[grp]

It's not possible to buy gears due to the "module" (I don't know how to translate it), it's a factor of scale that influence the spacing between gears.

The hardest parts are the springs (balance spring and mainspring) and the screws (there are so small, it's a specific norm, see NIHS for Normes de l'Industrie Horlogère Suisse). For the gear cutting, there is an example here [pdf link]: http://www.cowells.com/docs/cutter.pdf

There is a lot of custom machine tools, many of them are needed to resolve problems that an individual doesn't care. But there is always alternative ways and it's the fun and watchmaker's task to find them!

So there is not really indispensable tools as it depends a lot of the project. Expect if you want to make specific watch decorations, like perlage, guillochage, soleillage...

I can't find links but there is an operation named roulage where the guiding parts of the pinions are hardened, it's not indispensable if you don't want an industrial grade, kind of.

The simplest design is the roskopf movement.

[zokier]

> "module" (I don't know how to translate it)

fyi, it is called module in English too.

https://en.wikipedia.org/wiki/Gear#Standard_pitches_and_the_...