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    From: Clive Sutherland
    Date: 2005 Nov 21, 20:42 -0000

    Some years ago when I was servicing a sextant for a friend I noticed an
    excessive and variable (i.e. dependant on the part of the arc being used)
    backlash problem.My surmise was that at sometime during its life the index
    micrometer worm had been clumsily grated across the gear arc, and back lash
    was occurring wherever the worm did not now properly engage. This only
    only over certain parts of the arc, mostly near the parking position, as the
    sextant was put away in its box.
    This could occur on a sextant if the clamping lever had been insufficiently
    gripped as the index arm was rapidly moved and the teeth on the gear (which
    was made of a hard metal) come into contact with the softer arc and damage
    the tops of some the teeth. If you were to look at the shape of the damaged
    teeth in  profile their normal triangular shape would be turned into that of
    a breaking wave.
    My thoughts are that If you find backlash occurring on a old or second hand
    sextant or anytime on
    a plastic one, look carefully at the shape of the gear teeth. If any damage
    is not very bad It may be carefully repaired. But!  Beware, always look very
    carefully if you are buying second hand. Even a new plastic sextant is not
    immune, especially if "shop soiled!"  This, in my opinion, is the greatest
    risk with plastic sextants. They do need Extra Tender Loving Care!

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