I recently run into a threading problem and found the tool post out of alignment. After carefully aligning the tool post again so all tools including threading tools were perpendicular to spindle fixed the problem.
The programs run just fine and I couldn't see the alignment problem but the threads it made were scrap.
Not clear if this will help, But I would check. I dont have a rapid turn, but the tool post and tools are the about the same for the 8l lathe. .