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    Re: Problem with a sextant
    From: Joel Jacobs
    Date: 2006 Apr 25, 12:33 +0000
    Though posted to George's, my comment relates to all on this topic.
    My suggestion is to take the sextnat frame to a machine shop and ask them to use a "dial indicator" measuring the change in 1/1000 along the arc from its pivot point.
    I persoanlly doubt that normal use could cause a wear spot of such magnitude as to be notceable.
    Joel Jacobs
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    -------------- Original message from George Huxtable <george@HUXTABLE.U-NET.COM>: --------------

    > Bill wrote, about the problems Alex is finding with his sextant-
    > | > The only explanation I can imagine is that the arc is deformed near 0.
    > | > (Worn after so many index checks, as Bill joked:-)
    > |
    > | Not joking. After a large change in index error overnight while trying to
    > | use the sextant as a range finder by measuring parallax at 100 yards I
    > | discovered a speck barely visible to the human eye on an arc tooth. (Thank
    > | you Ken G.) I know I get a brass-alloy buildup on the arc near 0d from
    > | repeated IE checks.
    > |
    > | Theoretically I like the idea of adjusting the mirror so 2.0' on or off the
    > | arc is nearly 0.0' index error to check for that problem. Practical! ly ther e
    > | is still worm gear "residue" at 2.0' from 0.0' But a good start on reducing
    > | 2nd- or 3rd-order variables.
    > =====================
    > If there's wear near zero degrees, then presumably it's more likely to be on the
    > (softer) aluminium-alloy rack rather than on the
    > (harder) worm. Any error due to wear on the worm would be expected to repeat at
    > one-degree intervals, so could affect readings close
    > to an integral number of degrees (and zero minutes) anywhere on the arc. If wear
    > (or metal transfer between the surfaces) really is
    > the cause of the problem, then it would likely be worse near the zero-point,
    > because that setting is used so often.
    > I expect that Alex is quite accustomed to tweaking his mirror adjusting screws
    > and would be prepared to do so again. If so, may I
    > suggest the following, to test for whether there really is a w! ear-pro blem on
    > index-zeroing?
    > Why not really misadjust the index mirror, in a big way, so that it remains
    > perpendicular, but (in the other plane) the zero-setting
    > is shifted well away from 0 degrees? I don't know how much latitude the
    > adjusting screws would allow, but my guess is that a couple
    > of degrees (that is, 1 degree tilt of the mirror from its normal position) might
    > well be possible. Ideally you would want to shift
    > far enough so that the worm had moved to a completely different set of rack
    > teeth from the ones it normally occupies at the zero
    > setting, and that depends on how many teeth are in contact with the worm at a
    > time. Ideally, one might wish to shift the zero by
    > perhaps two and a half degrees, to use an unworn part of the worm as well as an
    > unworn part of the rack, when making an index-error
    > check. It wouldn't matter much whether the de! liberat e offset was on the arc or
    > off it.
    > Now, if Alex repeats his checks, allowing for that new, gross, index correction,
    > then if his discrepancies disappear, he can be
    > pretty sure that his problems result from local wear of his track/worm
    > combination. And if not, not.
    > It's a potential problem, that the older, Vernier sextants didn't suffer from.
    > Another disadvantage of progress, perhaps?
    > George.
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