Countersunk head screws are mostly used after installation, and the surface of the parts cannot have protrusions. The fastened parts are also thick and thin. The so-called thick ones mean that the thickness of the fastened parts is greater than that of the head of the countersunk head screw. Thickness, after the screw is tightened, some of the screw thread does not enter the threaded hole, as shown in the attached picture on the 8th floor, in this case, the countersunk head screw can definitely be tightened. There is usually another situation where the thickness of the part to be fastened is less than the height of the head of the countersunk head screw, which is common in sheet metal parts in mechanical equipment, such as the connection between the hinge of the chassis and the door and the box; the sheet metal of the equipment The connection between the housing and the equipment, etc. Due to the small thickness of the part, the fastened sheet metal part, the screw hole is completely a conical hole. In this case, when the countersunk screw is tightened, the head of the screw is not a conical surface to press the sheet metal part. , but the bottom of the screw head and the top of the threaded hole are squeezed. Although it feels that the screw is tightened, the sheet metal part is stuck instead of being pressed. In this case, although the screw feels tightened, the sheet metal The gold pieces were indeed not screwed down. This is a very common situation.