Components › Movement

THE BRIDGES

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1. General description

With the Main plate, the set of bridges constitutes the structure of the movement (referred to as the movement’s cage). It is between the main plate and its bridges that the moving elements of the movement are held (wheels, pinions, Pallets (lever), Balance, etc.). The lower and upper surfaces of the bridges also serve as mounting supports for numerous fixed components (springs, Regulator assembly, etc.).

Due to their structural function, the bridges must be perfectly rigid. This constraint calls for the use of a large number of Materials. Traditionally made of Brass, bridges are today frequently made of Gold, sapphire, Carbon, etc. The design of the bridges, their number and arrangement are structured around the position of the pivot points of the finishing train in order to represent a harmonious structure highlighting the succession of curves of the finishing train.

2. History

With the exception of monumental clocks (towers, bell towers, etc.) whose trains move while being held by an iron frame (chassis, cage), the wheels of watches and clocks always move between a Main plate and bridges or between two main plates. The most visible bridges of the earliest watches (16th and 17th centuries) were already often finely decorated (Openworking, engraving, etc.). Between the end of the 18th century and the beginning of the 19th century, the use of bridges dedicated to a component or a group of components became widespread. Initially raw in manufacture and of simple geometric shapes, bridges quickly became aesthetic elements (worked shapes) finely decorated (engraving, setting (stone/jewel setting), enamelling, skeletonizing, etc.).

With the era of industrialization, machined surfaces betray the use of machines. Efforts are made to eliminate machining marks, and this is achieved through processes also derived from industrialization (circular graining, Geneva stripes, etc.). Since the second half of the 20th century, new Materials are sometimes used for manufacturing Bridges (Sapphire, composites, etc.). Advances in technology now make it possible to manufacture very high-precision ball bearings of very small dimensions. Although their efficiency is not as good as a construction between Main plate and Bridges, they can, in some cases, allow a wheel to be held by only one end of its Axis, in the manner of a cage of Tourbillon flying. This option will be preferred for aesthetic reasons or for an ultra-thin construction.

3. Traditional manufacturing method

Several methods and different tools can be similar to the traditional method.

The first step consists of cutting out the outline of the Bridge shape. According to the traditional method, this cutting will be done using a Piercing saw (jeweler’s saw). The sides will then be filed to bring them to their final dimension and soften the surface.

Before machining the recesses of the Bridges, the Horologist (watchmaker) must carry out a fundamental step that requires absolute precision. For optimal performance of the Movement, the Axis of each component must be perfectly positioned in relation to the others. The marking is carried out using a marking machine. It defines the centers of the various drillings and turnings that will be carried out later.

With the centers marked, the Horologist (watchmaker) can then proceed with the various and numerous drillings (Axes of the wheels, pivots, screw holes, foot holes, etc.). These will now be perfectly positioned and will allow optimal indexing of the Fixed graver for the various turning steps.

The Fixed graver is the craftsman’s tool par excellence. It is a small Lathe bench that the horologist usually fixes in a vice. The horologist first manually sets the depth of the cut (pass). He then simultaneously operates a crank that drives the lathe and a second one that moves the graver (cutting tool). By combining the rotation speed of the bridge and the movement speed of the graver, the horologist adjusts the combination of these two speeds through his sensitivity in order to achieve a precise result in terms of dimensions as well as finishes that ensure aesthetics and proper functioning.

A bench lathe or a bed lathe (motorized) often replaces the fixed graver. The use of such a machine does not call into question the artisanal nature of the method. It offers a relative time saving compared to the fixed graver and allows the same technical and aesthetic quality as the fixed graver to be achieved.

4. Industrial production method

The emergence of numerically controlled machines and their widespread adoption from the end of the 20th century established them as the main production tool for Bridges and Main plates, among others. While a few craftsmen or prototypists still use a traditional method, the vast majority of Bridges are produced on computer numerical control machining centers. Generally, these machines can perform all the machining operations required for a Bridge (Milling, drilling, tapping, etc.). Depending on requirements and the technical specifications of the Bridges to be machined, the machines may have different capacities (3, 4, 5, 6 Axis). A “conventional” Bridge can generally be machined on a 3-Axis machine. This machining method, due to the setup and implementation time it requires, favors the profitability of large-series production. It is also appreciated for small series and prototyping for its versatility and the “standardization” of its use.

Depending on the watch range, some or all of the operations of decoration can be carried out during machining. Today, it is possible to diamond-polish an angle or mill Geneva stripes on the same machine that machined the bridge and during the same work cycle. High-end production will often use more artisanal decoration methods (deburring, beveling/angling, engraving, etc.).

Depending on industrial practices, once the bridge has been fully machined and decorated, it will receive a surface treatment to protect it from oxidation and enhance its aesthetics.

While electroplating treatments (galvanic treatments) (rhodium plating, gilding, etc.) remain widespread, treatments by vapor deposition (PVD, DLC (Diamond-Like Carbon), etc.) are increasingly gaining ground. These have reached an optimal level of quality, they are harder and therefore less vulnerable to shocks and scratches than an electroplating treatment, and offer a color palette that keeps expanding while still being able to reproduce electroplated colors.

Before it can accommodate all the Movement components, the Bridge must first be pre-assembled. All the “fittings” of the Bridge, such as the jewels, the shoulders, and the feet will be fixed to it. All these operations (industrially known as T0) can be carried out by the horologists (watchmakers) who will assemble the Movement, by specialized operators, or even be fully automated.

5. High-tech production methods

As with main plates, high-tech technologies can be used for certain stages of Bridge manufacturing. Electrical discharge machining (EDM), laser cutting may for example be useful for Openworking, of Skeletonizing or even of Decoration. In the case of using non-metallic Materials (ceramics, plastics, composites, sapphire), injection will sometimes precede the more traditional machining operations. In these cases, the machining operations will require high-tech machines and tools and expertise equal to the technical challenges (e.g.: material Hardness) and tolerances. Surface treatments (generally outsourced) largely stem from high technologies (PVD, DLC, CVD etc.).