Reasons And Precautions For Countersunk Screws Not Tightening

A Countersunk screw is also known as a flat machine screw or flat head screw. Its head is a 90-degree cone, and the mounting hole surface on the connector is also machined with a 90-degree conical socket, so as to ensure that the screw head is parallel to the connector surface, for the surface can allow a little protruding. In the process of using countersunk screws, why do we always encounter the phenomenon of not tightening?

As a professional fastener manufacturer, Xinchi Fastener Company has many years of experience in the fastener industry. We would like to share with you the reasons why countersunk screws are not tight and the matters that need attention.

Reason 1. The thickness of the secured part is greater than that of the countersunk screw head. When the screw is tightened, there is still a part of the screw thread that did not enter the threaded hole. In this case, countersunk screws can certainly be tightened.

Reason 2. 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, such as the connection between the hinge of the case and the door and the box; Or the connection between the sheet metal cover and the equipment and so on.

Due to the small thickness of the part, the screw hole of the sheet metal part is fastened into a cone hole. In this case, when the countersunk screw is tightened, the screw head is not conical compression of the sheet metal, but the base of the screw head and the top of the threaded hole. Although we feel that the screws are tightened, the sheet metal is stuck rather than pinned. In this case, the sheet metal is not tightened even though it feels like the screw is tightened. This is a very common situation.

Reason 3. The head circular vertebra of the countersunk head screw presented a 90° conical Angle. However, the top Angle of a newly purchased drill bit is usually between 118° and 120°. Some untrained workers are unaware of the Angle difference and often use the 120° drill bit to enlarge the hole. This causes countersunk screw tightening is not the head’s conical surface, but a line at the bottom of the screw head, which is also one of the reasons why the so-called countersunk screw is not tight.

Warm reminders need to pay attention to:

1. The taper of reaming should be 90°. It’s better to be less than 90 degrees than more than 90 degrees and that’s the key.
2. If the thickness of the sheet metal is less than the thickness of the head of the countersunk screw, you can change to a smaller screw, or prefer smaller reaming.

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