[3-16mm 1/2-20UNF] 3-16mm denotes that clamp the drill between 3mm and 16mm, and 1/2-20UNF represents a screw hole with 1/2-20UNF at the back of the chuck. [Speciall drill jaws] The clamping jaws are specially treated to improve wear resistance. [Perfect gripping] By using anti-thrust bearing to convert key-applied torsion into superior gripping. [With anti-rust paint] The housing is treated with electrophoretic paint, making the product anti-rust even in worst environment. [Tight attachment] High-accuracy thread and tapered holes can be tightly attached to the tool.