Thread marking (metric thread marking):
In general, a complete metric thread mark should include the following three aspects:
Thread type code indicating thread characteristics;
Thread size: Generally, it should be composed of diameter and pitch. For multi-thread threads, it should also include lead and number of threads;
Thread accuracy: The accuracy of most threads is determined by the tolerance zone of each diameter (including the position and size of the tolerance zone) and the screwing length.
Thread measurement
For general standard threads, thread ring gauges or plug gauges are used to measure.
Due to the numerous thread parameters, it is impossible to measure every parameter of the thread one by one. Usually, we use thread gauges (thread ring gauges, thread plug gauges) to make comprehensive judgments on the threads. This inspection method is a simulated assembly acceptance method. It is not only convenient and reliable, but also comparable to the accuracy requirements of ordinary threads. Therefore, it has become the most common acceptance method used in actual production.
Thread measurement (pitch diameter)
In threaded connection, only the size of the pitch diameter determines the nature of the thread fit, so how to correctly judge whether the pitch diameter is qualified is very critical. Based on the pitch diameter size, it is necessary to ensure that the most basic use performance of the thread can be achieved. The standard specifies the judgment principle for the qualification of the pitch diameter, that is: "The actual pitch diameter of the thread cannot exceed the pitch diameter of the largest solid tooth profile. However, the actual The single pitch diameter of any part of the thread cannot exceed the pitch diameter of the smallest solid thread."
There are currently two more convenient methods for measuring a single pitch diameter. One is to measure the pitch diameter with a thread pitch micrometer, and the other is to use the three-needle method (our company uses the three-needle method).
Thread fit level:
Thread fit is the size of looseness or tightness between screwed threads, and the level of fit is the specified combination of deviations and tolerances acting on internal and external threads.
For unified inch threads, there are three thread grades for external threads: 1A, 2A and 3A, and three grades for internal threads: 1B, 2B and 3B, all of which are clearance fits. The higher the level number, the tighter the fit. In the inch thread, the deviation is only specified for 1A and 2A grades, the deviation of 3A grade is zero, and the grade deviations of 1A and 2A grades are equal.
Metric thread, external thread has common thread grades: 4h, 6e, 6g and 6h, internal thread has common thread grade: 6G, 6H, 7H. (Japanese standard thread accuracy grades are divided into three grades: I, II, and III. Normally, it is grade II.) In metric threads, the basic deviation of H and h is zero. The basic deviation of G is positive, and the basic deviation of e, f, and g is negative.
