The faceting locator pin — also called simply the locator pin or detent pin — is the mechanical device on a faceting machine that locks the quill assembly into a precise, repeatable angular position relative to the mast. By engaging with a series of holes or notches machined into the mast at calibrated intervals, the pin ensures that the operator can return the stone to exactly the same cutting angle after lifting it from the lap, rotating the index wheel, or pausing work. Without a reliable locator mechanism, meet-point accuracy — the hallmark of well-executed faceting — would depend entirely on the cutter's ability to read and reset a scale by eye, introducing cumulative error across dozens or hundreds of facets.

Function and Mechanical Principle

In a conventional mast-and-quill faceting machine, the quill (the rod that holds the dopstick and stone) pivots on the mast through a range of angles, typically expressed in degrees from horizontal. The locator pin is spring-loaded so that it seats positively into the chosen hole or notch under light hand pressure, producing a tactile and sometimes audible click. This positive engagement distinguishes the locator pin from a simple friction lock: friction locks can creep under the lateral forces generated during grinding, whereas a properly fitted pin holds its position mechanically until deliberately released.

The spacing and number of holes in the mast determine the angular resolution available. Many production machines offer one-degree increments across the working range, though some precision instruments provide half-degree or finer positions. The cutter selects the desired angle, seats the pin, and proceeds to grind or polish; when all facets at that angle are complete, the pin is retracted, the quill is moved to the next setting, and the pin re-engaged.

Relationship to the Index Wheel

The locator pin governs the vertical angular position of the stone (the cutting angle, or angle of inclination from the lap surface). It works in concert with the index wheel, which governs the rotational position of the stone around the axis of the quill. Together, these two locking mechanisms define every facet's orientation in three-dimensional space. Symmetry errors in a finished stone can almost always be traced to slippage or misreading of one or both systems.

Variants and Alternatives

Not all faceting machines use a pin-and-hole arrangement. Some designs substitute:

• Friction or clamp locks — a lever or knurled nut that clamps the quill against the mast at any continuously variable angle, read off a graduated scale. These offer infinite resolution but require the operator to set and read the scale carefully each time.
• Vernier scales — a refinement of the graduated-scale approach that allows interpolation between major divisions, improving readability to fractions of a degree without discrete detents.
• Digital angle encoders — found on higher-end contemporary machines, these replace mechanical detents with electronic position sensing and a digital readout, combining the repeatability of a pin with the continuous resolution of a vernier.

Among hobbyist and guild faceters, the traditional pin-and-hole mast remains the most widely encountered configuration, valued for its simplicity, durability, and the positive feedback it provides.

Maintenance and Accuracy

The locator pin is a wear component. Over time, repeated engagement can enlarge the holes in the mast or round the tip of the pin, introducing small amounts of play — sometimes called slop in the trade — that degrade repeatability. Periodic inspection of pin tip condition and hole integrity is standard maintenance practice. Replacement pins are consumable items stocked by most faceting-machine manufacturers and suppliers. A worn pin that allows even half a degree of angular variation will produce visibly mismatched facets in stones cut to tight meet-point tolerances.