The countersunk head screw, also known as the fl […]
The countersunk head screw, also known as the flat machine screw, has a 90-degree cone on the head. Similar to a common wood screw, the head has a tool-tightening groove, a shape, a cross, an inner hexagon, and the like.
Countersunk screws are mostly used in places where the surface of the parts cannot be raised after installation. The parts to be fastened are thick and thin. The so-called thick, that is, the thickness of the parts to be fastened is larger than the head of the countersunk head screw. Thickness, after the screw is tightened, there is still a part of the screw thread that does not enter the threaded hole. In this case, the countersunk screws can certainly be tightened. There is usually also a case where the thickness of the fastened part is less than the height of the head of the countersunk head screw, which is common in mechanical equipment in sheet metal parts, such as the hinge of the case and the connection of the door and the case; the sheet metal of the device The connection of the cover to the device, etc.
Due to the small thickness of the part, the fastened sheet metal part, the screw through hole completely becomes a tapered hole. In this case, when the countersunk screw is tightened, the screw head is not a tapered surface pressed sheet metal part. Instead, the bottom of the screw head is crushed to the top of the threaded hole. Although the screw is felt to be tightened, the sheet metal piece is jammed instead of being pressed. In this case, although the screw is felt to be tightened, The gold pieces are indeed not tightened. This is a very common situation.
Let me talk about the reason for the processing: the head of the countersunk screw has a 90° cone angle. Usually, the top angle of the newly purchased drill bit is 118°-120°. Some workers who lack training do not know the angle difference. It is often directly reamed with a 120° drill bit, which causes the countersunk head screw to be not a force on the conical surface of the head, but rather a line at the bottom of the screw head, which is one of the reasons why the so-called countersunk screw cannot be tightened.