Self-tapping screws are used for nonmetal or soft metal, and there is no need to drill low holes and tap;
self-tapping screw
The self-tapping screw is pointed, so that it can be "self-tapping"; Ordinary screws are flat-headed and have the same thickness.
Self-tapping screw means: the hole drilled is a hole without tapping, and the screw used is different from the general one. The head is relatively sharp and the pitch is relatively large, which is a bit like chipless tapping, and it is screwed in directly without tapping. This method is usually used for metals and plastics.
It can use its own thread to "drill and squeeze" the corresponding thread on the consolidated material to make it fit closely.
Self-tapping screw is a kind of screw with drill bit, which is drilled, tapped, fixed and locked by special electric tools.
self-tapping screw
In one go. Self-tapping screws are mainly used to connect and fix some thin-plate parts, such as the connection between colored steel plates, the connection between colored steel plates and purlins, wall beams and so on. , its penetration capacity is generally not more than 6mm, and the maximum is not more than12 mm. Self-tapping screws are often exposed outdoors and have strong corrosion resistance; Its rubber sealing ring can ensure that the screw is watertight and has good corrosion resistance.
Self-tapping screws are usually described by three parameters: screw diameter series, number of threads per inch length and screw length. Screw diameters are 10 and 12, and the corresponding screw diameters are 4.87mm and 5.43mm respectively. There are three grades of threads per inch: 14, 16 and 24. The more threads per inch, the stronger the self-drilling ability.
1 evolution
Self-tapping screws have been commercialized since 19 14. The first design-mainly from wood screws-belongs to carburized steel taper tail type A thread forming screw. At that time, it was mainly used to connect iron sheets to the pipes of air conditioning systems, so it was also called iron screws. After more than 80 years of development, * * * can be divided into four periods-thread forming, thread cutting, thread rolling and self-drilling.
Thread forming self-tapping screw
(thread forming tapping screw)-it is directly developed from iron shell screw. When using thread to form self-tapping screws, it is necessary to drill holes in advance, and then screw the screws into the holes to forcibly squeeze the internal threads, while the materials originally located in the internal threads will be squeezed between the external threads. This is the so-called tapping screw. It can only be applied to thin and plastic materials, so it was developed again. ThreadCuttingTappingscrew)-Cut one or more notches at the end of the thread so that when screwing into the pre-drilled hole, the matching internal thread can be cut in a similar way to tapping by using the screw tail and teeth. Can be used for thick plates, hard or fragile materials, etc.
Thread rolling self-tapping screw
Threadrollingtappingscrew)── also known as TypeTT(TypeTai is still patented), is developed based on the principle of forming tapping. The thread-rolling self-tapping screw has specially designed thread and internal thread at the end, which enables the screw to roll into the fit by itself under intermittent pressure. At the same time, the material around the hole can more easily fill the space between the thread and the root of the self-tapping screw. Because its friction force is smaller than that of thread-formed self-tapping screws, it can be used in thicker materials, and the torque required for rotation is better controlled, and the combined strength is higher. The engineering standard definition of thread-rolling self-tapping screws in terms of materials, heat treatment and strength is higher and clearer than that of forming or cutting self-tapping screws, which makes thread-rolling self-tapping screws a real "structural" fastener.
Drill tail tapping screw
(self-drilling tapping screws)-Also known as Tec, in the process of assembling self-drilling screws, the most costly thing is the preparation of pre-drilling. The use of self-tapping screws requires drilling first, and the aperture must also be limited. There is no need for pre-drilling, which can save costs in some aspects. This is a combination of drilling, tapping and drilling self-tapping screw tightening in one operation. The surface hardness and core hardness of drill screw are a little higher than those of ordinary self-tapping screw, which is because there is one more drilling process for drill screw. In addition, the penetration test of the drill screw is needed to test whether the screw can drill and tap the thread within the specified time. The above is the design and development process of four main self-tapping screws, and the other two are screws with special thread design. The first type is: high and low self-tapping screws-used for plastics or other low-density materials. Double thread design, high thread (with larger tooth outer diameter) has flatter and sharper thread type, and the thread angle is 30 degrees. The thread angle of low thread (with small external diameter of teeth) is 60 degrees, and the height of teeth is only half of that of high thread. The combination design of high and low threads reduces the rotating torque, improves the pull-out strength, and greatly reduces the risk of fracture of the combined workpiece. It is also used for drywall assembly in the construction industry and has the following advantages.
Double-thread self-drilling tail horn head self-tapping screw
(Twin lead self-drilling point taping crews)-When assembling, you can easily screw into drywall, drill holes on steel racks and tap the matching internal threads. The horn head has a concave bearing surface, which will not damage the wallpaper or plaster surface when screwing in. Most self-tapping screws are used for commercial purposes. For example, when self-tapping screws are provided as assembly screws (SEMS), they can provide more compact services. However, screws with thick and thin threads are specially designed.
2 engineering standards
The dimensions, mechanical properties and functional characteristics of self-tapping screws have the following standards:
Ansi/asmeb18.6.4-thread forming and thread cutting thread and metal drive screws (English series)
Saej 933─-mechanical and quality requirements
Saej81-thread rolling screw
Saej1237-metric thread roller screws
Saej 78─-steel self-drilling tapping screw
FIP1000-Performance specification of tapping screws
Din 7500─-Thread Rolling Thread Metric Thread
Din 7504─-Dimensions, requirements and testing of self-drilling tapping screws
Iso 2702─-mechanical properties of heat-treated steel
JISB1055-Mechanical properties of heat-treated steel stainless steel tapping screws
3. Thread type
The identification characters of standard self-tapping screws include symbols representing the shape of thread and tail. Self-tapping screws have one or two characters representing threads to represent mechanical threads or wide threads. If there is an identification character "b", it represents a wide thread. If there is no "b", it represents mechanical thread. The tail characters are used to distinguish self-tapping screws from thread forming, cutting, rolling or self-drilling.
Mechanical thread is the same as Anglo-American unified thread, the thread angle is 60 degrees, and the pitch is the same as that of thick and thin thread. If the screws are lost or needed after removal, they can be replaced with standard threaded fasteners.
The thread angle of wide thread is 60 degrees, but the pitch is wide, and the thread lead is larger than that of mechanical thread because of the wide pitch and steep thread. There is also a thread specially used for plastics, with a thread angle of 48 degrees, commonly known as PT tooth screw.
At present, thread forming tapping screws A, AB, B, BP and C have been gradually replaced by thread rolling tapping screws, so it is not recommended to use them in the new design. AB, B and BP only have different tail shapes, AB has a threaded taper tail, B has a blunt tail, BP has a threaded taper tail, and the center of the pointed tail enters a pre-drilled hole to start forming threads. Thread cutting tapping screws include BF and BP.
The difference of a recognized character lies in its truncation. Each cutting tail is shaped to collect the cut waste. If the self-tapping screw is screwed into a blind hole (not a through hole), its fragments will remain and be sealed at the bottom of the hole. However, if it is tapped into the through hole, the waste will fall on the other side of the workpiece. Therefore, this must be considered when choosing this type of self-tapping screw. Waste may cause pollution, fall into moving parts or make electronic substrates inoperable.
All thread-rolling self-tapping screws are mechanical threads, just like the coarse threads of Anglo-American unified threads. Please refer to the provisions of SAEJ8 1. For metric threads, please refer to SAEJ 1237 and DIN7500.
There are two kinds of self-tapping screws at the drill tail: BSD for cutting wide threads and CSD for mechanical threads. In addition, there are special thread types, including high and low double threads, and various drill tails are also applicable. Because of the nature of drill tail and thread, drill tail screw is not suitable for blind hole operation. When assembling self-drilling screws, the drill tail must completely penetrate into the workpiece before cutting or forming threads, so there are usually some chips. If threads are used to cut threads again, there will inevitably be chips. Therefore,