Self-tapping screw, as the name suggests, self-tapping screw is a screw that can tap out the screw, the English name is self-tapping screw, which is a kind of threaded fastener that taps and drills the matched internal thread in the pre-drilled hole of metal or non-metallic materials.
Because it can tap the threads that fit it, it has a high anti-loosening ability when fastening, and can be disassembled and reused.
In small screw products, self-tapping nails have other screws in size, thread form, head shape and tapping and drilling, and the products have irreplaceable characteristics, and are widely used in engineering because of the characteristics of high strength, single piece, single body and single side assembly.
Today, Screw Jun introduces the knowledge of self-tapping products from the aspects of historical development, types, application scenarios, material selection, processing technology, surface treatment and screwing torque.
First, the development and application scenarios of self-tapping nails
Tapping threads, commercialized from 1914. It is of origin from wooden threads. At that time, it was mainly used for the combination of iron sheets on the ducts of air conditioning systems, so it was also called iron screws.
After a hundred years of development and changes, fastener products belonging to the category of self-tapping nails are constantly increasing and improving.
It can be roughly divided into 4 periods: screw forming, thread cutting, thread rolling, and self-drilling.
1. Thread forming self-tapping screws are directly developed from iron screws, thread forming self-tapping screws must be drilled in advance when used, and then the screws are screwed into the holes, and the materials on the original internal thread position are squeezed into the external threads, this self-tapping screws are called thread forming self-tapping screws, which can only be applied to thin and malleable materials, so thread cutting self-tapping screws have been developed.
2. Thread cutting self-tapping screws, cutting one or more cutting openings at the tail end of the thread, so that when screwing into the pre-drilled hole, the thread tail and teeth are used to cut out the matching internal thread in a similar way as screw tapping. It can be used in thick plates, materials that are not easy to shape such as hard or brittle.
3. The thread is rolled into a self-tapping thread, the triangular tooth is self-tapping screw, and the thread is rolled into a self-tapping screw, which has a specially designed thread and bottom end, which can make the screw roll into a mating internal thread under intermittent pressure. It can be used on thicker materials, and the torque of rotation is more controllable and the strength is higher when combined.
4. Self-tapping screws at the end of drilling, drilling, tapping and tightening in one operation to save costs. When using, it is necessary to do a penetration test to verify whether the hole is drilled and the thread is clicked within the specified time.
In addition to the above four types, there are also two special threads:
1. One is the high and low tooth self-tapping screw, double thread design, the high thread with a larger tooth outer diameter has a sharper 30-degree thread angle, and the low thread with a smaller tooth outer diameter has a 60-degree thread angle, and the tooth height is only half of the high thread, which is the NST thread below. These threads are particularly suitable for plastic parts and wood.
2. The other is the so-called TP tooth, in order to avoid the risk of cracking and slipping of plastic parts, a smaller thread tooth angle is used in the design, which is suitable for thermoplastic parts.
Second, the type, specification and related standards of self-tapping nail thread
1. ST thread.
The ST thread, which is the most common self-tapping thread – refer to GB T5280 and its profile diagram is as follows:
The specifications of ST self-tapping nails refer to GB T5280, and there are a total of 14 specifications of ST threads in GB T5280, but the types of self-tapping nails should be controlled in an enterprise's fastener library, and it is best to control them within 3 types, and not more than 5 specifications at most.
Specification of ST thread size commonly used in the automotive industry: ST29、st3.5，st4.2、st4.8 and st63。
There are a total of 3 threaded end forms in GB T5280, and it is best to use only 1 form in the company's fastener library.
Thread specification ST4Example of a thread for a thread with a self-tapping screw of 8: self-tapping thread GBT 5280-ST48。
2. NST thread.
NST thread, also known as high and low thread, is a self-tapping thread for plastic parts, with reference to the standard QC T713 "Self-tapping Screw Thread for Plastic (Nylon)", and its tooth type and structure are as follows
Specifications of the NST thread size.
Example of an NST thread marker: NST4
It is recommended to unify the form of threaded ends in the fastener library and choose only 1.
3. TP thread.
TP thread, referring to the popular standard VW60362, this kind of thread has many patented products, there are many differences in outer diameter and pitch, but the purpose is to solve the problem of cracking and deformation of plastic parts with self-tapping nails and plastic parts.
The tooth profile is as follows:
Specifications of commonly used TP thread sizes.
Example of TP thread marking: TP5
Third, the selection of self-tapping nail thread
The self-tapping screw is tapped by screwing into the connected plate to achieve the fastening function, and the plate has a great impact on the screwing performance of the self-tapping screw. The chemical properties and thickness of different sheets have different effects on the screwing torque and fastening properties of the self-tapping nails.
1. ST thread is used to screw into the metal sheet, and the metal sheet prefabricated hole is matched with each other, or screwed into the sheet metal stamping nut with the same thread, the typical fit nut is a leaf spring nut and a push-in leaf spring nut. In GB T5280, it is clearly written: "This standard specifies that the thread specification is ST1."5~st9.5. The thread and its end of the self-tapping screw (screw for metal sheet) must pay attention to the word "thin", and fully consider the selected thread specification, plate thickness and opening size.
When choosing metal plates, mild steel should be used, and it is important to pay attention to the matching of the mechanical strength and hardness of the metal plate and the strength and hardness of the screws. The surface treatment of the screw affects the coefficient of friction, if the coefficient of friction is high, the strength of the metal plate is high and the hardness is high, the phenomenon of screw slippage failure and inability to tap will occur. When choosing the fastening form of self-tapping nail and metal plate matching, it must be fully evaluated and verified, and do not think that self-tapping nail can be screwed into any kind of steel plate.
The sheet metal thickness and hole size in the following table should be tested to determine the reasonable screwing torque and feasibility.
The sheet metal thickness and opening size for ST threads can be referred to the following table: (unit: mm).
2. NST thread is a self-tapping nail thread for screwing into plastic parts, and plastic parts also need prefabricated holes, and the recommended opening size and depth are shown in the following table: (unit: mm).
3. In order to better solve the problem of matching between self-tapping nails and plastic parts, and ensure that the plastic parts are not prone to cracking, deformation and other failures while fastening the connection, TP threads can be selected.
According to the material properties and the size and shape of the connected parts, the size of the opening is also different, you can refer to the following table, the data in this table is only recommended, and the specific use must be fully evaluated and verified.
TP thread cut-out size reference (unit: mm).
Fourth, the selection and production process of self-tapping nail materials
ST thread and NST thread self-tapping nail performance grade according to GB T309085 The requirements of "Mechanical Properties of Fasteners Self-tapping Screws" are made of cold heading and carburized steel. The surface hardness of the screw after heat treatment is 450hv03. Core hardness: thread ST39: 270hv~390 hv5;Thread st42: 270hv~390 hv10。
The microstructure between the carburized layer and the core should not show bands of sub-eutectic ferrite. Commonly used material grades are SWRCH18A, SWRCH22A, etc.
After the thread is formed, the carburizing process is used to achieve the corresponding mechanical properties, and the carburizing depth of the screw should be in accordance with GB T309085 requirements.
TP thread because the tooth angle is 30°, so the thread is thinner, carburizing hardening will affect the strength of the thread, so the general use of quenched and tempered treatment, in order to achieve similar strength with ST thread, need to use alloy steel or low-carbon boron-containing steel production, such as SCMB21, etc., after quenching and tempering to achieve the corresponding mechanical properties.
The production process of self-tapping nails is basically the same as that of conventional fasteners, and the materials also need to be spheroidized annealing, drawing and lubrication, and then cold pier forming and thread rolling to make threads. This is followed by heat treatment.
Finally, the surface treatment is done according to the customer's requirements, and if the surface treatment has pickling and electroplating processes, effective dehydrogenation treatment is required to reduce the risk of hydrogen embrittlement.
Fifth, the head shape and wrenching form of the self-tapping nail
1. The head shape of the self-tapping nail is varied, and the commonly used ones are shown in the following table
2. The wrenching form refers to the way of twisting the screw head when installing and tightening the screw, and there are two basic methods: external wrenching and internal wrenching. The allowable torque of the external wrench is greater than that of the internal wrench.
The commonly used wrenching forms of self-tapping nails are as follows:
Sixth, the screwing torque of the self-tapping nail
According to a report by the Japan Institute of Screws and Threaded Fasteners, "On the Standardization of the Hole Size of Self-Tapping Screws", the optimal working condition of self-tapping nails should be in line with the ratio of failure torque to screwing torque 3, that is, the screw screwing torque is less than or equal to one-third of the destruction torque.
gb3098.The minimum moment of destruction of a self-tapping screw is specified in 5.
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