Cup washers, also known as cupped washers or finishing washers, are a type of washer with a curved or concave shape. They are designed to be used with screws or bolts and are commonly used to provide a smooth and finished appearance to the assembly, as well as to distribute the load evenly.

The cupped shape of these washers allows them to fit into countersunk holes, where the washer sits flush with the surface. This helps to create a more streamlined and aesthetically pleasing look in applications where a flat washer might not be suitable. Cup washers are often used in woodworking, metalworking, and other applications where a flush surface is desired.

In addition to their appearance, cup washers can also help prevent damage to the material being fastened, as the curved shape allows for a more even distribution of pressure. This can be particularly important in delicate materials or applications where a flat washer might cause indentation.

Cup washers come in various materials such as steel, stainless steel, brass, and nylon, depending on the specific requirements of the application. They are available in different sizes to match the corresponding screw or bolt size.