Gear polishing part 2

 

A little refresher in case you have forgotten part 1: The technical hurdle of polishing the teeth of the gears we make seems insurmountable. No one is around to help, our cries of anguish fall on deaf Swiss ears, the darkness approaches as we realise that our NH3 project cannot progress, alone in the void of watchmaking purgatory, we wait.... Enter James, our new toolmaker. A knight in shining... engineering overalls comes to our rescue. A micrometre in one hand, safety glasses in the other, he stands in front of the milling machine smiling - as if the impending doom of a failed project isn't looming. I explain the situation. "James, we can't polish the teeth of this gear, we can't do it manually, we can't buy a machine that will help us, and I don't know who to ask to help!" That's more or less as much as I said. A week of research: looking at photos of antique machines on the internet, scrolling through video tours of Swiss workshops frame by frame, looking through blogs and books and horological bibles. Even the great Daniels is remarkably silent about this dark art (read, he kept the secret to himself). James emerges with a concept, an idea. "Let's build this machine ourselves!" I've always wanted NHW to do this - building machines for individual, specialised tasks that would make the process of manufacturing a part easier. All the famous ateliers, from Pratt to Breguet, were littered with small desktop sized machines that helped their users execute one task and one task only, but to perfection. Some machines would only drill accurate holes, others would create a snailing pattern on a barrel lid, others would burnish the surface of a pinion to a mirror. In our case James proposed a modern take on a 1940's machine called a Prata Type 11. This machine would handle the polishing of the teeth with the ancient method of a split wooden disk that auto-indexes the gear whilst polishing the flanks with a fine abrasive paste. I felt relieved! Having a way forward meant that we could progress and make not only this specific set of parts, the ratchet and crown wheel for the NH3, but also all hardened steel gears for futures watches. But, the road would not be easy.... In fact it would only get harder. Stay tuned for Part 3. Josh

Gear polishing - The final frontier, Part 1

By Josh Hacko

With the NH3 project, and all future watch developments, there are some unknowns that keep me up at night. Things that worry me and that are seemingly beyond reach. But the night is darkest before the dawn, and routinely in hindsight I can say that I worried too much. Not the case with gear polishing. Gear manufacture at the best of times is an extremely complex process with multiple variables and great expense associated with it. All of the complexity and cost is driven by the requirements of the "form" of the teeth. They need to be accurately machined to comply within the strict mathematical limits of the theoretical tooth engagement. Too much clearance, and the gears will have too much backlash, and will lose efficiency. Too little clearance between the gear pair, and there will be excessive wear, a loss of efficiency and in the worst case, no power transmission at all (read - it don't work)! Watchmaking has even higher requests for this already complex procedure. The concentricity of the teeth to the centre of rotation, the form of the epicycloid profile, the taper and helix angle of the form as it progresses axially - all of these things need to measured, and kept within a very precise tolerance. The process up until this point for these NH3 ratchet wheels has been as follows: design, material selection, material and tool procurement, fixture design and manufacture, gear blank generation, inspection, lapping, inspection, gear cutting, inspection, hardening, tempering, inspection, lapping... and now - gear polishing.

One of the most important requirements in the list of demands that watchmaking imposes on it's gears is low friction. This means in practical terms that the teeth of the gear need to polished to an incredibly high level to reduce the friction during operation. This is very much "form follows function". Recently I've been seeing many collectors commenting on how "well" gear teeth are being polished, as if it were a cosmetic affair. Sorry to say, for the discerning horologist, the most important and driving reason for gear polishing is either a quest for chronometric accuracy or smooth auxiliary operation (think winding and setting, etc) So, why did gear polishing keep me up at night? The technical reason: Well, polishing is a material removal process - you are removing the "high spots" of a surface in the hopes that you will even out the surface and be left with something that is much smoother. Polishing without changing the form so much that it throws you out of the tolerance zone, or being able to polish the whole profile of the tooth evenly, or polishing to a high enough grade, are all difficult challenges that need to be addressed individually, but also in the context of the previous and future operations the gear will undergo. The personal reason: It's scary. Our initial thought was to head over to Switzerland to talk to the experts. The people that make the machine to polish the teeth of gears, wheels, and pinions. Throughout our journey we have learned that talking to the experts, paying them top dollar and humbling yourself in their presence is actually the cheapest way to move forward. We did the same with Kern, with Citizen, Affolter and Makino, to make and refine all the other parts of the watch. While COVID was a large contributor, it was not the biggest obstacle to the pursuit of this knowledge. The biggest obstacle was simply that this company, the keeper of the knowledge behind this tricky science of gear polishing.... didn't exist. Elevated from a well-defined, scientific process that could be traded for some hard-earned cash - gear polishing became black magic, art, mysticism and a maybe even religion. Our next step? We needed to become the people that built the machine. To be continued.