Function

An index plate is an auxiliary positioning plate used on faceting machines to provide angular increments different from those offered by the principal index gear. By substituting one index plate for another, the cutter accesses different sets of angular positions, allowing the same machine to cut faceting designs requiring different gear counts.

Construction

Index plates are typically machined disks with positioning notches, holes or stops at intervals corresponding to the desired angular subdivision. They mount onto the spindle in place of, or in addition to, the principal index gear. Some faceting machines use a stack of interchangeable plates, allowing the cutter to switch between configurations rapidly. Others use a single integrated index head with selectable subdivisions.

Common index plate configurations parallel the index gear configurations: 64, 80, 96 and 120 positions, with specialty plates for unusual cut designs. The 360-position plate, allowing one-degree increments, is used principally for protocol verification and for unusual asymmetric cuts.

Use in cutting

The cutter selects an index plate appropriate to the planned cut, mounts the plate on the spindle, and uses the index pin to engage successive positions during cutting. The principle is identical to the standard index gear and pin operation, with the plate substituting for the integrated gear.

Index plates are particularly useful in lapidary instruction, where students may cut a variety of designs on the same machine over short periods. Substituting plates is faster than swapping integrated gears.