Tolerance and fit of thread

Tolerance zone

The tolerance zone of the thread is determined by the position of the tolerance zone and the size of the tolerance zone. The position of the tolerance zone is determined by the basic deviation, which indicates that the distance between the starting point of the tolerance zone and the basic tooth type is the key to determining the mating nature of the internal and external threads. The designer should select the position of the tolerance zone according to the performance requirements of the threads.

The common thread standard specifies the lower deviation EI of the internal thread and the upper deviation es of the external thread as the basic deviation. The internal thread has two tolerance zone positions G and H; The external thread has eight tolerance zone positions a, b, c, d, e, f, g and h. The basic deviation of H and h is zero, the basic deviation of G is positive, and the basic deviation of a, b, C, d, e, f, and g is negative.

The tolerance zone of the internal thread is shown as follows:

The tolerance zone of external thread is shown as follows:

Tolerance class

The tolerance value is divided into several grades, called tolerance grades, to represent the size of the tolerance zone. The top diameter and middle diameter of the internal and external threads are provided with tolerances, and the top diameter tolerance is mainly used to ensure the contact height of the internal and external threads, while the middle diameter tolerance is an important size that determines the quality of the internal and external threads. The standard does not specify the tolerance of the bottom diameter of the internal and external threads, and the size of the bottom diameter is guaranteed by the tool. According to the different roles of various diameters in threaded connections, the standard provides a different number of tolerance levels for different diameters, which are specified as follows:

Spinning length and its grouping

The screw length affects the tolerance accuracy of the thread, the longer the thread, the more difficult the machining, the larger the tolerance value needs to be given. The common thread standard divides the screw length into three groups, using L to represent the long screw length group, N to represent the medium screw length group, and S to represent the short screw length group. If the actual screw length of the thread is not known, such as when selecting standard bolts, it is recommended to determine the thread tolerance by medium group (N).

Tolerance accuracy

The tolerance precision of thread is a comprehensive index to measure the quality of thread. It not only depends on the tolerance level of the thread, but also closely related to the screw length. The same precision thread, the degree of difficulty of processing is similar, in order to achieve process equivalence, it is often necessary to specify a larger tolerance value for long threads of the same precision (that is, low tolerance grade), and a smaller tolerance value for short threads (that is, high tolerance grade). Ordinary threads are generally divided into precision grade, medium grade and rough grade three precision levels. Precision precision for precision threads, when required to match the nature of the change is small: medium precision suitable for general use of thread; Rough precision is used in situations where precision is not required or manufacturing is difficult. Common thread tolerances and fits can be selected by reference to the table below.

Recommended tolerance zones for external threads are shown below:

Recommended tolerance zones for internal threads are shown below:

The black bold wire frame tolerance zone in the table is the recommended fit tolerance zone.

Thread marking

The complete thread marking consists of a thread feature number, a dimension number, a tolerance number, and other individual information where further explanation is necessary. The thread feature number is represented by the capital letter "M", which stands for the metric general purpose ordinary thread. The size units in the thread size code are all millimeters, and the size code of single thread is "nominal diameter × pitch". For coarse thread, the pitch can be omitted and not marked; For multi-thread threads, the dimension code is "nominal diameter ×Ph lead (P pitch)", if you want to further indicate the number of threads of the thread, you can add parentheses after the English description. The tolerance zone code consists of two parts: the middle diameter tolerance zone code and the top diameter tolerance zone code. Middle diameter tolerance zone code in front, top diameter tolerance zone code in back. If they are the same, only one set of codes is marked. Write after the size code, separated by a "-". The tolerance zone code of each diameter is composed of a value indicating the tolerance level and a letter representing the position of the tolerance zone. Uppercase letters are used for internal threads and lowercase letters are used for external threads to distinguish internal and external threads.