threads on a lathe

[Saltfever]

You should note that Marv carefully specified a sharp "V" thread. The 0.866 number is the cos of the sharp 30 degree angle. However, unless you are a steam fitter, that thread is hardly used anywhere. (neither Imperial or Metric) The sharp "V" is almost an interference fit (if not carefully set-up), or at least, is a very high friction fit. In addition, the sharp "V" at the top is easily damaged and the sharp "V" at the bottom may break or dull the threading bit. Common threads create a flat at both the top and bottom of the thread. It is more rugged, not damaged as easily, is a looser fit, and the threading tool will last longer and is more tolerant of various skill levels. The double depth dimensions on the fish above are based on 0.6495 x pitch which is the correct height (for Imperial threads) with the flat (.125 x pitch) at the top. I think metric uses 2 heights . Max depth (0.703 Pitch) and min depth 0.685 Pitch). They both have a flat at the top but the larger dim assumes no flat at the bottom.



If the pic won't enlarge just goggle something like "ISO Uniform thread". Both Imperial and metric use a modified thread form now days. The sharp "V" is about the only thread form that is leak resistant but it is so problematic, and there are such good sealers, it is rare to find it.