Working on the base

Uhren-Magazin 11/2005

The bottom plate: the foundation upon which the movement rests. Together with its colleagues, the bridges and cocks, it ensures that everything which turns, ticks, and spins in the watch holds together. It is also involved in the creation of the solid trio.

The beginning and end of the watch movement, its foundation and closing: the front and back plates encompass the watch’s movement. They are more than mere discs – they function like maps that set the stage for the assembly of the watch movement. They provide indication of the complex structure that will be built up by processes of turning, milling, drilling, and thread cutting. With precisely positioned drill holes, they provide firm grounding to all the staffs, bridges, and cocks in the motion work. Push-fitted jewel bearings facilitate the taking-up of pivots, recesses provide space for the gear wheels, pinned or bolted pillars fasten the front and back plates together and maintain the space between them.

Although it bears the watch dial, from the watchmaker’s point of view, the bottom plate is the foundation of the watch, upon which all the movement’s components and mechanisms will be constructed. The assembly of the watch movement always begins with the plate. It has the same shape as the entire watch movement and is generally made from solid brass (copper and zinc), although also occasionally from German silver (copper, nickel, and zinc.) For the most part, this material is plated with nickel, silver, gold, or rhodium. As a rule, the bridges and cocks are also made from brass.

Whereas the bottom plate has the same shape as the entire watch movement, the upper plate is fashioned with cut-outs for the balance wheel and escapement. Some sections can also be removed and replaced with individual bridges or cocks. These are the plate’s colleagues: bridges serve as bearing carriers for the movement’s wheel staffs. Moving parts such as the pinions of the motion work and the balance staffs must be secured on both sides. The bridges are secured to both ends of the bottom plate with pins and screws and span bridge-like (hence the name) over the plate and the wheels. They have recesses cut into their underside which carry the jewels that accommodate the upper pivots of the wheels. As a rule there are four bridges: one each for the barrel, the balance wheel, the train, and the lever. However, there are many other variations; for example the so called three-quarter plate can supplant three of these bridges.

Not to be confused with the bridges are the cocks, which lie on only one side of the plate, are usually fastened with only one screw, and project out freely over the plate. Like the bridges, they also serve as bearing carriers for the watch movement’s wheel staffs. Cocks are primarily used for the balance staff. Like the bridges, cocks are affixed to the bottom plate with alignment pins. If the watch movement is disassembled, these pins ensure that the bridges and cocks are sitting exactly in their original position upon reassembly. Alignment pins are fitted on the underside of the cocks and bridges and fit precisely into their corresponding holes in the bottom plate.

To ensure that everything turns out just as described here requires the utmost precision in the manufacturing of each and every part. This is especially true for the bottom plate, since every part in the watch movement connects to it. “The bottom plate is the most important part of the watch movement,” states Christian Gammel, production manager at NOMOS. Not only plates, but also bridges and cocks are produced directly in the studios of this Glashütte watch manufactory. This article only provides a description of how plates are made at NOMOS, since the individual procedural steps for producing each of these parts are nearly identical.

The raw material for plates is provided by sheets of brass, from which discs are blanked. These are greater in diameter than the finished bottom plate – the excess end serves as a retaining ring with which the blank can be secured while it is being processed. This is just the beginning of an extensive program for the brass disc. First it is grinded to the appropriate dimensions between two grinding wheels – this procedure is also called “lapping.” The goal is to reach the desired thickness within five-thousandths of a millimeter, while also achieving absolute parallelism and evenness. The blank is annealed both before and after the grinding in order to reduce internal stress acquired during processing. Furthermore, after every step in the production (including this one), the parts are meticulously cleansed in an ultrasonic bath. Likewise, repeated quality controls and measurement checks are carried out.

After the lapping comes the next step in the procedure: blanks are inserted into a CNC machine in stacks of 20 at a time. First contours are milled and holes are drilled on one side of the blank; these will later accommodate pins, jewels, and wheels. After one side is completed, the discs are removed. Only after they have been thoroughly inspected and measured are they placed back into the machine for work on the other side. Throughout this whole operation, the watchmaker must continually exchange instruments – quite an elaborate procedure. It takes about five hours for the ultra-modern CNC machine at NOMOS to process these 20 plates.

At this point, the plate no longer requires the retaining ring. The excess end of the blank is trimmed away until the plate reaches the correct diameter. Then it ends up on the artisan’s work table. Using a magnifying glass, the part is deburred; this means that the thin, sharp edge created during processing is grinded down. At the same time, metal shavings are removed and the plate is blasted. This process is analogous to sand blasting; however in this case a fine bronze powder is blown onto the plate using compressed air. The result is a smooth, velvety surface. Now that the plate has been perfected and beautified, the assembly can begin. Synthetic ruby jewels are placed into the drill holes by hand and pressed in with the help of a machine. The same is then done with the pillars and pins.

At this point, the only remaining concern is the plate’s appearance. A decorative finish is ground onto the plate with a hand-operated machine. At NOMOS the bottom plate is adorned with the rounded perlage pattern. This can be found on the dial side of the bottom plate as well as on the pallet cock and the balance cock on the side of the gear train. The three-quarter plate and the balance cock bear the Glashütte ribbing, analogous to Geneva stripes. Following the decorating, work on the plate is completed in a galvanizing bath. The parts are plated first with nickel and then with rhodium. This layer of rhodium not only makes the part more optically appealing, but also protects the brass. These new layers are razor-thin, measuring a few thousandths of a millimeter, thereby allowing the intricate decorative finish to remain clearly visible.

Now the plates, bridges, and cocks are ready to be sent to the watchmaker. By his or her skillful hands, this stabilizing trio will be complemented with gear wheels, a balance wheel, a barrel and all the other parts necessary for the watch movement to perform its job.

Iris Wimmer-Olbort