For the watchmakers out there, here's a thoroughly over-engineered solution to a problem you probably don't have. Have you ever tried to insert a new mainspring into a barrel, only to have it fight you coming out of the retaining washer? Personally, I have found that this can be a particular problem with vintage "new old stock" mainsprings that have been sitting in the washer for decades. Tired of having parts fly across the room, or pulling out the mainspring winders to deal with a spring that wouldn't go in the easy way, I designed this jig to make the insertion process as easy and foolproof as possible (well, easy if you don't count the 3D printing part...). As a bonus, the tool helps ensure that braking grease remains along the outside edge of the barrel and doesn't get scraped off by the new mainspring during insertion.

The jig consists of three parts:
- A base that has a deep recess for the mainstring barrel and a concentric shallow recess to hold the new mainspring-in-washer centered above the barrel.
- A lid that slides over the base, locking the washer & new mainspring in place above the barrel, while allowing access to push the new mainspring into the barrel.
- A simple pusher which is the perfect size to push in the new mainspring, preventing the outer coil from binding or otherwise getting hung up in the washer.

The base also has a hole to allow for arbor insertion after the new mainspring is in place.

I have incuded STEP files for a specific example of the jig that I used with an Omega 354 movement, but in order to work properly the parts need to be sized precisely for the particular mainspring barrel, mainspring and washer combination, so I have published this as a parametric model in OnShape. The model is publicly available here:

https://cad.onshape.com/documents/bb6b7c9f2ee01bf25d81fbaf/w/8bf836c25751f4949829f7a8/e/c191987dfef11bfeafc49767?renderMode=0&uiState=66dd1c713ffdfa79fcdab87f

You can create a free OnShape account, make a copy of the public model and modify the parameters (or other aspects of the model) as needed (look for the variables tab) to suit your application.

A few disclaimers:
- The parts need to be dimensionally accurate to work as intended, so I advise printing at high quality settings. I have been printing these for my own use in PETG, but PLA should work fine. Depending on the your particular printer or filament choice, your mileage may vary. All parts should print without support.
- I'm sure there are plenty of ways to improve this model, make it print faster & with less plastic, etc. I am frankly better at watchmaking than 3D model design.
- I have had great success with this tool but testing has been necessarily somewhat limited and I provide absolutely no guarantees as to the functionality.

Watchmaker Tool:  Mainspring Insertion Jig (Parametric)

Mainspring Insertion Jig - All Example Parts.3mf

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Watchmaker Tool: Mainspring Insertion Jig (Parametric)