Here we go. Hopefully this will make sense.
Although drawing threads could be done without it, I start by drawing a helix with a plugin which you can get
here.. The helix is used as a sort of path for the following steps and is later deleted. For this example I decided on a 1" diameter 8TPI thread. I scaled everything up by a factor of 100 to make it easier to work.
The helix drawn by the plugin is centered on the origin by default and is made into a group. You enter the radius (actually two radii because the script will make a tapered helix if you want.), the pitch and the number of turns. For this model I set the radii at 50", the pitch at 12.5" (100/8) and turns to 1. I kept the default 24 segments for the helix.
Now, from left to right in the image above,
With the helix drawn, I started off the bottom end of it and drew a triangle vertically so its apex is at the end of the helix and the face is vertical. This is the profile of the threads. Because I'm showing my work, I also drew some cross hairs at the origin which got copied down the line for the various steps. If you are doing this, just work at the origin and you can skip that part.
Select the profile and rotate/copy (Ctrl+Rotate) it 15° (360/24).
Move the new copy of the profile up so its apex is at on the helix at the intersection of the first two segments.
Draw lines between the profiles to fill the faces. You'll need a couple of diagonals to fill the faces on the outside of the thread. Delete the inside face. You'll also find a triangular face inside that needs to be deleted. Smooth (Ctrl+Eraser) the diagonals so they aren't visible. After that, make this little bit of the thread into a component.
Select the component and rotate/copy it 15°. Hit Enter. Move the copy up so the apex is aligned with the apex on the first.
Select both copies and rotate/copy them 30°. Hit Enter. Move them up as you did on the first.
Select all four copies and rotate them 60°. Hit Enter. Move them up.
Now here's a tricky one. Select the eight and rotate copy them 120°. Hit Enter and then *2, Enter. Move each new set of eight up.
A single turn of the threads is complete. Select one of the components and hide (Shift+Eraser) the end edges.
Select all of the thread components, make a component of them and copy them up. the distance to move the copies is, in this case 12.5". I then typed *7 to make a total of 8 turns of threads. Then draw a cylinder inisde the threads. Drawing it this way makes it a simple matter to adjust the length. Push/Pull for the length of the shaft and copy the threads up.
I didn't do anything to treat the ends of the threads. If they would be visible, you'd need to make the end component or two unique and edit them as needed.
If you want the SKP file, send me your e-mail address in a PM and I shoot it off to you.