Toolmaking 3: Hand Pushers

I had a bit of extra time in micromechanics, so I decided to make a "deluxe" version of our required hand pusher set.

Please forgive my greasy machinist's hands.

Please forgive my greasy machinist's hands.

Our hand pushers' original design is a knurled brass handle and acrylic pushers. The pushers screw on to either side of the tool, and it's used to manually press hands on to their pivots.

Since watch hands are frequently polished, and often made of soft precious metals like white gold on nicer watches, the soft plastic is a requirement to keep them from leaving marks or scratches. Our blueprints specified 7 tips, each with different inner and outer diameters.

I finished those two required parts of the tool early, so I decided to make a stand for them. Using a 25.00 mm round of brass from the scrap bin, I marked 12 holes using the dividing head on our lathe, and drilled them out. Each was fitted with its own friction fit pin (all zero tolerance operations), and the center hole was tapped to mate with the handle's threads. This leaves 5 pins available for future pushers, should I need to make more down the line.

Original, Mk. II, Mk. I.

Original, Mk. II, Mk. I.

The final part of the tool, as I envision it, is a finger rest for pressing down the hands with more precise force. Our screwdrivers have a rotating rest, and I tried to engineer a version of that design on an earlier tool, to limited success.

The 7750 eccentric tool's top was held in place with a rivet. The spread surface acted as both the axial and the radial limiting surfaces, but since it was a little off-centered, it wasn't ideal. Riveting, by its nature, is more chaotic and unpredictable than machining. I wanted a better solution.

I haven't taken apart the screwdriver to see how it's made (they use ball bearings and reassembly could be perilous), but I assume it's some sort of friction pin assembly. This Mk. II version of the finger rest uses the same design.

The pivot is a polished steel cylinder, which fits inside the top with minimal play. A hollow brass cap friction fits to the pivot to provide the axial bearing surface. This design was significantly more complicated to machine, but it provides much more controllable side and endshake. These are both held to just 0.01-0.02 mm.

The whole assembly is modular, and can screw on or off when needed.

Here's the whole tool set, put together. It's a bit excessive and complicated, but it was fun to successfully engineer the updated finger rest!

Watchmaking student at the Lititz Watch Technicum, formerly a radio and TV newswriter in Chicago.