C Plath probably set the fashion for round sextant horizon mirrors in the late 1930’s and the fashion, which is very logical, was copied by Tamaya in Japan, while British and American makers kept to rectangular mirrors. An exception was the Admiralty pattern sextants made by Hughes and Son pre-WWII and Kelvin and Hughes post-war for the Royal Navy. A round mirror intercepts a little more of the light from the index mirror but is rather more difficult to make and more expensive to replace. It is, however, easier to make in a sealed version in a watertight cell, which is what the Admiralty wanted
Amateur telescope makers have long known how to cut round sections of glass. Relatively thick mirror blanks 250 mm and more in diameter can be cut using only hand tools, grinding paste and a lot of exertion. Recently, I developed a method for sextant mirrors that cuts out the exertion, but does require access to a drill press or vertical milling machine. Though it could probably be adapted to the lathe, most lathe owners will also have at least a drill press. I make no claims for originality. The stimulus to develop the method arose when my friendly local glazier found his equipment could not cope with mirrors of only 56 mm diameter.
The glass used is standard 4mm thick mirror float glass. A piece about 200 x 300 mm costs about NZ$9. To cut out circular pieces, you need a sort of trephine, a tubular tool with a cutting edge on the end. The cutting edge is composed of grains of grinding powder that embed themselves in the softer material of the tool and slowly cut away the glass. A slurry of the powder in oil also widens the cut a little as the tool rotates, and prevents it from jamming in the cut. The better centred the tool is as it rotates, the less chance the workpiece, our mirror blank, has of oscillating from side to side and perhaps spoiling. Figure 1 illustrates the trephine that I made, but other improvisations are of course possible.
(Non-turners can skip this paragraph) It could have been turned from the solid, but as an amateur I dislike producing unnecessary swarf, so I fabricated it from three separate parts. I glued the spindle into the cross bar and turned the outside of the bar down to fit into a seat turned in the end of a piece of scrap, thick-walled tubing. At the same setting, I turned down the outside of the tube at the spindle end to form a fairly true seat, for holding the tube in the chuck for the relatively heavy turning required to form the diameters of the business end. When the business end was nearly to size, I fitted the collet chuck to the lathe, held the workpiece by the spindle and took the necessarily light cuts on the cutting end of the tool, to bring it to finished size.
This accomplished without disaster, the next step is to prepare the mirror blank, by cutting out a square that will provide the mirror with a generous margin of about 15 mm. If one simply grinds through the blank, there is a tendency for the edges to chip and splinter as the tool breaks through, so I glued a piece of ordinary glass to the underside using Araldite and ground through both. It is not of course necessary to go all the way through the sacrificial glass once it has done its job of supporting the edge of the mirror. Once the glue has set, a few pieces of masking tape provide some protection to the upper, silvered side of the mirror from stray particles of grinding grit. I have tried the classical glass-workers cement of 50 percent beeswax melted together with an equal quantity of rosin, but found that the heat generated by the grinding sometimes caused the cement to let go. Shellac flakes melted on the glass at about 180 degrees C in an oven are better, as the melting point of Araldite is very close to the temperature at which the mirror coating lets go of the glass. Note: I now use only flake shellac and heat the glasses on an upturned domestic electric iron.
The blank with its sacrificial underpiece of glass can now be attached to a piece of flat material such as medium density fibreboard (MDF) using a few nails hammered into it and bent over a little to grip the sandwich. If you are uncertain about how well-centred your tool is, leave the glass a little freedom to move from side to side. My tool is well centred, so I leave no freedom, and firmly attach the MDF to the table of my mill-drill as shown in Figure 2.
It is as well to protect the machine from stray particles of grit using sheets of newspaper in preference to rags, as the former tear and do not pull in hands if caught on the rotating machine spindle. Once set up, the tool is annointed with a thin paste of fine grinding grit in oil and the drill started. Most people will find it hard to buy grit on its own, but auto supply shops will have valve-grinding paste that can be thinned with light oil; and and 180 rpm seems about right for the speed. Update: fairly coarse (eg 30 micron) diamond paste is much quicker. The tool is fed downwards until it is heard to be grinding and then released, repeatedly raising and lowering, with the grit progressively working its way through the glass. If the soft swishing sound of fine grinding disappears, seizure may be imminent, so add more oil and, from time to time, more grit. It is hard to see through the edge of the glass when you have gone right through the mirror and a little further into the under-glass, so err on the side of going too far. It takes about half an hour to cut through 4 mm glass.
When you are certain that you have gone far enough, remove the blank from the machine and clean it thoroughly of grit particles using plenty of water and dish-washing liquid. The central mirror must then be separated from the rest. The melting point of Araldite is rather higher than a domestic oven, mine at least, can achieve (another reason to use shellac). You will have to place it on top of a layer of sand or brass turnings on a metal tray and heat the underside of the tray until you see the glue beginning to bubble, smoke and melt. Then slide the components apart and allow them to cool.
You can discard all except the circular mirror, which now needs to have residual Araldite removed. This is easily done by placing the mirror face down in a shallow dish of acetone which you cover and leave alone for half an hour, when it will be found easy to separate the glue from the glass with the aid of a safety razor blade. Update: If you use shellac, it chips off very easily when cold and can be cleaned up with alcohol. The edges can be bevelled with a few strokes of a wetted diamond lap held in the hand. I have explained how to remove paint and silvering from half of the mirror and how to paint the back in the previous post in this category. Figure 3 shows the mirror blank before grinding and the end result, which now resides in one of Mr C Plath’s sextants.