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  1. #1
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    Default Taps and Threading tools

    I am trying to find documentation whether specification or process on the basis of GH and LH designations for machine thread taps. I have used them for years but never have found the practice defined and described. If anyone has come across this I would appreciate your input.

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    From MSC Big Book page 273:

    Example - 1⁄4-20 NC H3
    In addition to the nominal size and pitch of a tap, there is another important
    dimensional factor to be considered in selecting a ground thread tap. This is
    the H limit of the pitch diameter of a tap. H means (high) above basic pitch
    diameter(A "D" corresponds to metric threads). These tap limits have been
    established to provide a choice in the selection of the tap size best suited to
    produce the class of thread desired. The difference in size from one H limit to
    the next is 0.0005􀁷 increments for taps through 1" diameter. Sizes over 1"
    diameter are separated by .001􀁷 diameter increments. If the threads in the part
    are too loose, smaller numbers such as H1 or H2 are used. If the threads are
    too tight, the H limit number is increased. Proper selection of the H limit
    number ensures that the threads are within the tolerance required by the part
    print. Best rule of thumb: always select the largest “H” limit possible to achieve.

    Tom

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    There is a good explanation in Mc Master Carr catalog for those that don't have a machinist handbook. I have used the oversize taps when heat treating is involved and with metals like Inconel and Colmonoy.


    McMaster-Carr

    Dick

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    Here ya' go..

    Presto Counsellor

    Best regards
    Søren

  5. #5
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    There's a lot of info on-line that's either insufficient or clear as mud. The big thing to remember is not to confuse individual part (screw and nut) specifications with fit. You can buy taps in incremental sizes, and any given tap may give different size threads in different materials. This says nothing about fit. You can look in Machinery's Handbook and get the size limits for external screw threads. That doesn't tell you anything about fit either. Only when you combine the thread size resulting from the use of a certain size tap, with a certain size external screw thread, do you get a fit.

  6. #6
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    Quote Originally Posted by Gordon B. Clarke View Post
    Great. If you're ever driving by stop in and we'll have a coffee

    Gordon
    Beware...

    I might hold you up on that one..
    Same here, if you ever get so far north as Viborg..

    Søren

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    Gordon, don't feel bad- things are more complicated than I thought too. I just went through this with a run of parts in a very expensive material where I missed the PD by a thou and it was too much.

    You can get the same or similar fits using all different size taps (the GL and GH numbers) if you combine the resulting threaded holes with different size externally threaded screws. It's the combination that matters. Fortunately people tend to stick with just a few popular combinations, but there's no law about it. For commercial hardware its a no-brainer, but for close fitted instrumentation screws and such the parts need to be specified correctly such that the resulting fit is what was desired.


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