Definition
A faceting machine is a precision bench tool used in lapidary practice to grind flat, angled facets onto a gemstone preform. Unlike a cabochon machine, which shapes rounded domes, a faceting machine produces the geometrically precise flat surfaces that characterize a cut gemstone.
Principal components
The machine consists of four principal assemblies. A horizontal spindle drives a flat lap plate at variable speed; the plate is the abrasive working surface. A vertical mast supports a movable quill, which holds the mounted gemstone. An angle-setting mechanism at the quill's base establishes the angle between the stone and the lap plate. An index wheel at the top of the mast controls the rotational position of the stone about its own axis.
Classification
Machines are classified by mast rigidity, stated spindle runout, angle readout type, and overall build class. Rigid-mast machines use a single machined column and are standard in mid-range and above; flexible-mast machines use a jointed arm and are found in lower-priced units.
Spindle runout values published by manufacturers range from approximately 0.0003 inch at the top of the market to 0.0015 inch at the entry level. Angle readouts are either analog vernier or digital encoder; digital encoders commonly provide 0.01 degree to 0.001 degree resolution.
Brands in common use
North American faceting machines produced in small series include the Ultra Tec, Facetron, Polymetric, and Mirage lines. Australian production includes the Gemmasta line. Graves Company of Pompano Beach, FL, historically produced entry-level machines.
Historical note
The modern mast-and-quill faceting machine derives from designs developed by Lee Graves, Jarvis Tool & Machine, and Ultra Tec in the mid-twentieth century. Earlier amateur faceting was performed on jamb-peg machines, a design that is still occasionally encountered in teaching workshops but which has been superseded commercially by the mast-and-quill format.