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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.



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