wear from 1/4 Whitworth versus 1/4 UNC

Discussion in 'Digital Photography' started by Matty, Jun 13, 2009.

  1. Matty

    Matty Guest

    I'm in the UK and want to get a few camera mounting bolts (1/4-inch
    Whitworth) for some projects.

    It's hard to get those 1/4 BSW bolts and I know a substitute is the
    American 1/4-inch UNC (also called "1/4-20").

    With repeated use over time, would a UNC bolt cause noticeably more
    wear or deformation on the camera's mounting socket than a BSW
    Matty, Jun 13, 2009
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  2. Whithworth and UNC are only equivalent in the size of hole they fit into.
    The actual threads don't match. The thread angle is different, Whitworth is
    55% UNC is 60% IIRR. You can't simply swap male and female threads around.
    See above.

    If 1/4 are unavailable all you need do is buy longer bolts and simply saw them to
    size. No real problem with a hacksaw and a drop of oil. If you had one, you could
    clean up the cut ends with the appropriate size die but otherwise you can clean up
    the thread with a file and test the cut ends in a nut to make sure they thread o.k.
    - rather than force them in the camera thread.

    michael adams

    michael adams, Jun 13, 2009
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  3. ....

    Oops sorry I was half asleep there I took the 1/4 to be the length you were looking

    As to mixing threads - as stated above, the thread angle is 55% for 1 and 60%
    for the other. If you want to believe it makes no difference all you need do
    is draw two sets of equidistant parallel lines (the pitch being the same) on two
    a sheets of paper right to the edge - one set at 55% to the side of the paper and
    the other one at 60% and then try and match them up. The longer the thread the
    greater the disaparity.

    Furthermore its probable that the female thread in the camera will be of softer metal
    than any steel bolt you might thread into it.

    A quick Google has revealesd that there are plenty of 1/4 Whiwrth bolts on eBay.uk

    michael adams
    michael adams, Jun 13, 2009
  4. Matty

    Rob Morley Guest

    I don't think you understand the difference between thread pitch and
    thread angle.
    Rob Morley, Jun 13, 2009
  5. Matty

    Rob Morley Guest

    Mixing the two thread forms means that most of the load will be on a
    small portion of the thread surface - because of the different angles
    the groove in a BSW nut is deeper than the ridge on a UNC screw, so the
    load is placed near the tip of the Whitworth thread. That's not too
    bad, because the Whitworth profile is rounded, unlike the Unified
    profile which has a sharp edge. You could wrap the screw in PTFE tape
    to improve the fit slightly. But why not just buy Whitworth screws?


    Rob Morley, Jun 13, 2009
  6. The thread angle is the angle of the sides of each thread against the
    centre line of the bolt. As in the angles of a pyramid etc. The pitch
    is the number of threads per inch or cm.

    Although admittedly as the bolt is being threaded one thread at a time
    rather than all at once the disparity doesn't increase as stated above.
    I don't know where that came from.

    Mixing threads may be OK for a one-off lash-up, but not where a camera may
    be subsequently used on a standard Whitworth tripod head.

    michael adams

    michael adams, Jun 13, 2009
  7. Matty

    tomcas Guest

    Camera mounting threads are typically American thread standard and not
    tomcas, Jun 13, 2009
  8. This is way off-topic, but on a side note:

    There is only one thread on a bolt (well, there are some speciality bolt
    which have two threads).

    And the thread on mass-market bolts is not cut with a lathe or die one
    revolution at a time but formed in one step by rolling, see

    Jürgen Exner, Jun 13, 2009
  9. Matty

    Rob Morley Guest

    Rolled threads are still formed "one revolution at a time", it's just
    that the material is pushed out of the way rather than being cut like
    a regular lathe tool or cutting die does, so there is no swarf to worry
    about (and a rolled thread has better characteristics). A thread
    rolling head is just like a thread cutting head, except it has hardened
    ridged rollers rather than hardened ridged cutting edges. :)
    It's often quite easy to spot the difference between a rolled and a cut
    thread - the outside diameter of a rolled thread is slightly larger
    than the stock from which it has been formed.
    Rob Morley, Jun 14, 2009
  10. Matty

    Rob Morley Guest

    The angles of a pyramid as seen from where - plan, elevation, normal to
    a face?
    That was the bit that particularly confused me.
    It depends on the sort of damage (if any) that might be caused. A
    plastic thread may be worn or broken, an aluminium thread may be
    deformed, and any of these may be a show stopper or negligible depending
    on degree.
    Rob Morley, Jun 14, 2009
  11. Matty

    Eric Stevens Guest

    There is no such thing as a 1/4-20 Whitworth thread. Its a 1/4-19

    Eric Stevens
    Eric Stevens, Jun 14, 2009
  12. Matty

    Eric Stevens Guest

    You are quite right. Now where did I get that from? Clearly it was a
    brain fart.

    Eric Stevens
    Eric Stevens, Jun 15, 2009
  13. The angle of elevation in relation to the horizon, which is then multiplied
    by two.

    Or if you prefer, if a camel is walking in front of the pyramid, then the
    thread angle would be equivalent to the angle between its humps.

    And just to be picky - the pitch is actually the distance between the tops
    of the humps, and if you had a mile long camel with 5000 humps then
    this would be 5000 humps per mile and the pitch would be 1/5000 of a mile

    If a mile long camel with a hump angle of 55% were to be turned upside
    down and lowered on top of a mile long camel with a hump angle of 60% it
    would be seen that the hump faces would only touch at one point. Hence
    the problem. This point would presumably change if this process were repeated
    and parts of the humps wore away. Hence the inadvisability of mixing

    michael adams

    michael adams, Jun 15, 2009
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