Mast — The Vertical Column of the Faceting Machine
Mast — The Vertical Column of the Faceting Machine
The structural backbone that holds the angle-adjustment mechanism and quill assembly
The mast is the vertical column of a faceting machine: the rigid structural element that supports the angle-adjustment mechanism and the quill (the rotating handpiece that holds the dop and the gemstone being cut). The mast is the backbone of the machine in the literal sense — the precision and rigidity of every cut depends on the precision and rigidity of the mast, and a worn or out-of-true mast will produce errors in facet angle and meet-point geometry that no amount of skill at the handpiece will correct.
Function
The mast accomplishes three things at once. First, it allows the cutter to raise and lower the handpiece relative to the cutting lap, so that the stone can be brought into and out of contact with the abrasive surface. Second, it carries the angle-adjustment mechanism — typically a protractor scale and locking mechanism, or in modern machines a digital readout — that sets the precise angle between the dop axis and the lap surface. Third, it provides the perpendicular reference against which the index gear (the rotational position around the dop axis) is measured, so that the cumulative geometry of the facets being cut is referenced consistently to a single vertical axis.
Construction
Mast construction varies among manufacturers. The principal types are solid round masts (a polished steel column, with the handpiece sliding up and down through a precision sleeve), square or rectangular masts (typically with a rack-and-pinion or similar mechanism for height adjustment, and a separate angle-adjustment mechanism mounted on the carriage), and cantilever masts (in which the handpiece is supported on an arm extending from the mast rather than directly above it). Each type has trade-offs in rigidity, ease of operation, and ease of cleaning.
Critical tolerances
Two tolerances on the mast affect cut quality directly. The first is perpendicularity: the mast must be vertical to the lap surface (or, more precisely, the dop axis at every angle setting must intersect the lap at the intended angle). Out-of-true masts produce systematic angle errors that vary with the height of the handpiece, making meet-points difficult to achieve. The second is rigidity: any flex or play in the mast under cutting pressure will produce angle errors at the lap surface that vary with the pressure the cutter applies. Both tolerances are addressed in the original design and manufacture of the machine; field adjustment is generally limited to fine angle calibration via the cheater (the small angle-adjustment mechanism on the handpiece) rather than to mechanical adjustment of the mast itself.
Variants and modern features
Modern faceting machines feature digital angle readouts (eliminating the parallax and reading errors of analog protractors), motorised height adjustment on some master-grade machines, fine-angle adjustment via a vernier or micrometer on the angle scale, and increasingly capable index systems. The mast itself remains a relatively conservative piece of engineering — the basic geometry has not changed substantially since the introduction of the modern faceting machine in the post-war period — but the supporting electronics and adjustments around it have been refined considerably.
Maintenance
Mast maintenance is principally concerned with keeping the sliding surfaces clean and lightly lubricated, the angle-adjustment mechanism free of grit and abrasive contamination, and the locking mechanism (which holds the handpiece at the set angle during cutting) in good working order. Abrasive contamination is the primary risk: faceting produces a constant fine spray of cutting compound and stone particulate, which can accumulate on the mast surfaces and cause wear or seizing if not regularly cleaned away. Most experienced cutters wipe the mast and angle mechanism after each cutting session.
In the trade
For faceters and lapidary equipment dealers, mast quality and condition are the principal indicators of the overall quality of a faceting machine. The standard references on faceting machine construction and use are the United States Faceters Guild's published material, the manufacturer documentation from Facetron, Ultra Tec, MDR, Polymetric, and similar makers, and the standard literature on faceting practice (Robert Long and Norman Steele's Introduction to Meet Point Faceting being a long-standing reference).