Characteristics and description of plating on threaded fasteners
1. Plating thickness of threaded fasteners
Regardless of whether barrel plating or rack plating is used in the electroplating process of threaded fasteners, the thickness of the coating obtained by each fastener in a batch of products is different, and the coating distribution on the same fastener is also uneven. of. For example, the thickness of the coating on the top of the thread is thicker than that of the middle diameter and the bottom of the tooth, and the thickness of the coating at both ends of the screw and bolt is thicker than that in 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 during nut electroplating, the thickness of the electroplating layer on the internal thread is very thin. Only the first button teeth at both ends are thicker than the coating in the middle part, which is equivalent to the hexagonal wrench surface. thickness.
2. How to describe the plating thickness of a batch of threaded fasteners
In view of the uneven distribution of plating thickness of threaded fasteners, four definitions of plating thickness are cited in the standard, namely "nominal plating thickness", "batch average thickness", "local thickness" and "effective plating thickness". "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 by "effective coating thickness". "Effective coating thickness" includes two aspects: "batch average thickness" and "local thickness".
"Lot average 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, each part in the same batch of parts cannot achieve the same plating thickness, but the thickness The change is subject to 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, which leads to the concept of "batch average thickness", which can be used to describe the whole batch. Coating thickness of batch threaded fasteners. The "batch average thickness" in Standard Table 1 is expressed as a range. The minimum batch average thickness is the requirement to ensure the anti-corrosion of threaded fasteners, while the maximum batch average thickness is to ensure the threadability of the threaded fasteners after plating.
"Local thickness" actually refers to the minimum local thickness in the standard, and the minimum local thickness describes the minimum specified value of the coating thickness that threaded fasteners should achieve on the specified local test surface. The data verified by the test shows that on the screws, bolt heads and the wrenching surfaces of screws, bolts and nuts, the local coating thickness values measured on these specified local test surfaces are all greater than their batch average thickness values, usually the batch average thickness The value meets the requirements, and the minimum local thickness also meets the requirements. The test results also confirmed that the numerical relationship of "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.