I want an edge at the intersection of faces

How do I create a perfect edge along the intersection line of 2 faces? Can’t find a solution on Youtube. I have a face that intersects an additional face at about 90 degrees. There has to be some way to use the knife tool to make that line.

I will clarify. What I think I should be able to do is select 2 faces that intersect or cross through each other like a + sign. I want a command that says “make edge at intersection.” Or is there another way to accomplish this?

There could be an addon that might do it (tinycad vtx, edge tools), but you really don’t want to make that kind of geometry connected.

3 faces connected to single edge is already a mesh error (interior face) and 4 is even worse.

I should clarify further. I am designing a mesh for 3D printing. I need the line so I can make new faces from the existing ones and then delete the new faces that would be inside the mesh or non-manifold.

Best way to clarify is to post screenshots/test file.

But yeah, knife tool can snap to vertices when cutting and with cut through option you could cut with it and use another shape as a guide. You probably have to join the objects (select, ctrl+J) first, don’t remember.
Can’t see what kind of geometry you’re working with so here are few tools that might help:
Knife project
Bisect tool (in upcoming Blender 2.69)
Boolean modifier and example

I’m learning tons with this Blender project. Now I can insert pictures. . .

This is for a C-130 Hercules project I want to print in 3D. In photo 1 you see the 2 faces that intersect. Note that there are sections of the face that extend inside the exterior of the mesh. Photo 2 shows where I want to create a new edge. Photo 3 shows the 2 new faces (yellow and green) that I want to delete. This would make that portion of the exterior of the tail section manifold without faces extending inside the exterior.

Thanks for your help.


I selected the cube’s intersecting edge, subdivided it, and slid the vertex to the plane (twice, once for the horizontal edge, once for the vertical edge.) Then I used ‘f’ to create an edge between the two new verts. I eyeballed this, but there might be a way to snap the vertex to the second object.

Anyway, the intersecting edge is part of the cube object, but it is simple enough to separate the edge from the cube and join it to the plane, if that’s needed.

In that I have a small plane intersecting with the cube and either is aligned with the world axes or each other. I first switched to orthographic view mode (numpad_5), selected front edge on the small plane, shift+numpad_1/3/7 (try which one) to get the view aligned with the plane, then first selected the plane object, then cube object, edit mode, and pressed knife project.

That creates one edge on the cube and I then used knife tool (K) in cut through mode (Z) to snap to those new vertices and got rest of the edges. Those edges can then be selected and filled with a face. Or you could extrude (E) the created edge to create new face on the outside.

As you might have noticed, polygonal modeling is not meant to be done by intersecting objects with other objects and tools lack in that respect, that’s why this is quite tricky.

Still no luck. I give up. Its just too hard. :slight_smile:

I’m not certain that this will cause problems for printing. I’m just trying to make everything as perfect as I can.

How about using the Boolean modifier? (it needs the two meshes to be separate objects though)

Can you use Connect (J) those 2 vertices.