spherize

Spherizing means forcing material into a ball or spherical form, typically during metalworking or plastic molding. The process is used to create ball bearings, spherical valve seats, pump impellers, and other rounded components that cannot be easily made by conventional turning or extrusion. Material is compressed or deformed under pressure until it assumes a spherical geometry.

In bearing manufacture, spherize refers to the cold or warm forming of steel wire or rod stock into ball blanks before grinding and polishing to final size. A typical bearing ball might be 10 mm to 50 mm in diameter depending on the bearing type. The blanks are fed into die sets that gradually reduce and round the material, often in multiple strokes. In some processes, two hemispherical dies close on a wire segment, severing it and forming the sphere simultaneously.

Spherizing also occurs in plastic injection molding when thermoplastics are forced into spherical cavities under high pressure and temperature. Polystyrene balls for packaging, thermoplastic check ball valves, and spherical lens elements are made this way. The plastic must reach sufficient flow temperature to fill the mold cavity completely without voids or flat spots.

Common Problems and Variants

Incomplete spherizing produces flat spots, ellipsoidal shapes, or surface rippling. This happens if die clearances are wrong, material temperature is too low, or pressure is insufficient. In metalworking, work hardening can prevent complete material flow, requiring intermediate annealing. Surface finish matters: rough input stock leaves tool marks that survive the forming process and must be ground away afterward.

The term is most common in bearing manufacture, where it has been used since the early 1900s. It distinguishes the shaping operation from swaging (which typically elongates) and deep drawing (which creates hollow forms). The word reflects the end result: the material is forced into a sphere, rather than merely shaped around a spherical form.