A lap arbor (or arbor, sometimes spelled arbour) is the rotating spindle assembly on a faceting machine or cabochon grinder that holds and drives the working lap. The arbor combines a precision shaft, bearings, a drive system (belt or direct drive), and a mounting flange or threaded boss to which laps are secured. Its concentricity, runout, and rigidity govern the flatness of the cut a faceter can achieve: a lap that wobbles or shows axial runout produces facets with a curved meet line and uneven polish.

Construction

Faceting-machine arbors typically run on sealed precision ball bearings or, in older heavy-duty machines, on bronze sleeve bearings. The spindle is manufactured to a tight runout tolerance – on a quality machine no more than a thousandth of an inch (0.025 mm) of total indicator reading at the lap face. Drive is by a flat or v-belt from a variable-speed motor, with electronic speed control on most modern machines. The arbor end carries either a threaded stud, a Morse taper, or a flange that accepts a master lap, and master laps in turn carry the working laps. Cabochon-machine arbors are heavier, often with multiple wheels mounted on a single shaft running through a tank.

Maintenance

Arbor health shows up in the work. Periodic checks with a dial indicator on the lap face, with the lap removed and the bare flange running, will show bearing wear before the cutter notices on the stone. Sealed bearings in faceting machines typically need replacement every several thousand running hours. Belt tension, drip-tray seals against water ingress, and the flatness of the master lap mounting face are the other recurring service items. A faceter who suspects arbor issues will compare results across known-good laps and a single test stone before disassembling the spindle.