirror(union=True)

[wentam]

Thanks! toPending() indeed allows for non-twisted extrusion:

Mirror doesn't fully achieve what is needed here, as asymmetric profiles need to be possible. It is a valid workaround for symmetric cases though.

I don't fully understand what pending vs non-pending wires are, and I can't find much information about it in the docs. Perhaps a candidate for the 'CadQuery Concepts' section?

[fedorkotov]

Asymmetric ones you can just glue together from two parts with union like this

result = \
    cq.Workplane('XY')\
      .polygon(6, 5.0)\
      .twistExtrude(-5,60)\
      .union(
          cq.Workplane('XY')\
            .polygon(6, 5.0)\
            .twistExtrude(5,-120))

[wentam]

Yup! - was just playing with union a bit as a workaround for this.

Thanks for your time! That'll be a good solution for now.

[adam-urbanczyk]

@wentam great project! As already discussed toPending is the way. Don't forget to use workplane too:

result = (
    cq.Workplane()
      .rect(5,5)
      .twistExtrude(5,20)
      .wires(">Z").toPending()
      .workplane()
      .twistExtrude(5,-20)
)

@fedorkotov see above, you need to use workplane to make twistExtrude wokr.

[wentam]

Aha: workplane does make it behave as expected:

Makes sense I supposed if extrusion is intended to occur relative to the workplane.

[adam-urbanczyk]

Regarding auxSpine: probably only C1 curves are supported. I did not check it though

[wentam]

@adam-urbanczyk the auxSpine solution doesn't seem to result in usable gears anyway, as the teeth on the opposite side are of a different shape than the front:

vs

[adam-urbanczyk]

AFAIK: your spine needs to be placed on a cylinder for this to work properly. You can take a look at how twistExtrude is actually implemented.

[fedorkotov]

Thanks a lot @adam-urbanczyk . Issues section of this project is a great supplement for documentation.

[adam-urbanczyk]

Yes, I try to add the question label for this kind of issues. One day we might have better docs and maybe even a FAQ.

[Jojain]

I am also working on gear and I made one here where you can specify the helix angle you want, (even though helix angle >3° make the filleting not working) It's still kindof WIP but maybe can give you some inspiration. https://github.com/CadQuery/cadquery-contrib/blob/master/examples/cylindrical_gear.py