How to create screw thread with a circular profile 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 :-)

EDIT

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.



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.
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 :



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.