Overhaul of Mk V/ AN5851 sextant bubble chamber

20 12 2008

This post is preceded by “Bubble Illumination of Mk V and AN 5851 Bubble Sextants” and “Refilling Mark V and AN5851 Bubble  Chambers”

The preceding post dealt only with refilling a bubble chamber that was otherwise clean and tidy. This post covers dismantling for cleaning and refitting the glasses. You will need an electric iron, a pencil, a screwdriver, some flake shellac (from a decorating shop or French polisher), some xylene, a syringe (preferably glass) and needle or a fine pipette.

Start by removing the bubble chamber assembly from the sextant as in the previous post. Remove the taper plug. Remove the air chamber (4 screws from underneath), carefully preserving the gasket if you can. The control unit unscrews by using a wrench on the octagonal metal rim. At this point, it is wise to be wearing an apron with its skirt clipped to your bench or table, as there is a ball bearing with a hole pierced through its centre just  waiting to fall onto the floor and be lost for ever. Dont let this happen to you. It is difficult to drill holes through small balls. Empty the control unit into the bubble chamber by repeated pressing and shake out as much xylene as you can. You should now have reached the state shown in the following photograph:


Also remove the four tiny countersunk screws on the underside and carefully prise the illuminating annulus free. The paint beneath is radium paint and, while it no longer glows in the dark, the radioactivity from the contained radium is still present and is best removed, before replacing it with modern photo-luminescent paint.

See https://sextantbook.com/2008/12/09/bubble-illumination-of-mk-v-and-an-5851-bubble-sextants/  for some advice on how to do this.


Meanwhile, your electric iron, carefully secured in some way with the sole upermost, can be warming up. The maximum setting seems to be just the right temperature. For those lucky people with laboratory hotplates, the temperature of my iron cycles between 160 and 182 celsius. Place the stripped down bubble chamber on the hot iron and do something else for five minute while it heats up and softens the shellac that is holding everything in place. You can check whether it is hot enough by touching the chamber somewhere with a flake of shellac to see whether it melts.

When the right temperature  has been reached, hold the chamber in one hand with a thick towel or heavy leather glove and unscrew the bottom retaining ring. For this you can use the correct tool, a pin wrench, or a piece of 1.5 mm metal strip previously filed to fit the slots. Then rap the chamber sharply on a wooden bench and with luck the bottom glass will fall out. When your hand has stopped hurting from holding a very hot object,  turn over the chamber and unscrew the other retaining ring. While everything is still hot, push out the top lens with a pencil from underneath.

Allow everything to cool down and then soak the glasses and retaining rings in alcohol while you chip away as much old shellac as you can from the chamber body. If you are patient, you could instead soak all the parts overnight in alcohol and rinse in the morning with clean alcohol. Everything should now look something like this:


There is a very tiny passageway between the air and bubble chambers and you should make sure that this is perfectly clear of shellac, as it enter the bubble chamber partly under the bottom glass and can easily be completely blocked by stray shellac.

Return the bubble chamber to the hot iron with the top of the chamber uppermost and when it is hot enough, carefully apply a ring of shellac to the flat lens seat. Do this by picking up flakes about 5 to 10 mm long in a pair of fine forceps and smear as you dab. When the ring is complete, align the etched line of the top glass with the fore and aft line of the chamber (the bubble control is aft and the air chamber to the left) and drop the lens into place. You will get perhaps one chance at realigning it if you get it wrong, before the glass gets too hot to touch. The lens goes in convex side up. The concave side has a flat annulus ground on it and this goes downwards.

The official manual instructs one now to apply pressure to the lens with a jig until the shellac has cooled somewhat, when the retaining ring is screwed into place. If you have the means, you can improvise a jig like mine, shown in the next photograph, or use a Mark I pencil as shown in the one after,  to apply a little downward pressure to make sure there is a complete ring of shellac beneath the glass. Having tried both on a variety of bubble chambers, I now use a pencil and screw down the retaining ring while everything is still nice and hot, and I bet the WW II technicians did too.



Allow the chamber to cool down and check the inside for loose flakes or little teardrops of shellac on the underside of the top glass. The latter can be chipped off easily and the job completed with a wipe of alcohol. The cleaner the inside is, the better the bubble will move and the clearer the view, so do it now, as you won’t be able to after the next step, which is to fit the bottom glass, following the same procedure as for the top. When applying the bottom ring of shellac, take special care to keep it away from the mouth of the capillary passage to the air chamber, but make sure there is some more peripherally to complete the seal.

When everything has cooled down again, chip away shellac from the outside of the glasses, clean up and refit the control and air chamber, the reverse of dis-assembly. Take care that the hole through the ball lines up with the passage into the bubble chamber from the control.

The last photograph actually shows an A10 vapour pressure chamber being resealed. The original seals were lead washers and don’t work for me when re-used. You can mess about with O rings that cause alterations in the optical path length and sometimes dissolve in xylene, or search for the ideal material with which to replace the lead washers, or you can make a really good job of it with shellac.

I hope to have an e-book overhaul manual for the A10/10A ready by the end of February, 2009. Meanwhile, you could enjoy reading The Naked Nautical Sextant and its Intimate Anatomy (but first you have to buy it).




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