Problems selecting extruded edges for fillet

[Craig Trader]

I'm using Python 3.8, CadQuery 2.1, and CQ-editor 0.2 on Ubuntu 20.04.

I'm developing models that will be used to produce bucks (molds) for thermoforming. In order to ease mold release, I need to avoid vertical or negative angles. To make a mold, I'll create a buck model, which has a positive draft angle, select the edges that aren't on the top or bottom, and fillet those edges. Then I'll create a well that has a negative draft angle and cut it out of the buck.

The problem I'm having is that for parts with a negative draft angle, I can't get the edge selector to select the critical edges.

Here's some test code to demonstrate the problem:

import logging
import cadquery as cq

LOG = logging.getLogger(__name__)

def extruded_fillet_test( x, y, z, r, d ):
    part = cq.Workplane().rect( x, y )
    part = part.extrude( z, taper=d )
    part = part.edges( 'not(|X or |Y)' )
    LOG.warning( '%d edges selected', len( part.objects ) )
    part = part.fillet( r )
    return part

f1 = extruded_fillet_test( 100, 100, 20, 5, 0 )
f2 = extruded_fillet_test( 100, 100, 20, 5, 15 )
f3 = extruded_fillet_test( 100, 100, 20, 5, -15 )

When I run this code, I see this in the log:

[2021-03-20 17:47:16.967398] WARNING: temp: 4 edges selected
[2021-03-20 17:47:17.001737] WARNING: temp: 4 edges selected
[2021-03-20 17:47:17.042074] WARNING: temp: 12 edges selected

And here's what you'll see in the preview:

F1: No taper, just fillet, as expected

F2: Positive taper, fillet good

F3: Negative taper, fillet on top as well as sides -- BAD

Can anyone suggest an edge selector expression that will select just the 4 edges that need a fillet, for all 3 scenarios I've shown?

Thanks!

- Craig -

[Jeremy Wright]

A couple of ideas to think about.

1. Use the not logical operator to select all edges that are not parallel to the X, Y and Z axes.

2. The selector syntax supports vectors instead of axis letters. If you know the vectors related to the edges you want to select, you could try that.

#2 Takes this form:

    stand.faces('>Z').edges('|(1,1,0) or |(1,-1,0)')

[Craig Trader]

Jeremy ...

Thanks for your response. I tried your suggestions, but they don't do the right thing in the first case (zero taper, where all edges are parallel to one of the 3 axes) or the third case (but not for the reason you'd think).

After further investigation, it appears that the extrude operation does completely different things when the taper angle  is negative. When I create a rectangle and do an extrude with a positive angle, what I get is a 6-faced solid that looks like a truncated pyramid (as expected):

cq.Workplane().rect( 100, 100 ).extrude( 20, taper=15 )

When I do the same thing, but use a negative angle, the end result looks like an inverted truncated pyramid, except that the new corners are rounded-off instead of sharp

cq.Workplane().rect( 100, 100 ).extrude( 20, taper=-15 )

Obviously, trying to fillet that is going to give me bad results.

I'll need to fix the extrude before I can fix the fillet.

- Craig -

[Adam Urbanczyk]

You could use loft:

res =(
    cq.Workplane()
    .rect(2,1)
    .workplane(offset=1)
    .rect(3,3/2.).loft()
    .edges('(not|X) and (not|Y)')
    .fillet(0.4)
)

[Craig Trader]

That's an interesting alternative, but the fillet still ends up looking wrong, because it's only rounding the top and not the bottom. 

But you've given me some different avenues to approach. Thanks!