Why does it do this, and how can I make the sweep work?

Ihateerrors

Matt_Shields

I can't exactly tell from your screen shot, but for best results, make sure the sweep path and sweep profile are touching.

eric_pesty

Can't tell for sure but it looks like you are trying to do sweep a "thread" along a helix and it might be struggling if the edge of the triangle is exactly coincident with the cylinder. You also have to make sure it is not trying to create "zero thickness" geometry where the subsequent threads meet. Extending the triangle inner face usually solves this.

However the easiest way to do this by the way is to use one of the custom features that can do this for you like "Threadlabs": https://cad.onshape.com/documents/5c0528b62c1fbb13a2a0e739/v/f86229e7f586f8517a2068a9/e/7cbb452d4d5e963a034ce616
Or "Thread creator": https://cad.onshape.com/documents/6b640a407d78066bd5e41c7a/v/845d049782179b9faee8b6e6/e/c953720c264ce001f1a82dc1

S1mon

It's hard to tell just from the screenshot, but it seems like the triangle is big enough that when it tries to sweep along the helix, it might self-intersect. 

If you share a public document, it will be easier.