Intersect faces with model weirdness



Consider a flat vertical plane being intersected with a house-wall in order to
"cut out" the windows and doors in order to skin-over the internal structure.

I always make sure that the plane is slightly “inside” the surface geometry
to ensure it is truly intersected. Coplanar geometry sometimes does and sometimes does not get intersected properly.

I observe the rectangle geometry being superimposed on the vertical plane, but when I go to “cut out” the opening faces, some of them are not separated from the rest of the whole plane. There are apparent leaks or non-coplanarities.

Sometimes I will try to bisect/triangulate the leaky openings, deleting those faces which are separated and that process can generate quite a convoluted and non-intuitive mess.
Sometimes I just draw another rectangle over the intersection-generated rectangle.
Sometimes that solves the problem for that particular opening, other times it does not.
I have resorted to just creating yet another new plane on the face or the intersected plane, manually re-drawing the openings inferenced to the intersected openings and abandoning the intersected plane/geometry.

Anyone run into this ? And have a workaround?
There is probably some addin to enforce coplanarity of selected geometry…
This has to be a bug.


It would help if you shared an example file in which the intersection didn’t work as you expected.

I think it’s a bug in your workflow.

Slightly inside might be the problem. Why only slightly? Why are you being stingy with the geometry?

Draw the rectangle making sure it is aligned correctly and then use Push/Pull to extend it fully through the wall on both sides. Don’t hold back. Add plenty of length to the extrusion. You want to make it easy to get at the ends of the hole cutter box. In this example the siding is a component as is each part of the framing. The box is drawn outside of the components and is just loose geometry. After putting it in the right place select all of the box geometry and use Edit>Cut to cut it to the clipboard.

Open the wall component/group for editing and use Edit>Paste in place.

Add the siding geometry to the selection set and use Intersect Faces>With Selection. Then drag right to left selection boxes around just the ends of the hole cutter box to select the ends and the unneeded faces up to the wall.

Hit the Delete key. The drag a right to left selection box around the part of the wall that needs to come out. Don’t select the perimeter edges.

Hit Delete and you should have an opening. Correct the face orientation in the opening and you’re finished.

If you have drawn other elements of the wall such as sheathing and insulation and made them components or groups, repeat the process for those elements to create the through opening. Until you copy something else to the clipboard, the hole cutter box will still be available for pasting into the components/groups.

If your wall elements are drawn as solid components or groups, you could make the hole cutter a solid and use Trim from the Solid tools to cut the opening. (You could use Subtract but if there’s more than one component/group to cut, you’ll lose the hole cutter after the first subtraction.


I used your approach early on when figuring it out - with one exception:
Instead of having the intersections floating geometry, I usually edit the
component and then intersect with model.
The cut-geometry is immediately integrated and i just select and zap the
faces. Done. The orthogonal faces are usually all there.

In this latest project, the wall structure/ window frames were already in
I thought it should work to lay a rectangle across the plane of the window
frames, extrude it in half inch or so and intersect it.
Curiously, the outside face was often but not always cut by the coplanar
model components while the inside face appeared to be ok. Why the
Ok, I’ll dance around that by pushing the extruded plane in a bit…
Not all the way in so as not to get any sporadic cuts from the wall surface
structure… Now the “stucco” extrusion is penetrated by the already extant
window frames.
Do the intersection and notice little anomalies in the face cleanup.
Ok… back up a step and dont extrude the stucco -leave it a plane and
extrude it after it is cut and de-faced. Still bizarre behaviour. My point
is that the cut geometry (appears to be) on the face and connected yet
sometimes does not define a new face

I will regenerate the geometry and post the relevant component set.


That’s what I was describing. I create the cutter geometry outside the component so there’s no worry about faces intersecting until I am ready for them to intersect.


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