Introducing the Audemars Piguet Royal Oak Perpetual Calendar Ultra-Thin

The Royal Oak RD#2 comes true.

A year ago, Audemars Piguet debuted the Royal Oak RD#2 Perpetual Calendar Ultra-Thin, a concept watch that claimed the title of the world’s thinnest automatic perpetual calendar by some margin.

Five years in the making, the RD#2 checked in at 6.3mm high, including the case, with the movement inside just 2.89mm – a third, or 1.42mm, thinner than the brand’s next thinnest perpetual calendar movement, the cal. 5134.

Audemars Piguet Royal Oak RD#2 Perpetual 4

The RD#2 concept watch unveiled at SIHH 2018

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The guilloche dial of the RD#2

While most concept watches typically have their technology streamlined when translated into regular production, the Royal Oak Selfwinding Perpetual Calendar Ultra-Thin is fundamentally the RD#2, with only a few cosmetic tweaks.

While the RD#2 was entirely in platinum, case and bracelet, the production model is made more wearable – and affordable – with a matte, brushed titanium case and bracelet but accented with the bezel and bracelet centre links in polished platinum.

But classic Royal Oak case finishing that combines brushed top surfaces and polished bevels, however, remains intact.

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar Ultra-Thin 4

And instead of the signature tapisserie guilloche of the Royal Oak that was found on the RD#2, the dial of the new perpetual calendar has been simplified, giving it a lighter and less cluttered aesthetic. The dial is dark blue and finished with vertical brushing, making the hands and hour markers more prominent.

It’s a wise decision, since the dial has more sub-dials than the ordinary Royal Oak Perpetual calendar, with the additions of a day and night as well as leap year indicator that overlap with the larger sub-dials.

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar Ultra-Thin 5

The day and night indicator at eight o’clock is now a single colour, instead of blue and white as on the RD#2

Despite the minor facelift, the watch is very much a Royal Oak. In fact, the dimensions mean it is physically reminiscent of the Royal Oak, the original ref. 5402 “Jumbo” from 1972.

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar Ultra-Thin 3

A new movement inspired by the past

The watch measures 41mm wide and 6.3mm in height, which is 3.2mm slimmer than the ordinary Royal Oak Perpetual Calendar. That’s all thanks to a newly developed movement that is ironically derived from one of the oldest, continuously produced movements in the industry.

The ultra-thin perpetual calendar is powered by the self-winding cal. 5133, based on the cal. 2120 found in the Royal Oak “Jumbo”. Consequently, it is one of the few ultra-thin movements with a full rotor, instead of a micro-rotor. In fact, the full rotor of the cal. 2120, and by extension the cal. 5133, is one of its signature features.

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar Ultra-Thin 8

In production for half a century, the cal. 2120 was originally developed by Jaeger-LeCoultre as the cal. 920, but is now produced in-house by Audemars Piguet. A record breaker when it made its debut, the cal. 2120 features a “suspended barrel”, which means it has no upper bridge and runs only on the pivot in the base plate.

Audemars Piguet Royal Oak Salmon white gold 15202BC 04

The cal. 2120 and its distinctive beryllium supporting ring

In addition, to reduce the thickness of the rotor, most of its mass is moved to its periphery. And to ensure greater stability, the rotor is supported by ruby rollers on its underside and its most distinctive element: a beryllium ring.


The new cal. 5133, again with a beryllium ring; also note the similarities in the winding, crown and barrel ratchet wheels

Not only is the new movement thinner, it is also simplified, being made up of 256 components as compared to 374 in the cal. 5134 inside the standard Royal Oak Perpetual Calendar (which is a cal. 2120 with a module on top).

Reducing the height of the movement so significantly meant the movement grew sideways. The cal. 5133 measures 32mm in diameter, as opposed to 29mm for the cal. 5134.

Audemars Piguet describes the movement engineering as akin to collapsing a three-storey building into one, spreading out the combined floor area over a large single storey.

Audemars Piguet Royal Oak Perpetual Calendar Ultra Thin

To achieve this, two patents were filed. The first concerns the date wheel which has an integrated end-of-month cam. It features a single irregular tooth that is deeper than the rest and advances the month disc by one increment at the end of the month.

Audemars Piguet Royal Oak Perpetual Calendar Ultra Thin 3

The date wheel and month wheel.

But what’s more impressive is the second patent, which redefines the 48-month wheel with a novel geometry using deep, curved grooves. This reduces the required tolerances of the levers, leading to a more reliable mechanism.

The deepest grooves represent the month of February with 28 days, while the shallowest grooves represent the months with 31 days. To indicate the leap year, the deepest groove has a corresponding notch that marks the February with 29 days. It is these two novel geometries that allow for a thinner calendar module.

The movement has a frequency of 2.75Hz and despite its thinness, offers a respectable, albeit shortish, 40-hour power reserve.

Key Facts

Diameter: 41mm
Height: 6.3mm
Material: Titanium case with a platinum bezel
Water resistance: 20m

Movement: Automatic cal. 5133
Frequency: 19,800 beats per hour, or 2.75Hz
Power reserve: 40 hours

Strap: Titanium bracelet with polished platinum links and titanium AP folding clasp.

Price and availability

The Royal Oak Selfwinding Perpetual Calendar Ultra-thin (ref. 26586IP.OO.1240IP.01) is priced at 140,000 Swiss francs, excluding VAT. It’s now available at Audemars Piguet boutiques, but in limited numbers so expect a long wait.


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The Latest in Silicon Hairspring Tech – From Hong Kong

Master Dynamic and its patented hairspring.

There’s a lot more than meets the eye in the hairspring pictured above.

At a glance it looks like just another ordinary hairspring, which makes up half of the regulating organ that keeps time in a mechanical watch, but this hairspring is the result of multiple innovations over the past decade – and it’s made in Hong Kong.

To start with, the hairspring is made of silicon. While the material in itself isn’t new, keen observers will notice the shape of the hairspring; it is silicon but features an overcoil.

Hong Kong high tech

Master Dynamic, a Hong Kong-based engineering outfit, developed the hairspring and received a patent for it in 2017 – United States patent US9658598B2. The patent covers the design of the hairspring, as well as the specific type of silicon used, which differs from the type used for Swiss silicon hairsprings.

Founded in 2010, the company employs over 130 engineers, whose work encompasses diverse fields, ranging from nano-engraving on diamonds to material analysis, and also production of micro-components in metal and silicon.

The silicon hairspring was Master Dynamic’s first major invention in watchmaking, making them one of the few companies anywhere able to develop a silicon hairspring from scratch. In fact, the players outside of Switzerland are just two: Master Dynamic is the only company in Asia making silicon hairsprings, with Firehouse Horology in New York City being the only one in the United States.

It was the Swiss, however, who pioneered the use of silicon in watch movements. Ulysse Nardin introduced the first wristwatch with silicon components in 2001, when the Freak made its debut. Thanks to Ulysse Nardin’s collaboration with Centre Suisse d’Electronique et Microtechnique (CSEM), the Swiss research institute responsible for developing silicon movement parts, the Freak was equipped with both its hairspring and escape wheels in silicon.

Ulysse Nardin Freak 1.5 sothebys 2

The original Ulysse Nardin Freak

Silicon hairsprings are now the domain of a consortium led by CSEM and backed by three watchmaking giants: Rolex, Patek Philippe, and the Swatch Group. Thanks to the patents filed by its backers, the consortium has a monopoly on the use of silicon hairsprings in Swiss watchmaking, helped by its willingness to defend its territory. That was demonstrated recently when Baume & Mercier, owned by luxury conglomerate Richemont, had to end its use of silicon hairsprings after being threatened with a lawsuit.

Why silicon?

Silicon boasts several inherent benefits over traditional metals. It does not oxidize or creep over time, thus maintaining its mechanical properties and performance over extended use. It is also non-magnetic, and resilient to temperature changes when coated with silicon dioxide, both tangible advantages in the daily function of a wristwatch.

The production of silicon components also allows for higher precision over its metal equivalents. Silicon parts begin their life as a solid, cylindrical single crystal ingot grown in a crucible, which is then sliced into thin wafers. Components are then chemically cut out of each wafer, typically by deep reactive-ion etching (DRIE).

In comparison, metal hairsprings are traditionally made by drawing thin wire through progressively narrower rollers to flatten them, till the desired thickness is achieved. This process results in a wire with tolerances within a few microns, versus the sub-micron precision of silicon parts made via DRIE.

Despite being only microns, such variances translate into a noticeable errors in timekeeping on the wrist, as unwanted variations in thickness along its length affects the stiffness of a hairspring, which in turn greatly affects timekeeping accuracy.

master dynamic silicon hairspring 2

A DRIE etching machine

Restructuring silicon 

While the production process is generally similar, the structure of the silicon differs between the CSEM and Master Dynamic hairsprings. More specifically, the difference lies in the angle of the planes that the silicon is cut from.

Silicon is an anisotropic material – in other words, it has different physical properties depending on its direction or orientation. A common example of anisotropy is wood, which is strong along the grain but weak across it. Silicon is anisotropic because it has a crystalline, diamond cubic lattice structure. Therefore, cutting the silicon wafers at different angles will lead to different overall properties of the silicon parts.

master dynamic silicon hairspring 3

The crystalline structure of silicon. Photo – Wikipedia

Various forms of silicon wafers each have their own internal crystalline structure. CSEM’s patent for silicon hairsprings is specific to the production of hairsprings from (001) silicon wafers, which is the commonly used orientation with the DRIE process.

Meanwhile, Master Dynamic was exploring an alternative – the potential of hairpsrings made from (110) wafers instead, which are angled offset at a 45° plane to the (001) silicon. Consequently, this led to a new patent – different from CSEM’s – that highlights the benefits of using (110) silicon wafers.

In its patent filing, Master Dynamic notes that (110) silicon is more directionally sensitive than (001) – a property it claims to be beneficial as it makes it easier to fine-tune the hairspring’s stiffness via adjusting the part’s shape. The adjustment is possible with the precision of the DRIE manufacturing process.

Another convenient benefit of (110) silicon hairsprings is that its edges can be chamfered, unlike its (001) counterpart. This is thanks to its orientation, which facilitates the creation of bevels along the parts via wet etching after the DRIE process. By using chamfers, stress concentrations along sharp edges can be avoided which otherwise leads to brittle fracture of the silicon parts.

Ultimately, why do planes and wafer types matter? It turns out all that might be an answer to an old problem in watchmaking – isochronism.

Variable stiffness for consistent time

Isochronism refers to the constant oscillation period of the hairspring as the balance wheel goes “tick” and “tock”. In other words, it should keep the same, accurate time regardless of its orientation (face up or face down, for instance) or the state of its power reserve.

The problem is that the hairspring “breathes” – as it oscillates back and forth, the geometry of the rotating hairspring deforms and bunches up along one edge as per the image below. This causes a shifting centre of gravity and also induces a radial force onto the balance wheel staff, resulting in friction.

And this compromises isochronism – on full mainspring wind, the balance wheel swings in a greater arc versus a near empty mainspring, thus this friction force will vary resulting in timing variations as the watch operates.

master dynamic silicon hairspring 4

The “breathing” of a regular hairspring. Photo – Master Dynamic patent US9658598B2

Of course, friction should also be avoided as it results in faster wear of the parts.

This is where silicon fabrication technology is the ideal solution. Unlike rolled metal hairsprings, silicon is etched from a wafer and therefore can be made into any planar shape or form. Thus, its thickness along different sections can be varied.

Specifically, Master Dynamic has created a flat hairspring that has a stiffer bifurcated section at its outermost end. This eliminates the lop-sidedness of the breathing hairspring and maintains its concentricity during oscillation, which minimizes the radial forces and hence preserves isochronism.

master dynamic silicon hairspring 5

A flat hairspring with a bifurcated section for stiffness adjustment. Photo – Master Dynamic patent US9658598B2

As a bonus, the ability to fabricate silicon into any planar shape allows optimisation of the collet mounting, the end point in the centre that attaches to the balance staff. Compared to a regular metal hairspring that has a separate collet, the silicon hairspring and its collet are a single part, simplifying construction.

But the biggest issue with silicon is its brittle nature – a typical, rigid collet design may fracture from stress when mounted onto the arbour of the balance staff. To address that, Master Dynamic devised a clover-shaped collet, essentially a collet with an integrated spring, that has some flexibility as it attaches onto the arbour. This eases assembly and reduces the risk of damage.

master dynamic silicon hairspring 6

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The flexible collet mounting. Photo – Master Dynamic patent US9658598B2

The pursuit of isochronism – a silicon overcoil?

The brittleness of silicon also means cannot be manually adjusted by watchmakers, unlike malleable metal hairsprings that can be carefully tweaked by hand. That is not usually a problem, since freshly etched silicon parts emerges perfectly formed and can be installed without any adjustments. And this leads us to why the topmost photo is fascinating – it’s a silicon hairspring with an overcoil.

master dynamic silicon hairspring 8

The impossible made possible, a silicon hairspring with an overcoil

The overcoil was the traditional solution by watchmakers to resolve the isochronism issue. By bending a metallic hairspring such that the spring passes over itself, it changes the geometry of its oscillation and thus retains its concentricity as the spring breathes.

A skilled watchmaker is needed to bend hairspring by hand to form the overcoil, making it a standard feature in high horology watches historically. But it is rarely seen in mass-produced watches today, being too labour intensive and also redundant since good timekeeping can be achieved with low-cost, mass-produced movements.

Given the antimagnetic and thermal stability properties of silicon, a silicon hairspring with an overcoil would, in theory, be the ideal chronometer hairspring. However, as silicon is brittle, it is impossible bend a flat hairspring into shape as one would a metal hairspring. And because silicon hairsprings are etched from a wafer, they are, by definition, flat to start with.

Breguet was the first watch brand to introduce an overcoil on a silicon hairspring. It was achieved not by bending the hairspring, but by separating the overcoil and hairspring into two parts. A connector joins the two and raises the overcoil so it can pass over the hairspring, thus not requiring any bending of the silicon parts.

breguet silicon hairspring overcoil

Breguet’s two-piece silicon hairspring with overcoil. Photo – Breguet

Master Dynamic’s approach however, is more straightforward. They form the overcoil by folding the hairspring over itself, like closing a book. The trick in bending the silicon without brittle fracture is to apply continuous heat, which relieves the internal stresses of the material as it is gradually bent into shape.

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Overcoil bending process for silicon hairsprings. Photo – Master Dynamic patent US9658598B2

Creating an overcoil via this method results in a “kink” in the overcoil (21 in above image). In the current Master Dynamic patent, this is unavoidable due to the folding process of the two halves of the hairspring, which is presumably automated and done by machine. Nevertheless, the patent does briefly describe the potential for bending the hairspring in a manner akin to the traditional method used by watchmakers for metal hairsprings.

The future?

Master Dynamic already has the capability to produce its patented silicon hairsprings, but they have not been adopted by watch brands yet, although the company is exploring supplying hairsprings to several Swiss movement manufacturers. According to a company spokesman, at the time of publication of this article, Master Dynamic just received automatic movements from a major movement maker as a trial run.

Even though widespread adoption of the Master Dynamic hairspring is some ways off, such developments are notable as they open a new avenue into practical performance improvements for mechanical watches. A promising and potentially significant breakthrough came to market recently when TAG Heuer unveiled its carbon composite hairspring, which promises all of the advantages of silicon but without its fragility.

Just like synthetic rubies or modern lubricants, the day might come when traditional watchmakers readily accept the use of silicon parts in high-end watches as an equal of traditional components.

Correction June 13, 2019: Master Dynamic is not the only company outside of Switzerland making silicon hairsprings; Firehouse Horology is also producing them.

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