Function

The index gear is the toothed wheel mounted on the spindle of a faceting machine, which fixes the rotational position of the stone relative to the cutting lap during each facet cut. By engaging successive teeth of the gear via the index pin, the cutter rotates the stone through known angular increments between facets, producing the symmetric facet pattern characteristic of standard cutting styles.

Construction

Index gears are typically machined steel disks with teeth cut around the periphery. The number of teeth determines the available angular increments. Common gear configurations are 64-tooth, 80-tooth, 96-tooth and 120-tooth, each suited to particular cutting styles. A 96-tooth gear, for example, allows angular increments of 3.75 degrees, useful for many standard cuts. A 64-tooth gear allows 5.625-degree increments, used principally for square and rectangular step cuts. A 120-tooth gear allows 3-degree increments, useful for fine detail work and for unusual cut designs.

Many modern faceting machines use a single gear with provision for higher or lower index counts via switching plates or by mounting different gears for different cuts. The index gear is one of the principal components determining the precision of the finished cut.

Use in cutting

The cutter sets the dop, with the stone attached, into the spindle and engages the index gear with the index pin to fix the rotational position. The cutter then makes the first facet at this position. The pin is then released, the gear rotated by the appropriate number of teeth, and the pin reengaged to fix the new position for the next facet. The process is repeated until all facets at this elevation are cut.

The cutter then changes the elevation angle, by adjusting the protractor on the faceting machine, and repeats the process for the next set of facets at the new angle. The combination of index gear positions and elevation angles produces the complete facet pattern of the cut.