The Nautical Sextant

In my post of 18 December 2010 I described the restoration of a Plath micrometer sextant and in Figure 8 of that post I showed how the friction of the shades mountings was adjusted. The adjusting nut needs a special tool to adjust it and recently someone asked me where such a tool could be obtained. As it does not appear in any tools catalogue that I have seen and the C Plath company that made the sextant no longer makes them, the only solution seems to be to make a tool or to have one made.

As the drawing in Figure 1 suggests, the adjusting nut is a pin nut, that is to say it is rotated by a tool that has projecting pins to engage in the holes of the nut. Usually, there are two holes on a diameter, but for some reason, Plath decided to have three holes in their nuts. The drawing shows the pins to be on a pitch circle diameter of 6 mm, but as will be seen, it is much simpler to use one of the nuts as a jig for drilling the holes into which the pins will be fitted.

Figure 1: Drawing of tool.

But first you will need to remove the nut, without having the tool to do so, a Catch 22.  I abused a pair of small spring-bow compasses, inserting the points into only two of the holes and tried to rotate the nut around its centre. It worked, and the dividers survived. If you encounter too much resistance, apply some releasing compound and leave overnight before trying again.

Start by turning a spigot on the end of a piece of 8 mm round bar stock (Figure 2) that closely fits into the 5 mm hole down the middle of the nut. Then mount the nut on the spigot in order to drill the 0.6 mm holes right through the nut and 3 mm into the bar (Figure 3). Drills of this size are both delicate and expensive, so it pays to make sure that the bar is vertical and that the drill enters the hole truly. Anything over 3 or 4 drill diameters counts as a deep hole, small drills have to be run very fast,  and they are particularly liable to jam on the swarf they produce and break. Apart from the expense, it may be impossible to extract the broken fragment, so take things slowly, withdrawing the drill frequently to clear the swarf. Once one hole has been deepened into the bar, insert a short length of 0.6mm piano wire into the hole to anchor the location of the nut for the other two holes.

Figure 2: Turn spigot to fit nut.

Figure 3: Nut used as jig to drill holes.

When the holes have been completed, cross drill the bar (Figure 4) and return the bar to the lathe to turn away the spigot and drill a 5 mm clearance hole (Figure 5).

Figure 4: Cross drilled for tommy bar.

Figure 5: Clearance hole drilled.

Then the bar can be knurled (Figure 6) , parted off, (Figure 7) and the end cleaned up to remove any sharp edges.

Figure 6: Knurling.

Figure 7: Parting off

You then glue three  5 mm lengths of 0.6 mm piano wire into the holes using a smear of industrial adhesive such as Locktite, though superglue would do as well. When piano wire has been cut with side cutters, sharp points are left, so these should be removed by holding the ends of the wires very carefully against a fine grindstone, or by using a diamond file or an old oilstone. Figure 8 shows the completed tool before the ends of the wires have been cleaned up and Figure 9 shows the tool in use.

Figure 8: The completed tool.

Figure 9: The tool in use.

If you are planning to make these tools for sale, it would pay to use a nut to make a hardened steel jig nut, but it would scarcely be worth the trouble, as the true cost of making the tool would probably be more than people are prepared to pay.