(09-28-2020, 03:54 PM)LoneStarGazers Wrote: Does anyone have experience with bubble sextants? I recently acquired one, but am finding it difficult to move the Slow Motion Knob. When I first inspected it, the knob seemed to moved freely. Then I mistakenly turned the winding crank only one click. After it wound down, the knob was extremely hard to turn, almost as if it is locked so the user can get a reading? Maybe there is a way to unlock it?
Any help is greatly appreciated!
Mike
I have experience of bubble sextants. I have repaired several.
Briefly to provide advice only: -
They are getting on 75 years old, and general deterioration takes its toll. Like all old equipment(s) they _can_ be restored if you are determined enough. It is unrealistic to expect them to work fully as they did when made in the 1940s.
Your problem is almost certainly hardened grease and lack of lubrication.
As a general rule they will need to be disassembled, cleaned of all hard grease on worm gears and mechanical systems; any defects repaired; lubricated with fresh grease on gearing; light oil as necessary; and optics cleaned.
Often the mirrors will be tarnished or completely opaque in which case re-silvering is the only way unless a modern mirror with thin glass thickness can be cut to size.
Re-silvering can be done with the Brashear process: involving chemicals including silver nitrate; 0.88 ammonia; conc nitric acid. And the resulting silver mirror protected with clear laquer.
The bubble chamber if intact is usually still good for working, if still fulll of fluid. British sextants used Heptane for the liquid to prevent freezing and have good mobility of the bubble.
All of this is a steep learning process for many people and possibly beyond what most people would want to do. It IS quite easy to do if you have basic engineering skills and determination though. The first one done is probably a nightmare for someone new - but the second one -easy !
Douglas Denny.
Bosham. England.
