Profile Drum — The Shaping Cylinder for Cabochon Sides and Backs
Profile Drum — The Shaping Cylinder for Cabochon Sides and Backs
Cylindrical sanding drum pre-formed to a specific cabochon profile, mounted on an expanding arbor
A profile drum is a cylindrical sanding drum pre-formed with a specific cabochon profile, used in lapidary work to shape the backs and sides of cabochons after the dome has been roughed out. The drum is mounted on an expanding rubber arbor and run at moderate speed, with the cabochon held against the drum to refine the curve and remove the grinding marks left by earlier coarser stages. Profile drums are stocked in various grits and diameters to match different cabochon sizes and are part of the standard equipment of any cabochon-cutting operation.
Construction
Profile drums are typically built around a hollow rubber expanding-arbor body, with an abrasive sleeve fitted over the rubber and held in place by the expansion of the arbor when the lapidary tightens the locking nut. The abrasive sleeve is the consumable; when the abrasive wears out or the grit becomes glazed, the sleeve is replaced and the underlying arbor continues in service. Diamond-impregnated sleeves are dominant for cabochon work, with grit ranges from 80 mesh through 3000 mesh and finer.
The drum diameter ranges from approximately one inch (25 millimetres) up to six inches (150 millimetres), with the smaller drums used for refining tight curves on small cabochons and the larger drums used for general cabochon work and for shaping larger stones. The choice of drum diameter affects the curve produced; a smaller-diameter drum produces a tighter inside curve when the cabochon is rolled over the drum surface, and a larger-diameter drum produces a more gradual curve.
Use in cabochon shaping
The profile drum is used after the dome of the cabochon has been roughed out on a flat or coarsely curved wheel and before the final polish. The lapidary holds the cabochon against the drum, rotating it to expose all of the dome surface to the abrasive. The action refines the curve, removes the coarser scratches from earlier shaping, and prepares the surface for the next finer grit.
For shaping the back and sides of the cabochon, the profile drum is the working tool. The cabochon is held with the back to the drum, the rubber arbor flexing slightly under pressure to follow the contour of the cabochon back, and the abrasive removes excess material from the back to produce the standard rounded or flat back profile required for the setting. The same drum is used to refine the sides where the dome curves down to meet the girdle.
Grit progression
The standard grit progression on profile drums runs from 80 mesh for initial coarse shaping, through 220 and 600 mesh for intermediate refinement, to 1200 and 3000 mesh for pre-polish preparation. The lapidary works the cabochon through each grit in sequence, rinsing the stone and the drum between stages to avoid contaminating the finer grits with debris from coarser stages. The progression is essentially identical to that used on flat lapidary wheels and is part of the standard cabochon-cutting workflow.
The final pre-polish surface from the finest profile drum is suitable for transfer to a polishing wheel charged with cerium oxide, alumina, or diamond paste. The polishing wheel removes the last fine scratches and produces the finished mirror surface of the cabochon.
Productivity and craft considerations
Profile drums improve consistency and reduce hand-shaping time, especially in production cutting and in calibrated-stone work where matched profiles are required. The drum's pre-formed curve takes the judgement out of the shaping decision; the lapidary brings the cabochon to the drum surface, and the curve produced is a function of the drum geometry rather than of the operator's eye.
For one-off custom cabochons in unusual shapes, the profile drum is less useful. Hand-shaping on a flat wheel allows the cutter to follow the contours of an irregular piece of rough or to produce an asymmetric design that no pre-formed drum will match. The choice between profile drum and flat-wheel hand-shaping depends on the production context and the design of the finished piece.
In the trade
Profile drums are stocked by every lapidary supply house and are part of the standard equipment in any cabochon-cutting workshop. The combination of a flat wheel for rough shaping, a profile drum for refining the curves, and a polishing wheel for final finish covers most of the workflow for standard cabochon production. The investment in drums and abrasive sleeves is modest relative to the productivity gain over hand-shaping alone.