The Nautical Sextant

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” ,  “AN5851-1 : jammed shades carrousel” and “A Byrd sextant restored”

Since writing the previous post about the Byrd sextant (or should it be the de Florez sextant?) I became disatistfied with the restoration and, for those interested in air navigation intruments I decided to add to the photographs. You will need to read the previous post first.

As noone followed my hint to donate me a set of index shades, I have had to make them myself. For the outline, it was simple enough to scribe around a genuine shade on to a sheet of 2.5 mm brass four times, drill and ream the mounting holes and saw out the blanks using a piercing saw. This tool is a little like a small fret saw and takes very fine blades having as many as 40 teeth per inch. It cuts on the down stroke and is used with the blade held vertically. Here is a picture of me cutting out a clock wheel blank. Note that it helps to have lots of light and vision…

I could then bolt the four blanks together with a close-fitting screw through the mounting holes and files the outlines to shape. It is actually easier and quicker to do the straight bits using a vertical milling machine – if it is already set up – and just to file the rounded corners by hand. The block of four could then be mounted and centred in the four jaw chuck of a lathe and all four drilled through and bored to 22 mm diameter. The outermost blank was then counterbored to 24 mm to a depth of 2 mm and removed, then the next counterbored and so on for all four, taking care to loosen and tighten the same pair of chuck jaws each time. This is not good practice, but it saves time when accurate centring is not vital.

It is hard to discover sources of neutral density glass, so I made a trephine out of mild steel and cut out discs of plastic Cokin filter material. This is used in photography and seems to be flat and parallel enough for a sextant likely to have observational errors of the order of minutes. I made the fit so the discs just popped into the brass frames and saved myself the trouble of having to swage them into place, though they might have looked more “genuine” if I had. Making the shouldered screw that holds them all in place was staightforward turning and I made the Belleville spring washer by making a thin brass washer, sitting it on the end grain of a block of hard wood and hitting it hard with a ball bearing. The finished set of shades shows well in the next photograph, as does the semi-circular lens that allows the level to be seen in focus.

I wasn’t happy with the fiducial line in my original restoration. It was simply a piece of fine thread wrapped around the level vial and secured in place with clear varnish. Since it is wrapped around a curved surface, it can be viewed only from one angle and still be seen as a straight line. In any case, it was rather too thick, so I scribed a thin line on some perspex and then cut and filed and drilled a tiny piece to size, securing it to the vial carrier with two 12 BA screws. These are only 1.3 mm in diameter and I was greatly relieved to have tapped the two blind holes without breaking my only tap of this size. The next photograph shows this small but important part. The flash has made it look dustier than it was in reality. I have no idea what was used in the original models. The Smithsonian Museum has two examples, but both are incomplete and I have only web photographs to look at.

Making the case from African mahogany needed only normally careful woodworking. Dovetail joints for the corners had by the 1920s given way to comb (finger) joints, but as some later American aircraft  cases used corner rebates, which are much easier to make without special machinery, this is what I used, with brass pins across the joints to prevent disaster if the very strong glue should fail:

I copied the hook latches from a Hughes and Son sextant case and the handle is a very close copy of the handle used for a Brandis Aeronautical Sextant Mark 1 Mod 4 of 1931. I am not good at sheet metal work, so will gloss over the battery box, with its belt loop. The pick for the two capstan headed screws was simple to make and the mirror-adjusting wrench required only the ability to convert a small round hole to a small square hole using a file. It remained  to dismantle the instrument to its component parts and spray-paint them using a satin finish paint that, while not perfectly imitating the original finish, at least has the merit of pleasing its owner.

The final photograph shows the sextant in its case with its furniture and fittings. It is certainly not an easy sextant to use on land, but since these latest retoration efforts, there has been little clear sky around for me to make a serious assessment. If there is some particular aspect of this instrument that you would like discuss or to see illustrated, do contact me.