If you are getting a rough thread with carbide there could be one of 100 reasons , but the first thing to look at is , coolant , are you getting any , next is everything tight to stop vibration , and then , depending on stock dia. and distance from the chuck you may need a steady rest . If you are already doing all the above you may just be getting some of the same ****** material we have been getting , we make a lot of threaded studs in the 2-4" dia range at work for machinery, you take a pc of 2" stock thread one end slick , flip it and turn the other end down to 1.125" and thread it , looks like a torch cut , it even looks rough where its turned down before threading. CHINA SCRAP or something, or you could be trying to take to much stock per cut , you cant make threads in two passes.