Under the Loupe/Jewelling

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Jewelling

Today, the functional jewels used in watch and clockmaking are made from synthetic corundum, most often in its "ruby" state. These synthetic, typically flawless, stones are created thanks to a method developed by Verneuil in 1892. In practice, synthetic ruby has many great advantages. It is very resistant to wear due to its hardness (9 on the Mohs scale). Its coefficient of expansion is negligible under the effects of heat and cold. It is by no means magnetic, and it is very chemically stable.

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Production of synthetic corundum is complex and expensive process, involving extreme temperatures, precision, and utmost cleanliness of the materials and tools. The level of quality and micro-proportions of these jewels, as demanded by the watchmaking industry, requires that these components be manufactured in a nearly perfect way. In addition to such demanding specifications, the jewels must also be highly polished, in order that friction may be reduced to a minimum where these parts come into contact with other components of a watch. Furthermore, in some cases, the surface of these pieces also undergo special treatment to augment the physical properties of the lubricants used on the contact surfaces.
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Types of Jewels in a Timepiece

There exists a variety of different forms of functional jewels in a watch, depending on their place and function. Below is a brief, though not exhaustive, list.

  1. Bushing (also spelled Bouching, from the French bouchon)
  2. Cap jewels
  3. Pallet jewels
  4. Roller jewels


How They Work

Jewelling takes on a variety of different functions in a timepiece. In the case of jewelled bushings, the jewels (also referred to as rubies) must ensure the correct centering, end-shake, and proper turning of the pivots. In modern watches, these jewelled bushings are friction set into holes pre-drilled into the mainplate and bridges. For this reason, the outer rim of these jewels are not as highly polished as the functional, lubricated surfaces. The correct positioning of these holes, and the corresponding holes in the center of the jewels, is essential to the proper running of a watch. Opposing holes must allow the wheels to run parallel between them and must also facilitate the correct depthing of the gear teeth from one wheel to another.

The roller jewel must be highly polished to reduce to a minimum the effect of friction in transferring energy from the pallet fork to the balance wheel. Its correct size and shape must also be considered in order to maximize this transfer of energy.

The jewels used for the pallets of the escapement must be highly polished, its surfaces perfectly flat, and the angles of engagement with the escape wheel precisely engineered to maximize the transfer of energy to the balance wheel. The surface of these jewels should be specially treated to promote the retention of lubrication at the contact surfaces.


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