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THE THIRD WHEEL

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Figure 1

Diagram of a Third wheel

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1. General description

The Third wheel (sometimes called small third wheel) belongs to the Counting and transmission mechanism. It is an essential element of the finishing Train (wheel train). It is located between the Centre wheel, which receives the energy directly from the Barrel, and the Fourth wheel (seconds wheel), to which it transmits this energy. By its intermediate position, it plays the role of a speed reducer and relay, allowing a slower rotation speed to be imparted to the Centre wheel than that of the Fourth wheel (seconds wheel), which is closer to the Regulating organ.

2. Pointing and rotation speed

As it does not carry any Hand, the third wheel is not required to rotate at a specific speed as is the case for the Centre wheel (1 rev/h) and the Fourth wheel (seconds wheel) (1 rev/min when it carries a seconds hand). Likewise, the position of its pointing is free, which allows, during the development of the Movement, it to be defined in order to optimize the size of the movement and, to a certain extent, the efficiency of the going train.

Its main function is twofold:

1. Energy transmission: it receives the driving energy from the Centre wheel and transmits it to the Seconds pinion.
2. Reduction of the rotation ratio: It contributes to the overall gear ratio of the finishing train by allowing, depending on the Frequency of the Regulating organ, to ensure the correct rotation speed for the Centre wheel and the Fourth wheel (seconds wheel) (when it carries a seconds Hand). (see calculations Counting and transmission mechanism).

3. Components

Like the other wheels of the finishing train, the Third wheel is made up of two components:

• A wheel, driving (driver) the pinion of the Fourth wheel (seconds wheel) (driven).

Figure 2

Diagram of a Third wheel

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• A pinion, which constitutes the Axis of the wheel set. It is driven (led) by the wheel of the center wheel (driving).

Figure 3

Plan of a third-wheel pinion

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4. Materials

Traditionally, the third wheel is made of Brass gilded. The pinion is made of hardened steel, mirror polished to increase, in particular, its mechanical resistance. The pivots are burnished (surface cold working). They pivot in synthetic stones (rubies) and are lubricated in order to minimize friction and wear.

The tandem formed by a Brass wheel and a Steel pinion is optimal in order to minimize friction in the Gears of a Movement. It applies to most of the multiplying or reducing Trains of a watch (finishing Trains, hand-setting, etc.).

5. Artisanal manufacture of a wheel (Third wheel)

In unit or small-batch production, the wheel blank is made of Brass of horological 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 the turning of a raw disc from a bar or strip of Brass. On a Lathe for Horologist (watchmaker), the turner successively performs the facing of both sides, the turning of the outer diameter (total diameter of the future gearing), and the drilling of 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 slippage. 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 evened out by hand. This time-consuming operation helps reduce the wheel’s Inertia and is also an aesthetic criterion.

The blank thus roughed out is then mounted on the chuck of a Lathe or the spindles of a rounding or cutting machine. If it is a Lathe, it is fitted 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 (around 0.07 to 0.15 mm), which requires highly precise Milling cutters and rigorous clamping of the part.

The finishing includes deburring of sharp edges, the Circling of the faces, the possible Polishing of the moldings, theBeveling / Angling if applicable, then a Surface treatments by galvanic Gilding. This deposit ofGold, of controlled thickness, improves corrosion resistance and gives the wheel its characteristic golden appearance. The riveting operation on the Steel pinion is performed using a riveting stake.

6. Industrial production of a wheel (Third 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 cutting (Stamping) performed on a combined tool press: 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 cutting, but it is generally subject to a follow-up Reaming. The dimensional tolerance of the inner diameter is tight in order to ensure a controlled and uniform tight fit across all pinions of the same series. The flatness of the plate is also checked at this stage, any residual warping being eliminated by straightening.

The gear cutting of the toothing is carried out on CNC cutting machines. For the common modules of the finishing train, the process used is cutting by Hobbing with the hob cutter (hobbing), which produces an involute profile of great geometric regularity. For the finest modules, less than 0.07 mm, some manufacturers useElectrical discharge machining (EDM) by wire or laser cutting, which achieve precision levels otherwise difficult to obtain through conventional material removal.

Depending on the quality level required, Deburring is carried out continuously by tumbling in vibrating bins loaded with suitable abrasive media, which removes cutting burrs without altering the tooth flanks. Traditional finishes (Circling, Beveling / Angling, Polishing) are the standard in fine watchmaking. The Surface treatments — usually a Gilding galvanic — is applied to loose batches of plates, ensuring a uniform deposit thickness across all parts.

Testing is integrated into the production flow. Vision systems measure the wheel profile, the gear tooth pitch, and the diameter of the fitting 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, the force-displacement curve being recorded for each Assembly, guaranteeing traceability and conformity of the fit.

7. Craft manufacturing of a pinion (Third-wheel pinion)

In single-unit or small-series manufacturing, the finishing train pinion is made of Steel Horologist (watchmaker) Profile turning, a high-machinability 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 with a rivet shoulder.

Turning is carried out on a Lathe precision horologist’s (lathe). The Horologist (watchmaker) successively turns the different diameters of the pinion and rough-machines the 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 shoulder — the rivet shoulder 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 (fine turning) and their surface is cold worked by rolling.

The cutting of the pinion Leaves is carried out on a Lathe equipped with a dividing plate. A pinion-cutting Milling cutter, with a profile suited to the module and to the number of Leaves (generally six to twelve for the finishing Train (wheel train)), cuts each Leaf through successive passes. This direct-division cutting process produces a profile close to the epicycloid or the involute, depending on the tooling used. The precise setting of the cutting depth is critical for the future meshing with the Brass wheel blank of the preceding wheel (here the Centre wheel).

After cutting, the pinion undergoes Heat treatments: Oil or air quenching followed by tempering, in order to give the Leaves the Hardness required for wear resistance. The roughed-out pivots are then finished by fine turning, after which their surface is Cold worked by burnishing.

The Assembly of the wheel blank onto the seat is carried out using a riveting stake: the shank is upset with a shaped riveting hammer, under visual inspection, until a regular rivet head flush with the face of the wheel is obtained.

8. Industrial production of a Third-wheel pinion

In series production, the pinion of the going train is manufactured by Automatic profile turning, from a bar of Horologist (watchmaker) Steel intended for profile turning. This process allows the turning of the pivots, the Arbor and all the diameters of the pinion to be carried out in a single setup, with high cadences and precise dimensional repeatability. Tolerances are defined at the programming stage, ensuring the Interchangeability of pinions in the Assembly lines and in 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 riveting boss is calibrated in diameter and height so that the subsequent riveting operation produces a uniform upsetting, without risk of deformation of the toothing or the wheel plate.

The cutting of the Leaves is carried out on CNC cutting machines using Hobbing by hob cutter (Hobbing), which produce an involute profile with a geometric regularity superior to gear cutting by direct division.

The Heat treatments — 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 Testing (dimensional and aesthetic) is generally integrated in-line. The riveting operation of the wheel plate is performed on an automatic press, whose force profile is recorded for each part: the force-displacement curve guarantees the conformity of the upsetting and constitutes a traceability element of the Assembly.

9. Technical specificities

• The wheel plate is solidly riveted to the pinion. Thus, the wheel forms a perfectly integral assembly.
• The gearing ratio of the Third wheel and of the entire finishing 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 minute hand and, through reduction, the hour hand. (see calculations Counting and transmission mechanism).