BS 93:2008 pdf free download – British Association (B.A.) screw threads – Requirements.
B.3.2 Internal threads
The tolerances on the effective diameter of internal threads, in millimetres, have been derived from the formula 0. 12P± 0.03.
B.4 Tolerances on major diameters
B.4. 1 External threads
The tolerances on the major diameterof close class external threads, in millimetres, have been derived from the formula 0.15P.
The tolerances on the major diameter of normal class external threads, in millimetres, have been derived from the formula 0.20P for sizes 0 B.A. to 10 B.A. and from the formula 0.25P for sizes 11 BA. to 16B.A.
B.4.2 Internal threads
No tolerance is specified for the major diameter of internal threads. It is recommended, however, that the radius at the major diameter of internal threads should be not less than one-half the standard radius, as defined in Figure 1, for the pitch of thread concerned.
B.5 Tolerances on minor diameters
B.5.1 External threads
The tolerances on the minor diameter of close class external threads, in millimetres, have been derived from the formula 2(0.08P + 0.02).
The tolerances on the minor diameter of normal class external threads of sizes 0 B.A. to 10 B.A. and of sizes 11 B.A. to 16 B.A., in millimetres, have been derived from the formula 0.20P + 0.050.
B.5.2 Internal threads
The tolerances on the minor diameter of internal threads, in millimetres, have been derived from the formula 0.375P.
NOTE These minor diameter tolerances are such as to permit a tapping drill ofsufficient size to be used to prevent binding at the root of the tap. If Jitil advantage is taken of these tolerances on the minor diameters of internal threads, the crests of these threads will be flat, as shown in Figure 2 and Figure 3.
If an external thread or an internal thread is inspected with a GO screw gauge made to the design size and a NOT GO gauge for the effective diameter made to the lower limit for the effective diameter of the external thread or the upper limit of the internal thread, the above conditions as regards the size of the simple effective diameter, and its size as influenced by any errors present in the pitch and flank angle of the thread, are safeguarded for practical purposes.
I(leally NOT GO effective diameter gauges should control only the simple effective diameter, and should not be influenced by irregularities of thread form. As ordinarily made, however, they necessarily have flanks of definite length and, therefore, do not strictly control the simple effective diameter of the product thread within the minimum material limit unless the flank angles of the product thread are correct. Such gauges will pass product threads with incorrect flank angles, the simple effective diameters of which can be outside the minimum material limit by an amount proportional to the flank angle errors and the length of the flank of the NOT GO gauge.
Flank angle errors and irregularities of thread form do not necessarily render a product thread unserviceable, but it follows that it is important to supplement the use of such NOT GO effective diameter gauges by some independent check that the form of the product thread is reasonably correct. This check can be made by supervising the thread form of the taps and other threading tools and also by examination of samples of the product thread.