An expanding drum — also called a rubber drum — is a cylindrical arbour, constructed from rubber or a synthetic elastomer, that is mounted directly on the motor shaft of a horizontal cabbing machine. Its defining characteristic is radial expansion under centrifugal force: as the motor accelerates to operating speed, the drum swells outward, gripping the inner surface of a sanding or polishing belt firmly and evenly across its full width. When the motor is switched off and rotation ceases, the drum contracts to its resting diameter, releasing belt tension and allowing the operator to slide a worn belt free and replace it in seconds — without tools, clamps, or mechanical adjustment of any kind.

Mechanism of Action

The self-tensioning behaviour arises from the elastic properties of the drum material combined with the physics of rotation. At rest, the drum's outer circumference is slightly smaller than the inner circumference of the belt it carries, so the belt sits loosely. Once the shaft reaches working speed — typically in the range of 1,725 or 3,450 rpm on standard single- or double-speed cabbing units — centrifugal force acts on the mass of the elastomer, causing it to deform outward. The resulting radial pressure is distributed uniformly around the drum, producing consistent belt tension with no hot-spots or slippage. The tension is therefore self-regulating: it increases proportionally with rotational speed and disappears the moment the drum stops.

This behaviour eliminates the mechanical belt-tensioning devices — idler wheels, spring-loaded rollers, or adjustable tracking arms — that would otherwise be required to keep a flat or sleeve belt taut on a rigid arbour.

Construction and Dimensions

Expanding drums are manufactured in standardised widths and diameters to accept the sleeve belts sold by lapidary suppliers. Common widths are 1 inch, 2 inches, 3 inches, and 6 inches; diameters of 4 inches, 6 inches, and 8 inches are the most frequently encountered on bench-top cabbing machines. The elastomer body is bonded or moulded around a rigid metal hub that mates with the motor shaft via a standard arbour thread or set-screw collar. Better-quality drums are balanced at the factory to minimise vibration at speed, which is important for achieving a smooth surface finish on the stone being worked.

The belts themselves — referred to as cabbing belts or sanding sleeves — are fabric- or film-backed abrasive sleeves coated with silicon carbide, aluminium oxide, or diamond compound in grits ranging from coarse (typically 60 or 80 grit for initial shaping) through progressively finer grades to pre-polish and polish. The same drum accepts belts of any grit in the matching sleeve size, making grit progression straightforward on a multi-drum cabbing unit.

Role in Cabochon Cutting

On a typical horizontal cabbing machine, a series of expanding drums is arranged side by side on a single motor shaft or on separate shafts driven by one or more motors. The lapidary moves the stone from drum to drum in order of decreasing grit, grinding the cab to its domed profile on coarser belts and refining the surface through successively finer grits before transferring to a separate polishing wheel or pad. Because belt changes require no tools, the operator can swap to a fresh belt of the same grit mid-session if the abrasive loads with swarf, maintaining consistent cutting action without interrupting the workflow.

The expanding drum's ability to provide a slightly compliant working surface — the elastomer yields fractionally under hand pressure — is considered an advantage when finishing curved cabochon profiles, as it allows the belt to conform marginally to the stone's dome rather than cutting a flat facet. This compliance is modest and should not be overstated; the primary shaping work is still performed by the abrasive grit, and technique remains the dominant variable.

Maintenance and Wear

Expanding drums require little routine maintenance beyond keeping the elastomer surface clean and free of embedded grit or adhesive residue from belt backings. Over time, the rubber can harden, crack, or develop flat spots, reducing the uniformity of belt tension; a drum in this condition should be replaced rather than continued in service, as uneven tension causes belt tracking problems and inconsistent surface finish. Operators working in dry climates or with aggressive coolants should inspect drums periodically, as both desiccation and chemical exposure can accelerate elastomer degradation.