Restoring a C. Plath Drei Kreis sextant

24 01 2010

The preceding posts covers “A C19 Sextant Restoration” and “Making a Keystone Sextant Case”

On Christmas Eve of 1974, The small city of Darwin in Northern Australia was struck by Cyclone Tracy, a very compact and intense cyclone with winds gusting to well over 220 kph (140 mph). Eighty percent of the houses in Darwin were totally destroyed. One of the casualties of the destruction was a sextant that in the aftermath found its way into a trunk with several other sextants and theodolites. The trunk was sold at auction for $20, the sextant was passed to a neighbour, a retired merchant sea captain, and he in turn recently passed it on to me. Such generosity must be very rare.

The sextant had escaped major damage. Though some screws were bent or missing, they could easily be repaired or replaced. Mild corrrosion with verdigris was  present, but the frame and index arm had been carefully cleaned without polishing the arc, which was in good condition. Fortunately, in the bottom of the trunk were a sighting tube and  two telescopes. The inverting ‘scope was missing  the objective lens, but my friend in Darwin went back to his neighbour’s trunk and found it in a corner. It was intact, but the balsam between the two parts of the achromatic lens had perished and the lens was unusable in that state. Apparently, it was this that had saved it from being converted into an air-rifle sight.

The next picture shows the sextant as I received it. The bottom screw that attaches the handle was missing and the clamp screw was bent. The lacquer on the telescope tubes had deteriorated to a powdery and patchy brown and the draw tube of the inverting telescope had seized completely. The clamp spring was absent and one of the dowel pins that held it in place was damaged. One of the legs had been replaced by an odd leg. The ground glass diffuser screen above the vernier scale had disappeared. There was, of course, no case, so I felt very lucky to have a complete kit of telescopes and sighting tube.

The famous name “C Plath, Hamburg” was clearly visible on the middle of the limb, and after a little cleaning of the bronze at the right hand end I found  the original tiny Plath logo of a little stick man with a sextant. The “S” is the inspection mark of “Seewarte” or naval observatory. Later, this became “D.S.” for “Deutsche Seewarte” or German Naval Observatory, some time after the formation of the German state in 1871.

Less easy to see was the serial number 5499 on the silver of the arc. The arc is divided from  – 5 to +157 degrees, but the scale is usable up to only 138 degrees 30 minutes,  so that although it was sold as the Drei Kreis (“three circle”) quintant, it falls a few degrees short of 144. The serial number places it a little before 1910 in date, on the basis that Plath made an average of about 320 sextants a year between 1910 and 1925.  The arc of about 175 mm radius is divided to 10 minutes, with the vernier allowing reading to 10 seconds, in principle if not perhaps in practice. Apart from the rather complex tangent screw mechanism, the construction of the sextant is perfectly conventional.  The next photo shows the rear view of the sextant as received.

The next step was to dismantle the instrument to its component parts, clean them in a solution of 50 percent household ammonia with washing up liquid added, rinse, dry and apply masking tape as in the next photo, prior to spraying them with black lacquer.  Although antique sextants are often found with all paint removed and the “brass”  (in fact, bronze) polished to a shine, they were never sold this way. Most often they were painted or lacquered black, or, with earlier ones, chemically browned or blackened. If the silver scale is unreadable owing to tarnish, it usually yields to the diluted ammonia and gentle rubbing with the pad of a thumb. On no account should it be polished, as the markings are very shallow and can be obliterated completely by vigorous polishing. I have one 1920s sextant in which only the numerals remain and am currently restoring one in which the markings over about 20 degrees of the scale are very faint.

Here are the parts looking distinctly refreshed.

I had to make a new leg and this slightly un-sharp photo shows the turning in progress to make the rather slender, tapered leg. Once parted off, a thread then had to be cut on the wider end. Fortunately, all the threads are metric ones.

Although the threads are standard, some of the screw heads are not, so I had to make a new screw to attach the handle. This photo shows it being parted off from the parent 12 mm brass bar.

The clamp screw was bent, but I was able to persuade it back to straightness. The tangent screw mechanism is rather complicated when compared to the standard nineteenth and early twentieth century arrangement, and I hope the next two photographs will help to make its workings clearer. It came into my hands just too late to be included in the print edition of my book, now in preparation.

The clamp is a thin slab of brass that is retained in the sliding block by two dowel pins that allow it some up and down movement, opposed by the leaf spring (ground to shape from a piece of clock spring and soft soldered to the clamp). The clamp screw locks it and the sliding block to the lower edge of the limb. The sliding block is hollow and contains a strong helical spring and its guide, the latter projecting from one end of the block. The other end is split and tapped for the tangent screw. A tongue, which is attached to the underside of the index arm expansion by a screw, is sandwiched between the tip of the tangent screw and a bronze block on the end of the guide. Two screws can be tightened to close the split in the sliding block a little, to ensure a snug fit of the tangent screw.  The sliding block is guided by two curved edges machined in the index arm expansion and is prevented from lifting away from these edges and the underlying surface by the sliding block spring, itself attached to an elbow bracket.

When the clamp screw is tightened, the sliding block is locked to the limb and when the tangent screw is tightened, it moves the tongue and the index arm to which it is attached. When it is slackened, the helical spring moves the index arm in the opposite direction. The two screws that are intended to remove backlash (lost motion) from the tangent screw are unnecessary because the opposition from the helical spring does this. They were a nuisance as far as I was concerned because they both lost their heads when I was trying to remove them and needed about half an hour’s work to drill them out and re-tap forslightly larger screws, which I  had to make from scratch. When a spring-opposed mechanism like this works, it is a pleasure to use, but dirt or a breakdown in lubrication can easily cause it to fail to return in one direction, and this is very annoying.

The rise and fall mechanism for the telescope is conventional:

Having overcome these various obstacles, it was time to reassemble the instrument and adjust the mirrors to ensure they were square to the plane of the arc and, when the scale reads zero, that they were parallel to each other. The paintwork is not quite as shiny as this flash photograph seems to suggest.

This still left the inverting telescope, with its seized draw tube and damaged objective lens. I tried various combinations of spirituous chemicals, releasing compound, heat and cold, but could not persuade the rather thin-walled tubes to release their grip on each other. Vigorous twisting was of course out of the question. In the end, I removed all the optics, pressed out the various light stops and machined a steel slug that fitted snuggly in the wider tube. I then used my larger lathe as a press jack to push out the narrower tube. In the next photograph, the tube is held loosely in the chuck against the shoulder and the steel rod held in the drill chuck is being pressed against the slug, which is out of sight inside the tube. It worked like magic.

The objective lens was even more of a problem, as it was swaged into a little brass cell. Swaging means that a lip of metal is bent over the lens to hold it in a recess in the mounting, and often the only way to get the lens out is to somehow hold the mounting in the lathe and carefully turn away the swaged metal. I managed to do this with only minimal damage to the edge of the lens, and still had enough of the mounting left to make replacement possible. Replacement lenses of the correct focal length and diameter are very hard to come by, so this was a relief.  Once the lens was out, I heated it slowly in a pan of water and, just short of boiling point, was able to slip the two components apart. A little xylene, in which Canada balsam is soluble, cleaned up the two part-lenses and I recemented them with Ultra Clear Araldite, rather than the now-obsolete Canada balsam or one of its very expensive modern replacements. I also used a tiny smear of the same adhesive to remount the lens in its cell. The telescope now gives a very clear view. The next photo shows the rear view of the sextant with restored telescopes, new adjusting pick and a tiny screw driver made from scraps of silver steel, brass and mahogany. The brush is C19 English, but does not seem out of place with this German instrument.

Some woodwork came next. Although I am slow at making dovetails, I can now do so without disaster. The sextant was supplied originally in a box with machine-made comb corner joints, but dovetails with narrow pins are also in period. These can just be seen in the next photo, which shows the interior fittings too. The wood is Sapele (Entandrophragma cylindricum), an African hardwood that closely resembles mahogany.

The photo following shows the instrument and its accessories in place:

The final photograph shows the exterior of the case. I don’t know what contemporary latches and handles would have looked like, so have copied English hook latches of the same period and made a handle that I hope looks slightly Germanic. It is made of five pieces of 6 mm  brass rod silver soldered into a whole. I used French polish for the final finish. I can recommend Zinsser Bulls Eye French polish. It is easy and quick to apply, and an amateur like me can get a good finish with it.

Do let me know if you would like more details of this or any other sextant I have covered on this blog.

If you have enjoyed this post, you might enjoy my book, now in print and available from the publishers or through amazon.com

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5 responses

24 01 2010
Jean-Philippe Planas

A very thrilling story, full of technical information (including pictures of machine tools) as I like them. I am just amazed at the speed at which you seem to work without taking liberties with quality or craftmanship.
You received the sextant less than a week ago and it is ready to go back to sea in a new freshly varnished mahogany box.
You are very modest about your woodwork skills. From the pictures I can judge that I am not sure a professionnal could do so well.
Something else I appreciate is the way you care for details such as the brush, the dedicated screwdriver, the adjusting pick…
BTW how were you sure the Cristal Clear Araldite would not spoil the optical path between the two glass lens?

Just a final word; congratulation for this outstanding job and article (just a shame it comes too late to be inserted in your book)
JPP

24 01 2010
engineernz

Thank you JPP. The refractive index of the Araldite is quoted as about 1.553, much the same as glass. I used it successfully before I knew this.

Bill

2 06 2010
Fabio Barbieri

With a light touch, Bill puts together a story with a rich technical vocabulary that also teaches.
His prose and the pictures help inexperienced people like me to improve their knowledge (and English). He could be a very good teacher. Fabio

20 03 2011
Brent

Fantastic information and a beautiful restoration. I am the proud owner of a very similar Plath, which I have been trying to date for years. As far as I can tell, it’s serial number is 123 (the only number on it). Think you might be able to help identify this sextant if I forwarded some pictures?

Brent

20 03 2011
engineernz

This vernier sextant has a three circle frame and has a rack cut on the back of the limb with a pinion and quick release mechanism for moving the index arm along the arc. Its serial number suggests that it may be a very early C Plath instrument. Number 2341 was inspected by Deutch Seewarte in 1877. The “Sunseeker” logo was registered as a trade mark in 1905, but may well have been in use before then.

I hope Brent will send me some more photos so I can add them to my post on the “Evolution of the sextant micrometer”.

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