Standard self-tapping screw identification characters, […]
Standard self-tapping screw identification characters, including symbols representing the thread and tail shape. Self-tapping screws have one or two characters representing the thread to represent mechanical or wide threads (SPACED). For example, with the identification character "B" For wide threaded screws. No "B" stands for mechanical thread. The tail end character is used to distinguish between self-tapping screws for thread forming, cutting, rolling or self-drilling.
The mechanical thread has the same 60 degree thread angle and coarse and fine thread as the British and American unified thread. If the screw is lost after the disassembly or if necessary, it can be replaced by a standard threaded fastener.
The wide thread has a thread angle of 60 degrees but its pitch is wider, and because of its wide pitch, its thread is steeper and its thread lead is larger than the mechanical thread. Another thread is specially used for plastic. Its thread angle is 48 degrees, commonly known as PT screw.
Thread forming self-tapping screws A, AB, B, BP, C, etc. have been gradually replaced by thread rolling self-tapping screws, so it is not recommended in the new design. AB, B, BP is only in the tail shape, AB has a threaded cone tail, B has a blunt tail, BP has a non-threaded cone tail, the center of the tip end enters the pre-drilled hole and begins to shape the thread. The thread cutting self-tapping screws are BF, BP, D, F, G And T and other forms, BF, BP is a wide thread, the rest is a mechanical thread.
The difference between the identification characters is the cutting tail. Each cutting tail has a shape that can collect the scrap under the cutting. If the self-tapping screw is screwed into the blind hole (the hole that does not pass through), the debris will remain and be sealed at the bottom of the hole. However, if you break into the through hole, the waste will fall on the other side of the workpiece. Therefore, you must 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 impossible. Working.
All threaded self-tapping screws are mechanical threads. Just like the British and American unified thread coarse teeth, please refer to SAE J81. For metric thread, please refer to SAE J1237 and DIN7500.
The self-tapping screws of the drill tail have two types: cutting wide thread BSD and mechanical thread CSD. There are also special thread types, including high and low double threads, and various tapping tails are also applicable. Because of the tapping tail and thread nature, the tail screw It is not suitable for blind hole operation. When assembling the self-drilling screw, the drill tail needs to completely penetrate the workpiece to cut or shape the thread, so there will usually be some debris. If the thread cutting thread is used, it is even more necessary. Chips. Therefore, it is necessary to consider this problem practically. For the BSD and CSD self-drilling screws, please refer to SAE J78 and DIN7504 (wide thread).
The most important part of the self-tapping screw thread is the outer diameter of the tooth. If it is too large, the assembly torque will increase. If it is too small, the thread shear strength will be reduced, and this characteristic can be easily determined by the centimeter card or the caliper. In addition to this feature, the self-tapping screws do not require any thread measurement. It is quite clear that the female thread is made of self-tapping screws and can be self-tapping, so there is no need for any matching measurement. However, the manufacture must still pay attention to the provisions of ANAI/ASME B18.6.4: C.D.F.G and T: its own size must not be less than the minimum effective diameter of the 2A thread, nor greater than the maximum outer diameter of the tooth. So sometimes The customer will comply with the requirements. When required, the ring gauge cannot be successfully inspected due to the limitation of the cutting groove. The thread indicator should be used for the measurement of the thread. Two other self-tapping screw thread characteristics are worth mentioning. :
The self-tapping screw of thread forming is extruded because the mating thread is required. Therefore, the bottom diameter of the screw must be slightly smaller than the pre-drilled hole, so that there is room for the material extruded on the workpiece, and the shear strength of the thread is also higher. Large. Thread cutting self-tapping screws are cut and formed because of the matching thread. Therefore, the screw bottom diameter must be slightly equal to the pre-drilling hole, and its preset torque is small, and the breaking strength and pulling strength are also low.