When presented with a calibre of watch movement which you have never worked on before, it is important to proceed with caution when it comes to dismantling the calendar mechanism.
Failing to do this may cause you to lose parts, or not be able to determine the original position of those parts.
Many calendar mechanisms will be secured with one or more cover plates and when a cover plate is loosened, any parts under tension will become unstable and the risk of them flying across the room will be that much greater.
With this in mind, the practice of using pegwood to hold certain parts steady as you dismantle the works would be highly advisable.
Let’s take a look at this Oris 715 movement. It’s a basic mechanical movement with the addition of a very simple calendar mechanism.
You may observe the cover plate which surrounds the calendar ring and secures the parts in place. The cover plate is secured with three screws.
Let’s assume I have never taken this calibre of movement apart before and I am not 100% certain how this calendar mechanism is constructed, and I do not have a service sheet or diagram for this movement, not an unusual situation to be in.
I am going to be very cautious because I know that some mechanisms have a jumper and a spring to hold that jumper under tension.
When I remove the cover plate, the tension of that spring may shift the calendar ring and jumper over and could potentially cause the jumper to be released from it’s position and ping away. If this happens then the spring itself will likely do the same.
Even when the parts are recovered, you will then have to determine how they go back in place. For some calibres this may be straight forward, but for others this can be a time consuming a challenge.
So as I lift the cover plate away, I want to maintain the position of the calendar ring as best I can and a good way to do this will be to hold it steady with some pegwood.
At this stage carefully release any pressure you may have applied with your pegwood and inspect the mechanism.
In this case I can see that there is a one piece jumper and spring, and removing the cover place has caused it to lose tension already and so I feel it is safe to continue dismantling without further worry.
This particular design has the calendar ring turning using the hour wheel as it’s axis, I can see the jumper and spring pivots on the click piece screw here, and it is only under tension when a particular screw for the cover plate is located.
Removing the calendar ring, I can see that the hour wheel drives two intermediate wheels which in turn causes the calendar driving wheel to rotate.
The calendar driving wheel has a peg which interacts with the teeth of the calendar ring once per every two full rotations of the hour wheel, and the jumper causes the calendar ring to always jump to a uniform position so that the numbers will be clearly displayed through the window in the dial.
Notice with this calendar mechanism, there is no function for setting the calendar manually. If the date is out of sequence then it can only be set by advancing the hands.
As mentioned, this is a very basic calendar mechanism and although the design is not entirely common, the principle of operation is actually extremely common, and the similarities will become more apparent to you with every new calibre that you work with.