I am using Blender 2.79b on a Mac.

I followed the steps in this tutorial to successfully make screw threads (a spiral): https://www.youtube.com/watch?v=g4NHORCniJY

There, in Screw modifier we can enter for "Screw" a low number for a sharp turn with threads close together, or a larger number to push the thread turns farther apart.

When I use a much larger number, the thread, which is based on a circle, is not circular throughout the thread. It flattens like a ribbon. I want those long, snakelike threads to remain the same diameter throughout for a full 360-degree turn.

How do I keep the object round throughout the turn when at a higher number?

Here are the steps in the video:

1. Delete cube.
2. Tool Shelf> Create> Circle.
3. R to rotate, X for x-axis, 9,0 for 90 deg, and Enter.
4. 3D View Header> View> Ortho view.
5. 3D View Header> View> Front view.
6. S to scale, .,5, Enter.
7. Drag red arrow to right from centerline.
8. Ctrl+A and select Location.
9. Properties Editor> Modifier icon.
10. Add Modifier> Screw.
11. Screw Modifier: Set Axis to Y. You see the edge of the object.
12. Rotate the screen down with middle mouse button to see over the top.
13. Screw Modifier: Set Steps to 32. Smooths the surface.
14. Screw Modifier: Set Screw to 3.5 for a sharp turn and larger number for a longer turn, with threads farther away.
15. Screw Modifier: Set Iterations. Incrementing this adds more threads.