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Figure 1
Assembled center wheel
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1. General description
the center wheel belongs to the counting and transmission mechanism of which it is the first element in the kinematic chain.
the center wheel is one of the fundamental components of the wheel train of a mechanical watch. It is so named because it is generally positioned at the center of the main plate (de facto of the movement). When it is off-center on the main plate, it is commonly called a third-wheel pinion assembly. Its axis passes through the movement to receive, on the dial, the hand minute hand via the cannon pinion. Its function is to transmit the energy from the barrel to the following wheels, while ensuring the main time indication, that of the minutes, and, indirectly, that of the hours.
2. Components
A center wheel traditionally consists of a pinion in steel and a wheel blank in brass:
- the centre wheel: Large in diameter, it occupies the central position of the Movement. Its number of teeth is calculated to establish the reduction ratio agreed upon with the Barrel and to guarantee a rotation in one hour. The Centre wheel (driving) drives the Third-wheel pinion.
Figure 2
Centre wheel
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- The Centre pinion: Made of Steel, it forms the axis of the mobile. It is driven (led) by the toothing of the Barrel. Its leaves are cut with precision in order to ensure smooth rolling and to limit friction.
Figure 3
Centre pinion
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3. Materials
Traditionally, the centre wheel is made of gilded brass. The pinion is made of hardened steel, mirror-polished to enhance, in particular, its mechanical strength. The pivots are burnished (surface cold working). They pivot in synthetic stones (rubies) and are lubricated to minimize friction and wear.
The pairing of a brass wheel and a steel pinion is optimal for minimizing friction in the gears of a movement. It applies to most step-up or step-down trains of a watch (finishing trains, time-setting trains, etc.).
4. Handcrafted manufacture of a wheel (centre wheel)
In unit production or small-batch production, the wheel blank is made of brass of watchmaking quality, typically a rolled copper-zinc alloy (CuZn37), chosen for its mechanical characteristics, its machinability and its compatibility with subsequent surface treatments.
Manufacturing begins with turning a raw disc from a brass bar or strip. On a lathe for horologists, the turner successively faces both sides, turns the outer diameter (overall diameter of the future toothing) and drills the central hole. This hole is then carefully reamed: its diameter is kept slightly smaller than that of the collet of the pinion in steel onto which the wheel will be riveted, so as to guarantee a tight fit and torque transmission without slipping. Depending on the profile and quality of the wheel, a molding may be turned on one face of the wheel or on both.
In order to open up the arms of the wheel (generally five in number), clearance holes are drilled in the intervals, then the material is hollowed out with the piercing saw (jeweler’s saw). The spokes are then filed and equalized by hand. This time-consuming operation helps reduce the wheel’s Inertia and constitutes an aesthetic criterion.
The roughed-out blank is then mounted on the mandrel of a Lathe or the spindles of a rounding or cutting machine. If it is a Lathe, it is equipped with a dividing plate, allowing precise indexing of each angular position corresponding to the desired number of teeth. A milling cutter module cutter — whose profile is matched to the module and pressure angle of the Gear teeth — cuts the teeth one by one in successive passes. For the finishing Train (wheel train), the modules are generally very small (in the order of 0.07 to 0.15 mm), which requires highly precise Milling cutters and rigorous clamping of the part.
Finishing includes Deburring of sharp edges, the circling of the faces, the optional polishing of the mouldings, the beveling / Angling if applicable, then a surface treatments by galvanic Gilding. This deposit of gold, of controlled thickness, improves corrosion resistance and gives the wheel its characteristic golden appearance. The riveting operation on the steel pinion is carried out using a riveting stake.
5. Industrial production of a wheel (Centre wheel)
In industrial production, the manufacturing of the wheel blank is organized in continuous flow from a strip of brass rolled (CuZn37 or similar) supplied in coils. The first operation is progressive blanking (stamping) performed on a press with combined tooling: in a single stroke, the strip is pre-pierced, openworked according to the wheel’s design, and cut to its final outline (including openworking of the spokes). This technique makes it possible to obtain several hundred blanks per minute with high reproducibility.
The central hole, intended for riveting onto the pinion in steel, is obtained during blanking, but it generally undergoes a follow-up reaming operation. The dimensional tolerance of the inner diameter is tight in order to ensure a controlled, consistent tight fit across all pinions of the same series. The flatness of the blank is also checked at this stage, with any residual warping being eliminated by straightening.
The cutting of the toothing is carried out on CNC gear-cutting machines. For the common modules of the finishing train, the process chosen is hobbing using the mother milling cutter (hob) (hobbing), which produces an involute profile of great geometric regularity. For the finest modules, below 0.07 mm, some manufacturers use electrical discharge machining (EDM) wire cutting (WEDM) or laser cutting, which achieve precision levels otherwise difficult to obtain through conventional material removal.
Depending on the required quality level, deburring is carried out continuously by tumbling in vibratory bowls loaded with suitable abrasive media, which removes cutting burrs without altering the tooth flanks. Traditional finishing (circling, beveling / Angling, polishing) is the standard in high-end watchmaking. The surface treatment — usually a gilding galvanic — is applied to loose boards, ensuring a uniform deposit thickness across all parts.
Quality control is integrated into the production flow. Vision systems measure the wheel profile, the tooth pitch, and the diameter of the press-fit hole in-line, and detect surface defects. The riveting operation on the steel pinion is then carried out on an automatic press with controlled force, with the force-displacement curve recorded for each assembly, ensuring traceability and compliance of the fit.
6. Handcrafted manufacture of a (centre) pinion
In single-unit or small-batch production, the finishing train pinion is made of steel horological turning stock, a highly machinable alloy whose composition is adapted to mechanical stresses and to heat treatments specific to movement pinions. The part comprises, on either side of the toothed body, two pivots and, at the location intended for the wheel plate, a cylindrical seat topped by a heel to be riveted.
Turning is carried out on a lathe precision Horologist (watchmaker) lathe. The Horologist (watchmaker) turns the various diameters of the pinion in succession and rough-machines its two pivots, whose geometric quality and surface finish directly determine the operation of the Train (wheel train). The seat intended to receive the wheel plate is machined with a sliding fit relative to the diameter of the plate’s hole, so that the wheel can be freely positioned there before riveting. Beyond the seat, a thin heel — the rivet itself — is left protruding; it is this that will be upset during Assembly to secure the wheel plate. The rough-machined pivots are then finish-turned and their surface is Cold worked by burnishing.
The cutting of the pinion leaves is carried out on a lathe fitted with a dividing plate. A pinion-cutting milling cutter, with a profile suited to the module and the number of leaves (generally six to twelve for the going train), cuts each leaf in successive passes. This direct-indexing cutting process produces a profile close to the epicycloid or the involute, depending on the tooling used. Precise adjustment of the cutting depth is decisive for the future meshing with the brass wheel blank of the preceding wheel (here the barrel).
After cutting, the pinion undergoes a heat treatment: oil or air hardening followed by tempering, in order to give the leaves the hardness required for wear resistance. The roughed-out pivots are then finely turned (fine turning) and their surface is then cold-worked by burnishing.
The assembly of the wheel blank onto the seat is carried out using a riveting stake: the collet is upset with a shaped riveting hammer, under visual inspection, until an even rivet flush with the face of the wheel is obtained.
7. Industrial production of a (centre) pinion
In series production, the finishing train pinion is manufactured by Automatic profile turning, from a bar of horological Steel for profile turning. This process makes it possible to chain in a single setup the turning of the pivots, the Arbor and all the diameters of the pinion with high cadences and precise dimensional repeatability. Tolerances are defined right from the programming stage, guaranteeing the Interchangeability of pinions in assembly lines and after-sales service.
The geometry of the wheel fit is machined with a sliding fit relative to the diameter of the hole in the wheel plate, so that the wheel positions itself without noticeable Set, but without force. The rivet stem is calibrated in diameter and height so that the subsequent riveting operation produces a homogeneous upset, without risk of deforming the toothing or the wheel plate.
The cutting of the Leafs is carried out on CNC cutting machines by hob Milling cutter (hobbing), which produce an involute profile with a geometric regularity superior to cutting by direct indexing.
The heat treatment — followed by a controlled tempering — is carried out in a furnace under a protective atmosphere to avoid any decarburization of the functional surfaces. The pivots are then ground on a CNC cylindrical grinder using diamond or synthetic corundum wheels.
Quality control (dimensional and aesthetic) is generally integrated inline. The riveting operation of the wheel plate is carried out on an automatic press, whose force profile is recorded for each part: the force-displacement curve guarantees the compliance of the upsetting and constitutes a traceability element of the assembly.
8. Technical specifics
- The wheel plate is firmly riveted to the pinion. Thus, the wheel forms a perfectly unified assembly.
- The tooth ratio of the centre wheel, and of the entire going train, is calculated to ensure a complete rotation of the centre wheel in sixty minutes, an essential condition for the direct driving of the cannon pinion and the hand of minutes and, through gear reduction, the hour hand.
9. Role in the movement
The centre wheel is both a transmission and counting component:
- Transmission: it directly relays the energy from the barrel to the rest of the wheel train (third wheel, seconds wheel, and escapement components).
- Counting: its axis, passing through the movement, forms the supporting component for the cannon pinion and the minute hand, the central pivot around which the entire time display unfolds.