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BS 93-2008 pdf free download
BS 93-2008 pdf free download.British Association ( B A ) screw threads Requirements.
Flank angle errors and irregularitiesofthread 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.
The normal methodof inspecting screw threads isby the useofthe limit gauges described above. The effective din meter limits ofthe workpiece are given mainly in order to provide a basis for dimensioning the
gauges. Only in cases where it is impracticable to use gauges for
economic orother reasons, should direct measurement cf the effective diameter by the two or three wire system be employed, and thenonly in conjunction with an examination of the thread form by optical
projection against a master profile in order to check the flank angles and thread form. It is essential that the pitch errors of the screw threads are also measured. In such cases the results need to be interpreted with care, as any surface irregularities, lack of uniformity of diameter, and errors of pitch and flank angles of the screw threads which are present make it very difficult to obtain a reliable estimate of the size of the virtual effective diameter.
Itisnot possible tolaydown definitetolerancesforthe pitchorthe flank angles of a screw thread of any particular size ortolerance class. In any particular case, the maximum permissible combined effects of pitch and flank angle errors would depend upon the difference between the simple effective diameter of the product and the maximum material limit of the effective diameter. In the case of an external thead having its simple effective diameter on the upper limit, no errors at all could be allowed in pitch or flank angle. The combined effect of the errors in pitch and flank angle can only attain a maximum value when the simple effective diameter ofan external thread is on its lower limit or when the simple effective diameterof an internal thread is on the upper limit specified.
It is important to realize, therefore, that the tolerance allowed on the effectivediameter should not be regarded as being available in full for variations in that element of the thread alone: part of this tolerance
should be considered as being reserved to compensate for the effects of errors in pitch and flank angle. which are invariably present to some extent.BS 93-2008 pdf free download.
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