Calculation and tolerance of internal thread minor diameter

a. Calculation of the basic product size of the small diameter of the internal thread (D1)

The basic size of thread minor diameter u003d the basic size of internal thread-pitch coefficient

Example: The basic size of the minor diameter of the internal thread M8 8-1.251.0825u003d6.646875≈6.647

b. Small diameter tolerance of internal thread 6H (based on the pitch) and calculation of the small diameter value

P0.8 +0. 2 P1.0 +0. 236 P1.25 +0.265 P1.5 +0.3 P1.75 +0.335

P2.0 +0.375 P2.5 +0.48

The lower limit deviation formula of internal thread 6H grade D1+HE1 is the basic size of internal thread minor diameter + deviation

Note: The lower bias value of 6H grade is 0

The calculation formula for the upper limit of internal thread 6H level u003d D1+HE1+TD1 is the basic size of internal thread minor diameter + deviation + tolerance

Example: The upper limit of the minor diameter of the 6H M8 internal thread is 6.647+0u003d6.647

The lower limit of the minor diameter of the 6H grade M8 internal thread 6.647+0+0.265u003d6.912

c. The basic deviation of the minor diameter of internal thread 6G (based on the pitch) and the calculation of the minor diameter

P0.8 +0.024 P1.0 +0.026 P1.25 +0.028 P1.5 +0.032 P1.75 +0.034

P2.0 +0.038 P2.5 +0.042 (Guide: The restrictions on the use of commodity fasteners in pipe flanges)

The formula for the lower limit of the minor diameter of the internal thread 6G level u003d D1 + GE1 is the basic size of the internal thread + deviation

Example: 6G grade M8 internal thread minor diameter lower limit 6.647+0.028u003d6.675

The upper limit formula D1+GE1+TD1 of the minor diameter of the 6G grade M8 internal thread is the basic size of the internal thread + deviation + tolerance

Example: The upper limit of the minor diameter of the 6G grade M8 internal thread is 6.647+0.028+0.265u003d6.94

Note: ①The tooth height of the internal thread is directly related to the load moment of the internal thread, so it should be within its 6H upper limit in the production of blanks.

②In the process of internal thread processing, the smaller the diameter of the internal thread will affect the use efficiency of the tool tap. From the perspective of use, the smaller the diameter, the better, but in general considerations, the smaller diameter is generally used. Between the middle limit and the upper limit, if it is cast iron or aluminum, it should be between the lower limit and the middle limit of the small diameter

③The small diameter of internal thread 6G can be implemented as 6H in the production of blanks, and its accuracy grade mainly considers the plating of the thread pitch diameter, so only the pitch diameter size of the tap is considered during the threading process without considering the minor diameter of the light hole .

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