The main categories of screws include ordinary screws, self-tapping screws and expansion screws; Ordinary screws: for special screws, non-standard screws, refers to the more common and commonly used, more widespread. Generally refers to the screws that are in stock. Do not need to go to the specific mold, order the screw plate to do; Self-tapping screws: Generally fingertip, coarse teeth, hard texture of wood screws, but also aluminum alloy, plastic. A special self-tapping screw for opening metal holes is called a tap. Self-tapping screws are used for non-metal or soft metal, without hitting the bottom hole and tapping, self-tapping screws are pointed, so as to "self-tapping", ordinary screws are flat-headed, the thickness is the same; Expansion screw: generally speaking, it is a metal expansion screw, and the fixing of the expansion screw is to use the wedge inclination to promote the expansion of friction gripping force to achieve the fixed effect. One end of the screw is threaded and the other end has a taper. The outer bread is a tin sheet, half of the tin cylinder has a number of cuts, put them together into the hole in the wall, and then lock the nut, the nut pulls the screw outward, pulls the taper into the tin cylinder, the tin cylinder is opened, and is tightly fixed on the wall, generally used for guardrail awning, air conditioning and other materials in cement, brick and other fastening.