The Nautical Sextant

The preceding posts covers “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” and “Heath Curve-bar sextant compared with Plath” .

Some good friends came for the weekend about ten days ago and one of them presented me with a wooden box that rattled ominously. An old uncle had been clearing out his house and had asked her to find a good home for the box and its contents. I recognised the box as belonging to a Hughes and Son sextant from the placing of the hook latches so that gravity keeps them both in place when the box is being carried, and on opening it, this is more or less what I found inside:

Figure 1 : Sextant as received.

As you can see, the key hole of the case is plugged with mud and some of the smaller parts inside were also encrusted with it. Most of the paint had disappeared from the frame and index arm and all the woodwork was loose, but there was no obvious major damage to the individual parts. In particular, the arc was all in one plane, the index arm was not bent and the index arm bearing was in good condition. All the screws, both great and small were present, but the lamp and its mounting for illuminating the scale were not and the electrical parts of the battery handle were the worse for wear. Of the wiring there was no sign.

This is the third valuable sextant I have been presented with as a gift and I count myself very fortunate to have such friends. I foresaw hours of pleasure ahead of me, though with some tedium involved in polishing screw heads and cleaning parts.

I began with the frame and index arm. The release catch was still attached to the latter, so I removed the parts and also removed the legs from the frame to make the clean-up easier. I left the index arm bearing in place and in general, this should never be removed, though in this instance, as there was no inspection certificate, there would have been no great harm in removing the bearing and re-calibrating the sextant. However, it would have required a special ring spanner and it seemed pointless to make one for an unnecessary task.  There is no harm in removing the tapered “journal” from the index arm, but it is seldom necessary to do so unless there is heavy corrosion around it.

I gave the parts a prelimary soak in diluted household ammonia, rinsed and then stripped off the remaining paint with a proprietary paint stripper. I then brushed all the mud and verdigris from the teeth of the rack, dried everything off and hung the frame and index arm up for repainting.

Hughes and Son at this time (1938) used a wrinkle finish paint on these parts, with a black lacquer for most of the others. The advantage of such paint is that it hides any minor blemishes in the finish of the underlying metal, it is not distractingly glossy and, having been baked on, it is reasonably tough. However, it had not withstood immersion in sea water very well. Happily, modern paint technology allows us to get the same finish without baking. I use PJH brand “VHT Wrinkle Plus” spray paint and find it gives a very good result. The wrinkles can be made tighter by heating with a hot air gun as the paint begins to dry and before the wrinkles have started to form. The next picture shows the finish I obtained, and it is quite close to the original (I have other Hughes sextants with which to compare it).

Figure 2 : Wrinkle finish paint.

Although the wrinkle paint appears to dry quickly, it is very easy to damage it, and, as a full cure takes five days, I moved on to some parts that I knew would take care and time to restore: the shades assemblies. I started with the easier horizon shades. These are mounted on to a bracket by means of a shouldered screw and locknut and separated from each other by washers. The washers have integral keys that engage with a keyway on the screw so that they cannot rotate. This ensures that when one shade is rotated, the others do not follow. Some resistance is applied by means of a Belleville washer, a dome-shaped washer that behaves as a short, stiff spring. You will see from the next photograph that the Belleville washer had not survived.

Figure 3 : Unrestored horizon shades

The index shades assembly uses a much older system. The shades and their separating washers together form a tapered hole. A tapered pin passes through the mounting bracket and  through the shades and washers, which must therefore be assembled the correct way around and in the correct order. An adjusting screw forces the pin further into the composite hole to adjust resistance to rotation. The tapered pin is prevented from rotating in the bracket by means of a cross pin. Any attempts to remove the tapered pin without first removing this cross pin are doomed to failure, so look for it carefully and punch it out. There are other ways to prevent rotation of the pin and I cover these in my book. Once the cross pin has been removed, brute force in the shape of a wooden drift and press or hammer may be needed to force the tapered pin free. The next photo shows the result.

Figure 4 : Unrestored index shades

If you cannot obtain a replacement Belleville washer, and living where I do, I cannot obtain most things, one can be made. I turned a washer from a scrap of phosphor-bronze bar, sat a scrap ball bearing in the hole, placed the two over a larger hole in a steel block and hit the ball with a hammer. It may sound crude, but it works.

As most of the paint had deteriorated to a mixture of paint and verdigris, I completed the destruction with wet  emery paper prior to repainting. In the past, I have done the painting by hand, but as spray painting gives a much better appearance, I worked out a simple way to mask the glass, one that does not attempt the near-impossible of punching out large discs of masking tape. The next photo shows the steps. The knife has to be sharp and have a fine point to be able to follow the curves.

Figure 5 : Masking shades.

If the shades are then suspended from a bar that just fits through the mounting holes, they can be sprayed with black lacquer without too much of it getting into the holes.

There are some tricks to remounting the index shades. It is best to have a trial mounting on the pin, to make sure that you have the shades and washers in the correct order and the right way around. The cross pin is not usually exactly on a diameter and will usually fit only one way round, so make some sort of mark on the head of the tapered pin and bracket to indicate the correct orientation. Then lay the parts in order  on the bench while you grease the tapered pin and place a tiny dab of grease on each face of the washers. You can then reassemble them on the pin and remove the pin without everything dropping apart. With the shades and washers on the pin, wangle them between the cheeks of the bracket (the right way around, of course). Once they are partially engaged with the bracket, you can withdraw the pin and carefully engage them further until the narrow end of the pin can be re-entered through the bracket, waggling the shades a little so that the pin passes deeper and deeper into the composite tapered hole. Make sure the cross hole lines up correctly and then press the head of the tapered pin home. Insert the cross pin, adjust the adjusting screw and you are home.

Much of the rest of restoring this sextant consisted simply in cleaning and repaint parts, so for much of the rest of this post I will illustrate, usually without commentary, parts that differ in Husun sextants from other makes. A lot of the ground is traversed in my book, together with details of other manufacturers’ approaches.

For example, the telescope mounting:

Figure 6 : Telescope in situ

Figure 7 : Detail of telescope mounting.

The micrometer mechanism:

Figure 8 : Micrometer mechanism and rack.

The battery switch:

Figure 9a : Switch button.

Figure 9b : Switch exploded.

The wire passes through an internal passage drilled in the handle and leaves it via a plastic grommet on the left-hand side of the handle. Thence it passes to the central contact of the lamp bulb. The plunger of the switch makes contact with a brass disc which forms the top of the battery compartment. Both sides of the disc need to be clean for the system to work. Earth return is from the lamp mounting to the frame. The spring in the bottom of the battery compartment is attached to the  lid of the compartment. The lid is connected through its hinge to the bottom handle mounting and thence to the frame. A slender spring-latch, not electrically connected to anything, holds the lid closed.

Figure 10 : Battery compartment lid.

The next photo shows the connections at the lamp. I have only partly followed Hughes’ design. Mine is simpler, as it omits an insulating bush which is not really needed, as the wire itself is well insulated.

Figure 11 : Lamp mount exploded.

But first I had to make a new lamp-holder, shade and mounting. A hollow pillar is attached to the index arm expansion by means of a 4 BA countersunk screw. The other end of the pillar is drilled to accept a spigot on one end of the mounting arm and split so that it grips the spigot. The lamp shade is mounted at the other end of the arm. The shade is threaded to take a standard torch bulb at one end and, at the other, the threaded cap through which the wire passes. To change the bulb, the shade and its mounting arm is simply withdrawn from the pillar to allow access to the underside of the shade.

Making the pillar is a fairly simple turning , drilling and tapping exercise:

Figure 12 : Turning lamp pillar.

So is the outside of the shade:

Figure 13 : Turning outside of shade.

Boring the inside of the shade is a little more difficult. As for  all boring operations, it is harder to see and to measure what you are doing:

Figure 14 : Boring inside of shade

Do not expect to be able to buy a tap to cut the internal thread, at least, not for any reasonable price. It is 14 threads per inch x about 0.38 inches major diameter, with a rounded thread form. Most people will attempt to cut it on a lathe, which is how I did it. I sometimes help out new friends who extravagantly praise my work.

Figure 15 : Internal thread of lamp shade

I have covered making things like cutting out connecting arms in  previous posts. Here is a reminder:

Figure 16 :Tools for cutting out arm.

The completed lamp mounting, together with wire and new retaining clips shows up well in this picture of the completed sextant:

Figure 17 : Sextant restoration complete.

After its near-death experience, I felt that the sextant should have a decent home, and so I gave the box  some attention. The base had a very large shrinkage crack in it, so I removed it, whereupon the rest of the bottom half of the box began to fall apart and had to be re-glued. I planed the edges of the crack straight and square, and glued in a strip of mahogany, which can just be seen in the final photograph. The front had also shrunk a little so that the lock no longer fitted, and this required some attention with a wood chisel. The lock had at some stage been forced, leaving a large chip and the ends of no fewer than three broken screws in one hole, in the rebate in the lid where the hasp had been. I made a fresh start at the damaged area and glued in a new block of wood before re-fitting the hasp.

Like many households, we have a large bunch of keys that fit nothing in particular, but I found one that nearly fitted the keyhole, so after washing out the lock and oiling it, I filed away at the key  with the aid of marking blue to guide me, until it operated the lock. The various mounting blocks had to be screwed and glued into place and the felts of course had to be replaced. Aftere cleaning of the brass “furniture” and making a new adjusting pick, here is the final result:

Figure 18 : Restored case.

If you would like information or advice about some detail that I have not fully covered, contact me and I will try to help.