The Nautical Sextant

The preceding posts cover : “C Plath sextant lives again”; “C Plath Micrometer Sextant”; “A Damaged Rising Piece”, “SNO-T Mirror Bracket Repair”,  “A Worm Turns”, “The case of the broken screw”, and “Worm with wrong thread angle?”

A new friend recently sent me a battery handle from a C Plath sextant to reconstruct. The sextant had been stored under a sink which had had  a long-term leak and when the leak was eventually discovered, the lady of the house banished the sextant, in its case,  to the garage. As she had no key to the case, there the sextant languished for twenty years until my friend rescued it. The index mirror frame had corroded to a few scraps of aluminium alloy and the horizon mirror frame was nearly as bad. Corrosion had also affected the battery handle, which, fortunately, had no batteries in it, but the screw cap at the bottom had seized and both switch buttons lay free. He could press them in, but there was nothing to stop them dropping out. Some makers, and C Plath was one of the worst culprits, made their battery handles in such a way that it was impossible to deconstruct them if anything went wrong. In this case, corrosion had done the deconstruction, leaving behind a mystery as to how the parts should be re-assembled. Figure 1 shows the exterior of the handle. There are two switch buttons which operate independently to supply current to two sockets on top of the handle (Figure 2). A plug from one goes to the scale lighting system while the other was presumably to supply current to an artificial horizon attachment.

Figure 1: General arrangement of handle

Figure 2: Lighting sockets.

At the bottom of the handle is a brass bush or socket that is threaded for the battery cap. It is held in place by the threaded end of the lower pillar, which passess into a threaded cross hole and which also serves to conduct current from the negative pole of the battery to the frame of the instrument. A plastic liner reduces the interior diameter to suit two AA cells (Figure 3). The positive contact of the upper cell is held against the head of a large brass screw at the upper end of the interior of the  handle.

Figure 3: Lower end of handle deconstructed.

Figure 4 is a cross section drawing through the switch buttons.  The reduced diameter of the switch button passes through a phosphor bronze spring and then through a cross hole in the lower end of the socket. This serves both to capture the socket in place and to make electrical contact with it. A C-ring the other side of the socket sits in a groove and captures the button. When the button is pressed, the tapered end of the button makes contact with the large screw at the upper end of the interior of the handle. No doubt someone was congratulated for the elegance of this design, but of course, once the C-rings are in place the buttons and sockets cannot be withdrawn, as there is no means of accessing the C-rings to remove them.

Corrosion had done the job of removing the rings for me so my problem was how to  put everything together again. I began by removing the large screw to give the buttons some more  freedom of movement Having found a couple of small rings I did a trial fitting outside the handle and then greased a ring and sat it at the bottom of the hole, where the diameter of the hole reduces. Taking care not to forget the spring (which would have been disastrous) I then passed the reduced diameter of the button through the cross hole in the socket and entered the tapered end into the hole in the C-ring. A squeeze in a vice forced the ring up the taper and into the groove, where it will remain for ever. The other one received similar treatment and then the large screw contact was replaced.

At the lower end of the handle the problem was how to unseize the screw cap without causing unsightly damage to it by, for example, using a pipe wrench on it. One approach is to bore a hole in a scrap of wood to fit the cover. Then a radial split is sawn into the hole, the cover inserted into the hole and the wood squeezed in a vice to grip it. My solution was to make a bush to fit the cap from a scrap of thick-walled  aluminium tubing which I then split and squeezed in the jaws of a lathe chuck. I had no compunction about holding the brass socket with a pipe wrench to force the screw cap to yield and once it had yielded it was the work of moments to wire brush the corrosion away and remove any marks made by the jaws of the wrench with a smooth file.

The electrons now flow satisfactorily from the negative of the battery to the spring inside the cap and thence to the brass socket and via the lower pillar of the handle to the frame of the sextant.  From there they make their way through the lamp and the return wire to the plug which is inserted into one of the sockets. When the correspondng button is pressed, its end contacts the large screw and the current of electrons flows through the socket and the button return spring to the button and thence to the large screw, completing the circuit by entering the positive pole of the battery.