Metric,ANSI／ASME,BS In machining and maintenance, many people run into the same problem:Two fasteners look similar in size and even thread in, but they won’t tighten, and when you remove them, the threads are already damaged. In most cases, it’s not poor machining or low-quality screws—it’s simply the wrong thread system from the start. In practice, most screw threads come from three major systems: Metric threads, Unified threads (ANSI/ASME), and Whitworth threads (BS). Their