Movement Stories: Triple Calendar Moon Phase Assembly
Triple Calendar + Moon Phase with a pointer date is a prized complication among vintage watches. I thought it would be interesting to go through how this rather complex calendar system is put together as part of a movement service. Strap in!
The subject of today’s exploration is a classic Cortébert 677, which was first introduced in 1945. Cortébert movements are somewhat rare these days, but the brand had a long history of premium watchmaking before being swallowed by the quartz crisis, including manufacturing movements for Rolex.
From the dial side, pictured below, it doesn’t look like there is anything special going on, and the 15-jewel design and layout is very “Standard Swiss Watch.” The only hint of something unusual going on are the chronograph-style vertical posts to fix the dial feet (circled in red in the photo), as opposed the more typical use of dial screws on the edge of the movement. We’ll see why this was done in a bit.
Coltébert 677 movement watchmaker side, with dial posts highlighted.
Flipping the watch onto the dial side, things still look normal at first glance, but careful observers will notice the 4 cutouts around the perimeter of the movement. These are for small pushers that allow the various elements of the calendar system to be individually quick set. The presence of these pushers, and the need to keep the perimeter of the movement clear for them, explain why dial posts were used instead of dial screws.
At this point, there’s still a pile of parts to assemble for the calendar works. Specifically, 5 lever arms, 5 driving wheels (including the hour wheel), 2 lever return springs, 3 jumper springs (one of which comes with a small spacer), 3 indicator wheels, and a bunch of screws. Mercifully, the screws mostly come in a couple of different sizes, with only a few unique variants.
Well, let’s walk through putting this all together. First step is to install the large calendar plate. This covers up most of the jewels on the dial side, including the balance setting, so it’s a really good idea to make sure all of these have been lubricated before putting the calendar back together. Once the calendar plate is in place, the levers for the quick set functions are installed next:
Beginning the triple calendar assembly - calendar plate and quick setting levers installed.
Next the return springs for the levers can be installed. These are what cause the quick set pushers to return to a neutral position when engaged. Note that each of these springs wrap around a post screw and engage with one lever on each side, and that, for each spring, one end sits underneath one of the large hold-down screws for the corresponding lever. When installing these, the easiest approach is to slip these ends of the springs into place under the corresponding screws, and then to apply tension arounf the post screw and set the other end with a plastic pointer:
Triple calendar assembly with lever return springs installed.
Next it’s time to install the jumper springs. These cause the calendar indicator disks and date pointer to snap from one position to the next. Note that the two sides of the “double sided” jumper, shown at the top of the photo below, are not identical, so it’s important to install this in the correct orientation. Similarly, the two other jumpers look very similar but are actually mirror imaged:
Triple calendar assembly with jumper springs installed.
So far the process has been pretty easy, but things are about to get much more complex as we install the driving wheels. The hour wheel goes on first - note that it has an extra inner set of teeth to drive the calendar mechanism - followed by the three calendar driving wheels which, going counter clockwise from the upper part of the photo, drive the date, day of week, and moon phase, respectively. These wheels look identical a first glance, but they are not. Each of them has an upward facing pin which is located at a different radius for each wheel: The pin for the day driving wheel is on the outside, the pin for the moon phase driving wheel is on the inside, and the pin for the date driving wheel is in the middle.
To make things more complex, each of these wheels is held down by a different screw: the date driving wheel uses the same “common” screw type used for the lever pivots, the day driving wheel uses a unique screw with the same diameter but a slightly taller shoulder, and the moon phase driving wheel uses a screw with a smaller diameter (to provide clearance for the post).
Once you have all of this sorted out, there’s one more thing to keep in mind - the posts on these driving wheels need to be aligned with each other in a specific way to ensure that the date, day, and moon phase all click over at or near midnight. This can be accomplished by installing each wheel so that the post is aligned with a line drawn from the center of the driving wheel to the center, or axis, of the driven indicator wheel. This alignment is shown using red arrows in the photo below:
Triple calendar assembly with driving wheels installed, showing proper alignment of the driving posts
Finally, with all of that done, it’s time to install the indicator wheels. Note that the month indicator wheel is not diven by the watch - the month can only be changed manually by engaging the corresponding pusher. Here’s what it all looks like when assembled, before the dial goes on:
Complete triple calendar assembly, with the indicator wheels in place.
One interesting design element to note here is that the post for sub second hand is coaxial with the moon phase indicator disk.
The last thing to do before putting the dial back on is to test everything, making sure that all of the pushers work, and that the pointer date wheel, day wheel, and moon phase wheel all click over properly when the time is turned past midnight: