NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Visit to Freiberg
From: Courtney Thomas
Date: 2005 May 23, 11:02 -0500
From: Courtney Thomas
Date: 2005 May 23, 11:02 -0500
Thank you Alex. Courtney On Mon, 2005-05-23 at 09:36, Alexandre Eremenko wrote: > On Mon, 23 May 2005, Courtney Thomas wrote: > > > What is backlash ? > > Backlash is the free movement of the worm screw with respect > to the arc. You can > rotate it back > and forth a little without moving the sextant arm. > The master who checked my sextant actually just tried to shake > the arm back and forth with his hand, apparently feeling whether > it moves or not. In other words, whether you can shake the arm > when the wormscrew is engaged, and without rotating the wormscrew. > > The usual procedure of measuring backlash is touching the > direct and reflected image of the sun twice, on the same > side (say Lover Limb), like you do when measuring > the index correction, but by rotating the > screw in two > different directions, first clockwise then anticlockwise. > The difference between your measurements is the backlash. > > But in the factory they do it with a special measuring device > which has apparently 1" precision. The same device measures the > arc error. > > The adjusting skrew is on the bottom side of the drum, > it is perpendicular to the plane of the arc. > (if the sextant lays on your table horizontally in mormal position, > the screw is on the bottom side of the drum). > He adjusted it with some special wrench (not with the wrench supplied > with the sextant. The strange wrench coming with the sextant is > for disassembling the drum. Both things are discouraged by the manual: > they say this should be done in a workshop. > > > Without going to Germany, how do you know if it's a problem with your > > sextant ? > > They said they would do everything by mail, > except that they do not accept credit cards, but only bank transfers. > > > Can you [yourself] approximately 'measure' it's significance with your > > sextant and correlate turning said 'screw', recheck and approximate a > > satisfactory adjustment ? > > I never knew that my sextant had this problem. I blamed the arc > excentricity, and brought it to Freiberger factory to measure this > excentricity precisely. > > Alex.