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I have a box that I have extruded to the shape I want. Then I placed a cylinder so that it intersects the box. After that I deleted the bottom faces of both the cylinder and box.

Now what I want to do is delete the portion of the cylinder that is inside the box and join the nodes so that it is one object. I have already joined the meshes so I can select points/edges/faces from either shape. It seems like the easiest way would be to just put nodes where the edges of the cylinder and box intersect and then delete the parts I don't care about. I can't find a good way to do that though.

What's the best way to accomplish what I'm trying to do?

outside view bottom view

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  • $\begingroup$ Hello :). This sounds like a perfect job for the Boolean Modifier set to Union. You'll have to separate the mesh into two objects first. Mesh > Separate > Loose Parts $\endgroup$ – Jachym Michal Dec 28 '19 at 19:39
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The Boolean is less destructive, but for a one-off, leaving more options in some cases:

  • Object Mode, CtrlJ combine the components if not already one object.
  • Edit Mode, all selected CtrlF Face Menu > Intersect (Knife)
  • Adjust Last Operation panel > Self Intersect

Followed by removing the faces you don't want, and AltM merging the selection of vertices you want to, By Distance.

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  • $\begingroup$ This actually seems more elegant than Boolean :). $\endgroup$ – Jachym Michal Dec 28 '19 at 21:34
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    $\begingroup$ @ᴊᴀᴄʜʏᴍᴍɪᴄʜᴀʟ I don't know.. I prefer Blender to 3DS for all sorts of reasons, but I do miss edit operations on the modifier stack.. maybe Everything Nodes is heading in a more procedural direction? Not all-the-way Houdini, but a happy compromise? $\endgroup$ – Robin Betts Dec 28 '19 at 21:43
  • $\begingroup$ This is exactly what I was asking! Thank you for this info. I tried it and it basically worked but I was left with the roof of the cylinder on the inside of the model. I'm planning on using the solidify modifier to make it 3mm thick for my 3D print so that won't work. I may come back to this if I can't get the boolean to play nice (it's not so far) $\endgroup$ – William Dec 29 '19 at 0:50
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    $\begingroup$ Ok figured it out. I was having some weird issues with the top face but I found that if I just deleted it then merged vertices by distance then I could select the edge ring and put a face on it. $\endgroup$ – William Dec 29 '19 at 3:04
  • $\begingroup$ @William Glad you found it.. In any operation involving Blender finding intersections, you have to watch out for exactly coincident faces / edges, leaving it unable to determine left / right, inside / outside.. floating point error.. division by 0... $\endgroup$ – Robin Betts Dec 29 '19 at 8:28
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You can use the Boolean modifier for this.

Add the Boolean modifier to one object, and target the other one.
Set operation to Union.

enter image description here

Apply the modifier, and hide/delete the target object.

enter image description here

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  • $\begingroup$ ok. I'm having a hell of a time with this. I had trouble with my original objects so I deleted them and make new ones. The box is a simple box with one extrude and the cylinder is a standard cylinder with no modifications other than changed dimensions. I left them whole objects (i.e. no deleted face). If I pick the box then select the cylinder using either "Union" or "Difference" I see only the cylinder. If I use "Intersect" then I guess I get what I'm expecting with "union" what the heck is going on? $\endgroup$ – William Dec 29 '19 at 0:54
  • $\begingroup$ But then if I do the same thing with the cylinder on the other corner (there are two I want to add this way) then I have to use "Difference" to get them to work like expected. Crazy! $\endgroup$ – William Dec 29 '19 at 0:56
  • $\begingroup$ The extrude has to be messing with it somehow. If I try a test with a stock box and two stock cylinders then union works as expected. I don't get it. $\endgroup$ – William Dec 29 '19 at 1:00

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