Cold heading is essentially "volume forming." The wire fills the die cavity
through plastic metal flow without cutting. According to the law of constant volume, any fluctuation in wire diameter is directly transferred into fluctuations of key finished-part dimensions such as head diameter and shank length. Each looser step of diameter tolerance enlarges the dimensional spread of the final product in parallel, easily pushing it beyond the fastener standard range and causing batch rejection.
Tolerance also directly affects the feeding stability of the cold header and die life. Modern high-speed cold headers form hundreds of pieces per minute. Inconsistent wire diameter causes fluctuations in feeding, gripping, and shearing pressure - at best causing jamming and flash/burr, at worst accelerating die wear or even cracking dies, significantly raising die costs and increasing downtime.
Volume consistency is the key to head forming quality. Common defects
such as head cracking, underfill, and flash largely result from fluctuations in the volume of individual pieces. The tighter the diameter tolerance, the more stable the volume per piece, the lower the forming defect rate, and the higher the finished-product yield - which in fact lowers overall manufacturing cost.
From the perspective of standards and customers, the diameter tolerance of cold heading wire must comply with GB/T 4232 (Cold Heading Quality Wire), GB/T 6478 (Steels for Cold Heading), ISO 4954, and DIN, among others. The difference between ordinary-grade and precision-grade tolerances can be several times. Precision fasteners in automotive, aerospace, and new energy fields (e.g., grades 8.8/10.9) impose extremely high requirements on dimensional accuracy, forcing wire tolerance toward the precision grade.
