Are there are any conventions for the clearance between the internal and external threads of BSW and BSF threads, or other threads using the Whitworth thread form?

Is the clearance achieved by modifying the thread diameter but not the thread form? Is the clearance applied to just the internal thread, so that the external thread retains its nominal major diameter?

  • $\begingroup$ What do the Zeus tables say? $\endgroup$
    – Solar Mike
    Feb 20 at 18:31
  • $\begingroup$ @SolarMike I'm not familiar with them but I will look into it. $\endgroup$
    – William
    Feb 20 at 18:46
  • $\begingroup$ Clearance on the helical surfaces of the thread, is achieved by controlling the pitch diameter vs the nominal, with the male thread's pitch diameter reduced and the female thread's pitch diameter increased. This is of course in addition to the tolerances for the major and minor diameters, which ensure clearance at the root and crest. $\endgroup$
    – Pete W
    Feb 21 at 0:26
  • $\begingroup$ ISO and American UNC/UNF threads both define classes of fit, which are tables specifying ranges of positive/negative deviation of the pitch diameter vs the nominal pitch diameter, for both male and female thread form in each thread size. Similar concept to the system of ISO cylindrical fits. No idea if Whitworth had such a system $\endgroup$
    – Pete W
    Feb 21 at 0:30


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