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I have an extruded rectangle that intersects the side of the cylinder. I need this extruded rectangle to cut a hole out of the side of the cylinder. Below is a picture:
(Ignore the middle segment)

I tried the Boolean modifier but it didn't work (yes, I tried all 3 options).

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Boolean

The Boolean modifier needs a closed mesh to work properly, see the wiki:

This means that this modifier will only work properly for the intersection of faces of the two meshes that will result in another closed loop of edges (filled with faces), creating a new resulting face topology.

(You can have an open mesh as long as the open parts do not intersect or interact with the boolean operation)

Removing the middle section of the cube should fix it:

enter image description here

You may also need to recalculate the normals (CtrlN)

Also ensure that:

  • The Modifier is applied to the cylinder.

  • The Operation is set to Difference:

    enter image description here

Knife

In addition to the Knife project tool mentioned by Haunt_House, You may want to use the Knife tool for this:

In edit mode, press K to start the knife tool and C to enable Angle Constrain. (you may need to hold shift at some points to place a cut, due to the view not being aligned perfectly to the topology)

enter image description here enter image description here

Modeling

Another simple way to do this is to use Loopcuts:

Press CtrlR and use the scrollwheel or the keyboard to set the number of cuts:

enter image description here

You can then use Edge slide (GG or CtrlE> Edge slide) to slide them into the correct position:

enter image description here

An advantage to this is that this does not produce N-gons (see this)

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  • $\begingroup$ Can you explain the knife method further? The selection doesn't snap to the intersection of both edges, even when the mesh is joined. $\endgroup$ – Keavon Sep 18 '13 at 1:58
  • $\begingroup$ @Keavon It's not snapping to the intersection of the cube and the cylinder, if that's what you mean. It's just snapping the angle to 45 degree increments. If you want to Exactly copy the shape of your cube, you are better off with Haunt_House's answer $\endgroup$ – gandalf3 Sep 18 '13 at 2:02
  • $\begingroup$ Ok. His answer didn't work perfectly, but your boolean operation tip did work, so you got the accepted answer. Thanks. $\endgroup$ – Keavon Sep 18 '13 at 2:11
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Knife Project Tool

You should be able to use Knife Project. This tool takes one mesh and cuts the second mesh according to the outline of the first.

Since the cutting mesh needs to be outside the cutted one, you need to select the cutting part and separate it with P. Then leave editmode, select your cutting mesh, then shift-select the main mesh and enter Edit Mode. Adjust the view, so you are looking in the direction of the cuts. Finally use the button in the Mesh Tools or find it in the Space menu

Then all you have to to is to go into Object Mode and join both meshes with CtrlJ and stitch them.

The workflow I'd recommend would be to make three loop cuts and extrude that. You can scale the front of the second mesh to zero to flatten it.

workflow

If your mesh is not for the game engine, I recommend giving the object some thickness.

topology advice

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I once had a very similar problem. Maybe this cool script by Witold Jaworski may become helpful. http://airplanes3d.net/scripts-253_e.xml

Basically it takes two meshes and creates an edge loop in the intersection. From that you can easily rework your meshes to get the desired effect. I find it very useful and in many cases much better then the original boolean operations (which create a lot of bad topology on the meshes).

The description about how to use it is available at the link above.

Hope it will help you and other Blender Users :)

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