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I recently found a loupe with a thread pitch reticle displaying common UTS thread pitches... except that rather than 40, 48, or 56 it has a 50. What would this be used for? It doesn't appear to date from earlier than the 1960s - the housing and dioptre lens are plastic, the reticle and retaining ring are glass/metal.

image of thread count reticle

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Fine threads (40 to 120 TPI) are used in adjustable mountings in fields such as optics and mirrors as well as other fields with small parts : watch making etc.

More detail can be found here: Fine adjustment screw

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  • $\begingroup$ But it's not an extraordinarily fine pitch, it's just an unusual one, and in particular seems like an outlier in the included set... $\endgroup$ Aug 2 '17 at 8:02
  • $\begingroup$ As pointed out fine is 40 tpi or more ... $\endgroup$
    – Solar Mike
    Aug 2 '17 at 8:03
  • $\begingroup$ Sure, and 40, 48, and 56 tpi are all fairly common thread pitches in the UTS set to which all the other ones shown belong... 50 tpi is not, hence my curiosity. :) $\endgroup$ Aug 2 '17 at 8:12
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    $\begingroup$ Used as specialist or proprietry threads see practicalmachinist.com/vb/antique-machinery-and-history/… $\endgroup$
    – Solar Mike
    Aug 2 '17 at 9:09
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    $\begingroup$ Whitworth Instrument Thread: "British fine thread for clocks and optical devices." includes 50 TPI: gewinde-normen.de/en/whitworth-instrument-thread.html $\endgroup$ Dec 31 '17 at 2:49

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