Master Lap — The Primary Cutting Disc on the Faceting Machine
Master Lap — The Primary Cutting Disc on the Faceting Machine
The dedicated cutting lap, charged with diamond compound, that handles the rough-shaping work
The master lap is the primary cutting lap on a faceting machine — the metal disc that remains mounted on the machine for production work, charged with diamond compound or other abrasive, and dedicated to the rough-cutting and shaping stages of the faceting process. The master lap is distinguished from the pre-polish and final-polish laps, which are typically separate discs (or separate sides of the same disc, on machines with reversible-mount lap systems) charged with finer abrasives and used for the later stages of the cut. The master-lap concept is fundamental to faceting workflow: keeping the cutting lap consistently charged and dedicated to one stage of the cut produces predictable cutting rates and reduces the setup time that would otherwise be required to switch between cutting and polishing operations.
Construction
Master laps are typically metal discs — copper, tin, type-metal alloy (a tin-lead alloy), aluminium, or steel — selected for the cutting characteristics that suit the abrasive being used and the stones being cut. Copper is the most common master-lap material for diamond-compound rough cutting because copper holds the diamond grit well and produces a consistent cutting action. Tin and type-metal laps are common for pre-polish and polish stages with finer compound. Steel laps are used for the most aggressive cutting and for materials that resist softer-lap cutting. The disc is mounted on the lap shaft of the machine via a central hub and is balanced for the rotational speeds at which it will be operated (typically 500 to 1500 RPM, depending on the machine and the operation).
Charging
A master lap is charged with abrasive — typically diamond grit suspended in a carrier (oil, water-soluble carrier, or a dry compound) and pressed into the surface of the lap with a charging tool. The grit becomes embedded in the surface of the metal lap, and the lap then cuts material from the stone as the stone is pressed against the rotating surface. Diamond grit sizes for master-lap rough cutting typically run from 60 mesh (very coarse, for initial shaping of large rough) through 600 mesh (fine, suitable for finishing of smaller facets), with intermediate sizes in between. The charging is renewed periodically as the cutting action wears the grit; experienced cutters develop a sense for when the lap needs recharging based on cutting rate and surface finish.
Workflow position
The master lap typically handles the first stages of facet cutting after the rough has been pre-formed (either by sawing or by pre-shaping on a coarser grinding wheel). The cutter establishes the table, then cuts the crown and pavilion main facets, then the star and break facets, in successive operations on the master lap. Once the cut is geometrically complete and the meet-points are established, the work moves to a pre-polish lap (typically a finer-grit diamond-charged lap) for surface preparation, and then to a final polish lap (typically a tin or type-metal lap charged with very fine diamond, alumina, or cerium oxide) for the optical-quality polish. The master lap is not used for polish; using a coarse-charged lap for polish would leave visible scratches on the facet surface.
Maintenance
Master laps require regular maintenance to keep them flat (avoiding the slight concave or convex profile that develops with uneven wear), free of contamination from other abrasive grades (which would compromise the cutting action), and properly charged. Re-flattening can be done by hand on a flat reference surface or by machining; periodic re-charging is part of the routine maintenance. Most experienced cutters maintain a small inventory of laps charged at different grit sizes for different operations, and reserve specific laps for specific abrasives to avoid cross-contamination.
In the trade
For practising faceters and lapidary equipment dealers, the master lap is one of the routine consumables of the bench. Costs vary with material and size, with quality copper laps in the 6- to 8-inch range running from approximately USD 100 to USD 300 each. The standard references for lap selection and charging are the United States Faceters Guild's published material, the manufacturer documentation from major lapidary equipment makers, and the standard literature on faceting practice (Robert Long and Norman Steele's Introduction to Meet Point Faceting).