The Mariner's Chronometer

A deck watch was used to carry time from the chronometer or chronometers to the place where sextant observations were being made. Ideally, the chronometer would be kept in a place where the motion of the ship was least, but in most vessels of the 20th century, it was to be found under a transparent window beneath the chart table. Deck watches do not have chronometer escapements, which are liable to be upset by sudden, especially rotatory, movements, but they were usually high class “chronometer grade” watches which could be relied upon to keep a steady rate to within a handful of seconds, not as good as a chronometer, but good enough to transfer the time.

A few months ago, I acquired a Soviet era deck watch. Unfortunately,  the upper balance staff pivot was broken, but I was able to buy a new staff and, for a trifling NZ$100, a local watch maker replaced it for me (I have since acquired the courage to do this myself). He also offered to clean and oil it for another $2 -300, but I declined this, intending to do the work myself. When collecting it, I asked to see his workshop, where he and his apprentice son work and was interested to see that turning on his lathe depended on his skill with a graver. I gave silent thanks that I had been able to obtain a replacement staff, as to have it made from scratch would have been even more expensive.

After a week or two of running, during which I worked at ascertaining its long-term rate of going, it began to stop after about 20 hours, signifying that something was amiss, perhaps a piece of grit jamming a wheel at the low power end of the train or a problem with the driving force. At this point I suspected a problem with the spring or its barrel, but was later to wonder about the stop work. Overhauling the watch gave me an opportunity to illustrate some of its features.

The watch looks like a large, open faced pocket watch with a centre second hand and is contained in a three part wooden outer case (Figure 1). While this particular version is stem wound and set, earlier versions were had a safety setting pin, and the space for this in the case is visible to the left of the stem.

Figure 1: Deck watch in case.

The watch is named “Polyet”  (“Flight”) while earlier versions bore the logo of the First Moscow watch factory. On each side of the 6 is found in Russian “Made in the USSR” (Figure 2). The face is protected by a heavy, flat glass and the bezel and watch case itself is made of chromium plated brass.

Figure 3 shows the placard on the front of the outer case. The first line reads in Russian “First Moscow Watch Factory,” and the second line “Named S M Kirov.” There is a certain irony here, as there is a strong suspicion that Stalin had Kirov murdered in Leningrad in 1934, because of his popularity, and following his death a tractor factory, a military medical academy and other institutions were named in his memory, including, as we have seen, a watch factory. The third line reads “Deck Watch”, while the last line reads GOST, an abbreviation for “State Standard” 17156-71. The outer case is very closely modelled on the outer case of deck watches made by Ulysse Nardin in the 1930s and 40s and, as we will see, so is the movement.

Figure 3: Front of outer case

The movement is revealed by removing an outer snap off back and an inner dust cover and Figure 4 shows the movement with the major parts labelled. A Ulysse Nardin Deck watch from about 1940 is shown for comparison in Figure 5.  There is a close general similarity, the main differences being that in the Nardin watch the seconds hand, borne on the fourth wheel arbor, is at the more usual six o’clock position and there is a safety setting pin.

Figure 4: The movement.

Figure 5: Movement of Nardin deck watch.

Both watches have more than the usual complement of jewels because the balance, the pallets and the escape wheels are all provided with end stones and the train is jewelled to the centre wheel. The extra jewel in the soviet watch is accounted for by the central seconds pinion pivot. The Polyet cocks, plates and wheels are gold plated, with the edges of the cocks carefully  bevelled, with a beautiful Geneva wave finish to the cocks. This latter has no function other than to demonstrate the high quality of the time piece, while remaining hidden from all except the watch maker or repairer.The “1-77” indicates that it was made in the first quarter of 1977. The split, bimetallic balance wheel and the lever escapement are conventional as is the train, except in the region of the seconds wheel. Figure 6 shows the area with the cock removed.

Figure 6: Seconds wheel and pinion.

Some watches with centre seconds hands have the jewel for the third wheel pivot mounted in the upper train plate or cock, with a projection through the cock on which the seconds wheel is mounted with a press fit. In this watch, the cock is cut away and the jewel is mounted in a separate cock, which is also jewelled for the pivot of the seconds pinion. The latter is held in its jewel by a delicate little leaf spring which presses against the underside of the pinion. Assembling the cock for the seconds wheel and pinion is difficult to manage without the spring slipping from under the pinion and I found the easiest way was to back off the little screw that holds the spring, to assemble the cock, to manoeuvre the spring into its proper place and then to tighten the screw.

The winding and setting mechanism are conventional but the stop work for the winding is a little unusual for a relatively modern watch in that it uses a Geneva mechanism (Figure 7).

Figure 7: Geneva stop-work.

The mechanism is accommodated in a figure 8-shaped depression in the lid of the spring barrel. A square on the spring arbor carries a disc with a tongue and the disc is cut away a little each side of the tongue. Another disc having six arms is mounted and rotates about a shouldered screw. Five out of the six arms have concave ends, but the sixth has a convex end. As the spring is wound, the tongue causes the Geneva to rotate up to five times, with the concavity on the end of each arm allowing clearance for it to do so.However, when it reaches the sixth arm, there is no clearance and winding is brought to a halt.

While it might be thought that the purpose of this stop work was to prevent “over winding”, most people with even a slight knowledge of watch and clock mechanisms will know that it is not possible to over wind, though it is of course possible by brute force to break parts of the winding mechanism. Rather, the purpose of the stop work is to prevent the last part of the spring in contact with the spring barrel from coming in to play, as its power is different from that of the rest of the spring and the power delivered to the escapement and balance wheel would vary too much for good time keeping, Of course, the balance spring is adjusted to be isochronous as far as possible, that is to say that its period of swing is the same whatever the amplitude of the spring, but nothing is perfect…

When I got to the spring barrel, I found that the square had partly disintegrated, perhaps because it had been left dead hard and cracked from a sharp corner or because of the application of brute force (not by me!). At any rate, I was obliged to graft in a new square, and after a thorough clean and oiling of the movement, it has since performed as it should.

I hope you have enjoyed reading about this brief exploration of a fine navigational time piece. If so, I am sure you will enjoy “The Mariner’s Chronometer”, obtainable through amazon.com and its associated world-wide branches.

Tim White, from down under in London, very kindly agreed to provide a “Guest Post” about his making a chronometer balance cock. I have added some notes in blue. He is in the process of copying a Hamilton M21 chronometer, using some of the original dimensioned drawings.  Most chronometer upper balance cocks are in the form of a Z (Figure 1). Traditionally, the folds were made by cutting a 92 degree groove in the metal almost the whole way through, bending to 90 degrees and running solder into the groove (See Post 8, Figure 7), but manufacturers later probably used extrusions that were then finished to size and contour.

Figure 1: Common form of balance cock (from The Mariner’s Chronometer).

However,  the chronometer example supplied to the Hamilton Watch Co. in about 1940, for them to assess, was by Ulysse Nardin (Figure 2), who milled it from a solid block and attached it near the edge of the upper plate, so Hamiltons did too (Figure 3). The Nardin method seems to be somewhat over-thought, needing many operations and several settings to complete.

Figure 2: Ulysse Nardin balance cock.

Figure 3: Underside of Hamilton Watch Co. balance cock

Tim began with a piece of 1½ inch (38 mm) square brass bar which he faced slightly over length in order to mark out and drill 6.5 mm for the 1/8 inch cock retaining screw and counter-boring for the screw head. The hole and counter-bore for the upper balance pivot and its jewel must be accurately located relative to this hole. All other dimensions are “air fits”, that is to say that they do not need as precise positioning and dimensioning, since they are not in contact with other parts. Note that the curves marked with green lines in Figure 3 are not necessary for the part to function and could have been omitted as an economy measure.

Tim continues:

Put in the small 4 jaw and face down to within .005” (0.13 mm),  removing to measure. Remove using 2 jaws only. Clean and replace.

Tighten lightly, then applying tailstock barrel and apply strongish pressure, re-tighten 2 jaws quite strongly. Remove barrel and face .001” at a time measuring after each cut and clean between cuts. Take three final cuts stopping the lathe between cuts. (A depth micrometer, measuring against the face of the chuck, is handy for getting the correct thickness without having to remove the workpiece from the chuck. I use a lump of copper to tap the first face into contact with the face of the chuck.)

Next, I coated one side of block with marking blue and traced the outline of the original cock. It’s difficult to draw the cock from the plans as they are not completely dimensioned, as well as having centres that lie outside the block. In order to cut the 3 curves I made 3 discs of the right diameters attached to a mandrel. Referring to Figure 4, which is modified from the original Hamilton drawing, for curve A, I mounted the block in the machine vice of the vertical slide in the lathe and then aligned the edge of the disk with the scribed outline of the outside diameter. I replaced the disc with a fly cutter with an adjustable cutter and adjusted the tip of the cutter using a calliper until it had the correct diameter. I then cut the curve.

Figure 4 : Outline curves (Hamilton Watch Co., modified)

Next I machined the straight part B, Fig 4.  I then machined the curve C , Fig. 4, mounting the block on the 3.4in (86.36 mm) disc so that the curve marking is exactly lined up with the edge of the disk placing it by carefully marking, drilling and tapping holes so that it can be screwed to the disk. NB I made a counter weight which was weighed and machined until it was the same weight as the block. I then screwed it to the other side of the disc opposite the block and machined it at around 600 rpm (Figure 5 ).

Figure 5: Set up for machining curve C

To machine curve D, Fig. 4,  I used the Sherline rotary table. The block could not be rotated by the lathe because the flat straight AB would hit the cutter. I attached the table to the vertical slide with the operating handle protruding towards me. I used the 2.8 in. (71.12 mm) disc (2 x 1.4 in) centred on the table using the table centre arbor and a spacer to plug into the disk. I secured the disk to the table using two sockets in the T slots. I then secured the block to the disk holding it with a small clamp while aligning the curve with the edge of the disk. I then centre popped through the two holes in the block using hardened centres of the correct diameter (transfer punches). I then drilled and tapped the holes and screwed the block to the disk.  Using a 7mm high helix end mill I then machined the short curve manually using small cuts of around .003in (0.08 mm) watching to ensure the mill never got near the flat area. This worked very well. The finish on all surfaces was good enough so that I could sand with 400 wet/dry and finish with 2000 followed by a Simichrome polish.

To machine the inside cuts I attached a round steel plate to the rotary table. In the middle of the disk (bored .4375” – 11.11 mm) I inserted a threaded pin screwed into the table centre. It has a protrusion which is the same diameter as the jewel hole. It protrudes only about 0.075in (1.90 mm) and is used to locate the jewel hole end during machining (Figure 6).

Figure 6: Locating pin for jewel hole, and face plate.

The other end is secured by an M3 screw secured into the disk. The table is attached to the vertical slide. The cock is centred using a pin, which has a reduced diameter end .188in (4.77 mm) diameter. Its OD is 3/8 in (9.53 mm) and is held in the headstock spindle using a 3/8in collet. The vertical slide and cross slides are adjusted until the end of the pin goes into the jewel hole. The cross and vertical slides are then locked. Machining is carried out with a milling cutter held in the lathe spindle and the cut is put on by advancing the lathe saddle. I used a dial indicator to monitor cutting depth. I started the cutting at the jewel hole end and progressed outwards. This method worked very well with the M3 screw and centre pin securing the work piece. Figure 7 shows the workpiece secured to the rotary table, ready to be mounted on the vertical slide of the lathe. Originally, the part would have been machined on a vertical milling machine.

Figure 7: Work piece mounted on rotary table.

The curve E, Figure 4, is formed by making a filing button set-up using an OD of .400in (10.16 mm) and an ID of .188in (4.77 mm) to fit the hole. These hardened buttons, one each side of the part, were classically called “cheaters”. When the part is to size and shape, the file skids of the buttons.  I ground off any excess then filed the remainder using fine small files. Then I used 2000 grade wet and dry and polished the work with Simichrome.

Figure 8 shows the part after machining, with Figure 9 showing the finished part alongside a Hamilton Watch Co, original. The latter was made of nickel silver, similar to 60-40 brass with about 20 percent of the zinc substituted by nickel, to give a hard, corrosion resistant, white metal. Holes remain to be drilled for securing the balance spring, the upper balance jewels, and the steady pins which ensure that the cock is correctly located on the top plate.

Figure 8: Part finish machined.

Figure 9: Hamilton original (left) alongside polished new part.

Turning the balance staff for a chronometer is very different from turning the same item for a watch. For a start, even for a large pocket watch, the staff is unlikely to exceed 8 or 10 mm in length, whereas my example is over 26 mm long, but not greatly thicker, so one has to be constantly aware that the work piece may flex under cutting loads. Since the late 1700’s, engineers’ lathes have been equipped with compound tool slide rests, a sort of “iron hand” to hold and guide the tool in a linear fashion, but for some reason, watchmakers, even today, often use the techniques of the wood turner, albeit on a greatly reduced scale, holding and guiding the cutting tool (“graver”) by hand. It may be that it was hard to justify the cost of a compound slide rest and most of the watchmaker’s lathes on the second hand market today do not have them. Those that do, attract much greater prices than those without.

I recently made slide rests for my Lorch watchmaker’s lathe since I do not have the years left to acquire the skills with the graver. In any case, I would no more think of using a hand tool at the small scale of the chronometer than I would for turning larger diameters on an engineer’s lathe. To put it another way, I proceeded as the amateur engineer of long-standing that I am, rather than as a craftsman watchmaker.

My first task was to take the dimensions of an intact staff and make a drawing. While having a staff with a broken pivot may seem to place one in difficulties, it is a fair assumption that the pivots on both ends will be the same. If both are broken off, one may have to cut and try in the chronometer for which the staff is destined. To measure the small diameters it is necessary to have a clean and well-adjusted micrometer and technique does matter.

Clean the faces of the anvil and spindle by trapping a piece of clean note paper between them and pulling it free. Then check for zero error. Old codgers in engineering shops used to be proud of their sense of “feel”, but I reckon that Mr Mitutoyo must know a thing or two about micrometers and his firm recommend use of the ratchet or friction device, so turn the spindle slowly until the faces meet and until there is one click. Make a note of any zero error or, better, adjust the barrel to read to zero. If you use exactly the same technique when measuring, the measurements will be consistent to within less than 0.005 mm. If you gaily twirl the spindle so the spindle face hits the anvil hard or you twirl so the ratchet sounds like a soccer rattle, they won’t be.

Measuring lengths is a bit more difficult than diameters, but on the whole, they are less important. A steel rule calibrated in half-millimetres and a hand lens may do. A hand lens equipped with a measuring reticule is better and will give measurements precise to 0.1 mm. Better still is a microscope with a moving stage. These usually have verniers that allow measurements to be made with ease to 0.1 mm. To measure over-all length a micrometer is best and the technique is to hold the instrument with the spindle vertical, to rest one end of the part on the centre of the anvil and, while slowly advancing the spindle, move the other end around, exploring the face of the spindle, as it were, for the moment of contact. When this happens, back off a little, centre both ends and take a final measurement using the ratchet for one gentle click. Figure 1 gives the measurements of a Soviet MX6 balance staff (all figures may be enlarged by clicking on them. Use the back arrow to return to the text). If you already have a bench micrometer able to measure over 25 mm, (Post no.5, Figure 3) you probably will need none of this advice.

Figure 1.

Sharp-eyed (or obsessional) readers will note that the reference length dimension does not equal the sum of all the sub-dimensions. This is because the sub-lengths were measured using a microscope moving stage, while the over all dimension was measured using a micrometer, precise to 0.01 mm. Note too that the seats for the rollers, the balance collet and the spring collet are all tapered. It is difficult to be exact about the amount of taper, as the differences in diameter between the ends of the tapers are so small, but the included angle seems to be around 0.8 to 0.9 degrees.

Pivot wire comes in various sizes, including a nominal 2 mm, which in my samples are 1.96 mm in diameter. It comes ready hardened and tempered to a dark blue colour, which is quite a lot harder than mild steel and which can be made much harder, glass hard if need be, but this is too brittle for our purposes. Since all the tapers on this staff are pretty well the same, the first task is to set over the top slide to turn a taper of 0.85 degrees. There is no hope of doing this by measurement and it must be done by cutting and trying on a piece of scrap material, adjusting the set-over until the difference in diameter at the beginning and end of a 10 mm length is 2(10 x tan 0.43) = 0.15 mm. While you are at it, check that the cutting edge of the tool is bang on centre height and that the tool is really sharp, by taking a facing cut on the piece of scrap and watching to see that there is neither a pip left behind (tool too low) or a pip is bumped off (tool too high). I have a large supply of discarded tungsten carbide engraving cutters that I convert to lathe tools for small work and they can be brought to razor sharpness with a fine diamond lap.

Once these adjustments have been made, turning can begin by cutting off a piece of pivot wire about a millimetre too long and holding it a collet chuck (or simply, collet) prior to facing both ends and measuring the length (Figure 2).

Figure 2: Facing the blank

If it is under-size, you will have to start again with another piece. Otherwise, using the graduation on the top slide thimble as a guide, reduce it to the desired length.

The next step is to turn a pivot on one end. The watchmaker of old would have blended the pivot diameter with the rest of the staff diameter by manipulating a graver to give a graceful curve. I use a tool ground flat on top (i.e. no top rake) and to the form desired on the periphery (Figure 3). Measuring the diameter of the pivot needs some care as there is only half a millimetre or so before the diameter starts to increase. I wear a pair of magnifying spectacles to make sure that only the parallel part of the pivot is being measured by the micrometer, and reduce it to 0.01 to 0.02 mm oversize to allow for loss in the burnishing.

Figure 3: Forming a pivot.

The next part might as well be turning for the discharging roller, by allowing just enough of the work piece to project from the collet chuck. If you have enough magnification you may be able to detect a slight wobble of the end of the pivot at this point. This seems to be because the pivot wire is not perfectly round, combined with tiny errors in the collet. For this reason, I rough turn the part to about 0.1 mm oversize and complete the turning between centres (see below). Figure 4 shows this rough turning completed, with enough of the blank protruding to allow the seat for the impulse roller to be rough turned.

Figure 4: Rough turning the seat for the discharging roller.

Next, the workpiece is reversed in the chuck to turn the other pivot and rough turn the seat for the spring collet. Figure 5 shows the staff with all rough turning completed. An idea of the scale may be had from the 3 mm diameter screw head visible in the bottom left corner.

Figure 5: Rough turning completed.

Figure 6 shows the set up for turning between centres. This ensures that all diameters are concentric with a line joining the two pivots, and also allows the part to be removed for measurement and put back between centres with an assurance that everything will still be concentric.  A brief explanation, referring to Figure 6, may help the non-turner.

Figure 6: Turning between centres for collet seat.

Beginners may have wondered why the pivots take the form that they do, with an elegant curve rather than just being squared off. This is because the hole through the female centre has an outer, conical portion and an inner parallel portion whose diameter is larger than the diameter of the pivot, so that turning forces are taken on a larger and stouter diameter than the pivot itself (Figure 7). The latter, being only 0.2 mm in diameter would likely break off at the first cut.

Figure 7: Diagram of piviot (left) in lathe female centre (right)

The live centre rotates with the spindle of the lathe as does the driver, which engages in a slot cut into the side of the dog, a little disc of brass secured to the work piece by a screw. The dead centre remains stationary and must be lubricated. Thus, firmly held between centres, the work piece rotates with the lathe spindle and as the tool is traversed by the top slide it takes a cut. Further cuts may be made in a controlled way by advancing the cross slide with its calibrated screw. The top slide also has a calibrated screw, so that the length as well as the depth of cut may be controlled. In Figure 6, the balance collet is literally hanging about, so its fit may be tried after each cut.

Since the staff is very long relative to its diameters and slender, cuts taken must be very light as otherwise the cutting force will deflect the staff away from the cutting edge of the tool, or even spring the staff out of the centres, with consequent damage to the pivots. The tapers are very slow, so that even a small cut will allow a part to slide a considerable way (relatively speaking) on to the taper. Figure 8 shows the balance collet on its seat. A small tap with a punch will slide it up to the shoulder and seat it firmly on to the staff.

Figure 8: Balance collet on its taper.

The taper for the spring is dealt with next and after that, the dog is placed on the other end, the staff turned end for end, and the remaining two tapers finished to size. All is not lost nowadays if the seat is made slightly too small as an industrial adhesive such as Loctite 603 may be used to provide a permanent join, provided the gap to be filled is not too large. This cannot apply to the discharge roller of course, as it has to be adjustable, and should not be needed for the spring collet as the tolerance is large.

Now the pivots need to be polished and tradionally this was done using a Jacot tool, though the work can be done in a lathe fitted with a suitable tailstock attachment. All my technical dictionaries are silent on who devised this tool, but it was probably Henri Jacot, (1796 to 1868) who worked in Paris in rue Montmorency from 1833 with his brother Julien. On his death, the business was taken over by his nephew Albert Jacot.

Figure 9: General arrangement of a Jacot tool.

The set up is similar to that used for turning between centres, as shown in Figure 9, except that there is no tail centre. Instead one pivot rests in a groove of such a depth that part of the pivot projects above the walls of the groove (Figure 10), while the other pivot rests in a dead centre, about which the driver rotates. Again, the curved part of the pivot plays a part in locating the staff in the device. The correct depth of groove for a given pivot is stamped in hundreths of a millimetre on the end of the tool. In Figure 10 it is 20/100 mm or o.2 mm

Figure 10: Close up of Jacot tool.

Note that most Jacot tools for sale are not large enough to accept a chronometer balance staff, so in mine I carefully milled a register parallel to the axis of the device in two planes. After cutting it into two pieces I could mount it adjustably on a bar to maintain alignment.

As the staff is rotated towards the operator, a burnisher that rests on top of the pivot is pushed away, smoothing and polishing the metal as well as keeping it in place in its groove (Figure 11). It is commonly stated that the process work-hardens the surface, but I doubt that the pressures attainable are sufficient for this. Polishing also reduces the diameter of the pivot slightly. The burnisher is a piece of dead hard steel in which very shallow transverse grooves have been made by dragging it across a piece of emery paper.

Figure 10: Burnisher in use.

The ends of the pivot are dealt with using a slightly different tool in the tail stock (Figure 11). Instead of grooves, the tool has a thin disc of steel with holes of various diameters to suit the pivot, so that as the staff is rotated, a burnisher can approach the end, to be manipulated in such a way as to slightly round the end. If this is overdone, a burr may be raised and if this happens, it can be simply removed by burnishing a slight chamfer.

Figure 11: Jacot tool set up to burnish end of pivot.

Figure 12 shows the finished staff, together with the parts to be mounted on it (the spring is slightly distressed and will need more work on it). It is important not to hammer the rollers into place as this risks splitting them, especially the tiny discharge roller which will probably need adjustment anyway, once it has been tried in the chronometer. If a roller will not slide far enough along its taper by pushing it it into place, it is a simple matter to replace the staff between centres on the lathe and remove a whisker or two with a burnisher or very fine file or oil stone.

Figure 12: Finished staff with parts to be mounted on it.

I hope my attempts will help to illustrate one way how to make a balance staff and that too many experts will not be aghast at my methods. I am always happy to receive kindly worded comments, suggestions and corrections.