FWIW, I am working on a model for which I need some left-hand threaded acme rod. Here’s the basic process I used to create it. I prefer this method over using a screw extension because I can ensure the threads are modeled correctly.
For this model I need 1 inch diameter rod with 5 threads per inch. I set the units to decimal meters before I started.
Using the Helix tool in Curve Maker I drew a helix with the following parameters: Radius 0.5, From Turns 0.0, To Turns 7.0, Sides/Turn 48, Height/Turn 0.2. Then I drew the profile of the threads based on information from the Engineer’s Edge website. Once the profile is drawn, I explode the group containing the helix.
I selected the helix and the face of the profile and then run Eneroth Upright Extruder. Correct the face orientation as needed and erase the original helix.
Next I cut the threaded rod into a suitable unit. In this case I wanted an inch of rod so I put rectangles 1 meter apart and intersected them with the geometry.
After that I trimmed of the waste and cleaned up the geometry so that I would have a solid component when finished.
After that the geometry in the component is scaled down to 1 in. diameter. I can copy it as many times as I need to get whatever length I want. 1 in. increments are done by making a linear array with the Move tool. Other lengths can be created by trimming a section. To make a single solid of whatever length, Outer Shell can be used.
For internal threads I would use the appropriate dimensions for the profile.
Admittedly this requires a little more work than the various thread-drawing extensions but I think I get a better result. For me it’s worth the little extra effort. Once the process is sorted out, it doesn’t actually take very long to do.
For the internal threads I did the same sort of thing and created a solid threaded shape with the appropriate dimensions. I used Trim to cut the threads in the block but Intersect Faces would work just as well. I also created a shape to chamfer the edges of the holes so the lead in to the threads is better.