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THE MAIN PLATE

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1. General description

The main support for the assembly of a movement’s components, the Main plate forms its frame and main element. With the bridges that are directly attached to it, they form the Movement frame (cage).

It is between the main plate and the bridges that the moving elements of the movement are freely fixed. These can pivot around their own axis between bearings (jewels) (Barrel, finishing train, Balance, etc.), or around a pivot machined or press-fitted into the main plate (Setting lever, Crown wheel, etc.). Some fixed elements (springs, plates) can be firmly assembled to it by screwing, press-fitting, or even welding or gluing.

To ensure perfect interaction between all the components fixed to it, a Main plate must offer maximum rigidity and the lowest possible Expansion coefficient.

The Main plate can be made from Materials as diverse as Brass, German silver, Gold, Sapphire, Carbon, etc.

Like the bridges, the Main plate can benefit from a wide range of artisanal decorations (hand Engraving, skeletonizing, beveling / angling, etc.) or industrial ones. Traditionally, brass main plates are protected from oxidation by electroplating treatments (galvanic treatments) (gilding, rhodium plating). Nowadays, hi-tech surface treatments (PVD, DLC (Diamond-Like Carbon), CVD) offer a wider range of colors and properties.

Although the main plate is usually round, it can come in all sorts of shapes, generally to match the design of the case it is intended for (tonneau, rectangle, oval, etc.).

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2. History

The technique for manufacturing main plates has evolved over time. Once made using fixed gravers, these were replaced during the industrial era by lathes and milling machines drawing their power from motive forces (hydraulic) and then from electricity. During the 20th century, these machines became automatic (mechanical programming of operation sequences by cams). Finally, the advent of computing saw the emergence of the first machining centres (versatile machines capable of performing milling, turning and drilling operations) with numerical control (digital automation of the entire manufacturing process). Auxiliary artisanal techniques (Stamping) or modern and industrial ones (laser machining, Electrical discharge machining (EDM), etc.) can, in certain cases, complement the manufacturing process, or even replace it entirely.

Historically, main plates are made of Brass or in German silver. Very close in their compositions, these two alloys offer ideal mechanical properties for their machining and decoration. Appearing at the beginning of the 19th century, German silver is an alloy of brass (copper and zinc) to which nickel is added. German silver has a silvery color and resists corrosion so well that it does not require electroplating treatment (rhodium plating or gilding, for example). From the end of the 20th century, many synthetic or composite materials have been used to manufacture main plates. These include synthetic corundum (synthetic sapphire), ceramics, carbon, etc.

The diameter of the main plate defines the calibre of the movement and is still measured today in lines (unit of measurement from the Middle Ages. 1 line = 2.255 mm).

Usually, it is on the Main plate that the Master’s hallmark (manufacturer), the Calibre number (model name) and the individual Movement number are engraved or stamped.

3. Craft production methods

Several methods and different tools can be related to the craft method.

Before machining the Main plate, the Horologist (watchmaker) must carry out a fundamental step requiring absolute precision. For optimal performance of the Movement, the Axis of each component must be perfectly positioned relative to the others. Marking out is carried out using a marking-out machine. It will define the centers of the various holes to be drilled and the turning operations to be carried out later.

With the centers marked, the horologist can then proceed with the various numerous drillings (axes of the wheels, tenons, screw holes, foot holes, etc.). These will now be perfectly positioned and will allow optimal indexing of the fixed graver for the different turning steps.

The fixed graver is the craftsman’s tool par excellence. It is a Lathe miniature bench lathe that the horologist usually secures in a vice. The horologist first manually sets the depth of cut (pass). He then simultaneously operates a crank that drives the lathe and another that moves the graver (cutting tool). By combining the rotation speed of the main plate and the displacement speed of the graver, the horologist (watchmaker) adapts the combination of these two speeds according to their sensitivity in order to obtain a precise result in terms of dimensions and finishing, guaranteeing both aesthetics and proper functioning.

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

4. Industrial production methods

The emergence of numerically controlled machines and their widespread adoption from the late 20th century onward established them as the main tools for producing main plates. While a few craftsmen, or even a few prototypists, still use a handcrafted method, the vast majority of main plates are produced on CNC machining centers. Generally, these machines allow all the machining operations required for a main plate to be carried out (milling, drilling, tapping, etc.). Depending on the needs and the technical specifics of the main plates to be machined, the machines may have different capacities (3, 4, 5, 6 axes). A conventional main plate can generally be perfectly machined on a 3-axis machine. This machining method, due to the setup and implementation time it requires, favors the profitability of large-scale production runs. However, it is also appreciated for small series and prototyping, for the versatility and “standardization” of its use.

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

According to industrial practices, the main plate, once fully machined and decorated, will receive a surface treatment to protect it from oxidation and enhance its aesthetics.

While electroplating treatments (galvanic treatments) (rhodium plating, gilding, etc.) are still widely used, treatments by physical vapor deposition (PVD, DLC (Diamond-Like Carbon), etc.) are increasingly imposing themselves. These latter treatments have reached an optimal level of quality, they are harder and therefore less vulnerable to impacts and scratches than an electroplating treatment (galvanic treatment) and offer a range of colours that keeps expanding while being able to perfectly reproduce galvanic colours.

Before receiving all the components of the Movement, the Main plate must first be pre-assembled. All the “fittings” of the Main plate, such as the jewels, pins, and feet, will be fixed to it.

All these operations (industrially called T-0) can be carried out by the Horologists (watchmakers) who will proceed with the Assembly of the Movement, by specialized operators, or even be entirely automated.

5. High-tech production methods

High-tech technologies can be implemented at certain stages of Main plate manufacturing. Electrical discharge machining (EDM) and laser cutting can, for example, be useful for Openworking, Skeletonizing, or even Decoration. When non-metallic Materials are used (ceramics, plastics, composites, Sapphire), Sintering or injection sometimes precedes traditional machining operations.

In these cases, machining operations will always require high-technology machines and tools, along with expertise equal to the technical challenges (e.g.: Hardness of Materials) and tolerances. Surface treatments (generally subcontracted) increasingly stem from advanced technologies (PVD, DLC (Diamond-Like Carbon), CVD, etc.).