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I'm still new to Blender so I don't know all of the terminology and can't seem to phrase my question properly. But I am learning every day to model for 3D printing. Mainly custom PVC fittings to build things.

My current project requires that I take a flat surface (the top of a cylinder) and move it (maybe extrude is a better term) to the curved surface of another cylinder to merge the 2 meshes. I don't want to go past the surface since the object is hollow. What I have done in the past is overlap objects and then complete union and difference Boolean operations to cut and merge objects. But when I use the 3D tools in this project to diagnose issues I can't seem to resolve the issues. I visually cannot "see" the issues to resolve them.

So I want to see if it is possible to manually move the vertices / flat surface to the curved surface "manually" so that I can see if this will be a better workflow or produce the desired results without errors (or minimal errors). screenshot of surfaces

Update: Here's what I did and the errors that I received in the 3D-Print panel.

I expanded the bottom cylinder into the upper "U" cylinder but I want the space in-between removed. enter image description here

So I copied the upper "U" cylinder and then completed a Bool Tool Difference to get this. enter image description here

Then I removed the upper section of the differenced mesh since I do not want that any longer. (I just selected and deleted the vertices - there may be a better way but not too sure how to do so just yet.)

enter image description here

I then brough back in the upper "u" cylinder. enter image description here

Then completed a Bool Tool Union then run the 3d-Print Check all and the is what I get. enter image description here

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    $\begingroup$ Which 3D tools give you errors? And what kind of errors? $\endgroup$
    – taiyo
    Sep 13, 2023 at 21:13
  • $\begingroup$ @taiyo, I updated my post with the errors that I am encountering. $\endgroup$
    – JoeFletch
    Sep 13, 2023 at 23:01
  • $\begingroup$ Hello please share your file: blend-exchange.com $\endgroup$
    – moonboots
    Sep 14, 2023 at 7:30
  • $\begingroup$ For some 3d printing processes you do not need a single connected mesh(no need for boolean operation at all). I use Cura with an FDM printer and it works fine with multiple meshes in a single object just overlapping as long as their normals point in the right directions. No point in doing work that is not needed for the purpose. In other situations, when you need bevels for example, the parts must be connected, but not for 3d printing. $\endgroup$ Sep 14, 2023 at 8:11
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    $\begingroup$ @JoeFletch Sorry for the delay. I wrote the answer. $\endgroup$ Sep 19, 2023 at 10:57

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Boolean operations might not be needed at all for 3D printing

Some 3d printing processes do not require joint meshes so boolean operations might not be necessary at all. For example if you are planning to print with an FDM printer, most slicers (like Cura for example) will deal with separate overlapping geometry just fine if the separate geometry is tidy and suitable for 3d printing (no holes, no non-manifold geometry, no inconsistent normals or problematic concave n-gons). This might also make editing easier, if modifications are needed in the future since you don't need to deal with complicated geometry at all. You can keep the disconnected mesh parts in one objects or you can even have multiple overlapping objects for this to work.

As you can see you can have 3 separate objects in Blender:

enter image description here

And once you export them in STL format and then import to Cura(or possibly other slicers), it will slice just fine:

enter image description here

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You can probably fix your current object, but you could use the Knife Project tool instead:

Create a new object > circle (here 16 vertices), put it above the cylinder, deselect, select the cylinder, switch to Edit mode, CtrlLMB to select the circle object, then Mesh > Knife Project:

enter image description here

Now you have the circle drawn on the surface of the cylinder, you can delete your circle object:

enter image description here

Remove the single vertices to get something like that:

enter image description here

Extrude up, flatten on Z:

enter image description here

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If you are on windows try importing the final mesh onto windows 3D builder. Its auto fix tool is pretty good at making the mesh manifold for 3d printing.

If you want a manifold mesh then projecting the top face of the bottom cylinder on the other wouldn't give you that, I think.

Instead, take both cylinders and join them into 1 object. enter image description here In edit mode, selecting only one cylinder, Face->Knife Intersect enter image description here This will create geometry where the faces intersect. enter image description here You can then go to Select->By trait->Interior faces, and delete them. Merge the rest by distance and recalculate normals just in case. enter image description here enter image description here

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Are you using the top cylinder without resizing it to cut the bottom? If you do that then the faces are going to overlap when you join them and there's a higher chance of having issues. Instead cut the top of the bottom object with a solid cylinder that has a smaller radius than the top one, but bigger than the inner hole

enter image description here

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Not sure if I understand the question correctly, but you can "manually" move the vertices to the other surface to have a look.

Select the top face of your cylinder and separate it from the mesh. Add a Shrinkwrap modifier to it and select the other cylinder as target. Set the Wrap Method to Project. This will move all vertices up to the other surface. Probably all the options of the Shrinkwrap modifier are also useful for you.

enter image description here

After applying the modifier, you can adjust the vertices to keep the mentioned errors minimal and for example Bridge Edge Loop the projected ring and the top edge loop of your cylinder again.

enter image description here

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  • $\begingroup$ This is a really great way to do this but I still get a significant amount of errors / issues in the 3D print toolbox. Keeping in mind that I have a low level of experience with this, it appears that when I union the 2 meshes, Blender is keeping the internal surfaces between the 2 meshes and identifying them as Non Manifold Edges. I may have to go back and do a difference of the surface of the upper cylinder to "cut" a hole in it and then union the 2 meshes to see what happens. It's late for me, so I will have to try this tomorrow. I much appreciate the potential solution and direction here. $\endgroup$
    – JoeFletch
    Sep 14, 2023 at 2:02

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