Thread Type
Standard self-tapping screw identification characters, including symbols representing thread and tail shape. Self-tapping screws have one or two characters representing the thread to represent a mechanical thread or a wide thread (SPACED). If it has the identification character "B", it means it is a wide thread screw. Those without "B" represent mechanical threads, and the tail represents characters to distinguish self-tapping screws as thread forming, cutting, rolling or self-drilling. The mechanical thread has the same 60-degree thread angle and the same pitch of the thick and thin threads as the British and American unified threads. If the screws are lost or necessary after disassembly, they can be replaced by standard thread fasteners. The wide thread has a 60 degree thread angle but its pitch is wider, and because of its wide pitch, its thread is steeper and its thread lead is larger than that of a machine thread. There is another kind of thread specially used on plastic, whose thread angle is 48 degrees, commonly known as PT screw. Thread forming self-tapping screws A, AB, B, BP, C and other types have been gradually replaced by thread rolling self-tapping screws, so it is not recommended to use in new designs. The difference between AB, B, BP is only in the shape of the tail. AB has a threaded cone tail, B has a blunt tail, and BP has a non-threaded cone tail. The center of the pointed tail enters the pre-drilled hole and begins to form the thread. Thread cutting self-tapping screws have BF, BP, D, F, G and T forms. BF and BP are wide threads, and the rest are mechanical threads. The difference between the identification characters is the cutting tail. Each cutting tail has a shape that can collect the scraps under cutting. If the self-tapping screw is screwed into a blind hole (a non-penetrating hole), its fragments will remain and be sealed to the bottom of the hole. But if the through hole is tapped, the scrap will fall on the other side of the workpiece. Therefore, it is necessary to consider this when selecting this type of self-tapping screw. The waste may cause pollution, fall into the running parts or make the electronic substrate inoperable. All threaded rolled self-tapping screws are machine threads with the same coarse thread as British and American unified threads, please refer to SAE J81 regulations. Refer to SAE J1237 and DIN7500 for metric threads. Drill tail self-tapping screws have two types of cutting wide thread BSD and mechanical thread CSD. There are also special thread types, including high and low double threads, and various tapping drill tails are also suitable. Due to the nature of the tapping tail and the thread, the tapping screw is not suitable for blind hole operations. When assembling a self-drilling screw, the drill tail needs to completely penetrate the workpiece before cutting or forming the thread, so there will usually be some chips. this question. Please refer to SAE J78 and DIN7504 (wide thread) for BSD and CSD drill-tail screws.
