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    Sextant frames
    From: Bill Morris
    Date: 2009 Apr 15, 19:46 -0700

    In a recent off-list exchange of e-mails about a sextant, Alex Eremenko 
    remarked to me that there seemed to be a perception that  "...aluminium alloy 
    frames are sort of inferior, cheap substitutes for the 'real' bronze frames." 
    A look back through NavList postings that mention sextant frames, seems to 
    confirm what Alex says.  In fact, aluminium frames ones made by pressure die 
    casting are superior on several counts..
    
    This process, which came into use around 1910, was well established by 
    the1930s, owing to impetus from the motor car and aircraft industries. In 
    sand casting, molten metal finds its way into a sand mould by gravity. In low 
    pressure die casting, molten metal is injected under pressure from below into 
    a metal mould, displacing the air from the mould. The pressure is maintained 
    until the metal has frozen and the mould, or die, is then split to release 
    the casting. 
    
    The process produces dense, homogeneous, stable  castings and, when used with 
    aluminium alloys, the strength and hardness of the product may approach that 
    of mild steel, depending on subsequent aging and heat treatment. As the mould 
    is made of metal, the finish of the product is as good as that of the smooth  
    inside of the mould, so that costs of finishing are much reduced. The moulds 
    are costly to manufacture, but are good for several thousand castings. This 
    is a very simple account of the process and ignores complications such as the 
    reactivity of molten aluminium which can quickly erode the mould without 
    special pre-treatment.
    
    Heath and Company described their Mark VI M. Duralumin Frame sextant in their 
    1937 booklet "My Modern Sextant" and C Plath produced many alloy framed 
    sextants during the Second World War. The USSR afterwards took over 
    production with confiscated machinery, producing essentially the same  
    sextant with the designation SNO-M (Marine Sextant for Navigation with 
    illumination).  Its frame bends about one third as much under load as do 
    bronze instruments of traditional ladder form. 
    
    Again during WW II, Tamaya produced an alloy-framed sextant with the same 
    ladder form as their traditional bronze framed instruments, which are nearly 
    indistinguishable from those made by Plath. The alloy Tamaya frame bends 
    under load about half as much as the bronze one and weigh about one third 
    less. Kelvin and Hughes also produced alloy framed sextants after they merged 
    in 1947. 
    
    The weight advantage would have been greater had not Tamaya hung on to having 
    a bronze rack moulded into the aluminium, unlike the SNO-M and its successor, 
    the SNO-T, which cut the rack directly into the frame. The SNO-M had a 
    stainless steel worm running in the rack, while the SNO-T used bronze for the 
    worm. In fast moving, heavily loaded applications, these combinations would 
    not have been ideal, but the worm-rack combination behaves as a lightly 
    loaded, slow-moving, reciprocating bearing and they seem to run very well 
    together.
    
    The corrosion resistance of the aluminium alloys used is not perhaps as good 
    as that of the traditional bronze, but the increased stiffness and reduced 
    weight give the alloy frames a clear advantage.
    
    The attached photograph shows a sextant frame clamped in a vice by the limb 
    and a bending force of about 1 kg being applied in the region of the index 
    arm bearing. The dial indicator shows that the frame has deflected only 0.1 
    mm.
    
    
    
    
    File:


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