Cross Recessed Countersunk Screws Applications and Installation

A cross recessed countersunk screw is a type of screw. Its head is a flat head, and it is also a countersunk head or a raised countersunk head. Countersunk screws can also be called flat head screws, countersunk screws, or raised countersunk screws. Generally, most countersunk screws have a cross recess.

Applications of Countersunk Screws

Countersunk screws are mostly used where there should be no protrusion on the surface of the parts after installation. There are two types of thicknesses for the parts being fastened.

The first is when the part being fastened is thick, which means that the thickness of the part is greater than the thickness of the countersunk screw head. After the screw is tightened, part of the screw thread has not entered the threaded hole. In this situation, the countersunk screw can definitely be tightened properly.

There is also another common situation where the thickness of the part being fastened is less than the height of the countersunk screw head. This is common in sheet metal parts in mechanical equipment, such as the connection between the hinge, door and enclosure, or the connection between the sheet metal cover and the equipment.

Because the parts are thin, the clearance hole in the sheet metal part becomes completely conical. In this situation, when the countersunk screw is tightened, the conical surface of the screw head does not press against the sheet metal. Instead, the bottom of the screw head contacts the top of the threaded hole and becomes stuck.

Although the screw feels tight, the sheet metal part is actually being trapped rather than pressed and clamped. In this situation, although the screw feels fully tightened, the sheet metal part is not actually tightened. This is a very common situation.

There is another reason related to machining. The conical part of a countersunk screw head normally has a 90° included angle, while the point angle of a newly purchased drill is usually 118°–120°. Some workers who have not received sufficient training do not know about this angle difference and often directly use a 120° drill to enlarge the hole.

As a result, when the countersunk screw is tightened, it is not the conical surface of the screw head that bears the load, but a line at the bottom of the screw head. This is also one of the reasons why a countersunk screw cannot hold the parts tightly.

How to Distinguish a Countersunk Screw from a Raised Countersunk Screw?

The top of a countersunk head is flat, while the top of a raised countersunk head has a curved surface. It looks more attractive, but raised countersunk screws are generally less commonly available as stock items.

Where Are Pan Head Screws and Countersunk Screws Used?

High-strength pan head combination screws for steel structures are mainly used in railway and highway bridges, boiler steel structures, industrial buildings, high-rise civil buildings, tower structures, lifting machinery and their steel structures where friction-type high-strength pan head combination screws are required.

A 90° conical recess is machined into the surface of the connected part. The head of the flat countersunk screw sits inside this conical recess and is flush with the surface of the connected part.

Flat countersunk screws are also available with a semi-round head in some applications. These screws are relatively attractive in appearance and can be used where a small amount of protrusion from the surface is acceptable.

Precautions for Cross Recessed Countersunk Screws

1. The Countersunk Angle Should Be 90°

The countersunk angle should be 90°. It is better for the angle to be smaller than 90° rather than larger than 90°. This is a key point.

2. Consider the Sheet Metal Thickness

If the sheet metal thickness is less than the height of the countersunk screw head, a smaller screw can be used. Alternatively, it is better to make the countersunk hole slightly smaller rather than enlarging the hole and increasing the diameter of the bottom hole, which may cause the part to fail to clamp properly.

3. Accurately Machine Multiple Countersunk Holes

If there are multiple countersunk screw holes on a part, the holes should be machined accurately. If the holes are drilled at an incorrect angle or position, assembly can be difficult and the final appearance may be affected.

However, as long as the error is not too large, the screw can still be tightened. When the screw diameter is relatively small, generally no more than about 8 mm, if there is some error in the hole spacing, the screw head can deform under load during tightening, allowing the screw to be tightened even with a certain amount of positional error.