(Above, the Piaget 20P/Lassale Caliber 1200, the thinnest hand-wound movement ever made.)
In Part One of this story, we looked at the history of ultra thin watchmaking – at why the earliest watches were so thick, and how advances in watch movement design, combined with the evolution of modern clothing styles, drove the development of thinner watches. We also looked at how and why ultra thin watches became, not only fashionable, but also came to be seen as a manifestation of the highest level of skill in watchmaking. Today we'll take a look at exactly why making an ultra thin watch is considered such a challenge – as well as at some of the ways in which modern fine watchmaking houses continue to develop the art.
Above is the JLC caliber 849. This is one of the classic ultra thin movements of the 20th century, and at 1.85 mm thick, it's an excellent one to look at in terms of why making thin movements is so challenging. There are a number of reasons it's harder to make a very thin watch, and make it work well, than it is to make a thicker watch. First of all, there's the question of power. If you want a watch to run accurately, it's essential to have adequate power fed through the gear train to the balance wheel, so that it can oscillate at a large enough amplitude (amplitude just means how much the balance swings, and is generally expressed in degrees) to maintain accuracy across the working power reserve of the watch. The amount of power you can get from a mainspring is generally dependent on the height of the spring, and of course, in an ultra thin movement there is considerably less height available. That means that the movement has to be made and assembled very carefully and precisely, in order to avoid excessive loss of power due to friction.
Another reason it's challenging to make a very thin movement is that as a rule, you have to make significant changes in movement architecture. As an example, in most watch movements, the mainspring barrel has two pivots – one running in a bearing in the mainplate, and the other in a bearing in the barrel bridge. The JLC caliber 849 has what's called a "hanging" barrel (which was originally invented by Lépine, who you'll remember from Part 1 of this story). A hanging mainspring barrel has no upper bridge; instead, it runs only in one pivot: the one in the mainplate. The construction is inherently less stable and again, requires a lot of care in manufacturing and design to work well, but getting rid of the upper barrel bridge saves precious millimeters and is essential, in the 849, in bringing in the height at under 2 mm.
Now let's take a look at a modern self-winding ultra thin movement. Above is the Audemars Piguet caliber 2121. (The simplest version of this movement is the caliber 2120, which omits the date wheel; the 2120 is 2.45 mm, and the addition of a date wheel adds to the height only slightly, bringing it up to 3.05 mm.) This movement was first produced in 1967 and at the time it debuted, it was the thinnest full rotor movement in the world. Like the JLC caliber 849, it's significantly different in many respects from a conventional self-winding movement, including its use of a hanging (or as AP calls it, a "suspended") mainspring barrel. One of the most interesting features of the caliber 2120 and its variants is the suppression of the thickness of the oscillating weight, by moving most of its mass to the periphery. The problem with this strategy is that the weight isn't very stable and in the 2120, there is an ingenious solution: the weight is supported by ruby rollers on its underside, which run in the circular rail you can see at the edge of the movement.
An interesting feature of the caliber 2120 and variants, is that despite its slim profile, it readily supports complications, such as a perpetual calendar. Below is the caliber 2120/2802, dial side – a perpetual calendar movement, as seen in the Jules Audemars Perpetual Calendar.
Now, the caliber 2120 (which is used today by Vacheron Constantin as the caliber 1120 as well) is the thinnest full rotor self-winding movement in the world, but there's a way you can trim even more height, and that's to use a micro-rotor. The advantages of the micro-rotor are obvious – the winding weight is in the movement, not on it. The problem is that because the diameter of the winding weight is much smaller than it would be in a full rotor movement, there's a major leverage disadvantage, so again, it takes very careful design, manufacturing, assembly and adjustment to get acceptable performance. One of the most impressive achievements in the use of a micro-rotor in an ultra thin movement was the introduction by Piaget (a name that's practically synonymous with ultra thin watchmaking) in 1960, of the caliber 12P.
Caliber 12P is only 2.3 mm thick and even today, it would be an impressive engineering feat – in 1960 it was an incredible accomplishment and did a lot to secure the reputation of Piaget as, in some respects, the watchmaking firm when it comes to ultra thin watchmaking – and this at a time when "elegant watch" would almost universally have been agreed to be three things: thin, gold, and automatic.
Movements like the calibers JLC 849, the Piaget 12P, and the Audemars Piguet 2120/Vacheron Constantin 1120 probably represent a certain practical limit on how far you can push ultra thin watchmaking using conventional materials. Attempts to go even further did not end well, in general.
Above is the 1.2 mm thick late (and largely unlamented) Jean Lassale caliber 1200, from 1976. In its time it was a technical miracle, and unfortunately, it was a mechanical technical miracle – thanks to the advent of quartz technology – produced at an inauspicious time for a new mechanical movement of any kind, much less a small, incredibly thin, and incredibly problematic one. By 1979 production (by Bouchet-Lassale, named for founder Jean Bouchet-Lassale) had ceased production. The name Lassale was bought by Seiko, who put it on the dials of ultra thin quartz dress watches, and the patents were bought by Nouvelle Lemania, who continued the production of the hand-wound caliber 1200 (and ultra thin Lassale automatic caliber 2000) for a few more years before production eventually ceased. Piaget used the Lasalle 1200 as the Piaget caliber 20P, which is the version you see here; Vacheron Constantin also used it, as the Vacheron caliber 1160, and how they managed to get Geneva stripes on them is a matter of wonder; you'd think there wouldn't be any surplus metal left to remove.
Alas, for Jean Lassale, the problem wasn't just bad timing; the movements were simply too thin to be reliable. The movements were so thin that they could be irreparably damaged just by opening the watch case and at both Vacheron and Piaget, servicing the watches when they came in often meant simply discarding the movement and installing a new one. The extreme thinness was achieved by putting all the train wheels in the baseplate itself – since there were no bridges, not only the mainspring barrel but all the gears as well, were "hanging" and had to be supported in ball bearings. This is generally a bad idea for watch movements, as the bearings can introduce undesirable variations in power flow. But it was a bold design, if ultimately doomed.
In our next installment, we'll look at how some of Lasalle's ideas, and other ingenious concepts, eventually bore fruit, and how modern watchmakers are using cutting edge manufacturing methods to take some initially unsuccessful ideas and turn them into resounding successes.
Check out Part 1 here.
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