Hi all,
I am not sure this belongs here, but....
I have not taken apart nor put together many watches, but I have never seen a LOOSE screw.
In my other life we designed every threaded fastener with either a lock washer, lock nut or some type of restraining device, cotter pins twisted wire thru holes in the heads etc....
In a watch there are from 10 to 30 screws ( not including the screws in the balance) and not one is ever with a locking device. Yet none are ever loose. Is it the thread pitch, better operator always tightening to a non loosening standard?
Just musing.
Maybe I should do something useful!!!

Melvin, this is a great question. I too come from a world of high tech fasteners and find the screws in these watches quite fascinating.

Anther observation is that they are super easy to get started and aligned... once I get the thing right side up. (not always easy when working with something about the size of flea poop).

Dave Abbe shared some interesting history on this with me one day in his shop. Dave, I'd love to hear you chime in on this.

Thanks a mil,

Paul

Generally the screws are made from steel ("Piano Wire") while the watch parts they hold together are made from softer Brass or Nickel. Brass and Nickel will retain some stress around close fitted threaded surfaces which tend to "lock" the screw thread and keep it from turning yet allow it to easily turn when it is rotated by force.

Unlike many other fasteners which will loosen under high duress of physical and thermal extremes, watch screws pretty much live in "peace" giving less cause for concern for their staying in. Exception is taken for case screws which often do loosen up largely because they are swaging or cutting the case down with the light thumping around the watch experiences in daily use.

Close tolerancing of screw threads to allow such a "fit" did not start until the mid 1800's, and before that you will see that most watches had "Pinned Plates" with maybe a hand-cut screw to hold down the balance cock.

An interesting point here is the type of "taps" used to make these early screw threads in the soft watch plates are what we would now call "express" taps", being that they had no "flutes" to allow for "chip clearance". These taps are made with a pointed end showing a spiral thread "growing out" from the tip. Express taps do not "cut" the threads. The tap drilled hole was finished to a diameter larger than the minor diameter by slightly less than 1/2 the thread depth. As the "express tap" entered the tap drilled hole, it would "squeegee" (Swage) out a thread that expanded IN towards the minor diameter of the express tap thread leaving a very smoothly finished tapped hole with no "chamfer" at the entry point.

That is why when re-assembling these watches, to avoid "cross threading", you have to be very careful to "pick up" the thread of the tapped hole before trying to turn in the screw.

Also we all find that smaller holes such as Jewel Screw size are sometimes poorly threaded simply because to maintain proper tap drill hole size was much harder with quickly dulled micro drills dragging out the diameter to where the tap was hardly cutting any thread at all.

Great Explaination and description of the way threads are fabricated.
Just had a further thought about the threads of a crown and its stem. Given that they are turned quite a bit, it is still a wonder why they don't unscrew.

Some more musing.
I think that the reason they do not loosen is more due to the pitch, ie threads per inch rather than any thing else.
It also seems to me that a few screws, will fit the same threaded hole. The correct one as well as one size smaller. They have the same pitch, just a smaller thread diameter.