Why can't Countersunk Screws be Tightened Easily?

Countersunk head screws, also known as flat head screws and flat head screws, have a 90 degree cone at the head, which is similar to common wood screws. There are tool tightening slots at the head, such as I-shaped, cross shaped, hexagonal, etc.

There are three reasons why countersunk screws cannot be tightened:

1. The thickness of the fastened part is greater than that of the countersunk screw head. After the screw is tightened, some screw threads do not enter the threaded hole.

2. The thickness of the fastened part is less than the height of the countersunk screw head, 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; Connection between sheet metal cover of equipment and equipment, etc.

Due to the small thickness of the part, the screw through hole of the fastened sheet metal part completely becomes a conical hole. In this case, when the countersunk screw is tightened, the screw head does not press the sheet metal part with the conical surface, but the bottom of the screw head root and the top of the threaded hole are squeezed. Although the screw is felt to be tightened, the sheet metal part is stuck rather than pressed. In this case, although the screw is felt to be tightened, But the sheet metal parts are not tightened.

3. The round cone of the countersunk head screw has a conical angle of 90 °. Generally, the top angle of the newly purchased drill bit is 118 ° -120 °. Some workers who lack training do not know this angle difference at all, and often directly use a 120 ° drill to ream the hole. This causes that the countersunk head screw is not hard to tighten the conical surface of the head, but a line at the bottom of the screw head. This is also one of the reasons why the countersunk head screw can not tighten.