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    Re: A-10 Sextant Manual
    From: Douglas Denny
    Date: 2009 Jun 9, 14:37 -0700

    Having just come onto this interesting forum I am amazed by the quality and 
    willing support from so many people.  Thank you all.
    Mr. Morris and Mr. Le Pook, thank you for your help and excellent quality pictures/diagrams
    I have not had time to take the sextant apart again to look into the problem 
    more carefully (in fact may do so after this e-mail exchange), but can make 
    some comments already.
    The sextant is in fine condition with no corrosion or any mechanical 
    deterioration at all. The inside looks as if made yesterday. Except for: 
    the bubble chamber was in need of reconditioning and filling (done as 
    described with silicone fluid);  the horizon view 45 degree glass was broken; 
    and the main index prism has a piece of glass shard broken away on its 'base' 
    - which is not affecting the optical properties at all.  The latter was stuck 
    back in position with cyanoacrylate glue and is of no consequence.
    All the ball bearings are in absolutely perfect condition and must have been 
    lubricated and still were covered in oil film. The index mirror holder 
    assembly is a perfect push fit onto the slotted 'drive' shaft.
    The index segment and worm gear are in perfect condition with not the 
    slightest pitting or corrosion anywhere.
    Even the black crackle finish is nearly perfect.
    This particular sextant was probably used and handled by Sir Francis 
    Chichester in the late wartime, as it was obtained from a gentleman who 
    worked at Henry Hughes (later Kelvin Hughes) as part of a whole collection 
    that had been put to one side in a storeroom, were to be thrown out, and he 
    'saved' them.  
    The collection was part of a number of sextants which were assessed for 
    performance by Francis Chichester when he was a consultant for H.Hughes, the 
    Air Ministry and the RAF on aircraft sextants, for presumably checking the 
    relative efficiency of them to assist in their designing their own.  There 
    are some interesting ones including a Japanese model which is, I believe, 
    literally a Japanese copy of an American design.
    back to the A10:
    The returning action of the spring on the index segment arc onto the worm is, 
    I am quite sure, acting properly as it should now I have ensured full tension 
    possible and mover the leverage further up the segment. I am quite sure now 
    it was working perfectly alright before my modifying it slightly.
    The backlash here is only in  the order of a few minutes of arc anyway when 
    the prism is rocked gently to test for backlash, and returns eactly and very 
    positively, whilst the variable element with the adjusting gearing backwards 
    and forwards is up to 25 or even 30 minutes of arc.
    The variable movement simply must be inherent in the worm shaft end-float. 
    There is no other explanation. I can see from your excellent photograph of 
    the A10 the left hand side upper bearing near the counter is the area of 
    interest. I rememember the helical spsring on the worm shaft but did not test 
    its action as I was concentrating on the index segment spring.
    Having not taken it apart yet again, I cannot say for sure,  but the diagrams 
    indicate a cone shape on the worm shaft, and the photo a flat end piece, so I 
    assume the bearing is a tapered (cone) plain bearing directly into the upper 
    bearing boss. If so, this is surprisingly poor qulity engineering for an 
    instument of this calibre, and it really needs a taper-roller bearing here 
    for efficient engineering and positive action.  A basic design flaw? - and no 
    wonder they have "backlash". This would have been done for cost-effectiveness 
    I guess as taper-roller bearings would have been expensive in wartime and 
    more costly to fit into the design.   Surprising because the Americans did 
    not usually stint on costs for their military equipment, but I suppose costs 
    eventually have to be considered somehwere eventually in manufacture.
    Even the shaft resting on a single ball-bearing at the end, with the spring 
    pushing the shaft onto the ball-bearing would have been a more positive 
    method in fixing the shaft position but allowing rotation - with low friction 
    too. A simple plain or ball-race at the upper bearing would then suffice.
    This is becoming more interesting. 
    Douglas Denny.
    Chichester. England.
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