The lead screw is mainly used for turning threads at a fixed distance, and the feed of the cart is relatively large. The function of the lead screw is to do servo drive. The function of the design screw is

Every time the spindle rotates, the tool rest can accurately move the corresponding distance to ensure that the pitch of the lathe is equal.

Therefore, the screw is used for threading;

When the polished rod is used in the common excircle, the feed is much smaller than that of the lead screw, and stepless speed regulation can be realized. The polished rod cannot guarantee the accurate transmission ratio from the structure. Therefore, it is generally used for turning excircles, end faces and inner holes. Generally, there is no need to turn the thread;

From the efficacy of the two, we know that we can't use only one screw to replace the polished rod and the screw at the same time.