Why can't I shell an object with a threePointArc in it?

[Charlie-83]

import cadquery as cq

l = 184
r = 10
w = 1.5

result = (
    cq.Workplane()
    .threePointArc((-3, (l - r) / 2), (0, l - r))
    .radiusArc((r, l), r)
    .line(10, 0)
    .lineTo(20, -r + w)
    .line(-10, 0)
    .radiusArc((0, 0), r)
    .close()
    .extrude(2)
    .faces("|Z")
    .shell(-2)
)

As you can see, the shell operation doesn't do anything.

There are two ways to make this work:

1. Change -2 in the shell to ≥ -1.45
2. Change the threePointArc to .lineTo(0, l - r)

I don't really understand why I am not able to shell the threePointArc with a thickness of more than 1.45

[jmwright]

@Charlie-83 Are you able to shell with a positive thickness, or will that make the resulting part too large for your use case? This is what I get with a thickness of 1.99.

That still does not explain why the 3-point arc does not work when shelling with a negative thickness, but it is a potential work-around.

[Charlie-83]

Hi, positive thickness works fine. However, I am measuring this against a real part: it is much more convenient to just construct the correctly size geometry and then have it hollowed out then having to subtract the thickness from all the measurements in constructing the geometry to counteract the object size increasing during the shell

I am also no longer convinced this issue is solely related the the threePointArc. If we replace it with .lineTo(0, l - r), then things work; until we change the selector to ">Z" (also bumping up the extrude value to make things clearer). With ">Z" we now get no shelling occurring.

result = (
    cq.Workplane()
    .lineTo(0, l - r)
    .radiusArc((r, l), r)
    .line(10, 0)
    .lineTo(20, -r + w)
    .line(-10, 0)
    .radiusArc((0, 0), r)
    .close()
    .extrude(10)
    .faces(">Z")
    .shell(-2)
)

This fails for all negative values for shell

I am wondering: is all of this some limitation of the cadquery? Or is this a bug?

I think I will try to use a positive thickness for now (adjusting the rest of the geometry) as you suggest

[jmwright]

There are CAD kernel issues with shell that users run into from time to time. Try the fix in this issue.

Add the kind attribute to the shell:

.shell(-2, kind='intersection')