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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.