Countersunk holes are drilled on the board with a flat-head drill or a gong knife but cannot be drilled through (i.e. semi-through holes) The transition part between the outermost/largest hole and the smallest hole is parallel to the surface of the pcb Yes, the part connecting the large and small holes is a flat surface, not an inclined surface. Through holes, as the name suggests, are transparent and through holes that can pass objects or liquids of a suitable size. The function is: a, used for connection, can be made into threaded through holes. b, as a cylinder of a piston engine. c. It is used to transport liquid, gas, dust-containing objects, etc. with pumps, etc. d. In life, the clothes and pants we wear are also examples of through-hole applications. The general crimping hole is a copper-plated plug-in hole. Because of the special design of the component feet, it can be firmly connected to the hole by its own structural tension after being inserted into the hole. The advantage is to reduce the wave soldering set up for soldering such components; if the hole is not soldered in the end, it will be easier to change parts. Of course, changing it multiple times will affect the reliability of the hole. The component feet that require crimping holes generally have an expansion function, rather than a threaded function. Some will be welded again later when other components have been welded by xx, while others will not be welded. Before the implementation of the GB-1984 standard, the counterbore was divided into three names: 1. The head of the fastener was completely sunk into the stepped hole of the part, called a countersunk. 2. The head of the fastener was not completely sunk The stepped hole of the part is called the countersunk hole 3. The head of the fastener basically does not sink into the part, and the hole that only faces the surface of the part is called a fisheye pit. The difference between a countersunk hole and a countersunk hole is that the upper part of the bolt hole is reamed. ：The counterbore is a straight cylinder structure; the counterbore is a forty-five degree structure, which is smoother than the counterbore. The upper part of the bolt hole can be reamed to accommodate the bolt head, so that the screw head is not higher than the surrounding surface. Countersunk and countersunk are just two different names. The difference is as shown in the figure below: Generally, the countersunk head is used in the place where the surface is required to be smooth and no protrusions after the connection, because the countersunk head can be screwed into the part. The round head can also be screwed into the part. The tightening force of the square head can be larger, but the size is large. In addition, in order to meet the needs of locking after installation, there are holes on the head and holes on the rod. These holes can prevent the bolt from loosening when it is vibrated. Some bolts are not threaded smooth rods to be thin, called thin waist bolts. This kind of bolt is conducive to the connection under variable force. There are special high-strength bolts on the steel structure, the head will be larger and the size will also change. In addition, there are special uses: T-slot bolts, the most used on machine tool fixtures, special shapes, and both sides of the head should be cut off. Anchor bolts are used to connect and fix the machine and the ground. There are many shapes. U-shaped bolts, as mentioned above. and many more. There are also special studs for welding. One end has a thread and the other is not. It can be welded to the part, and the nut is directly screwed on the other side.