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Thread: Measuring old lens threads

  1. #1

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    Measuring old lens threads

    I have several lenses missing their flanges. I have thread guages and callipers so I can measure the outside diameter of the threads and the threads per inch and check the thread angle. But many are significantly worn so the outside diameter of the thread is not as large as it once was. Has anybody got any suggestions as to how to adjust for or take into account this wear? I would like to specify to the machinist what I need as getting lenses there and back is fraught with potential problems.

  2. #2

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    Re: Measuring old lens threads

    Hello from France!

    Measuring an unknown thread accurately is a tricky business. As you say, you can't use the diameter at the top of the thread. With the tools you have, you can identify a standard thread, but measuring a completely unknown thread is much more difficult.

    What I have read in a machinist's thread-oriented tutorial on the 'net, at least for a male thread, is that you have to press small cylindrical wires (of precisely known diameter) against the thread flanks, and measure on top of those tiny cylinders and not on top of the thread itself. See attached "theoretical" diagram.
    I don't know how to do this in practice, I imagine that you need a small wire with a diameter close to the thread pitch.

    Usually when people contact a machinist to fabricate a retaining ring, the machinist's answer is often: "Don't try to mesure the thread, simply bring me the lens and I'll see what I can do."

    Good luck!
    Attached Thumbnails Attached Thumbnails 2022-03-13-measuring-a-thread.jpg  

  3. #3

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    Re: Measuring old lens threads

    Thanks for that - makes good sense. Fortunately the lenses that I have are all from one maker and are certainly all Whitworth profile threads. I'll look more into the wire idea - I can see that being a real possibility if only I can figure out (remember) enough maths! Perhaps someone will help on this?

  4. #4
    Tin Can's Avatar
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    Re: Measuring old lens threads

    There is a guy making flanges

    I recently bought one of his standards

    I think he is in Portugal, wonderful work

    The name may/will come to me
    Tin Can

  5. #5

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    Re: Measuring old lens threads

    I have eventually found another tutorial (in French) on precisely measuring threads.

    See also
    https://amesweb.info/Screws/metric-t...m-formula.aspx

    Summary, valid for ISO threads, angle of 60°, to measure a male thread

    Sorry, I do not have the formulae for a 55° Withworth thread.

    d = nominal diameter of the thread ; this is not exactly what you measure on top of the thread
    d2 = mid-diameter between the top and the bottom of the thread (according to the ISO standard)
    P = thread pitch

    d2 = d - 0.6495 P for an ISO thread, angle 60°

    Now take a pair of small wires of diameter dw, apply on both sides of the male thread and measure a diameter M against the top of those wires. The preferred diameter dw is dw ~= 0.6 P but can vary slightly, I am sure that dw = P could work as well.

    From M we find the nominal diameter d as follows
    1/ from geometical considerations about a circle of diameter dw touching a V-groove with and angle of 60° and a pitch P we get
    d2 = M - 3 dw - 0.8660 P

    2/ hence from M and P we get the nominal diameter d
    ==> d = M - 3 dw - 0.2165 P ; M = d + 3 dw + 0.2165 P

    Imagine that we have a M90x1 metric thread. We apply small wires of diameter dw = 0.6 mm
    the measured value on top of those wires will be: M = 90 + 3x0.6 + 0.2165x0.6 ~= 92 mm

    Now that was for a purely theoretical purpose

    In practice you can also send your lens to Jose Meneses in Portugal, as mentioned by Tin Can.
    https://customphototools.com/


    Usual disclaimer, I am not affiliated with Jose Meneses, I am simply a very satisfied customer, as many other French friends on our French MF+LF forum.

  6. #6

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    Re: Measuring old lens threads

    Many thanks both. I will look more into the measurements methods shown. I'm aware of several flange makers but am trying to avoid sending lenses off, especially abroad where duties can currently be very problematic going and coming back with Brexit and all (shipping seems to be a lottery and I know people who are struggling to deal with the customs who seem to be swamped - once in the 'system' chaos can ensue)!!! If I can specify the threads it will make things a great deal simpler!

  7. #7

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    Re: Measuring old lens threads

    Quote Originally Posted by pgk View Post
    Many thanks both. I will look more into the measurements methods shown. I'm aware of several flange makers but am trying to avoid sending lenses off, especially abroad where duties can currently be very problematic going and coming back with Brexit and all (shipping seems to be a lottery and I know people who are struggling to deal with the customs who seem to be swamped - once in the 'system' chaos can ensue)!!! If I can specify the threads it will make things a great deal simpler!
    And if you’re measurements are just slightly wrong you will still have a problem and bigger headaches. Can’t believe that there isn’t a machinist in Scotland.

  8. #8

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    Re: Measuring old lens threads

    Quote Originally Posted by Bob Salomon View Post
    And if you’re measurements are just slightly wrong you will still have a problem and bigger headaches. Can’t believe that there isn’t a machinist in Scotland.
    There are but I'm on an island where there are none and its not so easy to hop onto a ferry at the moment! So its ship lenses or specify threads.

  9. #9

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    Re: Measuring old lens threads

    Hmm. Photographer's machinists in the UK. Hmm. Have you asked https://www.srb-photographic.co.uk/ what they can do for you? They used to make custom adapters etc., might be able and willing to do what you need.

  10. #10

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    Re: Measuring old lens threads

    What Emmanuel has introduced here is what is known as measuring a screw thread's pitch diameter using the three wire method. This looks like a good description and covers calculations for the various thread forms including Whitworth: https://www.threadcheck.com/the-thre...technicalinfo/

    Full disclosure: I'm not a real machinist but play one in my home shop and once a month at a local historic industrial site. I've done this sort of measurement maybe twice in the last 30 years. It's rather fiddly, it's said that to do it one needs a set of thread measuring wires, a micrometer, and at least three hands...

    This forum sadly lost Leigh a few years ago. On one of the major machining forums he moderated the metrology section. He would have been able to explain the principle and importance of pitch diameter in screw threads,

    David

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