32: A chipped impulse jewel and another tale of woe

22 02 2018

In post number 30, I described how I made a new escape wheel for a Hamilton Model 21 chronometer and alluded to several other problems to be solved. In this post I will give a brief account of them, followed by a description of how I managed to avoid sourcing and buying a new impulse jewel. As the latter is tiny and rather difficult to photograph without sophisticated equipment, I apologise in advance for the sometimes poor quality of the photos, though I think they convey the details they are intended to convey.

As the fusee is one of the parts first to be removed, it revealed why I believe that the instrument had been dropped while out of its bowl (Figure 1).

Copy of Fusee damage

Figure 1: Damaged chain track of fusee.

Near the top of the fusee, the walls of the chain track had been bent out of straight so that the chain was wedged in place. I gently eased the chain out of the track and then, using a small screw driver with all sharp edges stoned off, I persuaded the walls upright again and filed off tiny burrs with a fine Swiss file. The damage is no longer apparent.

All of the pivots of the escape and balance wheels had been sheared off and the state of their hole jewels attested to the violence of the fall that had broken them (Figure 2).

Holestone damage

Figure 2: Destroyed hole stone of balance.

The plate jewels in the M21 chronometer are friction fitted into the plates and the throat of a staking set of normal size with the usual fitment for pressing jewels into place is generally too small to allow it to be used for a chronometer, so improvisation is needed to achieve a controlled pressure. I fitted the end stones first and then by careful and repeated measurement tapped the hole jewels into place with a brass drift until they were a scant 0.02 mm from touching the end stones. The official manual writes that they should be pressed into contact with the end stone, but it seemed to me that this might prevent the formation of an oil reservoir (if I am wrong, professionals please feel free to make a kindly worded comment). In retrospect I should probably have used a drift held in the tail stock of my small lathe as an improvised press with the plate resting on the  face plate.

I have described pivot repairs in posts numbers 6 and 7.

In early M21 chronometers, the plates and the detent were made of nickel silver, a brass in which half of the zinc has been replaced by nickel to improve its mechanical and corrosion resistance properties. There is no silver in nickel silver, but it is a shiny white metal. Columbia Metals UK describes it thus: “Nickel silvers are capable of providing a unique combination of strength, high modulus spring properties, corrosion and oxidation resistance, …. and numerous other attributes combined with ease of forming, machining, plating and joining.” Later chronometers have detents of beryllium copper. Happily, in the disaster, which seems to have included running away of the movement, my detent was “bowed but not broken” and the passing spring had survived . The two photos merged into one of Figure 3 were taken on different days in obviously different lighting, but are of the same detent from different viewpoints.

Detent merged

Figure 3: Bent detent.

The top image shows the detent in its support block and the bend at the base of the spring is rather obvious. Less obvious in the lower image is that the bend is not symmetrical and has introduced a slight twist. Only a fragment of the locking jewel had survived. By this stage, I had removed the passing spring. A few words of warning may be useful here. The screw that secures the spring in place is the tiniest part in the whole chronometer and can easily be treated as  a fragment of dirt and be lost, so it pays to store it safely in its own little container.

I straightened the spring by drawing it between the rounded jaws of special pliers, as described in post number 28 and eventually restored it to straightness. In an effort to avoid expense which I can ill afford, I attempted to make a new locking stone of tungsten carbide, as described in post number 24, for a Soviet MX6 chronometer, but the M21 locking stone is rather more slender at only 0.6 mm in diameter (versus 0.8 mm) and vibration from the belt of my tool and cutter grinder (since remedied) was enough to break it off when I attempted to grind the flats. I was thus obliged to buy a new locking stone and fit it as described in post number 15.

In early M21s the rollers were in one piece so that the mutual angles of the unlocking and impulse jewels was fixed, as in my example. Quite early on, the one-piece rollers were replaced by two separate parts. Figure 4 shows a view from a larger photo of the rollers from above, taken so I could replace them in their original position.

Copy of Rollers

Figure 4: Roller and jewels.

At first sight, all appears well, but as repairs progressed I discovered that the impulse jewel was quite badly chipped, so much so that a “replace and hope for the best” attitude would have been a poor plan. Figure 5 shows the jewel after removal.

17 Damaged impulse (2)

Figure 5: Chipped impulse jewel.

As I had not at that stage found a source of a new jewel, and doubted my ability to make a new one from tungsten carbide, I decided to reface it using a diamond file and laps, but first I had to devise a method of holding this tiny piece of synthetic ruby.  I filed a notch into a scrap of 2 mm brass sheet using a warding file until the jewel fitted snugly, and then secured it in place by melting a flake of shellac over it (Figure 6).

18 Impulse j set

Figure 6: Impulse jewel set into holder.

Note that this view shows that the damage extends quite a way down one edge of the jewel, so that in filing the new clearance the length of the jewel was quite considerably reduced. Looking through that most valuable of instruments, the retrospectoscope, I should have seated the chipped side in the holder and filed the new clearance on the other end. This would have maintained most of the length and made for a more secure re-fitting in the roller. A diamond file cut away the damaged face, much more easily than the same file cuts into tungsten carbide, and this underlines how much harder the latter is than ruby. As the clearance face comes into contact only with air, there was no need to achieve a very fine finish, but rather than leave a sharp edge, I rounded it a little with fine laps.

The roller is 6.35 mm (0.25 inch) in diameter, so another scrap of sheet brass with half of a 6.3 mm hole filed away provided the gauge to ensure that the tip of the jewel projected the correct length when being secured in place with melted flake shellac. This is described in post number 17.

Figure 7 shows the final position. Although the hold of the roller on the impulse jewel looks precarious, the shellac covers the base of the stone and I have left fillets of shellac on either face. If it does eventually give way, I will replace it with a carbide version.

20 Impulse j reset

Figure 7: Impulse jewel refitted.

The rest of the overhaul was routine and the chronometer showed no reluctance to resume its ticking with a very good action. However, I will end with a cautionary tale.

After about a week of rating it and finding that it had a very steady losing rate of about 1.7 seconds per day, its rate became erratic and then it lost two seconds overnight, so I stopped it and began to look for a source of the problem. As I was checking the balance and escape wheels for excessive end or side shake, the instrument suddenly declined to run at all. Suspecting that the impulse jewel had perhaps shifted, I removed the balance to check and found that it was still secure so I replaced the balance and turned my attention to the detent, to check what I already knew to be the case, that the escape wheel teeth were engaging about one third of the acting face of the locking jewel. The engagement was now much deeper. It was at this point that I discovered that, having at original assembly made a slight tweak to the depthing of the passing spring, I had failed to re-tighten the screw that holds the detent to the mounting block, labelled “detent clamp screw” in the original manual. Once I had tightened it, normal running resumed. Now I’m investigating its rate when half wound.

 

 

 





31: An eight day chronometer

14 02 2018

Some weeks ago, I received an e-mail asking about an eight day chronometer that the writer had recently inherited. He was able to tell me that it carried the name of John Carter of Cornhill, London and had been sold to the Admiralty in 1872. Its history between then and 1936 was not known, but in that year, it was sold by the Admiralty and in 1946 it had travelled to India with Ron’s uncle where it had remained on a bookshelf in a hot and humid climate until late in 2017.

After an exchange of e-mails, Ron sent me some photographs, which he has agreed I could share with others in my blog. Most of the photos can be enlarged by clicking on them. From Figure 1,  we can see that Carter did not actually make the movement. It was very common for retailers and even other makers to buy in finished movements and put their own names on the face. Makers would also buy from other makers if they had an order that they could not meet at the time. Parkinson and Frodsham as a company were active between 1801 and 1890. I cannot see a number anywhere on the movement that might give a clue to date of manufacture.

7 John Carter 738

Figure 1: Top plate inscription.

The form, size and layout of movements changed remarkably little from about 1800 until manufacture ceased in the 1980s and the same can be said for the bowls and cases. Two day chronometers of whatever make usually had three-tier cases forming a cube with sides of  185 mm, and heavy brass bowls of about 105 mm diameter. Almost always, the cases were brass bound with brass corners, presumably to hold them together in the marine environment if the glue and pins that held them gave way. Curiously, sextant cases, which would be a good deal more exposed to the elements than chronometer’s were almost never brass bound and the only one I have ever seen that was, other than so-called “reproductions”, is in my possession. Straight away, we see from Figures 2 and three, that my correspondent’s chronometer is an exception in the other direction.

1 john carter 738

Figure 2: Front of case.

2 John Carter 738

Figure 3: Side of case.

The front of the case has an interesting little escutcheon around the key hole, a bone or ivory lozenge, perhaps for the Admiralty to add their accession number to, and a broken button catch for the top lid. It is quite hard to find replacements for these little catches. The side view shows that the corners have rebate joints, another good reason for having the case brass bound, as they are less secure than dovetail or comb joints. We also see the two screws that secure the gimbals lock, the screw and rectangular washer that holds one side of the gimbals bearing and a standard drawer pull type of handle.

3 John Carter 738

Figure 4: Bowl in case.

On opening the case, we see a tipsy key  and gimbals lock whose forms did not change over time and between different makers of various nationalities. Unusual, however, is the way the tip of the gimbals lock engages with a brass band that nearly encircles the bowl, whereas usually it engages with a slot in a simple bracket that is attached directly to the bowl. The 15 mm thick walls of the case have dust seals of a darker wood, possibly ebony, let into the top of the bottom tier, except where the box lock and the hinges intervene.

4 John Carter 738

Figure 5: Face.

In an otherwise unremarkable 19th and early 20th century face is a clue that reveals the main interest of this chronometer (which is not to say that this venerable instrument is not desirable for other reasons). The state of wind indicator shows that the chronometer has a movement that runs for eight days. It is in a fully-wound state, while the minute hand is out of step with the seconds hand, whereas it should indicate a full minute when the seconds hand shows 60 (or zero, which amounts to the same thing).

Eight day chronometers are very uncommon and it is a little difficult to understand why any were made, since they would in practice be wound daily at the same time of the day to maintain a constant rate. The famous  and influential Captain Lecky, in his “Wrinkles in Practical Navigation“(ninth edition, London, 1884), writes “…it has been found that eight-day chronometers do not preserve altogether the same rate throughout the week; that is to say, that (though other conditions may be the same) their daily rate towards the end of the week will not agree with their daily rate at the commencement of it; notwithstanding which, the mean  rates of two consecutive weeks  may agree exactly.” He also adds “On account also of the lightness of the balance, the eight day chronometers do not go so well on board steamers which suffer much vibration from their machinery.”

Rupert Gould in his authoritative “The Marine Chronometer” confirms this when he writes (p.218) “...there is absolutely no advantage gained by making a chronometer go for more than two days between windings, and such machines are inferior both in principle and detail to the ordinary two-day pattern, although, if well-made, they may be found quite satisfactory in use at sea.

I reassured my new internet friend that it would not hurt to unscrew the bezel and tip out the movement. Figure 6 shows more details of the exterior of the bowl and the strange provision made for the tongue of the gimbals lock and the dust cover for the key hole. Note that the disc that houses it, attached with three screws, has the letters “H.S.”, followed by a broad arrow indicating government property, and the letter “I (or number 1). Perhaps we may tentatively wonder whether these letters refer to the hydrographic service of one of Britain’s former colonies.

6 John Carter 738

Figure 6: Exterior of bowl and GA of movement.

Turning to the general view of the movement, the most obvious difference from a two day chronometer is its exceptional height, necessary to accommodate a taller spring barrel and fusee, and that the rest of the movement, except for the centre wheel arbor, is sited between a sub-plate and the bottom plate. Figure 7 shows some more details of the spring barrel and fusee.

9 John Carter 738

Figure 7: The power source.

The the tall, large-diameter spring barrel accommodates about eight turns of  chain, while the fusee has about 16 turns, nearly twice as many as a two day chronometer, while the great or fusee wheel seems to have many more than the standard 90 teeth.

Another view of the power source (Figure 8) shows at lower left the elongated click of the maintaining power ratchet wheel attached to one of the pillars and, at top right beneath the top plate, the short pillar for  the fusee iron. This latter proved rather hard to photograph, so a verbal description will have to do. The fusee iron is shaped somewhat like a fork with two prongs. Its base is hinged to the short pillar and held off the top plate by a leaf spring. As the chain mounts the fusee during winding, it comes into contact with the underside of the fusee iron and raises it parallel to the top plate At about this point, a projection, called the snail, which is screwed on to the top of the fusee, enters the fork and brings winding to a halt. In the absence of  stop work, there is nothing to stop the heavy-handed from breaking the chain, with often disastrous consequences for the movement when the large amount of energy in the oversized spring is suddenly released. Also in Figure 8, the very large wheel for the state of wind indicator is seen at bottom right.

11 John Carter 738

Figure 8: Maintaining power.

This leaves the escapement, shown below in Figure 9. I have been able to show only the balance wheel and its associated parts The bi-metallic rim of the wheel is of course split adjacent to each timing weight which are situated as usual at the ends of the spokes, while large, wedge-shaped weights for temperature compensation are at roughly right angles to the spokes. The upper end stone seems to be of diamond mounted in a two-part balance cock. It is just possible to see the ruby locking jewel to the right of the spoke at seven o’clock and the ruby end stone of the escape wheel arbor to the left of the spoke. While in most modern chronometers the upper balance spring collet was attached to the top of the cock, in this instrument it has its own little pillar, and I wonder whether this might have been some form of compensation for middle temperature error, if the horizontal strip turns out on further examination to be bi-metallic.

Copy of 10 John Carter 738

Figure 9: Balance wheel.

I hope that this account may be of interest to those who, like me, have never seen an eight day chronometer “in the metal” as it were. Many more details of the structure of chronometers may be found in my book “The Mariner’s Chronometer”, available via amazon.com.

Post script 1 July 2018

Dave Murphy has kindly sent the following information about the markings on the chronometer bowl:

“You are correct on your reference to the H.S.3  markings on UK military chronometers.I can’t remember the exact details but the ‘arrow’ (correctly called a Pheon apparently) is used to mark items which are valuable and likely to be ‘pilfered’.

H.S. is Hydrographic Service and would have numbers 1 to 3 which references the timekeeping quality based on time trials.

H.S.1 is always a gimballed chronometer and would be permanently secured in the instrument room of large warships.  (typically in the outer box which was screwed down to a bench!)  It is the time reference for the ship and all clocks, watches etc would be set to it allowing for its rate.  

H.S.3 is typically used as a deck watch, referenced to the HS1 prior to taking sextant observations.

H.S.2 as I recall is usually a gimballed watch and was the main timepiece for smaller vessels such as MTBs, armoured tugs etc.”

 

 

 





30: A new escape wheel for M21 chronometer

6 02 2018

I recently acquired a damaged chronometer that seemed to have been dropped on to a hard surface while out of its bowl, or perhaps the owner had thrown it at a wall in frustration. At any rate, the walls of the fusee track had been squashed in at one point and had trapped the chain. By carefully using a small screwdriver whose edges had been rounded off, I was able to open out the track and a fine file removed all other traces of the injury to the fusee. On further inspection I found a large range of problems that would need to be fixed before the chronometer could be brought back to life, some of them probably due to the shock of the fall and others probably consequent on the initial damage.

The most obvious was that all the pivots of the balance and escape wheels were broken, which I could fix, and two of the escapement hole jewels were shattered, which I could not, so I was committed to having to buy replacement jewels. In the Hamilton M21 chronometer, these jewels are friction fitted in the holes in the plates and so are rather more difficult to fit than those of most other chronometers. The locking jewel was broken off flush with the top of the detent, but happily, enough remained for me to be able to photograph, so that its expensive little replacement could be replaced at the correct angle of 8 to 12 degrees of draw. The detent spring was also bent out of shape, but I was able to straighten it using the method outlined in Post 28.

To the naked eye, the escape wheel looked fine, apart from the disaster to the pivots, but on closer inspection its teeth looked worn and when viewed under a low power microscope, the tips and sides of the teeth looked worn, as shown in Figure 1.

00 Worn teeth

Figure 1: Worn and damaged escape wheel teeth.

Normally, only the tips and front faces of the teeth come into contact with anything, i.e. the locking jewel and the impulse jewel. Its manufactured diameter would have lain between 13.14 and 13.18 mm, but its measured diameter was only 12.73 mm, a relatively enormous disparity, so I guess that something had caused the escape wheel to “run away”,  the while battering itself against a jewel, eventually breaking the locking jewel and, as I found later, badly chipping the impulse jewel. Perhaps, when the first escape wheel pivot broke, perhaps it was still able to run, albeit drunkenly, and so damage the sides of the teeth.

I searched for a new escape wheel, but could not find one to buy. Most chronometer escape wheels seem to have fifteen or thirteen  teeth, but Hamilton chose sixteen, so the readily available Soviet MX6 escape wheel (a bargain at US$ 235 with pinion?) could not be substituted. I had to bite the bullet and try to make one myself. A quick trawl of the internet showed that two people had made escape wheels by laboriously making a punch and a die, so that the wheel could be punched out of sheet metal with the dimensions and crossings all ready formed for machining the teeth. This method did not appeal to me, and anyway, for me part of the pleasure of restoring these instrument comes from using old techniques. A punch and die are fine if you want to make lots, and Hamilton eventually made over 11,000, but for a one-off, it seemed to me to be overkill.

Figure 2 is a posed photograph of how to remove an escape wheel from its arbor.

Figure 2: Removing wheel from arbor.

Note how a slot in a scrap piece of brass protects the collet from damage, while the arbor is punched out. Beginners in the use of a staking tool perhaps need to be reminded to check the depth as well as the diameter of the hole in the punch, so that they do not inadvertently damage the pivot. The photos in what follows were taken while developing a method over several days, so the keen-eyed reader may notice certain lapses in continuity…

Some makers start by cutting out or buying a circle of brass, mounting it on an arbor, turning it to size and then cutting the teeth. This works well with larger wheels, but because of the small size of the hole in the middle, this makes securing the blank a bit uncertain for turning and the set up lacks rigidity when it comes to cutting the teeth. I used the more wasteful but safer method of machining the teeth on brass rod and then parting off slices, so I could have several goes at developing my method. After turning the rod down to the outside diameter of 13.16 mm, I transferred the chuck from the lathe to my dividing head in a bid to retain concentricity. Figure 3 shows the faces of the teeth being cut.

1 Cut front face

Figure 3: Cutting the front of the teeth.

Although the photo shows the work piece in a three jaw chuck, I found that the set up would not hold concentricity between lathe and dividing head to any better than 0.03 mm, so I eventually used a four jaw chuck and centred the piece with a dial indicator against a register machined on the bar. To cut the teeth, I used a fly cutter filed from a piece of 6 mm silver steel and then hardened, tempered and polished on the cutting faces. Run at 1,500 r.p.m this gave an excellent finish that needed little polishing. Tools for cutting brass need to be sharp. Note that the acute angle of the teeth of about 60 degrees means that the front edge of the tool needs to be lowered below centre by just over 2mm.

It was somewhat more difficult to machine the curved back faces of the teeth, but I eventually managed to file a radius of 2.5 mm on the end of a piece of silver steel and to form the all-important relief behind the cutting edge.

2 Cut back face

Figure 3: Back faces of tooth being cut.

Setting the tool height to get the tooth looking correct needs to be combined with rotating the work piece to get a tooth that is robust enough and to leave a narrow land at the top of the tooth, specified as 0.13 to 0.14 mm wide. Of course, this cannot be easily measured and for myself I modified that specification to be “land present and just about visible”. In this photograph of an early attempt, the land is a little too wide, but the form looks fine. If you take too much off, the diameter inevitably has to be reduced to restore the land.

7 Set parting off

Figure 4: Parting off.

Most turners will agree that “Parting off is such sweet sorrow”. The tool needs to be sharp with its face square to its axis, and the axis has to be square to the axis of the lathe. The cutting edge needs to be exactly at centre height or, with a back tool post, a minute amount above. With the work piece transferred back to the lathe begins the task of parting off slices while ensuring that no slice is thinner than 1.32 mm. As the conditions above are difficult to meet exactly, it is better to part off oversize and then face the slices down to the desired thickness. Measuring small distances in a confined space is best not done with a ruler, and I use a depth micrometer, as shown in Figure 4. Parting off is easier if there is a central hole and I finally remembered to drill this before parting off.

I then turned a holding fixture which would not leave the lathe until all the slices had been reduced to the correct thickness, though I eventually realised that Hamilton’s specifications and tolerances were to ensure interchangeability and that 0.1 mm either way was of little importance as long as it is no wider than the impulse roller. Figure 5 shows the simple fixture being bored to produce a recess 1 mm deep that would just accept a slice of embryo escape wheel.

3 Bore fixture

Figure 5: Boring fixture.

Knowing the depth of the recess from the outer shoulder, it was then possible to set a facing tool back from this shoulder, using the graduations on the tool slide, by an amount to give an over all thickness of 1.27 to 1.32 mm.

At first, I used shellac to secure the wheel in place, but found that the fixture combined with the mass of the chuck formed such a large heat sink that it was difficult to reach a high enough temperature with a small flame, a problem not eased by relieving the end of the fixture with a deep slot, seen in some of the following photographs. Eventually, I used superglue. It melts at a much higher temperature than shellac, so I was obliged to remove the fixture from the lathe and soak it in acetone overnight to release the wheel.

4 Counterbore wheel

Figure 6: Initial counterbore.

Having cemented the part into place and faced it to the correct thickness, the counter bore in the wheel can be started using an 8 mm end mill or slot drill, its depth controlled by the graduations on the tail stock quill. This counterbore is then opened out with a boring tool (Figure 7) to its correct depth and diameter.

5 Counterbore enlarge

Figure 7: Counterbore enlarged.

The correct diameter removes just a little of the root of the tooth to give the finish shown in Figure 8. To ensure concentricity of the hole in the centre and the tips of the teeth, I ran a small but rigid reamer, with one of its two end teeth ground back to make of it  a small but rigid tool. A reamer is usually used to size a hole and will normally follow the existing hole, but in this case, only a whisker was removed, and in any case, the hole is too small for a conventional single point boring tool to enter.

6 Counterbore finished

Figure 8: Counterbore completed.

After giving it a good soak in acetone the wheel could be removed from the fixture so that marking out for the crossings could begin. I faced the ends of a piece of wooden dowel and glued to each end a piece of emery paper, one of 800 and one of 1200 grit and rotated the dowel against the floor of the counterbore to remove most of the turning marks prior to marking out. In a scrap of brass, I faced, drilled and reamed a 3 mm hole and made a close fitting removable spigot, one end of which was turned down to a close fit in the hole. Before parting off the spigot I made a minute centre mark in it with a sewing needle held in the tailstock chuck (Figure 9).

8 Marking out 1

Figure 9: Marking out jig.

From this centre I scribed a circle of radius of about 18 mm and divided it into 6 parts by the well-known method of stepping the dividers around it at the same radius. I marked out centres on three of the radii at a radius of 13 mm and from these centres scribed the outlines of the spokes, adjusting the radius by trial and error to give sufficient metal at the joining of the spokes with the periphery, which has a radius of about 5.5 mm. Midway between the spokes and the periphery I made punch marks and then removed the wheel from the jig to drill 2 mm holes at these points. Figure 10 shows a wheel at this stage.

9 Drilled

Figure 10: Ready for crossing out.

The purpose of the holes is to allow entry for the blade of a piercing saw, in this case a new 4/0 blade, but before beginning to saw, some filing makes subsequent sawing and filing much easier. I long ago made myself a mini four square file by grinding away two adjacent sides to make safe edges and I used it to file in and out to the marked lines, so the the saw could start right next to the line (Figure 11).

10 Extend hole

Figure 11: Preliminary filing to lines.

The wheel is much too fragile to be held in a vice and Figure 11 shows how it is held horizontally between fingers and a horizontal surface while the file moves up and down. It helps greatly to be able to see exactly where the file (or saw blade) is going and I have a binocular microscope mounted on a boom on my work bench. Note too the piece of leather between my fingers and the wheel. The teeth of the wheel by this stage are sharp! A later photo (Figure 13) taken before this one shows my fingers before I learned this important lesson. Figure 12 shows a wheel prior to sawing.

11 Drilled and filed

Figure 12: Preliminary filing completed.

Sawing could now commence, again holding the wheel horizontal, as is usual when using a piercing saw (Figure 13). The saw “table” is simply a strip of metal cantilevered from a small vice to give the hand room to move up and down beneath it, and the wheel is rotated to keep the blade tangential to any curve as the cut progresses. When progressing around tight curves or into corners, the blade must be near-vertical, but around shallow curves or in straight lines the blade seems to follow the lines better if canted forwards a little, as shown in the figure.

12 Sawing

Figure 13: Crossing out with saw.

It will, I hope, be obvious after a little thought that only half of each crossing can be sawed this way and for the other half the wheel must be transferred to the other side of the table and the saw held in the left hand. This needs more ambidexterity than I have, so I simply reversed the blade in the frame, so that the teeth faced inwards towards the frame, and sawed backwards towards myself. (See Fergus’s comment) Figure 14 shows the results in an early, practice attempt to assess the practicality of making the crossing by hand.

13 Sawed

Figure 14: Ready to file again.

The rest involves filing to the lines. The closer one can saw to the lines, the less filing is required. Swiss needle files are needed and there is a particular form used for crossing out and it is called a crossing file;  both surfaces are curved with different radii and tapering to a point, so with care, finely rounded internal corners can be cut. A crochet file is useful for getting into sharp corners as it is tapered in width and in length. Again, a powerful aid to vision is very helpful.

A close inspection with a microscope in good light will show all manner of burrs and the simplest way of removing them from corners is to lightly draw the blade of a small craft knife across them, taking great care to avoid the faces and tips of the teeth.

Each face of the wheel is easily polished by rubbing against a piece of wet and dry emery paper resting on a scrap of plate glass under water to which a drop or two of washing up liquid has been added. I start with 800 grit and finish with a piece of well-worn 1200 grit, taking the polishing no further, as these faces contact only air.

The acting faces of the teeth were left with a very fine finish by the fly cutter, but I felt that the locking and impulse jewels would have an even smoother ride if polished (Figure 15).

14 Polish teeth

Figure 15: Polishing acting surface of tooth

The figure shows how I held the wheel, by now with its collet in place. In a block of wood that I could hold comfortably in my fist while it rested on the bench, I let in a piece of pivot steel and held the wheel stationary with an index finger, while polishing the faces under direct vision with a scrap of diamond-impregnated film glued to an old feeler blade. I went from 9 to 3 micron film and decided that was far enough.

Figure 17 shows the original wheel with a couple of trial wheels, with the one on the left nearly good enough, but as the rim at the top is a little irregular, I decided to finish with a spare one of the dozen or so that I had parted off at various stages of my trial. I had fitted this to the chronometer before I thought to photograph it, and as the chronometer has now run for over 24 hours with a gain of 1.7 seconds with an excellent action and the mainspring set up only two turns, I am not about to take it apart for a photograph. The final version is seen in Figure 15.

15 Group finished

Figure 16: A compendium of escape wheels.

If you compare the finish of punched out edges, as shown in Figure 1 with the edges left by filing in Figure 15, there is surprisingly little difference (Figure 17).

21 Teeth compared

Figure 17: New (right) and old (right) wheel finish compared.

My teeth probably have more mass than the originals , but the rim is a little finer and the mass of the spokes is concentrated nearer the centre, so that the inertia, mr², of my wheel is probably about the same.

Making the collet was a simple turning operation, albeit at a small scale and industrial glues have made interference fits and riveting of collets unnecessary. Finally, Figure 17 shows a Soviet MX6 escape wheel being refitted to its arbor.

16 Replace on arbor

18: Fitting an escape wheel to its arbor.

 

I hope you enjoyed reading this post and if you haven’t already done so, I encourage you to buy my book, available from amazon.com. It may well tell you more than you wish to know about the structure of the marine chronometer.