Hi Mike,

The problem is you have overlapping surfaces, when you swept your profile Boolean union is getting confused because it's trying to union the spiral body to it self as well as the cylinder.

When you look down the centre of the spiral body the lines indicated there are underlying surfaces within the body.





Hiding the front face reveals the overlapping surfaces inside.




I've attached a cleaned up version where I removed the inner spiral overlapping surfaces, extruded another cylinder surface, trimmed it up and joined it to make a solid, it should union to the cylinder now.

~Danny~

Thank you, Danny.

Any ideas on how to avoid this problem in the first place?

Hi Mike,

> Any ideas on how to avoid this problem in the first place?

You'd probably want to have a smaller piece for just the thread, so that the part that's spiraling around doesn't run into itself. Surfaces that cross back over themselves (even just barely grazing) will tend to cause a lot of problems with intersection calculations.

So for example you'd want the inside part to be just a straight extruded cylinder surface and not a spirally type thing in that spot.

Some examples:

http://moi3d.com/forum/index.php?webtag=MOI&msg=767.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=490.36
http://moi3d.com/forum/index.php?webtag=MOI&msg=4623.1

- Michael

Thank you, Michael.

I trimmed 0.1mm from the top of my screw thread profile (as shown below) and had no further problem.