Thread fit is the size of looseness or tightness between screw threads. The level of fit is the prescribed combination of deviations and tolerances on internal and external threads.
For unified inch threads, there are three thread grades for external threads: 1A, 2A, and 3A grades, and three thread grades for internal threads: 1B, 2B, and 3B grades, all of which are clearance fits. The higher the level number, the tighter the fit. In inch threads, the deviation is specified only for 1A and 2A grades, the deviation for 3A grades is zero, and the deviations for 1A and 2A grades are equal.
Metric threads: There are three thread grades for external threads: 4h, 6h, and 6g, and three thread grades for internal threads: 5H, 6H, and 7H.
In metric threads, the basic deviation of H and h is zero. The basic deviation of G is positive, and the basic deviations of e, f, and g are negative.
1. H is the position of the tolerance zone commonly used for internal threads. Generally it is not used as a surface coating or a very thin phosphating layer. The basic deviation of the G position is used for special occasions, such as thicker coatings, which are rarely used.
2. g is usually used to plate a thin layer of 6-9um. If the product drawing requires a 6h bolt, the 6g tolerance band is used for the thread before plating.
3. The screw fit is best combined into H / g, H / h or G / h. For refined fastener threads such as bolts and nuts, the standard 6H / 6g fit is recommended.
Complete marking of fasteners
According to regulations, a complete fastener mark contains the category, standard number, thread size or nominal size, other diameters or characteristics, nominal length, thread length or rod length, product type, performance level or hardness or material, product level , Wrench type, and surface treatment