I was experimenting with creating a threaded rod using the Swept Cut feature and the Twist Along a Path option, defined by Turns.

See attached part. I had the rod 2" long, with a thread width of 0.125" and the path length of 2.125". I wanted 9 turns per inch, so used 18 as number of turns.

I got the error message "Smooth path is need for sweep with twist."

Strange. What could be smoother than a straight line?

After some experimenting, I discovered that if I reduced the number of turns (to 16), the sweep worked.

I realized that it was all in the math. Dividing the path length (2.125) by the thread width (0.125) gets 17 as the theoretical maximum number of turns.

I hope this helps others who get stumped by this error message.

The error message should probably be change to say "LONGER path is needed for sweep with a twist".