1. Electroplating thickness of threaded fasteners

Whether barrel plating or rack plating is used in the electroplating process of threaded fasteners, the thickness of the plating layer obtained by each fastener in a batch of products is different, that is, the plating layer distribution on the same fastener is also uneven of. For example, the coating thickness of the thread crest is thicker than the pitch diameter and the bottom of the tooth, and the coating thickness of the two ends of the screw and bolt is thicker than the middle, and it becomes more significant as the length to diameter ratio increases. The same is true for nuts. Due to the shielding effect of the nut electroplating process, the thickness of the electroplated layer on the internal thread is very thin. Only the first buckle at both ends of the thread is thicker than the middle part, which is equivalent to the hex wrench surface. thickness.

2. How to describe the plating thickness of a batch of threaded fasteners

In view of the uneven distribution of the plating thickness of threaded fasteners, the standard quotes four definitions of plating thickness, namely "nominal plating thickness", "batch average thickness", "local thickness" and "effective plating thickness". "Nominal plating thickness" refers to the nominal plating thickness of threaded fasteners, and cannot represent the actual plating thickness. The actual coating thickness of threaded fasteners is described by "effective coating thickness". "Effective coating thickness" includes two aspects of "batch average thickness" and "local thickness".

The "average batch thickness" is defined in Chapter 3 of the standard text. The plating thickness of threaded fasteners cannot be represented by the plating thickness of a part. Regardless of the threaded fasteners processed by rack plating or barrel plating process, each part in the same batch of parts cannot reach the same plating thickness, but the thickness is different. The change obeys a normal distribution. We assume that the coating thickness is evenly distributed on the surface of the batch of parts to calculate the average thickness of the coating. This leads to the concept of "batch average thickness", which can be used to describe the entire batch. The coating thickness of the batch of threaded fasteners. The "batch average thickness" in the standard table 1 is a range to indicate. The minimum batch average thickness is to ensure the anti-corrosion requirements of the threaded fasteners, and the maximum batch average thickness is to ensure the screwability of the threaded fasteners after plating.

"Local thickness" in the standard actually refers to the minimum local thickness, and the minimum local thickness describes the minimum specified value of the plating thickness that the threaded fastener should reach on the specified local test surface. The test verification data shows that the local coating thickness values measured on the screw, bolt head and screw, bolt, nut screwing surface, these specified local test surfaces are greater than the average thickness of the batch, usually the average thickness of the batch The 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 plating thickness of threaded fasteners.