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.

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6. Re-pivoting a Balance Staff, Part 1

9 06 2013

In Post  number 5 I showed how to replace a damaged balance staff and in this post I show how to re-pivot the badly cut staff. Figure 1 shows the pivot together with its dimensions.

Figure 1 : Cut pivot

Figure 1 : Cut pivot

The traditional way of re-pivoting would use a lathe with the staff held in a collet and the staff steadied using a Jacot tool. A graver would be used to turn off the damaged portion and a centre for a drill started using the point of the graver. A hole would then be drilled using a spade drill and a slightly tapered piece let into the hole until it jammed in place in the hole. The pivot could then be cut off to length and finished to size using a graver. This process has scarcely changed in two hundred years, but small machine tools and their attachments are now much better constructed than they were even fifty years ago and it seems to me that we should take advantage of advances. In any case, I am nearing the end of my days and have not the time to learn skills which have been made largely obsolete by modern tools. Professionals who read what follows may disagree with my method, but it does use the skill that has been built into the lathe and employs the lathe slides that can guide a tool far better than I could by hand. It is important, however, that the cutting edge of the lathe tool should be exactly on centre height and this is best achieved by making trial cuts on a piece of pinion wire, closely observed with a magnifying lens.

The first step is to soften the end of the staff by heating. To prevent the heat from travelling too far from the end, it is convenient to hold the staff in a small tool-maker’s clamp as a heat sink and to pass the end of the staff briefly through a gas flame until tempering colours have made their way 3 or 4 mm down the staff. By clicking on the figure to enlarge the image, these colours can be seen in Figure 2 , which shows the staff held in a modern collet chuck and steadied in a home-made Jacot tool (most Jacot tools were made for watchmaking and are too small to accommodate a chronometer staff).  The latter has a variety of holes of different sizes drilled very accurately on the centre line of the lathe.

Figure 2 : Facing end of staff.

Figure 2 : Facing end of staff.

Once the end of the staff has been faced square using the cross slide of the lathe, the Jacot tool can be replaced by a drill chuck and a centre started. Even the smallest standard centre drill is too large for this task, but modern small solid carbide drills are ground by the four facet method and so are self centring.  I used a relatively large 1 mm diameter drill to remove the merest shaving from the centre to provide a true start for the smaller drill to follow (Figure 3). If this step is omitted, the end of the smaller drill is liable to wobble about off centre and eventually break off.

Figure 3: Centring end of staff.

Figure 3: Centring end of staff.

Once a true start has been made, the smaller drill can follow, in this case of 0.45 mm diameter , as shown in Figure 4. A very delicate touch is needed. Marvin Whitney recommended holding the drill between the fingers in a small pin chuck, but if care is taken to keep the drilling under constant observation with a magnifying glass and to frequently withdraw the drill to clear away swarf, the normal feed arrangement provided in the tailstock can be used safely. A depth of three to four drill diameters should be deep enough.

Figure 4 : Drilling to size.

Figure 4 : Drilling to size.

A start can now be made on the insert. Whitney recommended English sewing needles whose temper had been drawn to a “wine” colour. This corresponds to a temperature of about 280 degrees Celsius, a little lower than for normal pinion wire which tends to be blue and therefore a little softer. However, English darning needles are hard to come by so I used Swiss pinion wire, hardened and re-tempered to a dark purple (wine?) colour, turning it down to fit the hole drilled in the staff (Figure 5).

Figure 5: Turning down pinion wire.

Figure 5: Turning down pinion wire.

Modern industrial adhesives have made interference fits almost a thing of the past. I used Loctite to secure the new pivot in the hole and after letting it cure, returned to the set-up of Figure 2 to turn it down to length and diameter, followed by burnishing. This is a finishing process which, as well as smoothing and polishing the pivot, to some extent may also work harden it (I seriously doubt that work hardening takes place at the pressures possible). A glass-hard burnisher is drawn across emery paper to make very fine transverse grooves and the burnisher is then pressed on to the pivot and drawn across it as it rotates. The pivot has to be supported while this is being done and my home-made Jacot tool is provided with a disk which has a variety of semicircular and vee grooves cut into the periphery (Figure 6). Prior to burnishing the end, I rounded it using a fine file. Note the use of a ball race to support one end of the burnisher and to keep it parallel to the axis of the workpiece. It then remained only to polish off the tempering colours from the staff, purely for cosmetic reasons

Figure 6: Support for the pivot while being burnished.

Figure 6: Support for the pivot while being burnished.