Centrifugal casting (silversmithing)

Centrifugal casting in silversmithing is a casting technique where a small mould is poured, then spun on the end of an arm. The centrifugal force thus generated encourages a successful pour.

Processes

Centrifugal casting, or centrifuging,^[1] is used as a means of casting small, detailed parts or jewelry.^[2] An articulated arm is free to spin around a vertical axle, which is driven by an electric motor or a spring. The entire mechanism is enclosed in a tub or drum to contain hot metal should the mold break or an excess of metal be used. Single use molds are prepared using the lost wax method. A small amount of metal in a crucible (a sort of ceramic pan) next to the mold is heated with a torch. When the metal is molten the arm is released, forcing (by centrifugal force) the metal into the mold. The high forces imposed on the metal overcome the viscosity, resulting in a finely detailed workpiece. A similar advantage may be obtained by vacuum casting or pressure casting.

For industrial casting of small parts using poured hot metal, a disk shaped mold is contained within a rotating drum, and molten metal is poured into the center.

Glass

The technique is known in the glass industry as "spinning". The centrifugal force pushes the molten glass against the mold wall, where it solidifies. This cooling process takes anywhere between 16 to 72 hours depending on the impurities or volume of material. Typical products made using this process are television picture tubes and missile nose cones.

The term is also applied to the fabrication of large telescope mirrors, where the natural curve followed by the molten glass greatly reduces the amount of grinding required. Rather than being cast by pouring glass into a mold an entire turntable containing the peripheral mold and the back pattern (a honeycomb pattern to lighten the finished product) is contained within a furnace and charged with the glass material used. The assembly is then heated while spun at slow speed until the glass is liquid, then gradually cooled over a period of months.

Applications

Centrifugal casting is commonly used to shape glass into spherical objects such as marbles.

Machinery

Many machines are available which can perform centrifugal casting, and they are relatively simple to construct. All that is required is an arm which rotates with an adequate amount of centrifugal force, a container on the end of said arm to hold both a mold and the material to be cast into the mold.

See also

• Centrifugal casting (industrial)
• Spin casting
• Spin casting (mirrors)

References

1. Efunda site page with centrifugal casting fundamentals
2. Untracht, Oppi (1985). Metal Techniques for Craftsmen. Robert Hale. ISBN 0-7091-0723-4.