Ring Mandrel — The Jeweller's Sizing and Forming Tool
Ring Mandrel — The Jeweller's Sizing and Forming Tool
A graduated steel cone for measuring, forming, and adjusting ring shanks
A ring mandrel, also known as a triblet, is a tapered steel rod marked with graduated ring sizes, used to measure, form, round, and resize ring shanks. The tool is the working bench's reference standard for ring size and the principal forming surface against which a shank is hammered into a true circle. It is one of the indispensable items at any bench that fabricates or repairs rings.
Form and markings
The mandrel is a long, slowly tapering cone — typically 230 to 280 millimetres in working length — turned from hardened tool steel and ground true. Engraved or stamped size markings run along one side, calibrated to the standard in use. UK mandrels carry the alphabetical UK sizing (A through Z and beyond, with halves), US mandrels run a numerical scale (typically 0 through 16 in halves and quarters), and European mandrels carry millimetre circumferences. Some mandrels carry both UK and US scales on opposite sides.
Cross-sections are most often round, though oval and square mandrels are made for forming non-round shanks. A grooved mandrel — with a longitudinal slot cut along its length — allows a stone-set shank to be sized without the mandrel pressing against the stone or table.
Hammering and sizing
The mandrel's principal forming use is in stretching, rounding, and truing shanks. A shank is slipped over the mandrel and tapped along its length with a rawhide or rubber mallet to round it; tapping with a steel hammer stretches the metal and increases the ring size, with corresponding work-hardening. Repeated stretching requires periodic annealing to prevent cracking, particularly in alloys such as platinum and palladium-rich white gold.
Sizing down requires removal of metal — cutting and resoldering — rather than the mandrel alone. A mandrel can verify the new size after sizing operations are complete, but cannot itself reduce a ring size.
Quality and care
Hardened tool steel is essential: a soft mandrel deforms under hammering and quickly loses its true taper, after which size readings become unreliable. Quality mandrels are hardened through their working length, with a visible heat-treatment colour change toward the tang. The working surface should be kept free of pitting and rust; a light oil wipe after each working session prevents corrosion.
The mandrel is held by its tang in a bench vice or in a dedicated mandrel holder. Free-hand use is possible for measurement but not for hammering, where the mandrel must be securely fixed.
In the trade
For repairs and bespoke fabrication, the mandrel is the bench's measurement reference. Sizes called out by retailers should always be verified against the mandrel actually in use, since sizing standards drift slightly between manufacturers and across decades of stock. Most working benches keep at least one round mandrel and one grooved mandrel; busier shops will have additional oval and square forms for unusual shank geometries.