A Byrd Sextant Restored

30 05 2009

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” and “AN5851-1 : jammed shades carrousel”

I recently acquired  a Brandis nautical vernier sextant without case, telescope, or any shades. It appeared to have an extra mirror in front of the horizon mirror and I recognised it as an early bubble sextant of the type used by the then Commander Richard Byrd on his claimed flight to the North Pole in 1926. There are several magazine photographs extant that show Byrd in posed pictures, using a similar sextant, this one, for example:


K Hilding Beij, writing in the Bureau of Standards Report 198, Astronomical Methods in Aerial Navigation in about 1926, refers to the sextant as a “Byrd sextant”, though Luis de Florez, a prolific  inventor, claimed priority. He had filed for a patent for exactly this type of bubble sextant in March 1919 and he was granted US Patent number 1,536,286 in May 1925. My sextant looked like this when I received it:


The one Byrd is using is a full-size Brandis vernier quintant with an arc of 180 mm radius reading to 30 seconds, whereas my example is unusually small for a vernier quintant, having an arc radius of only 140 mm, also reading to 30 seconds, so my hopes of possessing an historic instrument were disappointed (but see postscript). Nevertheless, it is a rare and early instrument dating from about 1920 and I felt it was worthwhile  to restore it to working order. As I have no access to original instruments, I did not set out to make exact copies of the attachments that make a nautical quintant into an aeronautical one, but I did follow the same principles, while retaining all the original parts. Needless to say, I dismantled it completely to begin with, and cleaned all the individual parts. A photograph of the restored instrument will perhaps best help to explain its workings.


The optical path for the heavenly body is as usual, via the index mirror and silvered half of the horizon mirror. An ordinary spirit level vial is held in a carrier and viewed via an auxiliary mirror set at 45 degrees above it, through the plain half of the horizon mirror. The image of the bubble would be out of focus viewed directly through the x 2 Galilean telescope and so an extra, semicircular, lens is interposed in the light path to bring it into sharp focus. The auxiliary mirror may be swung downwards to allow direct view of the natural horizon by pressing a spring-loaded catch.

The sensitivity of the vial has to be carefully chosen. If too sensitive, it is never at rest when the instrument is held in the hand and if not sensitive enough it is not possible to get meaningful results. I settled for one where the bubble moves 2 mm for 6 minutes change in level. This is of the same order of sensitivity as most other bubble sextants.  The bubble is illuminated from one end and, to try to get even illumination, the vial is painted white over most of its surface, including the end distant from the lamp. This has parallels in the lighting of some circular bubble cells, where an attempt is often made to conduct the light around the periphery of the cell with some sort of light guide. The next photograph shows a view of the lighted vial through the telescope (the view is somewhat more extensive than this in reality).


In sextants with circular bubble cells, one usually aims either to centre the body in the bubble or to align it with its equator, but this cannot be done with a linear cell, so a more-or-less central datum line is used and the sextant adjusted so that when the bubble is centred, the datum line lies on the horizontal. As is usual for a Galilean telescope, each half of the objective lens “sees”  its own half of the field of view (compare this with the inverting or astronomical telescope, where obscuring half the objective simply cuts out half the total light). Thus, the auxiliary, semicircular lens in the telescope attachment sees the left half-field and  is used to bring the bubble into focus by sliding back or forth.

The heavenly body is seen on the other half-field and, as is usual, there is a narrow band of overlap where both the sky and the horizon or bubble may be seen together. I have not made any significant trials as yet, but it is relatively easy on dry land to align a star, the moon or the sun with the datum line while trying to keep the bubble centred. However, the instrument gives no indication of lateral tilt and in an aircraft the results must have been very uncertain.

The scale lighting is particularly good. A shaded lamp shines via a standard diffusing screen on to the scales, which are viewed through a simple magnifier. Unlike many such systems it gives a very even illumination so that the main and vernier scales are seen with equal contrast, making reading relatively easy and rapid.


The handle seems to be a modified early Brandis battery handle. Power is now from an external source. Scale lighting is via the original press switch, while the bubble unit is switched by a rather crude rotary switch on the front of the handle, seen in the general arrangement photo above. The following photo shows the wiring layout, pretty well as found except for the decayed silk covered wire.

Byrd handle

I have coated the parts that I have made – telescope and attachment, and vial carrier – with modern paint. The original paintwork on the rest of the instrument is careworn and I cannot help but feel that it would look better for a fresh coat of paint. I would repaint a more modern instrument in the same condition and wonder how readers might feel about that. Is it sufficiently “historic” to preserve it as found? Would repainting it devalue it in some way.”? After all, noone is likely to mistake it for a modern fake, repainted or not.

The index shades, by the way, I borrowed from another Brandis sextant, so that if anyone has a spare set of Brandis or US Navy Mark II index shades that I can beg or buy, I should be glad to hear from them.

Post script, 11 September 2015.

When I wrote this post, I thought Byrd was holding a larger sextant than mine, but an enquiry led me to look more closely at the structure of the sextant he is shown holding and I now believe that he is holding the same type of sextant that I describe, viz.a survey sextant of 140 mm radius.

The Case of the Broken Screw

24 05 2009

Many years ago, when I was a student, an emminent, but possibly not-very-gifted surgeon said to me, once he had stopped sweating, “Any fool can get into trouble. It’s getting out of it that counts.” A fellow enthusiast, wishing to do a complete overhaul on his vintage Hezzanith sextant, broke a vital screw when it was only part of the way out. He couldn’t get the rest of it out, nor could he screw it back in, and in this condition, the micrometer of the sextant was useless. He sent me the index arm with attached micrometer mechanism to me from England, together with an A10-A bubble sextant frame with a seized index prism shaft, but that’s another story.

Heath and Co.’s sextants with their Hezzanith Endless Rapid Reader Automatic Clamp differ in several ways from most other micrometer sextants: the rack is cut on the back of the limb rather than on the edge; the worm is swung out of the plane of the limb when the release catch is operated, rather than in the plane of the limb; and the worm shaft runs between two  screws, the 60 degree coned ends of which rest in the same centre holes on which the shaft rested when originally being turned. The screws have to be locked and a locknut is a simple way of doing this, though care has to be taken to check that the setting of the screw is not disturbed by tightening the nut. The chassis on which the worm shaft bearings are mounted, which I have chosen to call the swing arm since noone else seems to have given it a name at all, is also mounted between centres.

Mounting a shaft between centres is a very good way of maintaining concentricity, the centres can be adjusted so that for practical purposes there is no unwanted play (looseness) in the bearings, and they are very cheap to produce. The centres cannot sustain a heavy load at high speed for any length of time, as any turner will bear witness, but in a sextant, parts are always lightly loaded and slow moving. Running centres are hardened. The following picture shows the layout I have just explained in a rear view of the index arm expansion.


The hardening was the undoing of my friend’s sextant, since hardened parts have a tendency to crack, especially if they have sharp internal corners, as do screw threads. There are various ways of dealing with broken screws. If the other end is accessible and can be gripped in some way, it may be possible to back it out, but in this case, only the 60 degree point was projecting. If a hole can be drilled in the broken end, an easyout may be used. This is a tapered hardened tool with a coarse left hand thread that jams in the hole as it is screwed in, left-handed, and with luck then unscrews the offending broken screw; but in this case the screw was only 5/32 inch (3.96 mm) in diameter and anyway, it was hardened. For this reason too, it was not possible to try option 3, which is drill out at tapping size, leaving only the thread to remove with a tap. In desperation, one can trephine out the whole screw. A trephine is sort of tubular drill with cutting teeth on the business end. There are other methods not usually found in the home workshope, like spark erosion.

As I have no small carbide drills, I elected to try to soften the screw prior to drilling it out,  by heating it with a small blow lamp. As I did so, I noticed a bead of solder appearing, more or less at the same time that the local paint disappeared and it dawned on me that the screw was running in a brass bush that had been soldered into the bronze swing arm frame. If I could remove the bush, I could replace it with a new one and choose any size of thread that was close to the original for the cone-ended screw.

I had no idea about the exact form of the bush until I had removed it, so thought it best to remove it undamaged. The bush resisted my initial blandishments so I made up a gripping tool that would spare my blistered finger tips. This sort of tool, seen below, is very good for gripping delicate objects that have to be unscrewed without damaging them, like telescope lenses and threaded parts. It can be made of fine-grained hard wood or soft metal and, as can be seen, for a tool that would be used only once, I wasted no time on precision and finish apart from getting the size of the hole correct.

Bush repair

Back to the blowlamp and within minutes the bush was free. It then took only about twenty minutes to make a new one, identical to the old, except that the central hole was tapped 5/32 x 40 tpi instead of the original 48 tpi. It was the work of moments to solder it into the frame. A few more minutes saw the production of the cone-ended screw. The original had been hardened all through, the source of all the trouble. I hardened only the end and left the rest soft.

I do not, by the way, undertake sextant repairs for gain, but people occasionally send me calls for help and post me bits of their sextants. I do my best to help them, especially if they have bought copies of my restoration manuals or my e-book, The Naked Nautical Sextant.

AN 5851-1. Jammed shades carrousel

14 05 2009

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” and “Overhaul of MkV/An5851 bubble chamber”

I have been asked about the carrousel that holds the various shade glasses in the AN5851 bubble sextant. This is one of my least favourite bubble sextants, heavy (2.5 kg), awkward to hold and the optical path complicated by the inclusion of a horizon prism. The near indestructability of the case was bought at the expense of more weight (6.4 kg!).

My correspondent reported that his carrousel was jammed and wondered whether its associated spring could be the cause. Here’s how you get at it.

1) Remove 4 screws (circled in white, below) from the black plastic cover and remove it. Take the opportunity to clean the copper electrical contacts inside it, that carry current to the scales-illumination lamp.

Carrousel 0

2) Loosen, but do not remove, the two screws that hold the black shade cover over the carrousel. Then dislocate it upwards and pull it outwards.

Carrousel 1

Carrousel 2

 This reveals the shouldered screw that holds the carrousel on to a complicated little alloy casting that also mounts the astigmatiser. There is a brass spring washer between the head of the screw and the alloy carrousel body. Removing the black shade cover also loosens the leaf spring’s attachment, and in the unlikely event that a disordered spring is the cause of stiffness, this should allow the carrousel to revolve freely.

Carrousel 3

3) Corrosion between the central screw and the carrousel is much the most likely cause of stiffness. It is not easy to get at the screw slot without removing the index prism (best left alone) or improvising a bent screw driver from a piece of steel strip or grasping the head of the screw in a narrow pair of pliers. However, if you swing the astigmatiser out of the way, you will be able to get at the two screws that hold the casting into which it is inserted, to the frame of the sextant. If there is corrosion, and you need to use a lot of force, you may break off the screw where it enters the casting or damage the casting. It is much safer to apply a little thin oil between the screw head and the carrousel. Allow a few hours for it to penetrate and then gently work the carrousel free. Be prepared patiently to repeat the process several times. Remove the central screw only if it loosens easily.

Replacement is the reverse of dis-assembly.

Sounding sextants 2

6 05 2009


The Survey Sextant Mark III, by Kelvin and Hughes, was designed specifically as a sounding sextant, rather than being an ordinary sextant without index or horizon shades. It has a cast aluminium alloy frame with a stainless steel worm running in the rack, which is one with the frame. The limb is divided to 140 degrees. The micrometer reads to single minutes. There are no shades nor is there anywhere to put them.

In the preceding post about sounding sextants, I pointed out that most of them were not provided with handles appropriate to their normal use with the frame horizontal, nor with legs placed so the instrument could easily be set down without changing hands. This sextant has a solid mahogany wooden handle attached only at the top by four very substantial screws, but it is still better adapted to vertical rather than horizontal use. The legs, however, can be unscrewed from the back and replaced in tapped holes in the front of the frame, circled in white in the photo above, so the instrument can easily be put down.


The Galilean telecope has a large objective lens of 45 mm diameter to give a large field of view, and a magnification of   x3.  It is very solidly attached to the frame via a spigot and three screws, and there is no rising piece nor provision for adjusting collimation. At the level of precision required of this class of sextants, the telescope mounting would have to have been very badly made indeed to have been significantly out of collimation, and Henry Hughes and Sons always produced instruments with fine workmanship.

The large, circular, sealed index mirror, 55 mm in diameter,  has no provision for adjustment either, but machining surfaces that are truly at right angles to each other is one of the more elementary operations in the engineering workshop. The  circular “horizon” mirror, 35 mm in diameter, is also sealed and has provision for adjusting out side and index error, more details of which will be found in my book, The Nautical Sextant.

Hughes and Son were granted a patent in 1928 (UK Patent 309,648) for a method of engaging and disengaging the worm from the rack, by swinging the worm out of the plane of the rack. They do not seem to have used it prior to WW II, but did make use of it in this sextant and in their handy little double sextant.

Its light weight certainly makes it easy to hold, but it has an annoying feature in use when taking angles greater than about 80 degrees: the horizon mirror “sees” past the edge of the circular index mirror over one’s right shoulder, as well as providing the image reflected from the index mirror, so that there are three images to sort out. With practice, though, one soon learns the position and form of the three images and to ignore the over-the-shoulder one.

This instrument’s former home was in the Solomon Islands and may indeed have been used by one of my uncles when he was Marine Superintendent in Honiara in the 1950s. It was not in good condition when it reached me and it had no case. It seemed to me fitting to make the latter out of the tropical hardwood kwila, which is found in the Solomons, and I followed Hughes’s style in putting the handle on the side and having the hook latches (cut by hand from 2.5 mm brass) arranged so that gravity keeps them both in place. The whole instrument has been repainted. The graduations and labellings were then cleaned out and re-filled with white paint.


If you love detail or simply would like to know more about the structure of your sextant, you will find much more along these lines in The Nautical Sextant . Next in this series will be an account of a double sextant by Henry Hughes and son.

Worm with wrong thread angle?

5 05 2009

Just lately, I was  pleased with myself at having acquired and restored a rather corroded Heath and Co sextant. The last act was to have been to lubricate the rack, and a good way of distributing the oil to where is is needed is to wind the index arm the full length of the rack. On this occasion, as I did so, I noticed a slight periodic roughness each time the micrometer read around 55 minutes. Sometimes this happens when the worm shaft is slightly bent and the micrometer drum rubs against its index periodically, but in this instance, all was well in that area. I decided to have a very close look at the worm.

In Heath micrometer sextants, the worm shaft rotates between conical and adjustable centres that rest in conical holes in the ends of the shaft. This is a very good way to ensure concentricity of the worm, as the shaft rotates on the same centres that were used to produce it, and it is possible by careful adjustment to eliminate all backlash. To remove the worm shaft together with the micrometer drum, it is necessary to undo the locking nuts and back off both centres, when it is just possible to wiggle the shaft free. If necessary, the micrometer drum can then be removed from the shaft. The next pictures show the removal sequence.



I had already examined the worm using magnifying spectacles, but this time I used a stereo microscope. It was then possible to see fine burrs at one point on the crest of the threads and I became curious about how this had come about. I then noticed that the angle of the thread was unusually sharp and that the crests were not truncated as is usually the case.. It is perhaps just possible to see that the thread angle seems wrong in the next photograph. The lower, stainless steel worm is the original and the upper one is a bronze replacement.


In screws and nuts generally, it is important that the load is borne on the flanks of the threads and not on the crests. This is even more important in leading screws, and the worm and rack of a micrometer sextant is equivalent to a short screw rotating against a small part of the circumference of a very long nut. While the pitch of a thread is important, it is also important that the flank angles of the screw and nut are the same. The following sketch illustrates what happens when they are not.


In the upper half, the thread angles match and the threads are truncated to remove any possibility of their bearing on their crests. In the lower illustration, however, with mis-matched thread angles, the crest of the worm bears on the bottom of the rack and all wear and damage is likely to be concentrated on the crest of the worm.

At first, I thought that at some time in the past, the worm had been damaged. Someone had made a perfectly competent job of producing another one, except that he had made a wrong guess at the thread angle and had left the crest sharp, so that it bore on the bottom of the rack rather than on the flanks of the teeth. Without rather special measuring gear, it is not easy to measure the thread angle, but I made the assumption that in an English sextant made by a conservative maker around 1967, it was probably of Whitworth form, with an included angle of 55 degrees, but the rack angle seems to be about 60 degrees . I used some of my diminishing stock of drawn phosphor bronze bar to make a new shaft and worm with this angle, seen in the pictures above. The micrometer then moved the index arm with a pleasing smoothness and absence of periodic roughness. It will be interesting to re-calibrate the sextant and compare the results with those of the original certificate, which showed no more than 12 seconds of error.

A few days later, I was able to examine another Heath micrometer sextant and discovered that the thread angle of the worm was just as mine had been, unusually sharp. I conclude that Heath delibertely arranged for the crests of the worm to bear on the flanks of the rack teeth.

In the type of release catch used in this brand of sextant, some post-war Kelvin and Hughes sextants and the USSR SNO-T sextant, the worm is swung out of the plane of the rack to disengage it. The majority of sextants swing  the worm shaft and its bearings in the plane of the rack about a hinge on one end of the chassis (swing arm chassis) on which they are mounted. In the former type, it would be pointless to have a worm that was free of backlash unless the swing arm was also free of backlash. The final photograph shows Heath and Co.’s way of mounting it between centres to achieve this.


You can read much more about the fine detail of the nautical sextant in my book The Naked Nautical Sextant and its Intimate Anatomy

Postscript: Together with several other instruments, I calibrated the worm of this sextant on 5th July. You can see the results in the posting “A Worm Turns” (above).