Micrometer Female Thread Damage
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  1. #1
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    Default Micrometer Female Thread Damage

    I bought an older Starrett No. 224 Mic set, and I took it apart because it turned fairly stiff. Iíve cleaned everything thoroughly, but it still turned quite stiff. I looked at the internal spindle threads with light and a magnifier, and I could see a few spots that had a raised lump on the flank of the thread. They were shiny, which tells me thatís where it had been rubbing. I was considering buying a 1/4Ē-40 tap and running it through there, very gently. Unless the piece with the internal threads can be replaced, I think that may be my only option. What do you guys think?

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    Is the nut adjustable most ???? mics I've seen have an adjusting nut ring around the od of the sleeve nut. - usally adjusted with the pin spanner key.

    If you run a tap through you will ruin the mic - ordinary taps are no where near the accuracy of a mic thread

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    Quote Originally Posted by Limy Sami View Post
    Is the nut adjustable most ???? mics I've seen have an adjusting nut ring around the od of the sleeve nut. - usally adjusted with the pin spanner key.

    If you run a tap through you will ruin the mic - ordinary taps are no where near the accuracy of a mic thread
    That is without the nut entirely, for the first 1/4Ē of thread it goes fine, then it tightens right up. It isnít a hard stop but itís difficult to turn. Iíve already checked resistance on the spindle shaft by itself, so it must be the threads

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    Quote Originally Posted by Everett45 View Post
    That is without the nut entirely, for the first 1/4Ē of thread it goes fine, then it tightens right up. It isnít a hard stop but itís difficult to turn. Iíve already checked resistance on the spindle shaft by itself, so it must be the threads
    Unless there not square.....
    throw it away and get a new one, or send it to a COMPETENT repair place and get it fixed correctly.
    the threads and spindle on a mic is lapped in, like sami said dont touch it with a tap.

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    Sounds like that mic might have been in a grinding shop (which can be notoriously hard on mics etc ) .......or (and more likely) has been dropped and landed on the end of the barrel (where you turn it) which damaged the threads.

    Either way and has been said already, proper professional repair or bin it.

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    Part of me wants to say that the accuracy in a micrometer is primarily in the male thread, not the female threads it fits in. Inaccuracy seems to depend on that male thread and it's consistency along all of it's length. The length of the male thread should be the range of the micrometer (usually 1" or 25 mm) PLUS the length of the female thread. So, as long as the fit of the male thread is consistent along all of it's length, it should work equally well at all points in the female thread.

    But another part of me says that we are talking tenths and even hundredths of a thousandth of an inch and things can go wrong in unexpected ways.

    On the use of a tap, I would be more worried about the class of fit after using one instead of a loss in the pitch's accuracy. Taps come in different classes of fit and you positively do not want to open up the female thread so much that it would result in a poor fit with the male thread. But since there is an existing thread which is probably good except at those few spots, using the tap probably would not result in any interference due to any small differences in pitch. What it could/would do is make the existing, female thread somewhat looser, at different places along it's length. So only those places that are less changed would provide the working part of that fit with the male thread AFTER adjustment for the fresh cut. And those remaining areas that do all the actual work will wear faster than others in the female thread so it will need adjustment more often as it wears in.

    I do wonder how the damage you describe could have been produced. Just knocking it around or dropping it would have applied equal amounts of pressure all along the thread, not in just a few, small places. Perhaps some kind of contamination got in there and lodged in those spots? You may want to look at the male thread under magnification.

    It is a Starrett so it should be of pretty good quality and therefore worth saving. I would call Starrett and ask if they can repair it. Using Starrett is a definite suggestion if it is a model that reads to tenths. If it only reads to thousandths, you may want to risk the tap. But in any case, I would talk to Starrett FIRST.

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    Quote Originally Posted by Delw View Post
    Unless there not square.....
    throw it away and get a new one, or send it to a COMPETENT repair place and get it fixed correctly.
    the threads and spindle on a mic is lapped in, like sami said dont touch it with a tap.
    @Delw tried to get in touch with you via PM and email about your Freeport surface grinder. Can you PM me, thanks....Nick
    Sorry to jump in on this thread folks.


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