When a lathe is used for thread cutting, the number of threads per inch produced is determined by the speed relationship between the __________.

Correct Answer: Option A — lead screw and headstock spindle

Thread cutting requires a fixed, positive mechanical relationship between the headstock spindle rotation and the lead screw travel. The change gears determine this ratio, ensuring the carriage moves a specific distance per spindle revolution, which defines the threads per inch being cut on the workpiece according to standard machine shop practices.