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    Re: Manufacture new Bygraves?
    From: Brad Morris
    Date: 2009 Jul 9, 16:04 -0400

    Hi Geoffrey
    
    I must politely disagree here.
    
    Within the navigation community, when the original Bygrave was being used by the RAF, the
    complaint was that the tubes would slide relative to each other, leading to erroneous results.
    I have encountered this very problem on the 'real' MHR-1, when I don't apply enough force to
    the friction locking mechanism.  I catch this by solving the problem twice, and if the solutions
    are different, then it slid.  It is highly unlikely that an un-desired slip will be by the same amount.
    
    Whilst Gary may have hit upon a good combination of friction that permits good referencing, I remain
    uncertain as to how long that fit will last.  A slip fit permits 0.002 inches,
    but this means that we cannot lock the tubes at all.  A 0.0005 inch fit would work, but will
    readily wear, soon leading to a .001-.002 inch fit, especially in the plastics defined.  In
    other words, the friction that causes it to lock also causes it to wear out.  (Sorry that this isn't in
    metric, but here in the US, the normal terminology for fit is in inches, which I what I am used to)
    
    The MHR-1 solution to this problem is to allow a very loose slip fit with a locking mechanism (creating
    an interference fit).  How loose?  The aluminium tube over which the scale is glued is clearly oxidizing
    on my instrument. Therefore, the scales are growing little hillocks of 
    corrosion under paper.  As the instrument is
    used, the tops of those little hillocks are wearing out the paper.  However, the fit is loose enough
    where most of the hillocks remain without wear.  This makes the slip fit at least 0.025 inches
    
    I don't believe this to be over-engineered, I believe it to be a good solution to a simple problem.
    Of course, that is just my opinion and you are quite welcome to yours.  Both 
    solutions (Bygrave's Original
    and the German MHR-1) use friction.  The Bygrave is under continuous friction, leading to wear.  The
    MHR-1 uses intermittent friction, with solid locking.  Given the choice 
    between the two, I would clearly go
    for the design of the MHR-1, as it does not suffer from that troublesome 
    erroneous results and will have very
    life.
    
    ------
    
    One thing we must agree upon as a consortium is what we are to build.  The easiest approach will be to
    copy (exactly) either and existing Bygrave or an existing MHR-1.  That is, 
    copy the pointers, copy the tubes, etc.
    No thought is required and for this exercise, that would be a good thing.
    
    A harder approach, and certainly more costly, would be to develop a 
    Bygrave/MHR-1 ++ device as I alluded to.
    We then need original engineering with prototyped parts and the full gamut of 
    product development. That may be
    too much for our (theoretically) tiny consortium.  Then we also get into 
    issues about just how far do we go?  It
    just becomes too difficult to manage.
    
    Best Regards
    Brad
    
    
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