I am new to blender and this might be a duplicate thread, so please point me to a similar question because I don't know the right vocabulary to formulate my question.

I try to make a screw thread with a circular profile. So I create a cylinder and next to it a circle. I use the Screw modifier to create a spiral around my cylinder.

Now I have the problem that I can not unify my thread with my cylinder. Normally I use a Boolean modifier and operation Union to do so. This time it complains.

What I recognise is, my thread is not a solid object but actually more like a pipe. I can see an opening there and I wonder if this is the reason.

I tried the Solidify modifier but this didn't help ether.

In general the spiral and the cylinder are touching each other, so I don't really see the problem. Hope somebody can point me to a solution. Thanks in advance...

By the way, I want to print this screw in a 3d printer. So in general these objects needs to be solid or so :-)


So here is an image of my result so far. I guess the boolean is not working because of this "pipe". It is not a solid object but only vertices. How can I make it solid so I can unify it with my cylinder.

enter image description here

When I then try to boolean unify both I get the following error. This happens also when I select the cylinder first or the circle first.

enter image description here

  • $\begingroup$ Hm, I don't have a screenshot of the desired result so far. I added a picture how it looks so far and try to understand the boolean fails. I really guess it come from my two objects. The circle is not solid as you can see in the picture and maybe thats why it can not unify the two objects?! $\endgroup$ – drmariod Jan 22 '16 at 12:00
  • $\begingroup$ Screw is applied, yes. When I select the boolean modifier, I then define the cylinder as object and union as operation. When I click apply it says that the boolean operation can not be executed $\endgroup$ – drmariod Jan 22 '16 at 12:26

The main problem is from known limitations for Boolean - non-manifold geometry.

If you select non-manifold geometry by Ctrl+Alt+Shift+M you'll see these boundaries selected :

enter image description here

So to apply Boolean without problems in this case fill the boundaries of the screwed circle.

You can do that by pressing F while they're still selected. Then add Boolean to the cylinder and choose Union.

  • $\begingroup$ This looks like what I am looking for... How do I easily fill the boundaries :) As I said, I am a total newbe... $\endgroup$ – drmariod Jan 22 '16 at 12:44
  • $\begingroup$ @drmariod In this case it's enough to press F after selecting them because boundaries are rather simple. This will create one large face per boundary (this face will be Ngon). Note that this will not create accurate topology but it's enough in order Boolean to work $\endgroup$ – Mr Zak Jan 22 '16 at 12:49
  • $\begingroup$ this seems to solve my problem or at least brings me a big step further :-) $\endgroup$ – drmariod Jan 26 '16 at 12:04

If I'm understanding correctly, you want something like this - single mesh with circular thread: enter image description here

To do this, instead of applying the screw to a circle with a separate line of vertices, instead start off with a mesh similar to this and then hit the screw button. In this case booleans aren't necessary: enter image description here

Just maybe remove doubles & Control-N to fix normals afterwards.


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