Intersect faces" tool not working how i expect. please help

Working on my c6 model again, and trying to build a rear hatch with a lambo style window. i’m trying to get these two pieces to intersect, so i can remove the excess and cut out the windows
however it seems like no matter what order i “intersect faces - with model” or “with selection”, but what it will do most often is make edges where the two parts intersect that are not attached to either party, so they continue to exist un-cut.

Or when they do actually intersect, i will loose seemingly random parts of my geometry, which could be fixed by hand, but i’d rather not. also in that instance when fixing the losses, i will find random edges lifted slightly off of the surface of my model causing problems lining up replacement edges and faces.

i want all of the grey geometry to remain intact on the inside of the window but i will lose pieces like in the above image because the intersect will not cut half of the faces.i read a few other posts about scaling the model up to help with the issue, but it doesn’t seem to have helped in this case.

any help would be appreciated

i could only post one image. so insert the following images in-between the paragraphs to help clarify

It does matter which you use. When you pick Intersect Faces with Model, the intersections plotted are between whatever is selected and whatever isn’t, if any. When you pick Intersect Faces with Selection, the intersections plotted are between whatever is selected and itself.

Context also matters (context is the editing environment in which the command is invoked). The lines of intersection appear in the editing context from which the Intersect command is invoked. If you are inside a group or component context when you invoke Intersect, the lines appear inside the group context and cut whatever geometry they encounter in that group. They do not affect any geometry outside the group. Similarly, when you invoke Intersect from outside a group, the lines of intersection are plotted on the outside of the group and do not cut any geometry inside the group. If you want everything cut, everything must be in the same context; that is, all in the same group or component or in none at all.

If you’re losing odd, unpredictable faces or or pieces of faces of extrusions, it’s likely you’re dealing with too-small geometry, but since scaling up doesn’t seem to have helped, we can set that aside. Other problems could include gaps in the outline, segments out of plane, or…[insufficient data].


i will give the grouping a try, but what do you mean by “segments out of plane”?

edit- tried grouping the objects together, received the same result, so either thats not the problem, or i’m doing something horribly wrong. at this point i would believe either scenario…

I didn’t say to do that.

The raw geometry (edges and faces) must be in the same context. Putting two groups in a higher-order group doesn’t accomplish that. The raw geometry you want to interact is still divided into the same two groups as before.

A segment out of plane is one (or more) of the linear elements making up the outline not sitting flat on the same plane as all the other linear elements. They must all be in the same plane–coplanar.


Switch OFF hidden geometry.
Let’s assume the magenta surface is inside a group and the gray profile is all raw geometry and it is in another group.

  1. Edit it the magenta-group.
    Select all and use intersect with model.
    The cut-lines should appear in on the surface [same context].
  2. Exit that edit and edit the gray-group.
    Select all and use intersect with model.
    The cut-lines should appear on the gray surfaces [same context].
    2a. Still in the edit, select the unwanted parts of the gray geometry [the outside parts] and , use Erase to tidy up as needed.
  3. Exit that edit and re-edit the magenta-group.
    Select the parts of the surface you want to be ‘holes’ and .
  4. Optional: If desired re-edit and form faces on the ends of the gray-cut-parts.
    To help you see what you are doing… consider toggling View > Component Edit > Hide Rest of Model…

Here’s a simplified step-sheet:

-tig. fantastic. I’ll try that tonight after work. Also you assume correctly, however both the magenta and grey builds are individual componets. I don’t beleive they are even set as groups in my model.

-gully. sorry, I’m understanding a little less than half of what your talking about haha, you’re speaking a bit outside of my skill level. I still need instructions written for dumb folk.

I’m also thinking that I may have over complicated my model a bit (trying to change the attributes of components after they’ve already been set. I think this may have somehow compounded some of the attributes, but this is just a guess.) I’m hoping there is a way to clear all of the current settings on each component without starting over so I can set both up properly. Then perhaps that will simplify things. Again I could be wrong. I’m still trying to wrap my head around the majority of this software.

Here is a much older version of what I’m trying to achieve. (Many of the faces were bad, missing, or otherwise incorrect. Hence the re-do)

  • TIG, tried that method once i got home. the “cut lines” are still appearing in a different context if I’m understanding the terminology correctly now. also having new problems.

after scaling the whole model up by 200x and attempting the cut again, after i use the intersect faces i can see its making cut faces that aren’t even close to where i want them. i’m about to scrap the whole project.

as you may be able to see in the image below, its splitting the grey geometry in seemingly random places, and doing a right ugly job of it too. as well as in the background on the magenta geometry the cut line is far from where the grey intersects. i’ve no idea why this is happening, and i’m close to finding some new software. is there a patch or update for this?

Can you attach the model, or at least the two parts you want to intersect.
Copy them into a new model and attach that.

You need to be inside a component [editing it] and select all and intersect-with-model to get the cut lines inside that ‘context’.
You need to do that twice - in each component - so that they are both ‘cut’.

How big is this model ?
If you have some small facets/edges then “intersect” will fail to form very tiny edges [SketchUp’s tolerance assumes points <1/1000" apart are coincident!].
Make copies of the two component parts - off to the side.
Scale up the two copied parts x1000
Perform the two intersects.
The cuts will appear in both instances of the components.
Delete the unwanted geometry within the components etc.
When you are done, erase the two giant versions - the real sized ones will keep the tiny facets/edges - they can exist but cannot be created from scratch.

[quote=“TIG, post:10, topic:6541”]
You need to be inside a component [editing it] and select all and intersect-with-model to get the cut lines inside that ‘context’.You need to do that twice - in each component - so that they are both ‘cut’.
[/quote] I’ve done that a hundred times with the same result. I’ll post the model tonight. It just isn’t working for me.

Although I’ve just realized the magenta geometry was built in inches and then imported. while the grey geometry was built in metric. Would this cause the model to have issues?

rear window~.skp (521.6 KB)
maybe someone can take a crack at it if they want

Thank you for sharing. :slight_smile:
(Followed this tread for days)

Here is a try. Take only the outside faces of the WindowComponent/Gray and delete rest of Window to get a simpler cut i the “rear window”/Magenta.(Remember hidden geometry off, for one-click delete.)

For the cut in the Gray part, there was a bit of repair afterwards thought.
Some lines didn’t connect properly, so there was missing faces.
Hope this can be useful.

rear windowOutsideFaces.skp (1.9 MB)

Context and scale.
rear window~.skp (344.7 KB)

well thanks. not sure what i was really doing wrong, but oh well.

heres an image for your efforts. going to do a new render soon i hope.

Complex intersections are problematic in Sketchup. In my experience the main source of the problem is this:

A face does not have to be perfectly flat, that is, all points defining the face do not have to be on the same plane; a certain tolerance is allowed. On the other hand, in intersect, the intersection edges come from intersecting the face planes and since a face plane is only an approxiamtion (or a best fit of all points) the intersect edges may not fit the actual face if the face is non flat close to the tolerance. That is at least my best guess to what’s going on.

Since a triangle by definition is perfectly aligned with the face plane, the solution is to triangulate problematic faces. If you triangulate the red faces (in the image) you will obtain the situation to the right and this should make the intersection work. Also, scale up a large factor.

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Well thanks for the help again guys. Something about the large intersection that didn’t want to cooperate. On smaller intersections I haven’t had any problems, see below. I’ve updated sketch up, cleaned up the lines, and have been having less problems. so i’ve made a few more modifications to the model, and violá!

in the next year or so this will be a real car, hopefully, so i appreciate your time.

Wow man, that’s incredible looking! Best accessory for a C6 Vette I’ve ever seen. Hope you get it to production!

Anyways, first time posting on the forums. I’m glad I found this thread. I’m having the same trouble with cutting angles and window openings in some metal siding (which is ribbed and dimensional). I’ll try these suggestions when I get home.

Please do! And if you run into difficulties, please start a new topic so your discussion doesn’t get lost amidst this one!

I have the same kind of problem This discussion has been really helpful. Thanks all. I am bing notified tha tthie topics 450 days old and that by entering this reply it bumps it back into the front queue. I odnt mean to be kicking a dead horse but frankly have the above experience of non intersecions compounded with a crash of the program and or a spinning wheel indicating its locked up in cyberspace which requires me to force quit the file and sketchup. Thank the force for auto backup !
If anything new has been learned in the interim, I would appreciate learning more about the issue.