NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bubble sextants on e-bay
From: Douglas Denny
Date: 2009 Jun 8, 15:25 -0700
From: Douglas Denny
Date: 2009 Jun 8, 15:25 -0700
I am grateful for your assistance with this sextant. I have now opened the main casing and dissasembled the index sector gear and re-adjusted the anti-backlash spring to it's maximum rotation consistent with it not binding as fully wound; and lengthened the application point of the spring further along the index sector to increase the lever arm of the force applied. I am now satisfied the maximum returning force is being appleid by the spring. I have now checked the sextant against a collimated light source for backlash and have proved to myself the index prism sector arm and worm gear is _not_ the cause of the problem. By gently rocking the index prism whilst looking at the (pinhole) light source, one can easily see the slight shift in position of the light (its only a few minutes of arc anyway) and the very positive returning motion to exactly its same position each and every time. The backlash in the index sector/worm gear is indeed eliminated. There is still variability however when the light source is approached from the bottom or from the top. It is quite consistent in value (most of the time) when a series of checks is made from one way then the other. It is still in the order of 20 minutes of arc. This means there can only be one other source of movement causing the apparent shift of reading and that is movement of the whole worm shaft itself slightly up or down with the force applied by the worm gear when approaching from one way or the other. In other words there must be "end-float" of the whole shaft in its mountings with the worm gear pushing it one way then the other. I shall have to investigate further to see if this can be eliminated by some engineering 'fix'. If not, then the sextant has a serious fundamental flaw built-in to the design from the very start and indicates very poor design engineering - which is a pity if so as it seems to be a nicely built instrument otherwise. Douglas Denny. Chichester. England. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---