STAMPING PRESSES

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Since the 16th century, jewellery, coins, and mechanical parts have been shaped, formed, and/or cut using stamping presses. Stamping allows for identical reproduction of complex shapes and patterns at lower and decreasing production costs. Stamping requires the manufacture of specific cutting tools for each part design: the stamp. The stamp consists of two elements: the die and the punch. The die is a plate of hard metal (usually steel) in which the outline of the component to be manufactured (negative) is cut and hollowed out. The punch is made of the same metal as the die and is formed (machined) positively according to the same outline. The material is punched (cut), stamped, or bent by the pressure of the punch through the die.

The manufacture of cutting tools (punch and die) requires great skill and involves long hours of often artisanal work. This technology, therefore, involves heavy tooling costs. These costs logically decrease rapidly as production progresses. Moreover, dies can have lifespans that can be counted in tens or even hundreds of years and reach almost unlimited production volumes. Compared to other production methods (automatic lathes, machining centres, wire EDM machines), stamping machines require few adjustments, and the implementation of production is significantly faster. Furthermore, even automated machines capable of sometimes producing tens of components per minute do not require numerical programming.

Most metals used in watchmaking can be stamped (gold, platinum, steels, brass, aluminium, etc.) and stamping is used to produce movement components (wheels, springs, pallets, etc.) as well as case components (cases, dials, bracelets, etc.). The stress exerted on the material during stamping creates structural tensions. Stamping involves heat treatments (annealing) to eliminate these tensions.

Highly developed until 1980, the production of watch components by stamping has since declined sharply with the appearance of the first machining centres and CNC machines. Paradoxically, it is these same machines that now allow for the manufacture of more complex and less expensive dies to manufacture, thus revitalizing stamping activity.

Still present in many watchmaking workshops, the screw press is a manually operated mechanical cutting, stamping, or bending machine. Depending on their construction and size, screw presses can be floor-mounted or bench-mounted. The body of the machine is typically made of cast steel. A flat horizontal table occupies the centre of its body. It is on this table that the die is positioned and fixed. The table is topped by an arch vertically traversed by an endless screw. The punch is fixed to the lower end of the endless screw. It is positioned and guided by vertical slides. A large horizontal lever is fixed by its centre to the top of the endless screw and driven by a square adjustment. Weights are usually placed at the ends of the two arms of the lever to increase its inertia.

The material to be stamped is positioned at the centre of the vertical axis. The artisan rotates the toggle formed by the lever and its weights. By the action of the endless screw, the punch descends vertically to the desired depth (through the material and the die for cutting). Depending on the pitch of the endless screw, the angle required for the toggle to strike, and the speed of movement of the punch can be adapted to the type of operations for which the press is intended.

The pressure exerted on the material here depends on the inertia of the toggle and the force applied by the operator. Most screw presses used today have power ratings ranging from a few hundred kilograms to several tons.

The operating principle of a hydraulic press is similar to that of a screw press. Instead of the endless screw, it is cylinders that actuate the punch. Hydraulic pistons allow for a considerable multiplication of the energy supplied to the machine and can achieve considerable power. Hydraulic presses used in watchmaking have pressure forces ranging from 1 to 200 tonnes. Some presses require manipulations between each strike (removal of the cut piece, repositioning of the raw material for the next strike). Other machines are automatic. In this case, an animation (generally hydraulic) allows for the ejection and collection of the cut component and for advancing the strip of raw material or a magazine (through perforations, like a film reel) to perform the next strike. This principle allows some presses to receive dies sequencing several successive striking operations. This is called progressive stamping.