The thread is a shape in the solid outer surface or the inner surface of the cross section, there is a uniform spiral convex. And screw fastening means that the degree of tightness between the screw threads of each other. Class refers to the combination of internal and external deviation and tolerance.
For the thread, the thread level has the following three: 1A, 2A, 3A. The grade of internal thread has the following three: 2B, 3B, 1B, all belong to the gap, the higher the level, the more tight. For the thread, the deviation of 3A level for 0,1A and 2A are the same.
The 1A and 1B are very loose tolerance levels, suitable for use in the external thread and the tolerance between the internal thread. 2A and 2b is imperial mechanical fasteners the general tolerance grade; Grade 3A and 3b, can spin synthesis, the tight fit, generally used for tighter tolerances for fasteners, and a key safety design.
The tolerance exists between the 1A level and 2A level of the external thread, 3A level not. Generally, 1A level of tolerance is larger than the 2A class 50%, greater than the 3A level 75%. Internal threads of the 2B level of tolerance is 30% larger than the 2A class. 1B level is 50% greater than the 2B class, 75% larger than the 3B class.