Hi Thomas,
Yes, I am sure that the purpose of stop work is to prevent the barrel being further wound at the end of winding.
This is to avoid the high torque caused when the mainspring is wound tightly around its arbor. If the spring is wound fully and tightly around its arbor, the very high torque that this creates can cause "over banking" - the balance is impulsed so much that it turns further than it should and the impulse pin strikes the outside of the lever fork. This not only results in inaccurate timekeeping but can also damage the escapement.
Of course, the stop work also works in reverse and will stop the watch before the spring is fully unwound. However, this is a consequence of the design rather than its primary purpose, and rather than preventing a loss of accuracy, this would result in a total loss of accuracy and be an inconvenience to the owner.
As the mainspring of a going barrel (i.e. no fusee) unwinds, its torque reduces and the amplitude of the balance also reduces. With a manually wound watch this usually happens every 24 hours, between when it is fully wound and when it is run down before it is next fully wound. The effect of this variation in amplitude between fully wound and run down on timekeeping is minimised by making the balance and balance spring assembly as near to "isochronous" as possible, within the normal operating range of the mainspring.
Although a watch would maintain its rate less accurately, and therefore keep time less well, if it was allowed to operate with the spring run down and outside the range for which the balance is sensibly isochronal, it would still be of some use to its owner, and could be wound and corrected to time when the opportunity presented itself.
However, a watch that has stopped has totally lost accuracy, even if it was within one second of the correct time before it was stopped. As each second passes, the watch becomes one second less accurate. (Until 12 hours have elapsed, when it is right again - although of course the owner cannot know this from simply looking at the watch.)
A stopped watch is of no use to its owner whatsoever. In fact is a positive nuisance because, in addition to winding it, the owner must find a clock or sundial to set the time correctly against, which in earlier times might not be so easy.
In case this discussion does not convince you, I quote George Daniels (Watchmaking, p284) "If the barrel is without stop work then some form of resilient or recoiling pawl is necessary to relieve the pressure of the fully wound spring." Also in Britten (Watch & Clockmaker's Handbook, 16th edition, p38) "To avoid high torque when the mainspring is tight wound about its arbor various stop work devices can be used."
As George Daniels says, manually wound watches without stopwork have a recoiling pawl that allows the barrel to turn backwards slightly, to "recoil", and let the mainspring uncoil a little from being fully wound. All modern manually wound watches have recoil clicks. Automatic watches allow the mainspring to slip in the barrel at the end of winding so that the watch is not wound too much, either manually or automatically.
A recoil click of course does not stop the spring running down. To avoid the very low torque when the spring is near fully unwound, the spring is sized so that a manually wound watch will go for around 36 hours, on the assumption that the owner will wind it every 24 hours and hence the last portion of the unwinding will not be encountered in normal use.
There is a longer discussion about stop work on my web site at Watchmovements: Stop work.
Kind regards - David
