A cab arbor — also termed a cabbing arbor or, in older trade usage, a gallant arbor — is the rotating spindle or shaft that forms the mechanical core of a cabochon-cutting machine. Driven by an electric motor through a direct-drive or belt-and-pulley arrangement, the arbor accepts a succession of grinding and polishing wheels, allowing the lapidary to progress through coarse shaping, fine grinding, sanding, and final polish in a single workflow. Without a true-running, properly balanced arbor, consistent cabochon geometry is impossible regardless of wheel quality or operator skill.

Construction and Dimensions

Cab arbors are machined from steel and are designed to precise dimensional tolerances. The two standard shaft diameters in widespread use are 5/8 inch (approximately 15.9 mm) and 3/4 inch (approximately 19.1 mm); the 5/8-inch arbor is the more common standard on smaller hobby-grade and mid-range machines, while 3/4-inch arbors appear on heavier production units where larger-diameter wheels impose greater radial loads. Wheels are secured to the arbor by means of flanges — typically a fixed inner flange and a threaded outer flange — tightened with a locking nut. The thread direction on the nut is conventionally left-handed on the operator-facing end of the arbor, so that the rotational force of cutting tends to self-tighten rather than loosen the assembly.

Arbor length varies by machine design. A multi-wheel unit may present four to six wheel positions along a single extended shaft, enabling the lapidary to slide laterally from one grit station to the next without removing and remounting the workpiece. Shorter, single-wheel arbors are found on compact or modular machines where individual units are arranged in a row.

Orientation: Horizontal and Vertical Arbors

The vast majority of modern cabbing machines employ a horizontal arbor, with the shaft running parallel to the work surface and wheels spinning in a vertical plane. This orientation allows water — used as a coolant and swarf carrier — to be delivered to the wheel face efficiently, and it presents the grinding surface to the lapidary at a natural wrist angle. A small number of machine designs, particularly older or specialist configurations, use a vertical arbor in which the wheel spins in a horizontal plane, much like a potter's wheel. Vertical-arbor machines can offer certain advantages for flat-lapping but are less suited to the curved profiles required for true cabochon work.

Balance, Alignment, and Safety

Arbor runout — the degree to which the shaft deviates from perfect concentricity during rotation — is the single most important quality criterion. Excessive runout causes vibration that translates directly into chatter marks on the stone surface, accelerates wheel wear unevenly, and places cyclic stress on bearings that shortens machine life. Reputable machine manufacturers specify runout tolerances measured in thousandths of an inch. Bearings supporting the arbor are typically sealed ball-race units; their replacement is the most common maintenance task on a heavily used cabbing machine.

Wheel mounting balance is equally critical. An out-of-balance wheel — whether from uneven bonding matrix, an off-centre bore, or accumulated swarf — amplifies vibration at the arbor and can, in extreme cases, cause wheel fracture. Lapidaries working with resin-bond or sintered diamond wheels are advised to inspect the bore fit before mounting and to ensure flanges seat squarely against the wheel face.

Wheel Compatibility

The arbor is the mechanical interface between the machine and its consumable tooling. Diamond grinding wheels, silicon-carbide wheels, expandable drum sanders, and felt or leather polishing laps are all mounted on the same arbor, provided their bore diameter matches the shaft. Adaptor bushings are available to fit wheels bored for one standard diameter onto an arbor of the other. The lapidary's typical progression — from a coarse diamond wheel (often 80 or 100 grit) through medium and fine grits (220, 600, 1200 or finer) to a polishing lap charged with cerium oxide, aluminium oxide, or diamond compound — is entirely dependent on the arbor's ability to present each successive wheel at the same centreline height and angle.

In the Trade

Replacement arbors and arbor-bearing assemblies are stocked by lapidary suppliers as standard service parts, reflecting the component's status as a wear item in any production cutting environment. When evaluating a used cabbing machine, experienced lapidaries check arbor condition first: a worn or bent arbor renders the entire machine unreliable regardless of the quality of its wheels or housing. The arbor diameter should be confirmed before purchasing replacement wheels, as mixing 5/8-inch and 3/4-inch components without the correct adaptor bushing is a common source of frustration for those new to the craft.