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Bill and Jared,
One has to distinhuish the worm/drum excentricity
from the arc/arm excentricity.
The errors increasing towards the end of the arc must be due
to the second kind of excentricity.
Drum/worm excentricity is something very different,
and they cannot be reflected in the usual certificate.

Alex.

On Mon, 16 Apr 2012, Bill B wrote:

>
> On 4/16/2012 2:56 AM, Jared wrote:
>
>> I struggle to understand the geometry, how in some instruments very
>> recently cited on the List, the error owing to eccentricity can
>> accumulate so regularly as the worm travels the limb from O deg to 120
>> deg, that when plotted the change resembles a sine wave. If this trouble
>> is described in Bill Morris's book, I can't find it.
>>
>> Is the worm defective? Are the teeth on the limb spaced improperly?
>> --Jared
>
> Thank you Jared for raising some excellent questions.
>
> About a half decade ago Frank Reed proposed a method for testing
> drum/worm gear eccentricity. I had tested his method extensively, and it
> seemed quite viable.
>
> In my limited experience the errors along the arc are not a sine--or any
> other wave.  They come and go at their own pace.  For example, my Astra
> IIIB easily beats its advertised plus/minus 20" along-the-arc-error
> specs consistently--except near the 90d position--where it is almost a
> minute off.  Both Alex and I have verified this with multiple
> star-to-star measurements against predicted distances calculated the nth
> and the worlds best sextant--his SNO-T ;-)
>
> Past possible worm gear problems, if I understand you seem to add two
> other variables, stated or unstated. Is the arc a perfect section of a
> circle (eccentricity)?  If it were, are the teeth cut into that
> arc--making it a gear--spaced evenly (or of uniform height?
>
> Bill B
>
>
>
>
>
>
> : http://fer3.com/arc/m2.aspx?i=119090
>
>
>

   

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