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Thread: Asme b1.1-2003 unified thread - calculating external minor diameter?

  1. #1
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    Asme b1.1-2003 unified thread - calculating external minor diameter?

    ipt>
    Hi all,

    i am in desperate need of computing the limits of size for the MINOR diameter of an EXTERNAL unified thread.

    To preface this...

    1.) I understand the manufacturability of an external screw thread is not necessarily concerned with the minor diameter, however, I am still interested in solving for this.

    2.) In reference materials, such as M.Handbook. External threads contain a table spec that is described "UNR Minor Dia Max". This, I interpret as the MMC of the minor diameter to still allow functionality of it's internal counterpart. This is the value I am interested in calculating.

    3.) You will find references to external minor diameter tolerance in the standard which is described in sec. 5.8.1.
    "To intersection offlat root with flanks of threads (see Figs. 2 and 3), equalspitch diameter tolerance for class of thread specified,plus 0.21650635P (see Table 5). " Although this information is useless, when a limit of size is unknown (which is not specified anywhere in the standard).

    4.) You can references specs from this calculator, which appears to solve/pull the matching value for max minor diameter, but I've found the tolerance range to NOT match whats in the standard in sec. 5.8.1


    Link removed... (Administrator)

    Any help with this would be greatly appreciated. Thank you

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    The size of the thread minor diameter is calculated from the pitch diameter - formulas are provided in Machinerys.

    Not sure why you need the tolerance of variation only on the minor diameter of an external thread in that the thread go and no-go gauges verify the pitch, major and somewhat the minor diameter.

    https://www.engineersedge.com/screw_threads_chart.htm
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Kelly,

    Why are you assuming I "only" need the tolerance of variation on the minor diameter?

    It appears you have assumed the nice formulas in M.Handbook allow one to arrive at table specs for a given thread class, but you would be mistaken. The formulas in the design profile don't account for allowance. And should one use them to establish precise sizes from, you'd better know how to use them.

    I invite you to try and compute the maximum minor diameter UNR, as I explained above from the formulas. You will not arrive at the table spec from any of them.

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    Quote Originally Posted by theconsumer1 View Post
    Kelly,

    Why are you assuming I "only" need the tolerance of variation on the minor diameter?
    Did not assume anything - that's what you wrote.. You would appear to have the rest of the information.

    "i am in desperate need of computing the limits of size for the MINOR diameter of an EXTERNAL unified thread."

    UN External Inch Size Screw Threads Formulas and Calculator

    Screw Threads Calculator for Internal Unified Inch Screw
    Last edited by Kelly_Bramble; 09-21-2022 at 10:30 AM.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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