TIMING OPERATIONS
Timing is one of the most critical stages in the manufacturing — and more rarely, in the servicing or restoration — of a mechanical watch. It is also among the most complex and delicate. As with the escapement adjusting process, respecting the proper sequence of operations is essential. For better understanding, please refer to the video accompanying this chapter.
- Riveting the Balance Wheel to Its Staff
The first step in assembling the regulating organ consists of riveting the balance wheel onto its staff. It is vital that the balance wheel be firmly secured so it cannot rotate around the staff due to inertia. Moreover, the balance wheel must be perfectly concentric with its staff, and its plane must be perpendicular to the staff’s axis to ensure that it runs true and flat.
- Fitting the Roller
Although the roller belongs to the escapement, it is mounted onto the balance staff, just beneath the balance wheel. It is friction-fitted, which keeps it firmly attached during operation while still allowing for adjustment or removal without damaging the staff. When fitting the roller, the roller jewel must be positioned correctly — usually along the median line between two balance arms.
- Static Poising
The accuracy of a watch depends on the regularity of the oscillations of its regulating organ — the balance wheel. As an inertia wheel, it must be perfectly poised so that its center of gravity coincides precisely with the axis of rotation. Any imbalance creates variations in the oscillation period due to gravity when the staff is not perpendicular to the ground. Static poising must be carried out before the hairspring is installed.
- Temporary Fitting of the Collet
The operations that determine the attachment points of the hairspring to the collet and stud, the shape of the terminal curve, and the total length of the spring — as well as assembling these parts to the balance staff — are collectively referred to as the construction of timing. The first of these steps is to temporarily fit the collet onto the balance staff.
- Temporary Mounting of the Hairspring
In this stage, the hairspring is temporarily positioned around the collet. This allows the inner end of the raw hairspring to be shortened and provisionally secured in preparation for the next step: the first timing.
- First Timing
In a watch equipped with an index (as in our example), the active length of the hairspring is the portion between its attachment to the collet and a theoretical point located between the stud and the index pins, near the latter. The regulating organ’s frequency depends on this active length, combined with the hairspring’s torque and the balance wheel’s inertia.
The first timing determines, as precisely as possible, the active length required for the desired frequency of oscillation.
- Determining the Attachment Point to the Collet
Using the previously measured active length, this step defines where the hairspring should be attached to the collet — the point where the spring makes contact with it. Once identified, the inner end of the spring is cut approximately 60° beyond this point, then bent to form a small hook that will fit into the hole in the collet, securing the hairspring in place.
- Pinning the Collet
In modern industrial production, the hairspring is sometimes glued to the collet. Traditionally, however — and in our example — it is secured using a small brass pin. This method allows the assembly to be disassembled later if necessary, particularly to correct the centering of the hairspring on the collet.
- Centering and Flattening the Hairspring at the Collet
Once the hairspring is attached to its collet, its position must be adjusted so that the coils expand concentrically with the balance staff. This step involves both centering and verifying that the plane of the hairspring is perfectly perpendicular to the staff — ensuring that it is also perfectly flat.
- Second Timing
After the hairspring is firmly secured to the collet, a second timing operation is performed to determine its active length — and thus its frequency — with greater precision. This step is crucial for forming the terminal curve correctly.
The active length is fine-tuned by clamping the outer coil in the timing machine and adjusting it until the nominal frequency is reached as accurately as possible.
- Forming the Terminal Curve
The terminal curve of a flat hairspring prevents the penultimate coil from striking the stud when the spring expands during oscillation. The distance between the penultimate coil and the terminal curve, along with the shape and position of the two bends in the curve, ensures that the hairspring is correctly centered within the balance cock and between the index pins.
To determine the 60° angle, the balance and hairspring assembly is placed on a jig with an angular protractor (or a perforated plate with a graduated paper scale). The 60° is measured from the point marked during the second timing toward the outer end of the spring, which is then cut. The two bends of the terminal curve are then formed, maintaining perfect concentricity.
- Attaching the Stud
The final stage in constructing the adjustment is known as studding, which involves fixing the stud to the hairspring. As with the collet, industrial production often uses adhesive for this step. In the traditional method shown in our example, the stud is secured using a small brass pin.
- Dynamic Poising
During operation, the hairspring does not expand perfectly concentrically around the regulating organ’s axis, causing a slight shift in its center of gravity with each oscillation. Dynamic poising rebalances the assembly under actual running conditions. Proper dynamic poising ensures accuracy and minimizes positional rate differences.
Rate measurements in the six standard testing positions reveal any imbalance, which can then be corrected by removing material (milling) from the underside of the balance wheel rim at the heavy spot.
- Final Inspection
Once dynamic poising is complete, the rate is checked again in all six positions. If necessary, fine corrections are made — either by adjusting the active length of the hairspring or by modifying the moment of inertia of the balance wheel.