1. The thickness of the fastened part is greater than the thickness of the head of the countersunk screw. After the self-tapping screw is tightened, there is still a part of the screw thread that does not enter the threaded hole. In this case, the countersunk screw can definitely be tightened.
2. The thickness of the fastened parts is less than the height of the head of the countersunk screw, which is common in sheet metal parts in mechanical equipment, such as the combination of the hinge of the chassis and the door and the box; the combination of the sheet metal cover of the equipment and the equipment etc. Due to the small thickness of the part, the screw through hole of the fastened sheet metal part has completely become a conical hole. In this situation, when the countersunk screw is tightened, the screw head is not a conical surface to compress the sheet metal part. , But the bottom of the head of the Tapping screws and the top of the threaded hole are squeezed. Although the screw feels that the screw is tightened, the sheet metal part is stuck instead of being pressed. In this situation, although the screw feels that the screw is tightened, The sheet metal parts are indeed not tightened. This is a very common situation.
3. The head cone of the countersunk screw has a cone angle of 90°. Usually the top angle of a newly purchased drill bit is 118°-120°. Some workers who lack training do not know this angle difference, and often use 120° directly. When the countersunk head screw is tightened, it is not the conical surface of the head that is strenuous, but the line at the bottom of the tapping screw is strenuous. This is also one of the reasons why the so-called countersunk screw cannot be tightened.
