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Bill,

I have now disassembled the sextant again; and also the worm assembly. (pictures enclosed.

I think I have the problem determined. If not actually solved in terms of 
curing it completely, I am fairly sure what the problem is: and it was as I 
expected some degree of end-float problem.

With reference to the disassembled worm assembly: it immediately becomes clear 
that the bearings at both ends are plain bearings and the main worm/shaft is 
removed from the larger upper bearing hole.  The shaft is free to move in 
either direction except for the stops which are the small gear at the bottom 
end acting against the washer of the lower end of the lower bearing stub, 
stopping it moving upwards; 
and the stop downwards is the change in diameter of the worm shaft at the 
upper side of the lower bearing onto the washer. The spring on the worm 
shaft, plus the spring of the index prism sector  are the only devices 
pushing downwards to keep the worm shaft in contact with the lower washer. 
This position must be maintained in all rotations and movements of the worm 
shaft. For an absolute positive position determining method this is poor 
design, as there are too many variables to alter things with wear, or other 
possibilities such as the washer not being exactly flat.

There is a small amount of end-float possible. It is only in the order of a 
few thou, but amplified with the leverage of the sector arm to the prism, 
this no doubt will easily represent the 20 minute of arc variability found, 
and explains why there is a difference from moving the prism to the object 
one way then the other.  There is considerable friction from the adjusting 
knob through the helical gearing to the worm shaft, so moving the adjust knob 
one way will tend to shift the worm shaft slightly against one stop and hold 
it there, and vice versa the other way.  The only way to try to overcome this 
with the existing set-up is with a much stronger spring to overcome the 
friction between adjust knob and worm shaft;  and also to ensure the minimum 
friction in teh worm shaft itself in its bearings so there is no 'stickage' 
involved in the worm shaft itself during rotation.  I shall try it.

The only final way to solve this in an engineering sense is to incorporate a 
better method of keeping the worm shaft in a fixed position pressing onto the 
bottom bearing stub with the springs and eliminating any friction that can 
hold it one way or the other against its stops. A small taper-roller bearing 
here might be one solution.


Douglas Denny.

Chichester. England.
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