1. Electroplated layer thickness of threaded fasteners
In electroplating processing of threaded fasteners, whether using roll plating or hanging plating processes, the coating thickness obtained for each fastener in a batch of products varies, and even the coating distribution on the same fastener is uneven. The coating thickness at the top of the threaded tooth is thicker than that at the middle diameter and bottom of the tooth, and the coating thickness at both ends of the screw and bolt is thicker than that in the middle, and becomes more significant with increasing length to diameter ratio. The same applies to nuts. Due to the shielding effect during nut electroplating processing, the plating layer on the internal thread is very thin, with only the plating layer on the first teeth at both ends thicker than the plating layer on the middle part, which is equivalent to the thickness on the hexagonal wrench surface.
2. How to describe the plating layer thickness of a batch of threaded fasteners
Due to the uneven distribution of plating thickness on threaded fasteners, the standard cites four definitions of plating thickness, namely "nominal plating thickness", "batch average thickness", "local thickness", and "effective plating thickness". The nominal coating thickness refers to the nominal coating thickness of threaded fasteners and cannot represent the actual coating thickness. The actual coating thickness of threaded fasteners is described as "effective coating thickness". The effective coating thickness includes two aspects: batch average thickness and local thickness.
The plating thickness of threaded fasteners cannot be represented by the plating thickness of a single part. Regardless of whether threaded fasteners are processed using hanging plating or rolling plating processes, each part in the same batch cannot achieve the same plating thickness, but the change in thickness follows a normal distribution. We assume that the plating thickness is uniformly distributed on the surface of the batch of parts to calculate the average plating thickness, which introduces the concept of "batch average thickness", The average thickness of the batch can be used to describe the coating thickness of the entire batch of threaded fasteners. The average batch thickness in Standard Table 1 is represented as a range. The minimum batch average thickness is a requirement to ensure corrosion resistance of threaded fasteners, while the maximum batch average thickness is to ensure the thread fit of threaded fasteners after plating.
"Local thickness" in the standard actually refers to the minimum local thickness, which describes the minimum specified value of the coating thickness that threaded fasteners should achieve on the specified local test surface. The experimental verification data shows that the local coating thickness values measured on the specified local test surfaces of screws, bolt heads, and screws, bolts, and nuts are greater than their batch average thickness values. Generally, the batch average thickness value meets the requirements, and the minimum local thickness can also meet the requirements. The test results also confirmed that the numerical relationship between "nominal coating thickness", "batch average thickness", and "local thickness" is correct, which constitutes the theoretical basis for the detection and acceptance of the electroplating coating thickness of threaded fasteners.
