# How to introduce an edge where two faces intersect [duplicate]

I'm looking for a way to introduce a cut to a plane where it is intersected by another plane.

The plane is a single face for simplicity.

Here's the setup:

• Add another plane and rotate it by 90 degrees at X or Y so it is not coplanar

• Join the two meshes together using Ctrl + J and enter edit mode
• Notice that in edit mode face select allows selecting one or the other face
• Notice that no edge has been introduced where the two planes intersect

The first solution I came up with was to just use two loop cuts, one on each plane. In this setup the plane dimensions were left unchanged as was their location so it just so happens that the intersection edge coincides with what you get if you loop cut both the planes.

The second solution I found was to use a boolean modifier on both planes, selecting the other plane as the object for the boolean operation in both cases. This correctly introduces an edge where the planes intersect. The can be joined after both boolean operations have been applied, giving a single mesh with four faces as desired.

I am wondering about the general case, though. What if one plane was twice the size of the other, or shifted by half the size?

My solutions do not work in those cases. Loop cut needs to share origin in order for the cut offset (half the face size) to coincide with the intersection edge. Boolean doesn't work if the faces are different sizes or shifted against one another so that their edges do not connect anymore.

Is there a general purpose tool in Blender for selecting two faces which may or may not intersect and saying "introduce edges where they do"? Obviously some sort of logic needs to be applied when the faces are shifted so that their edges do not connect with each other, my thinking is in that case both faces should be cut as if the other face was infinite in size and not finite? Or some other logical fallback behavior.

Is there such a tool? This question is primarily for Blender 2.8.

• One way is you could use just one plane, cut it in the middle and then extrude both sides, AFAIK you can't insert cuts when two or more faces intersect without have any vertex in common Commented Apr 22, 2019 at 18:42
• What you describe is a nice solution #3 for the case I describe where the faces share origin (although I am having trouble avoiding skew when extruding the edges), but unfortunately doesn't generalize either. Unfortunately if what you are saying about having to share a vertex is correct it looks like there might not be any general solutions to this problem. Commented Apr 22, 2019 at 18:50
• As I said, AFAIK you can't create a vertex/edges where two or more faces intersect, anyway using edge loop/cut or modifiers to solve this kind of situations is not bad, I mean not always there is one-click solution. BTW what are you trying to do? Commented Apr 22, 2019 at 19:42
• Commented Apr 22, 2019 at 22:11

The simplest solution would be to select one of the planes and use Intersect(knife) that can be called from the search menu(F3/space) or Face menu(ctrl+f):

• Just want to point out, on my fresh install of 3.4.1, F3/space doesn't bring up the search menu, and ctrl+F doesn't bring up the face menu. Commented Feb 19, 2023 at 6:16
• Space is used to start and stop animation by default in 3.4.1. F3 and Ctrl+F still work fine. Commented Feb 19, 2023 at 12:01
• perhaps it's cos I'm new to blender, but I selected a face in edit mode then pressed F3, and the viewport changed to "Top Orthographic", and I don't see any changes when hitting Ctrl+F. Edit: never mind, I reset the keymap to default and it started working. Thanks! Commented Feb 23, 2023 at 23:04

I've looked through all Duarte's duplicates (phew!) ... but can find no mention of the Knife Intersect tool, found in the CtrlF Face menu.

you can use the tiny cad addon, to enable it write tinyCad in blender preferences and check it to enable it and then save user settings, then go to you model in edit mode, select both parts that you want to interset then press w and select tinycadand select the XALL option.

• That's an interesting add-on, thanks for mentioning it. If I press 'w', all I do is cycle through selection tools. How do I invoke the add-on? Commented Jul 14, 2021 at 8:24
• enable it first in the blender preferences... Commented Jul 14, 2021 at 21:12
• Thank you. Yes, I had it enabled :) I believe the 'w' key is maybe for older versions of Blender? Commented Jul 16, 2021 at 12:14
• yep W in 2.79 keymap... Commented Jul 18, 2021 at 22:47

Your Boolean operation method works with a few extra steps.

1. Add a Solidify Modifier to a copy of both faces.
2. Do the Boolean operation method with these 4 faces.
3. Now use ALT+M to merge the verities together.

hmmm... imo, the easiest way to do this is simply with a bolean modifier... place your planes as shown on the pic. select 1 plane (plane 1) and (in object mode) add a boolean modifier. in operation box select union. in object box select plane 2. hit apply. now half of plane 1 will disappear. first select plane 2 and move or remove it. now select plane 1 (wich is now 1 and a half plane) and go to edit mode. select the edge that binds the 2 planes and hit E+dir+1 (dir= x,y or z depending the direction in wich you want to extrude). now the 2 planes are 1, with an edge connecting the 2... hope this helps

• But how? Using the boolean modifier removes half of my plane. Commented Apr 23, 2019 at 13:50
• Perhaps you could expand this answer with a simple explanation on how to use the boolean modifier and show an example cropped screenshot of the setup and result. Commented Apr 23, 2019 at 13:58
• if half your plane is removed, extrude it again... the edge is there now, so... Commented Apr 23, 2019 at 14:29
• @FrancoZero Hello and welcome. We tend to prefer fuller answers which explain why the proposed solution works, and describes at least some rough steps about how to achieve it. You can edit your post and add some more details in Commented Apr 23, 2019 at 15:10