As the most important mechanical fasteners, the surface of the thread is mostly used for anti-corrosion and decoration. Controlling the size of the thread before plating is the fundamental guarantee for the qualified size after plating, and it is also a major problem for many thread processing enterprises in the production and testing process. In this paper, the control methods and detection methods of dimensional accuracy of thread products before plating are summarized.

1. Effect of electroplated layer on thread geometry parameters

Assuming that the thickness of the coating is evenly distributed on the thread, the coating has different degrees of influence on the diameter of the thread, and the increase in the diameter of the external thread is usually 4 times the thickness of the coating. By the same token, after the internal thread is plated, the diameter reduction is also 4 times the thickness of the coating.

In fact, because the protruding parts of the parts are prone to excessive concentration of power lines, the so-called "tip effect" is produced, so it must be considered when formulating the electroplating process.

2. Control of thread size before plating

In order to make the size of the threaded parts after plating qualified, can be through the following way.

1) Appropriately reducing the coating thickness under the conditions allowed by the corrosion resistance can reduce the influence of the coating thickness on the matching size.

2) Under the condition that the performance of the parts allows, the parts shall be treated in advance by pickling and chemical polishing, and the thickness of the parts to be plated shall be reserved to ensure that the size of the parts after plating is within the scope of the drawing requirements.

3) in the parts machining process to reserve sufficient thickness of the coating size.

4) Choose a base material with better corrosion resistance to reduce the thickness of the coating, or choose a high corrosion resistance coating (in the case of a lower coating thickness can achieve the same protective performance) to ensure the dimensional accuracy of the parts.

3. Dimension detection of plating thread

In the testing of plated threads, according to GB/T5267.1, the dimensions before plating shall comply with the relevant standards unless the threads or other parts are allowed to be coated thicker than the standard threads as far as possible in order to meet the functional requirements. External thread: g, f, e; Internal thread: G or H. These tolerance bands are preferred for electroplated coatings.

After plating, the common thread shall be in accordance with the provisions of GB/T3943: the external thread or internal thread shall be detected by the general gauge with the tolerance band position of h or H.