Turning a straight shaft and threading a relatively "long" shaft are two quite different exercises. The straight shaft is much easier because it is easy to measure and you can greatly reduce the tool push by using a pointy tool. With threading, the concern is the pitch diameter, which others have done a good job of pointing out. When threading it is nearly impossible to eliminate the tool push. Making numerous passes at the same setting will not address the spring of the tool and work piece. The issue is that the tool shape is generating the form of the thread, so there is a lot of the cutting edge in contact at once. For a long thread, you have to work out a follower rest. Also, make sure the tool is very keen, use lots of back rake, consider cutting only half the flank at a time. Another point is that you only need the follower rest near the end - most of the thread can be cut without the follower rest (the pitch diameter will be barrel shaped, but so what, you are still roughing - that makes it much easier to manage the brass tips of the follower rest).
