NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Freiberger Trommelsextant errors
From: Bill B
Date: 2012 Apr 16, 04:35 -0400
From: Bill B
Date: 2012 Apr 16, 04:35 -0400
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
