Spring Cages Spring Cages enhance die spring life by providing a flat, hardened die pocket for spring operation. Cages are available for metric and inch spring diameters from 3/4″ – 2″ (20mm – 50mm). Accommodates standard metric and inch guiding rods and drill diameters. Material: 0.049″ / 1.245mm case-hardened, cold rolled steel with black oxide finish. Conforms to NAAMS standards.