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Each of the three steps in the Bygrave solution involve the same 
manipulations and each step takes three movements. First you line up the 
cursor on a value on the cosine scale. Second, you move the cursor in 
unison with the cosine scale so that the other end of the cursor (or the 
pointer on the original Bygrave) lines up with a value on the cotan 
scale. The third movement requires moving just the cursor to another 
value on the cosine scale and then your are able to read out the answer 
from the cursor on the cotan scale. It is only this third movement where 
there is a  problem with inadvertent movement of the cosine scale. 
Thomas Kleeman suggested printing the cosine scale on clear plastic 
obviating the need for the cursor and also eliminating the problem since 
the third movement is eliminated. In fact, the last two movements are 
eliminated since you only make one movement, lining up a value on the 
cosine scale with a value on the cotan scale and then immediately 
reading out the answer on the cotan scale opposite the second value on 
the cosine scale. This would be like a rolled up version of my flat 
Bygrave since this is the simple manipulation used on the flat Bygrave. 

This points out another advantage of the flat model, no problem with 
wearing of the scales causing loosening and loss of friction allowing 
the inadvertent movement of the scales. Only one movement and the scales 
stay in place due to friction or pressure from your finger.

The same holds true for my recreation of the cylindrical models. The 
cosine scale is on plastic and, as you hold it in you hand, you only 
need to give it a little squeeze to hold it in place as you rotate the 
cursor tube for the last movement if friction is reduced with wear.

gl


Brad Morris wrote:
> 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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