Identifying Thread Form
There are many different thread forms in use today. The forms most widely used for power transmission screw threads are illustrated in Figure 43. An optical comparator is the easiest method of determining thread form. Profile gages, if available, and visual methods can also be used.
Great care must be taken as many forms are almost identical. The Acme form (29 degrees included angle) is only 1 degree different from the ISO Metric Trapezoidal form (30 degrees included angle).
Many thread forms such as Unified, Metric ISO and Acme are subject to published standards while others, including Ball Screw and Worm threads, are not defined in detail by any standards organizations.