There are many kinds of combined screws, including two combined screws and three combined screws (flat washer and spring washer or separate flat washer and spring washer) according to the type of combined accessories; According to the head type, it can also be divided into pan head combination screws, countersunk head combination screws, external hexagonal combination screws, etc; According to the material, it is divided into carbon steel, stainless steel and alloy steel (Grade 12.9).