The lapping wheel is a rotating abrasive disc used in lapidary and gem-cutting work for grinding, smoothing, and polishing gem materials. It is closely related to the lapping plate (the two terms are often used interchangeably) and is distinguished principally by emphasis: 'lapping wheel' is sometimes used to describe smaller-diameter, faster-rotating discs used at the bench, particularly in faceting and finishing work, while 'lapping plate' tends to describe larger flat-bed discs for slab work. The distinction is not rigid in trade usage.

Construction and materials

Lapping wheels are produced in cast iron, copper, lead, tin, zinc, and various ceramic and composite materials, each suited to different stages of the cutting and polishing process. The hardness, surface finish, and chemical compatibility of the wheel material with the abrasive and the gem material are all considerations in selection. Cast iron and copper wheels charged with diamond grit are standard for most modern faceting work; tin and lead wheels with diamond or cerium oxide compounds are used for the finest polishing stages. The wheels are mounted on a vertical or horizontal arbor and rotated by an electric motor at speeds appropriate to the work, typically 500 to 3000 rpm for general lapping work and slower for finishing work.

Use in faceting

The lapping wheel is the principal tool of the faceting machine. The faceter mounts the gem material on a dop (a holder cemented to the stone) and brings the dop to the rotating lapping wheel under controlled angle and orientation. The wheel grinds and polishes each facet of the stone in turn, with the angle and azimuth set precisely by the machine's index and angle controls. Different wheels are used for different stages: a coarse wheel (typically 600 grit) for the rough cutting of facets, a medium wheel (1200-3000 grit) for intermediate work, a fine wheel (8000-14000 grit or finer) for pre-polish, and a polishing wheel (with cerium oxide, alumina, or diamond compound) for the final finish.

Use in cabochon work

In cabochon work the lapping wheel is used principally for the back finishing of the stone, the doming of the cabochon being done on a curved-surface cabbing wheel. The flat back of a cabochon is finished on a sequence of lapping wheels of decreasing grit until the desired surface finish is achieved. For inlay and intarsia work, lapping wheels are used to prepare the flat surfaces that will be glued or set into the substrate.

Position in the lapidary toolkit

The lapping wheel sits within the broader lapidary toolkit alongside the rock saw, trim saw, cabbing wheel, faceting machine, polishing buffs, and various specialised tools for particular materials and techniques. The choice of wheel for any given operation depends on the gem material, the stage of work, and the desired surface finish. Modern lapidary practice has standardised much of the equipment and consumables, and detailed specifications for wheel-grit-compound combinations are available in the major lapidary manuals. For the working bench jeweller and lapidary, the lapping wheel is an everyday tool whose proper selection and use is part of the foundation of the craft.