Feature: How On Earth Does A Minute Repeater Work?

If you’re looking for a complicated watch to beat all other complications, perhaps you’d consider a tourbillon. Bold, dramatic and more fiddly than inserting an HDMI lead without looking, it’s surely one of the most impressive visual displays of high watchmaking. And the perpetual calendar, with its ability to tell you your precise location in time for many, many decades without adjustment, is surely one of the most staggering feats of micro engineering achievement. How about this little Cartier, then? It seems tame, but looks can be deceiving, because inside it hides the most incredible complication ever made.

Behind the dial of this Cartier Tortue lies a secret. Not a mysterious, intriguing, exciting secret, but one of a more practical, human kind. We envisage the great master watchmakers of Switzerland to have an almost mythical status, wizened men of great age assembling the finest complications with ease.

That’s true, to a point, and that point is the minute repeater. This mechanism, that chimes the hours, quarters and minutes on demand, is so notoriously complex, so delicately appointed, that it has earned itself a reputation amongst watchmakers of being an utter b*****d to make. In fact, watchmakers at Jaeger-LeCoultre have dubbed its vertically stacked variation of this mechanism ‘The Infernal Tower’ because it is so difficult and frustrating to construct.

So how can such an unassuming little watch be such a tremendous pain in the backside? Imagine building a Rube Goldberg machine, with all its levers, ratchets, springs and switches, but it’s the size of a penny. It’s a mechanism of over one hundred parts that all have to operate to tolerances of fractions of a millimetre; if a single piece is just a hair out of place or misshapen by the finest sliver of material, the machine will stop, and the chimes will cease. Even the very act of construction can deform the components by enough of a margin to prevent the mechanism from working properly.

Understanding what a minute repeater has to achieve and how it has to achieve it begins to unravel the reasoning behind this professional frustration. This digital age where code can be used to ask and answer questions of an operation makes visualising this analogue approach alien and unfamiliar, but it’s worth understanding just what makes a minute repeater so much more complex than, say, a chronograph.

For example, a chronograph is also an on-demand function, but it is still simply just a secondary timekeeping mechanism, connected to the primary train at request via a clutch. Think of a car, the constantly running engine connecting with the wheels to provide motion on demand—it’s simply a case of on or off, an input that’s provided manually by the driver. A chronograph user requests the same actions of a chronograph, asking for it to start, stop and reset by the push of a button.

A minute repeater advances this decision-making process a step forward, taking an input and accomplishing a calculation. With a chronograph, the user’s request generates the same response every time; with a minute repeater, it is always different. The mechanism is required to sample the time and read it back, requiring a mechanical process that is somewhat smart. It’s like a car with an automatic gearbox, making its own decisions dependant on conditions once it is put into action—only this mechanism has to fit inside a watch.

But that’s enough fluff—how exactly does a minute repeater do what it does? How does a pile of a hundred parts have the brains, when assembled, to do what it took me the first six years of my life to learn and read the time? To be honest, the complexity is so great that to understand the whole is the work of a lifetime, such is the number of systems at play all at once, but we can at least break down the major elements of the minute repeater and attempt to bring clarity to them.

The major aspect of the minute repeater is the sampling mechanism itself, the elements that read the time and report it to the chiming system. Stepped snail cams, that rotate with the hour and minute hands, begin the translation of the analogue time display into a digitally reported sound. Upon activation of the minute repeater, levers—called racks—are pivoted against the cams, one for the hours, one for the quarters and one for the minutes. The steps in the cams allow the tips of the racks, known as beaks, to fall within varying degrees of the centre of the cam, with each step corresponding with a specific time.

As the rack resets, a row of teeth on its opposing side trigger the chime one by one. So, the closer to the centre of the cam the rack gets, the more teeth on the opposing side return past the chime, the more chimes are sounded. This happens three times: once for the hour, whose cam has twelve steps, one for each hour; once for the quarters, whose cam has four steps, one for each quarter; and once for the minutes, whose pinwheel-shaped cam offers four quadrants of fifteen steps to cater for the minutes between the quarters. And this is only the beginning.

What you’ve got to understand is that everything we’ve just discussed thus far is the very barest of bones of the minute repeater mechanism. We still need to cover how the mechanism powers its timing, regulates itself, stops itself from jamming and prevents misuse—all aspects that have been addressed within this miniscule space. We’ll start with the power: the lever on the side of the case has a resistance when activated, and that’s the force of the mainspring being wound. And not the main mechanism’s mainspring, no—the minute repeater is such a power hog that it gets a mainspring all of its very own.

And then to regulate the timing of the mechanism, to prevent it spilling all its chimes at once, is the governor. This is a spinning mass with weighted arms that extend as it spins, much as a figure-skater would to control the speed of rotation. Then there’s the surprise piece, so-called for the speed of its appearance when called into action. The surprise piece hides between the quarter and minute cams—which turn continuously, unlike the hour cam, which is driven in increments—only popping into action to extend the very topmost step between quarters to prevent the rack from slipping and falling back onto the step previous.

And then there’s the ‘all-or-nothing’ piece, a sprung lever that blocks the minute repeater mechanism until the lever has travelled its complete distance, and also whilst the minute repeater is in action. All these processes occur in sequence, perfectly every time. Never mind trying to make one—just talking about it is making my brain hurt.

The old adage, ‘never judge a book by its cover’ has never been more apparent than with the minute repeater. It’s a monumental undertaking, an incredible mechanism and a complete pain in the backside to work with. To enjoy the clear and sweet chimes with perfect accuracy is nothing more than the slide of a switch for the user—for a watchmaker, however, it’s a lifetime of learning and one massive headache.

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