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Odd threads

Discussion in 'Locomotive Engineering M.I.C' started by Dag Bonnedal, Oct 14, 2017.

  1. Dag Bonnedal

    Dag Bonnedal New Member

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    Being located outside the UK (or US), one problem we face in maintaining old steam locomotives is the mixture between different screw threads. Standards are good, but many standards are not necessarily better.
    Most of our Swedish, German and British built locos are built with Whitworth standard threads, but there are exceptions. And these days, BSW screws are hardly obtainable in Sweden, only UNC.
    https://en.wikipedia.org/wiki/Östra_Södermanlands_Järnväg

    Right now I have one odd problem. In the machinery of our two Swedish built superheated 2-6-2T Nos. 4 & 9 there are fitted screws for the bearings of the coupling and side rods. As you may see in the detail of the drawing below these are mostly 3/4" fine treads with Whitworth profile. But the odd thing is that they are 14 TPI!!!
    As we know 3/4" BSF is 12 TPI and UNF is 16 TPI. The Motala variety is right in between.
    No. 4 is in reasonably good shape but No. 9 is in need for a number of new screws, some of them have been replaced by plain W3/4" by the previous owner.

    When I Googled for such threads I came across references to the "Admiralty Fine Threads" which happens to be 14 TPI for 3/4". Does this standard exist today and are taps and dies available anywhere?
    The alternative is of course to have these tools custom made. I found a few companies that claim they make such tools, but is there any one you would recommend?
    L4365 Vevstake det.jpg
     
    Last edited: Oct 14, 2017
  2. Steve

    Steve Part of the furniture Friend

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    Within the British Standards there was a series of fastening threads using the Whitworth form. These were incremental from 1/16" up to 2¼" diameter. 1/16" & 1/8" used 28 tpi; 1/4" & 3/8" used 19 tpi; 1/2" -7/8" used 14 tpi and 1" upwards used 11 tpi. In my experience the railways and loco builders used these quite a lot for screw threads other than nuts and bolts.. At the moment I can't lay my hands on the obsolete BS or even remember the number.
    If you are making new bolts and nuts it wouldn't be a big job to machine them to these threads in the lathe, especially as 14 tpi 55° is a standard tip size. Certainly that's what we do at Middleton.
     
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  3. Dag Bonnedal

    Dag Bonnedal New Member

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    Thanks,
    of course we also cut threads in the lathe. But in this case we would also like to have a set of thread tools available.
    Thus, my question remain.
     
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  4. The Saggin' Dragon

    The Saggin' Dragon Part of the furniture Staff Member Moderator

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    Wouldn't the modern version be to have a CNC program made and have the bolts batch produced as required? Maybe a project for a local technical school?
     
  5. huochemi

    huochemi Member Friend

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  6. Sheff

    Sheff Well-Known Member

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    But what about the tapping side of things?
     
  7. Steve

    Steve Part of the furniture Friend

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    It is easy enough to work out. Just subtract twice the thread height (0.0457 from memory) from the nominal diameter.
     
    Last edited: Oct 15, 2017
  8. Eightpot

    Eightpot Well-Known Member Friend

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    Dag Bonnedal likes this.
  9. Dag Bonnedal

    Dag Bonnedal New Member

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    Thanks for the link, I have still not done anything about it, but I will coming workshop season.
    UNS is a new "non-standard" to me ;).

    Dag B
     

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