What are the thread forms
Standard self-tapping screw identification characters, including symbols representing thread shape and tail. Self-tapping screws have one or two thread characters to indicate mechanical threads or wide threads (SPACED). For example, the identifying character "B" stands for a wide thread screw. Those without "B" represent mechanical threads, and the end represents characters to distinguish whether the self-tapping screw is self-tapping screw, thread screw, cutting screw, rolling screw or self-drilling screw.
The mechanical thread has the same 60-degree thread angle and the same thick and thin thread pitch as the Anglo-American unified thread. If a screw is lost or needed after disassembly, it can be replaced with a standard screw fastener.
Wide threads have a thread angle of 60 degrees but a wider pitch. Because of its wider pitch, it has a steeper thread than a mechanical thread, and it also has a larger screw lead. There is also a thread specially used on plastic, its thread angle is 48 degrees, commonly known as PT screw.
Self-tapping screws A, AB, B, BP, C and other types have been gradually replaced by rolling self-tapping screws, so they are not recommended for use in new designs. The only difference between AB, B and BP is the shape of the tail. AB has a threaded cone, B has a blunt tail, and BP has a non-threaded cone. The center of the pointed tail goes into the pre-drilled hole and begins to form the thread. Thread cutting self-tapping screws have BF, BP, D, F, G, T and other forms. BF and BP are wide threads, and the rest are mechanical threads.
The distinguishing feature is the cut tail. Each type of cutting tail has a shape that collects debris from the cutting. If a self-tapping screw is screwed into a blind hole (a hole that is not passed through), its fragments will remain at the bottom of the hole and be sealed. However, if a through hole is tapped, the scrap lands on the other side of the workpiece. Therefore, such self-tapping screws must be considered when selecting such self-tapping screws. Scrap can contaminate, fall into functioning parts, or render electronic substrates inoperable.
All rolled self-tapping screws are machine threads, the same as the Anglo-American Unified Thread coarse pitch thread. Please refer to SAE J81 regulations. For metric threads, please refer to SAE J1237 and DIN7500.
tighten
There are two types of self-drilling and self-tapping screws: cutting wide thread BSD and mechanical thread CSD. There are also special thread types, including high and low double threads, and various tapping and drilling tails are also suitable. Due to the nature of tapping and threading, end drill screws are not suitable for blind hole operations. When assembling self-drilling screws, the drill tail must fully penetrate the workpiece to cut or form threads. Therefore, there are usually some fragments. Chipping is unavoidable if thread cutting is used. Therefore, practical considerations must be made. this problem. Please refer to SAE J78 and DIN7504 (wide thread) for BSD and CSD self-drilling screws.
