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    Re: Manufacture new Bygraves?
    From: Thomas Kleemann
    Date: 2009 Jul 08, 23:21 +0200

    Brad Morris schrieb:
    > This is the method used by the Germans.  They wrapped the scale around aluminium tubes.
    
    Here my 2 cents (we are EUR):
    
    I thought about some dimensions of an imaginary BSR (Bygrave slide rule):
    
    To see what is practical and achieveable, I rummaged for my school time 
    slide rule. Its scales are 250mm long (about 10in).
    
    In the area where the graduation is smallest, the tick marks are 0.4mm
    apart (10 marks per 4mm). I can read that easily without magnifiers, but
    some hyperopic may have difficulties.
    
    Inspecting the functions that form the scales of the BSR one can clearly
    see, that the closest ticks appear at 45� at the cotangent scale and at
    the smallest value of the cosine scale.
    
    By allowing 0.5mm spacing in the 45� area one can mark every single
    minute of arc on the cotangent scale. Using the original scale markings
    running from 0�20' to 89�40' it gets an overal length of 8847mm
    or 8.85m! Impressive, isn't it?
    
    Wrapping it around a tube of 50mm diameter (2in) with 8mm pitch (1/3in)
    one gets 56 turns distributed over 450mm length (18in).
    Why 8mm pitch? A dimension of my slide rule again. A pitch of 7mm gives
    400mm in length (16in).
    
    On my slide rule the scales are paired with the upper one having the
    readings above (like Gary's) and the lower one having the readings below
    the ticks. This way one can adjust the marks of both scales without the
    numbers overlapping each other. Only the tube material for the cosine
    scale has to be transparent.
    
    68�
    .|....|...
    '''|''''|'
       35�
        62�
    ....|....|
    ''|''''|''
      29�
    
    With this configuration there is no real need for a 'Cursor tube'.
    nor interpolations. One can directly read the cotangent scale to the
    nearest minute of arc against the other scale.
    
    The cosine scale is another story:
    Using the same spacing as for the cotangent scale one can tick off
    every min of arc from	89� and 60�
    every	 2nd 		60� to 45�
    	 5th		45� to 20�
    	10th		20� to 10�
    	20th		10� to  5�
    	30th		 5� to  3�
    and then every degree.
    
    Interpolation between the ticks is clearly needed here.
    
    So far it looks feasible and 50 x 450mm (2 by 18in) is not too
    cumbersome.
    
    /Thomas.
    
    
    
    
    
    
    
    
    
    
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