7. Repivoting , part 2 : an escape wheel arbor.

22 07 2013

A few weeks ago, I received a Russian MX6 chronometer that had not travelled well in a journey half-way around the world. Despite having been packed with care, partly wound and with the balance wedged, it arrived with the upper balance and the upper escape wheel pivots broken. As I had a spare balance staff by me, replacing it was without problems (see post 5: Replacing a balance staff), but re-pivoting is for me always a little nerve-racking and I am in good company as one authority on repairing has remarked that “…replacing a pivot is never a pleasure.” There is more than one way to skin a cat and in the preceding post I described how to replace a pivot by drilling down the existing arbor and inserting a new piece. The drill used is necessarily slender and if it breaks off in the hole it is practically impossible to remove or to turn away, so the surrounding arbor has to be turned away, shortening it in the process. In this post, I describe a method used by clockmakers of old to replace worn pivots, but scaled down for the much finer chronometer pivots.

In this process, a new piece is fitted over the old arbor using what was picturesquely called a “muff”. It is rather difficult to illustrate photographically and I have shown it diagrammatically in Figure 1.

Pivots 2 drwg

Figure 1: Fitting a muff.

A in Figure 1 shows the broken pivot which in B has been reduced in size by turning, for about one and a half diameters. The MX6 arbor is about 1mm in diameter and I turned it down to 0.7 mm using my improvised Jacot tool as shown in Figure  2 of post 6. Turning is made much easier by annealing the end of the arbor by heating it beyond blue to a black heat and letting it cool down, since the hardness of the arbor itself is no longer important. The heat is prevented from travelling down to the pinion by holding the shaft in a small toolmakers clamp or a crocodile clip. The muff shown in C must now be made from pivot wire or silver steel and although the latter is usually supplied annealed, it is best to make sure of it by annealing, as occasionally hard batches are encountered.

Holding the steel in a collet chuck, it is centred, in my case by using a 1 mm carbide drill which has been sharpened by the 4-facet method and is therefore self-centring. This is followed by a 0.7 mm drill to a depth of a little more than the turned down length of arbor, taking great care to back out the drill frequently, so that swarf does not accumulate in the hole and jam the drill. If it does break off, it is not a disaster of the same order as if it were to break off in an arbor, but tiny drills of this size are not cheap. The outside of the muff and the pivot is then rough turned to, say, 0.2mm oversize for the body and 0.1 mm for the pivot, before cutting off perhaps 0.5 mm over-length, reversing in the chuck and facing off to exact length by repeatedly trying it on the arbor and measuring the over-all length using a micrometer with care.

Drills seldom drill exactly down the centre line of a work piece, so to ensure that the hole and the finished outside are concentric, it is necessary to make a tiny mandrel, turned down at its end to the same diameter as the arbor and the hole, and to fit the rough-turned muff to the mandrel without removing the latter from the chuck, unless you can be sure that your chuck is accurate to very close limits. On this occasion, I heated the end of the mandrel with a soldering iron and applied a flake of shellac until there was sufficient heat to melt it, at which point I fitted the muff to the mandrel and allowed everything to cool down. This takes us to point D in Figure 1 and is shown in Figure 2, when the muff can be turned down to its finished size and the pivot burnished. Burnishing is often described as a process that both smooths and work-hardens the surface of the pivot, but with the sort of pressures that can safely be applied, it is unlikely that any work-hardening ever takes place, so the finished muff must be heat-treated to increase its hardness without too much reducing its toughness (or increasing its brittleness, which amounts to much the same thing). Since the muff, being close to the chuck, is well supported, it can be burnished without the use of Jacot tool and it is helpful to hold the burnisher under the pivot so that progress can be monitored more easily until the pivot appears to have an even polish. Application of the soldering iron then releases the pivot from the mandrel.

Figure 2: Finish turning muff.

Figure 2: Finish turning muff.

As the pivot is now only 0.2 mm in diameter, heating it directly in a flame to harden it may cause it to flare up to white heat and disappear, so I buried mine in a little pile of case-hardening compound (Kasenit) on a fire brick and heated it slowly until it melted all around my muff, when I brought it to red heat and decanted the bleb of compound into cold water. The compound protects the steel from oxidation and in this instance also refines the grain structure near the surface, reducing the likelihood of cracking in service. It is now very hard, but brittle, so must be tempered to increase its toughness. The easiest (and safest) way is to use a domestic oven turned up to 260 degrees Celsius (500 F). I polished the end of the mandrel to witness the tempering colours and fitted the muff to the end as a convenient way of not loosing a tiny piece of steel barely 2 mm long and 1 mm in diameter. As a precaution, I also monitored the temperature with a thermocouple thermometer. After tempering for 30 minutes and allowing the parts to cool, I then cemented the finished muff to the arbor, again using shellac. I have used Locktite in the past, but to make it release its grip it needs to be heated to a much higher temperature than shellac, which melts at about 140 Celsius, so any heat treatment of the adjacent metal is put at less risk with shellac.

Figure 3 shows the finished product. The tempering colour is just visible, a dark brown verging on purple, so it is harder than ordinary blue pinion wire and than the deep wine colour recommended by Marvin Whitney in his “The Ships Chronometer”, but as it shows the degree of toughness and hardness used in the past for punches and reamers, it should stand up well to service in the chronometer. I have left the tempering colours on the body of the muff as a witness to the repair for any future servicer of the instrument.

Figure 3: Hardened and tempered muff fitted to escape wheel arbor

Figure 3: Hardened and tempered muff fitted to escape wheel arbor

Although the muff appears to be of larger diameter than the tapered rest of the arbor, it is in fact 1.01 mm in diameter, and the escape wheel boss fitted over it without problems. It remained only for it to be fitted to the chronometer. It was a trifle over length, so I reduced it by about 0.01 mm using a diamond lap so that there is now barely discernible end play in a freely running wheel and the chronometer is performing as it should.





36: Usher and Cole’s Finest.

18 09 2019

By double-clicking on them, most of the photos can be enlarged. Use the back arrow to return to the text.

While I have been fairly busy since January restoring sextants and chronometers, I have not been very good at writing blog posts and have some catching up to do. In early June, I heard from a new friend in Sydney who had in 2009 acquired a chronometer made by the makers Usher and Cole some time before July, 1897. The chronometer had never run since Captain Dave first owned it. A clock maker in Sydney had managed only to break off the minute hand. Could I help? I was happy to agree on my usual terms of every care taken but no guarantee given and no reward accepted, so later in June the instrument arrived at my home in Pukenui, New Zealand.

We know something of its earlier life, as it had been on trial at the Royal Observatory in Greenwich between July 3rd 1897 and 22nd January 1898 prior to being accepted for purchase by the Admiralty. It was described as having “Auxiliary to balance acting in heat and cold. Palladium spring”. Over the six month trial the difference between the least and greatest rate (a) was 19.4 seconds and the greatest difference in rate between one week and the next (b) was 5.6 seconds, giving it a trial number (a + 2b) of 30.6. Its least losing rate per week was -1.6/week and its greatest gaining rate was +28.1/week, not great by modern standards, but considered adequate in the days before the arrival of the Invar group of alloys. Bear in mind that by 1897, all Naval ships were steam powered and unlikely to be for a prolonged period away from ports where chronometers could be checked.

The chronometer duly acquired its broad arrow and disappeared from view until after the Second World War, when it was in use by the Australian Civil Aviation Department to check air navigation aids. Figure 1 shows its face before restoration to health.

Face before

Figure 1: Face on arrival.

The silvering was in poor condition. Note for future reference that it has stopped at 48 hours.

Problems

I quickly had it apart and found that as well as the broken minute hand, the upper escape wheel pivot was broken, the detent was in several pieces, the oil was green and of the consistency of thick glue and the mainspring had broken into three pieces at the barrel end.

Dead pivot

Figure 2: Dead pivot.

Trials of re-pivoting

What should have been a relatively simple task of making a muff and applying it to the arbor (https://chronometerbook.com/?s=Repivoting+part+2) became a major problem when, during turning down of  the arbor, it snapped off flush with the top of the pinion. I dealt with this by drilling right through the pinion, taking extraordinary care to ensure that the hole was well-centred and straight, and then making a complete new arbor to which I then glued the pinion with Locktite.

Carbide drills sold for circuit board work are sharpened by the four facet method which means that in principle they are self-centring, but if the tail stock of the lathe is not truly concentric with the axis of the lathe spindle, being brittle they will wobble and then break as the hole deepens. I use a stereo-microscope permanently mounted on my medium-size lathe and if there was the slightest sign of wobble, I adjusted the set-over of the tail stock until there was none, before drilling through. I have increased the rigidity of the drill to some extent with epoxy putty. Figure 4 shows the result before polishing and assembly.

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Figure 3: Starting centre.

 

Esc wheel arbor exploded

Figure 4: New arbor with pinion and escape wheel.

Of course, those handy with a graver might have used one to make a true centre before using a spade drill held in a pin vice and guiding it by hand through the pinion.  I prefer to use the “iron hands” of the modern lathe to do the same thing, as I have not enough years left to spend them learning old techniques. Happily, both the pinion and the escape wheel ran truly after assembly.

New detent and support block

The detent posed a complex problem for an amateur like me, as the makers provided no means of adjusting its position on the top plate, but attached it directly to the plate. Figure 5 is a sketch from Marvin E Whitney’s “The Ship’s Chronometer” to show the general layout and dimensions of an English detent.

English detent

Figure 5: Means of mounting “English” detent.

Instead, and with Captain Dave’s permission, I elected to make a detent of more modern pattern and attach it to a support block so that its depth into the escapement could be adjusted by means of the screw seen on the left of Figure 6. See posts 22, 33 and 34 for details of making a detent.

Detent lab

Figure 6: Detent and support block.

Locking stone and passing spring

The locking stone had perished in the disaster that had destroyed the detent and the passing spring was absent, so I made the stone from tungsten carbide (see post 24) and the passing spring from 0.05 mm brass shim stock by the crude method involving fine scissors and finger nails described in the later part of post number 22.

Support block

I carved the support block out of a scrap of brass to the dimensions and shape of one from a Soviet MX6 chronometer (Figure 6). When it came to fitting it , I failed to notice that it overlaid the position of the third wheel arbor, and so had to file a cutout for the arbor. Once I had settled the position of the block, with the detent pointing to the centre position of the Balance wheel arbor (Figure 7), I clamped it to the top plate with a roughly made little clamp before spotting through for the steady pins and attaching screw (Figure 8).

Align detent horn

Figure 7: Aligning horn of detent.

Align support block

Figure 8: Mounting the support block.

Minute hand

Fortunately, both parts of the broken minute hand were present, so it was the work of only minutes to soft solder them together, leaving a generous fillet underneath where it cannot be seen.

Mainspring troubles

I next cleaned all the parts during which I discovered that the mainspring had broken into three parts (Figure 9). Until more modern steels were developed, this was a moderately common occurrence, so suppliers stocked a very large inventory of springs.

Broken mainspring

Figure 9: Mainspring, barrel and arbor.

I was interested to find the signature of the makers on the inside of the broken middle part and wondered whether this had perhaps been a stress raiser that contributed to the spring’s eventual failure (Figure 10). The whole of the inscription reads “Geo Cotton & Sons. Feb 1902”.

Usher and Cole 009

Figure 10: Spring maker’s mark.

I was able to obtain what was possibly the last spring of identical dimensions on the planet.  I checked the length of the original spring with a tape measure and double checked by using the usual formula that takes into account the thickness of the spring, the internal diameter of the barrel and the external diameter of the arbor. They agreed with each other and so I cut the new spring to length with a few extra centimetres for good luck.

When it came to winding the spring to fit it in the barrel I was perplexed to find that I could not get the arbor hook to engage, try as I might to shape the inner end of the spring, and eventually discovered that the hook had lost its edge. There was not enough of the old hook to file it to a new hook, so I filed away the old hook and drilled a hole at 90 degrees for a new one. Once fitted, the outer end kept slipping of its hook, so I was obliged to remove it and fit a new one there as well.

All the parts were now ready to be assembled (Figure 11), I fitted them all together, set up the mainspring a cautious single turn and the tired old chronometer sprang into life at once. However, when I tried to set it up the usual six plus turns, I found that only five and a quarter were available and had to adjust the chain to make the stop work act earlier (about which, more later), lest a future owner wind it so hard as to rip the barrel hook out of its moorings. It then ran for a maximum of 48 hours, rather than the more usual 56 plus. In practical terms, this is insignificant, since two day chronometers were always wound daily at the same time.

Top plate GA

Figure 11: View of top plate.

It seemed to be a pity to leave the dial as shown in Figure 1, so I stripped the old silver off,  graining the dial in the process, using fine emery paper and refilled the engravings with sealing wax stained black (See post number 9). I used a proprietary substance , probably silver chloride, to re-silver, and stabilised it with cream of tartar. Lacquering  clock faces is a skill that I have not learned, so I finished the face by polishing with silicone wax polish. I did the same with two clocks I made about fifteen years ago and the silver has not yet tarnished, though I live far away from any industry or busy roads, so this is perhaps not a good test of its efficacy. Figure 12 shows the finished face. Captain Dave learned a lot about polishing brass when an apprentice, so I left the bowl for him to do.

Face under bezel

Figure 12: Re-silvered dial.

Now for some points of interest that do not appear in more modern chronometers.

Stopwork

If there were not some means of bringing winding to a halt, a ham handed person might well continue winding until the chain or the barrel hook gave way, so all clocks fitted with a fusee have some means of stopping the winding. Figure 13 shows the top of the fusee. The comma-shaped object is called a snail.

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Figure 13: Fusee snail.

Turning now to Figure 14, which shows the underside of the top plate, we see the fusee iron, which is spring loaded to rest against the chain as it approaches the top of the fusee during winding. Eventually, the chain raises the iron to a position where the end of the iron butts against the projecting snail, bringing winding to a positive halt. Notice in passing that even the undersides of the plates have beautiful decoration, though only an overhauling chronometer maker would ever see it.

Stop work lab

Figure 14: Fusee iron.

Auxiliary temperature compensation.

As the temperature rises, the material of the balance spring becomes less elastic and the chronometer tends to run slower. Meanwhile, the material of the balance rim has become larger, also slowing the chronometer., so the rim is made of brass on the outside and steel on the inside. Brass expands more with heat than steel and the effect is for the ends of the balance rim to move inwards, decreasing the mean radius of the rims and tending to compensate for the slowing effects of temperature.  However, it was discovered that if the compensation is correct at two temperatures, the chronometer runs slower  midway between them, the so-called middle temperature error. In other words a graph of compensation versus temperature is convex downwards.

Much ingenuity was expended in correcting for the middle temperature error to, as it were, flatten the curve to make it more linear. Figure 15 shows two such auxiliary compensations in the same chronometer. Poole’s acts at a given temperature at which it interferes with further expansion of the rim at lower temperatures, while Mercer’s comes into action at higher temperatures, when the short bi-metallic strip moves the weight inwards and reduces the moment of inertia a little. A much fuller description can be found in Rupert T Gould’s magisterial book, The Marine Chronometer: its History and Development.

Copy of Aux comp 1

Figure 15: Auxiliary compensation.

Charles-Edouard Guillaume’s studies of nickel-iron alloys led to the invention of Invar, which has a very low coefficient of thermal expansion and of Elinvar, which has a practically constant elasticity at temperatures likely to be survived by human beings. Combining Invar in the balance rim and Elinvar in the spring nearly eliminated the middle temperature error. For his studies, Guillaume deservedly won a Nobel prize in 1920. The Hamilton Watch Company’s invention of the ovalising balance further reduced compensation errors to a practical minimum.

Rate

How well did this old chronometer perform? Figure 16 shows its very creditable  rate over 5 days, winding every 24 hours. There is a small deviation from the mean, a maximum of about one second at 70 hours, which would translate into a quarter of a nautical mile error in longitude at the equator. However, when allowed to run down to 48 hours, the losing rate changed to a gaining rate at about 30 hours, no doubt because the short mainspring began to deliver less power at this stage.

U and C rate 2

Figure 16: Rate over 5 days.

Maybe some of my methods would not gain the approval of all professional restorers, but I can say with Galileo Galilei “E pur si muove“, “And yet it moves”. And Captain Dave is happy too.