Sometimes watch crystals do not quite fit the standard shape, which also makes it difficult to find an original replacement, especially for discontinued and vintage watch models. In this review we will go through the basic steps of cutting non-standard crystal shapes, and provide you with some tips in cutting and polishing specially shaped mineral glass crystals in a watchmaking workshop.
The work is done in three stages:
- Rough cutting.
- Grinding the edges of the shape.
- Polishing the edges.
Note that the crystal blank should be protected with yellow polishing tape from both sides, as the next stages use a lot of abrasive particles and the glass will get scratched without proper protection.
Next, we grind off material to achieve the desired shape. This is the most time consuming part of the operation. For the rough grinding it’s OK to use a drilling machine (Saeyang or Dremel style) with a soft diamond disc at high speed, followed by aluminium oxide 400-500 grit sand papers and water. Glass edges should only be touched using wet sandpaper, and should have no contact at all with dry sandpaper. We shave off the glass edges little by little, shaping up one side perfectly, then adjusting the other side. Undercut the protective film millimetre by millimetre while you shape the glass closer to the required shape.
The more time you spend on measuring, fitting and adjusting each edge, the more precise the end result will be. More water means better quality, too – rinse the glass shavings/abrasive sandpaper residue from your glass stock, using soft paper towels to dry. The cleaner the work is kept- the better the quality at the end.
The last stage is polishing. Glass polishing is quite a dirty task and requires a separate room. Sandpaper and glass particles will contaminate the regular horological workshop and any of the exposed watch parts and tools therein, so we have a separate room for these kinds of dirty and dusty operations.
We bevel up the edges with 1000-1200 grit abrasive paper (water is mandatory again), after which we use Cerium oxide and water on a hard wool base. It’s good to use a separate machine for glass polishing, but for this particular crystal it wasn’t necessary as everything could be done by hand. Now all that was left to do was to rub the glass edges with wool to give them a mirror shine.
Cerium oxide particles, glass particles, aluminium oxide particles, diamond particles – these are left everywhere after the project has been finished. Some of these abrasive particles will inevitably settle on tools and watch parts, causing extra wear and mechanical problems. It’s normal to do a full clean after every glass cutting operation.
More tips and other projects can be found in our Restoration Services section.