What Is The Reason Why The Countersunk Head Screws Cannot Be Tightened?

Countersunk head screws are mostly used when there should be no raised areas on the surface of the parts after installation. However, countersunk head screws sometimes cannot be tightened. The possible reasons are as follows:

1. The thickness of the fastened parts. Generally, there are two types of thickness for the parts that need to be fastened. Thick means that the thickness of the fastened parts is greater than the thickness of the countersunk screw head. In this case, after the screws are tightened, some of the screw threads have not entered the threaded holes, and the countersunk head screws can be tightened. Thin means that the thickness of the fastened parts is less than the height of the countersunk screw head, which is common in sheet metal parts in mechanical equipment. Due to the small thickness of the parts, the screw holes of the fastened sheet metal parts have completely become a conical hole. In this case, when the countersunk head screws are tightened, the screw head does not press the sheet metal with the cone surface, but the bottom of the screw head is squeezed with the top of the threaded hole. In this case, although the screws feel tightened, the sheet metal is not tightened. It is just stuck instead of pressed.

2. There may be deviations in the angle of the drill bit used. The cone of the countersunk screw head has a 90° cone angle. Usually, the top angle of a newly purchased drill bit is 118°-120°. Some workers who lack training are not aware of this angle difference and often use a 120° drill bit to expand the hole. As a result, when the countersunk head screw is tightened, it is not the conical surface of the head that is strained, but a line at the bottom of the screw head that is strained. This is one of the reasons why the countersunk head screw cannot be tightened.