Self-tapping screw

Self-tapping screw

Although self-tapping screws have been used for a long time and generally know how to make them, it may not be very clear about the standards and requirements stipulated in each specification. Here, we only use the collected specifications to sort out a suitable one for everyone to read, so that we can have an additional understanding of self-tapping screws. Self-tapping screws are one of the threaded fasteners that self-tap and drill the mating female threads in pre-drilled holes in metal or non-metal materials. With high pull, single piece, unilateral combination features. Because of its self-forming or tapping its mating thread, it has high anti-loosening ability in combination and can be loaded and unloaded. The size, thread type, head type, and tapping performance of small screws are almost unlimited in engineering applications.

1 Development (Evolution): Since 1914, self-tapping screws have been commercialized. The first design-mainly derived from wood screws-is a carburizable steel cone tail type A thread forming screw. At that time, the main use was to join the iron sheet on the duct of the air conditioning system, so it was also called iron sheet screw. After more than 80 years of development, it can be divided into four periods-thread forming, thread cutting, thread rolling and self-drilling.

Thread forming self-tapping screws (Thread Forming Tapping Screws)-are directly developed from iron screws. When using thread forming self-tapping screws, they must be drilled in advance, and then the screws are screwed into the holes, and the female threads are strongly extruded. The material on the position of the female thread will be squeezed between the male threads, which is called the thread forming self-tapping screw. It can only be applied to thin and plastic materials, so it has been developed; Thread cutting self-tapping screws (Thread Cutting Tapping Screws)-cut one or more cutting holes at the tail end of the thread, so that when screwing into the pre-drilled hole, Use the screw tail and teeth to cut the matching female thread in a similar way to screwing. It can be used in thick plates, which are hard or fragile and are not easy to shape,.

Thread rolling self-tapping screws (Thread Rolling Tapping Screws)-triangular self-tapping screws, also known as Type TT(Type Tai currently has patents), are developed based on the principle of forming screw tapping. The thread rolling self-tapping screws have special design The thread and tail end enable the screw to roll into a matching female thread under intermittent pressure. At the same time, the material around the hole can more easily fill the space of the thread and tooth bottom of the self-tapping screw. Because its friction force is smaller than that of the thread forming self-tapping screw, it can be used on thicker materials and the torque required for rotation is better. Control, and the strength is higher after combination. Thread rolling self-tapping screws have a higher and clearer definition in terms of material, heat treatment and strength than forming or cutting self-tapping screws, making thread rolling self-tapping screws a real "structural" fastener.

Drill Tail Self-Tapping Screw (www.tqbjz.com)-Also known as Tec, in all the processes of assembling self-tapping screws, the most costly is the preparation of pre-drilling. Self-tapping screws must be drilled first. And the aperture must also be limited. The need for pre-drilling can save costs in some ways. This is the self-tapping screw that integrates drilling, tapping and screwing in one operation. The surface hardness and core hardness of the drill tail screw are a little higher than those of the general self-tapping screw. This is because the drill tail screw has one more drilling operation. In addition, the drill tail screw needs a penetration test to test that the screw can be drilled and tapped within the specified time. The above is the design and development process of the four main self-tapping screws, and the other two are screws with special thread design. The first is; high and low tooth self-tapping screws (High - Low Tapping Screws)-used in plastic or other low-density materials. Double thread design, high thread (larger tooth diameter) with flatten and sharper thread angle of 30 degrees. The low thread (smaller tooth diameter) has a thread angle of 60 degrees and the tooth height is only half that of the 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 rupture of the combined workpiece. Another type of assembly application for drywall (Drywall) in the construction industry. Self-tapping screws with such thread design are especially suitable for plastics, part plates and wood.

The other is; Double-threaded self-drilling tail horn head self-tapping screws (Twin Lead Self Drilling Point Tapping screws) -can be easily screwed into the dry wall during assembly, self-drilling a hole in the steel tooth strip and tapping out the matching female thread. The recessed bearing surface of the horn head can be screwed in without damaging the wallpaper or plaster surface. Most of the self-tapping screws are for commercial use, for example, when self-tapping screws are provided as combination screws (SEMS), they can provide closer service. The special designers of screws with thick and thin threads are focused on preventing loosening after assembly.

2 engineering standards: self-tapping screw size, mechanical, functional can have the following standards:
ANSI/ASME B18.6.4─Thread Forming And Thread Cutting Tapping Screws And Metallic Drive Screws (Inch Series)
SAE J933─Mechanical and Quality Requirements For Tapping Screws
SAE J81─Thread Rolling Screws
SAE J1237─Metric Thread Rolling Screws
SAE J78─Steel Self-Drilling Tapping Screws
FIP 1000─Tapping Screws Performance Specifications
DIN 7500─Thread Rolling Screws For ISO Metric Screws Thread
DIN 7504─Self-Drilling Tapping Screws Dimensions, Requirements And Testing
ISO 2702─Heat-Treated Steel Tapping Mechanical Properties
JIS B1055─Mechanical Properties For Heat-Treated Steel and Stainless Tapping Screws

3.1 thread type: standard self-tapping screw identification characters, including symbols representing thread and tail shape. Self-tapping screws have one or two characters representing threads to represent mechanical threads or wide threads (SPACED ). If it has the identification character "B", it means wide thread screw. If it does not have "B", it means mechanical thread. The characters at the end are used to distinguish self-tapping screws as thread forming, cutting, rolling or self-drilling.

Mechanical threads have the same thread angle of 60 degrees and the same pitch as coarse and fine threads. If the screws are lost or necessary after disassembly, they can be replaced by standard thread fasteners.

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 therefore its thread lead is greater than that of mechanical threads. Another kind of thread 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 and other types have been gradually replaced by thread rolling self-tapping screws, so they are not recommended in the new design. AB, B and BP differ only in the shape of the tail. AB has a threaded cone tail, B has a blunt tail, BP has a non-threaded cone tail, and the center of the tip tail enters the pre-drilled hole and starts to form the thread. Thread cutting self-tapping screws are in the form of BF, BP, D, F, G and T. BF, BP are wide threads and the rest are mechanical threads.

Identifying characters differs in their cutting tails. Each cutting tail has a shape that collects the scrap material under the cutting. If the self-tapping screw is screwed into a blind hole (a hole that does not pass through), its fragments will remain and seal at the bottom of the hole. However, if the through hole is penetrated, the scrap will fall on the other side of the workpiece. Therefore, when selecting this type of self-tapping screw, it is necessary to consider this, the waste may cause pollution, fall into the running parts or make the electronic substrate unable to operate.

All thread-rolling self-tapping screws are mechanical threads, the same as the common thread coarser, see SAE J81. Refer to SAE J1237 and DIN7500 for metric threads.

Drill tail 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 tails are also applicable. Because of the nature of the drill tail and thread, the drill tail screw is not suitable for blind hole operation. When assembling the self-drilling screw, the drill tail needs to completely penetrate the workpiece before the thread can be cut or formed, so there will usually be some debris. If the thread cutting type thread is used again, there will inevitably be debris. Therefore, it is necessary to consider this issue in practical terms. Refer to SAE J78 and DIN7504 (Wide Thread) for BSD and CSD Drill Tail Screws.

The most important thing about the thread of self-tapping screws is the outer diameter of the teeth. If it is too large, the assembly torque will increase. If it is too small, the thread shearing strength will decrease, and this feature can be easily determined by centimeter or caliper. In addition to this feature, self-tapping screws do not require any thread measurement. The reason is very clear. The matching female thread is made by self-tapping screws and can be self-matched, therefore, there is no need for any matching measurement. Nevertheless, attention should still be paid to the provisions of ANAI/ASME B18.6.4 in manufacturing: c. d. f. g. and t-type: their own size shall not be less than the minimum effective diameter of 2A thread, nor shall they be greater than the maximum tooth outer diameter. So sometimes the customer will according to this regulation request. When required, the ring gauge cannot be inspected smoothly due to the restriction of the cut groove, and the thread indicator gauge must be used for thread measurement. Two other self-tapping screw thread characteristics are worth mentioning:
Since the thread forming self-tapping screw is extruded, the tooth bottom diameter of the screw must be slightly smaller than the pre-drilled hole, so that there is space to accommodate the extruded material on the workpiece. And the shear strength of the thread will be greater. Since the thread cutting self-tapping screw is cut and formed, the tooth bottom diameter of the screw must be slightly equal to the pre-drilled hole, and its preset torque is small, and the breaking strength and pull-out strength are also relatively low.

Ratio of thread diameter of self-tapping screw to aperture diameter of test iron plate
Type Hole Size Hole Size/d
A AB F,TT d A AB F TT A AB F TT
#2-32 #2-32 #2-56 0.086 0.076 0.076 0.073 0.075 0.884 0.884 0.849 0.872
#3-28 #3-28 #3-48 0.099 0.081 0.081 0.081 0.087 0.818 0.818 0.818 0.879
#4-24 #4-24 #4-40 0.112 0.086 0.086 0.096 0.098 0.768 0.768 0.857 0.875
#5-20 #5-20 #5-40 0.125 0.107 0.107 0.101 0.110 0.852 0.852 0.808 0.880
#6-18 #6-20 #6-32 0.138 0.116 0.116 0.120 0.120 0.841 0.841 0.870 0.870
#7-16 #7-19 … 0.151 0.129 0.129 … … 0.851 0.851 … …
#8-15 #8-18 #8-32 0.164 0.136 0.136 0.147 0.147 0.829 0.829 0.896 0.896
#9-14 … … 0.177 0.149 … … … 0.842 … … …
#10-12 #10-16 #10-24 0.190 0.159 0.159 0.173 0.166 0.837 0.837 0.911 0.874
… … #10-32 0.190 … … 0.177 0.172 … … 0.932 0.905
#12-11 #12-14 #12-24 0.216 0.188 0.188 0.199 … 0.868 0.868 0.921 …
#14-10 … … 0.242 0.217 … … … 0.895 … … …
… 1/4-14 1/4-20 0.250 … 0.217 0.228 0.219 … 0.866 0.912 0.876
… … 1/4-28 0.250 … … 0.234 … … … 0.936 …
#16-10 … … 0.268 0.238 … … … 0.888 … … …
#18-9 … … 0.294 0.261 … … … 0.888 … … …
#20-9 … … 0.320 0.290 … … … 0.906 … … …
… 5/16-12 5/16-18 0.313 … 0.272 0.290 0.277 … 0.870 0.928 0.886
… … 5/16-24 0.313 … … 0.295 … … … 0.944 …
#24-9 … … 0.372 0.344 … … … 0.924 … … …
… 3/8-12 3/8-16 0.375 … 0.328 … 0.339 … 0.875 … 0.904

The more teeth per inch, the more material needs to be squeezed or cut during assembly. Therefore, mechanical threads generally require more torque than wide threads. However, the more teeth, the greater the shear strength after assembly, and the better the resistance to thread breakage and screw pull-out.

3.2 head type:
The bearing surface formed by the enlarged shape of one end of the fastener. ANSI/ASME B18.6.4 specifies 13 standard head types for self-tapping screws. Please refer to paragraph 1.2 of B18.6.4. For thread forming and thread cutting self-tapping screws, 5 of the 13 types-flat top countersunk (Flat Countersunk ), oblate countersunk (Oval Countersunk ), pan head (Pan ), hexagonal and hexagonal Chinese head (Hex and Hex washer Head) are the most important. These five head types account for almost 90% of all self-tapping screws. These five head types should be considered first when using self-tapping screws.

The other five other types are flat top concave bearing surface (Flat Undercut ), flat top whole edge (Flat Trim), flat round concave bearing surface (Oval Undercut ), flat round whole edge (Oval Trim) and cylindrical head (Gangshan top Fillister) are relatively minor head types, while mushroom head (Truss ), round head (Round) and 100-degree flat top countersunk are not taken for the new design, and other cover types can replace them, pan head can replace mushroom head and round head, 82 degree flat top countersink can replace 100 degree flat top countersink.

The choice of thread rolling self-tapping screw head type is the same as above. The main consideration for use is flat top, flat dome, pan head, hexagonal head and hexagonal Chinese head.

Flat top, flat round top, pan head and hexagonal head are the most common screws for drill tail. The reason why hexagonal head is not used is that pressure needs to be applied to the tightening tool to support the tool for drilling during drilling. Therefore, all use cross flat top, flat dome, pan head or hexagonal head.

Although the U.S. national standard only specifies 13 standard head types, other head types can still be used for commercial purposes, such as horn heads used in drywall screws, pancake heads and other special head types used according to design, which can be manufactured by manufacturers according to other information.

Bearing surface-The part that supports or locates a knot, usually through which screws are loaded and unloaded. There are two basic types of bearing surfaces. The flat bearing surface (perpendicular to the rod part of the knot piece) and the tapered bearing surface (forming an angle with the rod part of the knot piece) serve to bear the load force acting on the knot piece in most cases. The latter can also be used for positioning in addition to continuing the same function of the flat bearing surface. A knot with a tapered face is usually referred to as a countersunk head type. The different applications of the head type initially depend on the function of the bearing surface and the ability of the head to transmit torque.

Common types of bolts and bolts with flat bearing surfaces

Flat head: It can replace the new design of round head and mushroom head. The head has a low diameter and a large diameter. The circumference of the head is connected with a high-profile edge indicating the characteristics, so that it can play a driving role for high-strength torque. There is a slight difference between the head type and the cavity head.

Round head: It is the most commonly used head type in the past.

Top stigma: the diameter of the standard flat round top stigma is smaller than that of the round head, but it is relatively high due to the relationship between the groove depth. The smaller diameter increases the pressure acting on a small area and can be closely combined with the flange and the increased surface layer. They can be successfully applied to inner drilled holes because they are headed in a drilling die set to ensure centrality.

Dome wide-brimmed head: Because the head is cut under the bondage and reduces the wear of the wire components, it is most commonly used in electrical appliances and tape recorders, providing a more attractive design type for the mid-bow type with its more sufficient bearing surface.

Large round head: also known as oval top wide edge head, is a low type, clever design of large diameter head type. When the combined tolerance of the additional action allows, it can be used to cover metal plate holes with larger diameters. Flathead may also be recommended as an alternative.

Slot head type: This is a creative anti-loose head type, once the combination is not easy to release, but it can be started with a general standard screwdriver. This simple design can usually solve the cost problem in the combination, increase the number of production, and create amazing economic benefits for the manufacturing process.

Hexagonal socket head: a knot with a wrench head height and hexagonal head size. The hexagon is completely cold-formed by a counter-hole mold, with an obvious depression at the top of the head.

Hexagonal socket head: such as the standard hexagonal socket head type, but at the same time, there is a socket surface on the base of the head to protect the completion of assembly so as not to damage the wrench. Sometimes the role of things is more important than appearance.

Hex head: This is a standard type of torque applied to the hex head, with the feature of trimming the sharp corners to close to the tolerance range. Can be recommended for general commercial use, can also be applied to various standard models and various thread diameters. Because of its necessary second process, it is more expensive than a normal hexagonal socket.

The head of the cylinder has a deep hexagonal bearing hole, and the common one has a high-strength Cap Screw. The deeper bearing hole makes the high torque act on the knot.

Twelve o'clock: The high cylinder head is covered with twelve outer points, with a Chinese surface to bear the load of the knot. Because the design facilitates the provision of a high degree of torque, it is often used in high-strength knots.

Plum head: a medium-height hexagonal lobular head with a Chinese head. Its design includes a straight face perpendicular to the wasi face, and a torque bearing surface that transmits torque in the circular protruding part (in the leaflet). This design makes the highest use of the external drive system, allowing the torque to be exerted to the extreme. The transmission of high torque does not change the head itself.

Common types of bolts and bolts with tapered bearing surfaces

Flat-top countersunk-standard angle is 80 to 82 degrees, used for fasteners with tight surfaces. The bearing surface provides good centrality.

Flat countersunk-known as "Oval Countersunk", this head shape is similar to a standard flat-top countersunk, but is more widely used. In addition, a round, neat upper surface, the design is also more attractive.

Small flat top countersunk-like standard flat top countersunk and flat round countersunk, the head angle is 80~82 degrees, but the bearing head position is 1/3 smaller, which is used for simple products or especially short length. In more detail, it is used for standard counterbores and is very suitable for tight fittings.

Flat-top countersunk (countersunk 100 degrees)-This special flat-top countersunk screw is gradually being applied to surfaces that require close fit. It is recommended to use it on soft materials to decompose pressure on surfaces with larger and smaller angles, especially suitable for aluminum, soft plastics, etc.

3.3 Drive System:
The function of the drive system is to drive and transmit torque to join and loosen the fasteners. Adequate transmission of torque throughout the system is the most important point that makes the fastener practical. For threaded fasteners, there are two basic drive systems, an external drive system and an internal drive system. External drive system its drive elements are in the entire head, the wrench works outside. The internal drive system is driven by the fastener head, and the wrench operates inside. Generally speaking, the internal drive type allows higher torque for screws.

Head shape of external drive system: hexagon head, hexagon drive system, twelve-edge drive system, etc.

The slotted groove is the oldest type of groove, and it is also the most common for all drive systems. There are two ways to make the groove: one is to cut the groove mechanically on the screw of the complete head type. The groove, the other is formed at one time during forming and forging. One-time forming of the cutting groove is more economical because it does not require secondary processing, but there are still problems in some aspects. For example, if the hexagonal head or hexagonal head is directly formed, the depth of the cutting groove will be difficult to measure due to the depression (Indented). What is more serious is that the joint surface between the screw and the screwdriver will be reduced. When direct forming is used in the round head, the joint surface will remain unchanged, but the forming pressure will force the head diameter, especially on both sides of the cut groove, the head size is quite difficult to control when using direct molding for some head types.
The cutting groove is a type of depression. It is a standard type for all head types except flat top whole edge and flat round top whole circle. The cutting groove size of each head type is specified in B18.6.4. The cutting groove is especially suitable for manual combination, but not suitable for semi-automatic or automatic assembly. The effect of this drive system depends on the height and flatness of the head, such as flat head and hill top. This is because the higher the head, the deeper the cutting groove, and the smoother the head, the driving force will be closer to the head. The outer edge of the torque is more effective. If higher torque is required in practical applications, shearing is a problem. Even with a deep bond, it's hard to find a good fit between the driver and the slot. The gap that currently exists between the driver and the fastener can cause non-perpendicularity. When the driver is not vertical under external force, the screwdriver will damage the edge of the slot and cause shear. The smaller or rounder the head, the more likely it is to occur.

A slot is not suitable for quick installation. For example, on an assembly line, the drive screwdriver will slide from one end of the slot to the other. If the center of the drive screwdriver is basically aligned with the center of the fastener, the drive screwdriver is effective. If it is not aligned, it will definitely cause head damage. Similarly, the drive screwdriver may also fall to the surface and directly act on the fastener, causing damage. With the need for increased torque, loading is also required to prevent shearing.

There is no production problem with the slotting, but in most cases, it does require a second slot forming, and there is no problem with the effectiveness of the driver driver at present. The slotted slot is best used where high torque is not required, especially those that need to be handled and adjusted in many different environments. The best example is the adjustment screw on the carburetor. The same groove type is also commonly used on consumables that require repair and disassembly of fasteners, such as lawn mowers, rotating equipment, etc.

In response to the large number of automated assembly and the development of some recessed head type, which retains the high torque, easy operation and high-speed assembly, while a considerable number of head type use is quite common, b18.6.4 recognized three types as the standard. All are cross holes, which are respectively type I cross (Philips ), type IA meter (Pozidriv) and type II ( Frearson ). Please refer to 18.6.4 for dimensions. Type II was first developed, followed by type I, and type IA is an improved type of type I, of which type I and type IA are the most common, while type II is less and less in demand.

Cross grooves are derived from the category of cross-shaped grooves, like intersections. The sides of the cross slot intersect obliquely in the middle of the bottom of the slot, usually in the center of the fastener head. It is relatively deep and can help drive the driver to correct. Sometimes the groove depth has reached the neck of the fastener, in which case the strength limit of the fastener is potentially weakened. It is only suitable for torque needs from low to medium, because of its unavoidable ease of slipping, which is also caused by the hypotenuse of the groove. In general, a torque of 1.7 pounds -1 pound per inch will not cause slip-out, but when the torque exceeding this value acts on the cross groove, it will cause slip-out, a vicious circle. Many times, this vicious circle can make the groove of the fastener completely incapacitated. This vicious circle will also cause the loss of the drive driver, in many cases the life of the tool will be shortened, increasing the cost. The cross groove has good correction and is suitable for automatic assembly lines. There is no problem in production. When the head is formed, it is formed once and does not need to be processed a second time. Tools are widely used. Suitable for low torque requirements on manual and automatic assembly lines, such as ultra-thin steel plate to thin steel plate, thin steel plate to cork, soft plastic.

FREARSON RECESS is another cross groove, commonly known as "reed & price" groove. (Refer to 18.6.4 Type II) Its design is very similar to a cross groove, but the sides of the groove are vertical and the bottom is pointed. Because of these subtle differences, the FREARSON has a better driving performance than the cross groove, but there will also be some shortcomings, there will be a slip-out phenomenon. This drive system can be used for low to medium torque requirements, the greater the torque, the greater the degree of groove and tool damage, the current production without problems.

Since this drive system is not widely used, the corresponding drive supply is limited, which needs to be taken into account in the evaluation. Where cross grooves can be used, FREARSON can also be used, and some places that are not suitable for cross grooves can also be used.

The rice groove is a cross-shaped groove hole, which is developed to overcome the shortcomings of the above two grooves. The basic design is similar to a cross groove, but with additional grooves, commonly known as "ribs". Suitable for low to medium torque requirements, compared to the first two groove shape has a great improvement, can greatly reduce the probability of sliding out. The meter slot also requires an end-loading to prevent slipping out, but can withstand higher torque without harming the slot and drive screwdriver. Because its basic design is similar to the cross groove, it also has good centrality and is suitable for automatic assembly lines. The forming process of the rice groove is similar to the cross groove, and it is also formed once when the head is formed, and no second processing is required. Tools are easy to obtain, but to get the best results, you should use a special tool for the meter slot. Some people try to use the cross slot to drive the tool, but the effect is not good. Due to the superiority of the rice groove, it can be used whenever the above two groove shapes can be applied. It can also be used in larger torques, such as heavy-duty thin plates and sheets, thread cutting machines and thread forming machines, fasteners made of plastic materials, and automatic machines that require low torque screws.

Hexagonal socket drive system as his name indicates, there is a hexagonal hole in the head of the fastener, commonly used Socket head cap screws ", a high-strength fastener. The hexagonal hole is suitable for high torque. For this drive system, slip-out is not a problem, but due to the nature of the bond between the drive wrench and the fastener, the cavity and the wrench will deform after only a few times. In order to ensure bonding, the dimensions of the cavity and the tool have a common tolerance, but this can only reduce the actual surface contact, and equipment loss. This type of fastener is expensive and will greatly increase the cost if used in those that require frequent disassembly. There are no major problems in the production process, it is a one-time forming. Before that, the hexagonal hole needs to be formed through two processes-drilling and punching. Suitable tools are called "hex wrenches" and are divided into two categories, short arm and long arm. Hexagon wrench is a hexagonal rod steel bent into L shape. For fixed size, the length ratio of long arm wrench is larger than that of short arm wrench, and its effectiveness is no problem. On the automatic assembly line, it is also driven by a hexagonal screwdriver. Hexagonal holes are usually used for high torque. Whether the use condition is ideal depends to a large extent on the number of repeated use. For fasteners that need frequent disassembly, it is not economical because the groove and tools are easy to deform and increase the cost. Hexagonal holes are mostly used for high-strength fasteners for assembly on medium and heavy equipment.

The SPLINE RECESS is essentially circular and has six right-angled ribs in the socket in a direction parallel to the fastener axis. The application of the toothed hexagonal head is the same as that of the hexagonal head, which is used for high torque applications. It is designed to minimize the wear of the hexagonal head and tools. The main disadvantage of the toothed hexagonal head is in the manufacture of the head and tools. Because of its complex design, the head must be shaped when the fastener is headed. Due to its many sharp angles, it is extremely difficult to control within the required tolerances during production. The life of the tools used to make the heads in production is extremely short, so the fastener cost is generally increased. The same problem applies to production-driven tools. Due to limited suppliers, the toothed hexagon drive tool is more difficult to purchase than any of the aforementioned drive tools. Toothed hexagonal head for high torque applications
Occasions, but supply problems often arise due to manufacturing difficulties-especially when demand is high. Supply shortages should be taken into account when selecting drive systems.

TORX RECESS (Plum Blossom Hole): The Plum Blossom Hole is a Camcar design patent. Its design solves all the problems of the above-mentioned head drive system. The plum head is a hexagonal blade design with a straight inner edge and a shallow head. The torque transmission of this design is a surface support rather than a point support like most heads. Therefore, the efficiency of torque transmission can be improved. Straight edges eliminate the tendency to slip and end loads during torsion. These are the best driving combinations in practice to transmit torque. In fact, because plum holes are generally shallower than other holes, this means that the tightening force will not be lost due to the depth of the hole. This design has excellent features that make it an ideal tool for automatic assembly lines. No problems in manufacturing. The head is formed in the process of the head. Because the plum head has many arcs, rather than right angles, the manufacturing tool wear is not obvious. Tight tolerances ensure maximum bonding.

In addition, there are a number of patent providers to provide users with a variety of sources. Tools are available from many sources. Plum Blossom Hole is suitable for any torque application requirements, whether manual or automatic assembly. It is designed to eliminate slippage, so it can transmit greater torque, longer head life and manufacturing tool life. These advantages can reduce the cost of fasteners. Plum holes can be used in high torque applications, especially for repeated use, such as heavy machinery and equipment. The plum holes can be used for automatic assembly. This is because the head does not need to be reworked due to deformation, the tool has a long service life, and the operation errors caused by worker fatigue are also reduced. In the application and automatic industry, plum holes are becoming more and more popular. The plum head is suitable for almost any high-speed assembly occasion.

Outer hexagon: The outer hexagon drive system is a universal head type and is widely used. It is used for automatic assembly of various torque requirements. It is suitable for medium and low torque applications. In the case of high torque, the same problem as the hexagonal head. The fastener head will deform and the driving tool will wear out. Because of cracks caused by pressure, the life of the sleeve used to drive the hex head is often very short. This will greatly increase the cost of the fastener.

There are two types of hexagonal head drive systems. They're made differently. INDENT HEX is an economical head type. It is formed at the same time as the head, without the need for two processes. The full-edge hexagonal head (TRIMMED HEX) is one of the more expensive. After starting, the hexagon is machined in the second step. This makes the angle sharper, which is good for performance and appearance. In addition, the two types of hex head sometimes the head will be slotted, so that when necessary can also be driven by a screwdriver.
Drive tools are quite popular. The outer hexagon is suitable for manual and automatic assembly of medium and low torque applications. The rated torque must be remembered. When the fastener and its driving tool are deformed, the rest of the system should be re-checked. External hexagon head fasteners are very useful in the assembly of various steel plates.

Twelve-edge head: The twelve-edge drive system is a high-strength external wrench system mainly used in the aircraft industry. The twelve-edge design is based on a cylinder with a moderate apex-as the name implies-twelve edges. When used in the aircraft industry, holes are drilled along the center of the head to 2/3 of the height of the head to reduce weight. The twelve ribs are mainly used for high-strength fasteners. This type of fastener is used in high torque applications. This head is driven using a sleeve of the same construction. The twelve-pronged system is generally superior to the external hexagonal system, but the disadvantages are the same. The fastener drive bond is primarily on the edge rather than on the face. When used repeatedly, the ribs tend to wear and become rounded, causing the fastener to twist. The drive sleeve has a further disadvantage. Because the structural fasteners are subjected to torque reaction on the inner wall of the sleeve and cause cracking. The fastener itself is expensive, and this disadvantage also increases its cost. The twelve-edged structure is more difficult to manufacture than many other types of structures, but it is also molded in the head. Drive tools are generally available. Twelve-pronged drive systems are commercially used in high-intensity applications. Especially for COUNTERBORE applications, heavy machinery and equipment and aircraft.

External Torx head: Torx drive system is suitable for all external wrench drive occasions. The plum-shaped drive design is hexagonal lobed, with a straight edge parallel to the fastener axis and a moderate height. The lobes are driven in surface contact rather than point contact unlike most other drive systems. This makes the driving torque transmission more efficient. The plum external drive system can be used for any torque requirement, but it is most suitable for high and medium torque occasions. Its fast, convenient and automatic combination with the driving tool makes it very suitable for automatic assembly of the external plum head type like other external head type with sleeve drive. Because it is to use the support surface to effectively transmit torque, the sleeve is basically not damaged, thus saving the cost of driving tools, thereby greatly reducing the cost of high-speed, large-volume applications of fasteners. The fastener head will never deform even if it is used repeatedly. This saves fastener costs during assembly, service and rework. The plum design head is easy to form within strict tolerances, and all manufacturing is no problem. Tools are also available from several large, well-known suppliers. This head type is a solution to automatic assembly problems including tool damage and fastener deformation. The plum drive is suitable for most occasions where high torque is required for manual or automatic assembly. The plum head can be applied to assembly, automobile, heavy machinery and equipment, etc. Torx head is suitable for thread cutting and thread forming self-tapping screws. This is where excess torque is necessary. Plum heads are used in a variety of ways.

3.4 screw length: It is very important to ensure that the screw thread matches the thickness of the joint during assembly. The average screw length should be equal to the combined length (total thickness of the assembly) plus the screw end. The end of the screw is not completely threaded and the length of the taper tail of the thread forming screw AB or BP type or the length of the drill tail screw. These different taper tail or drill tail lengths are specified in each specification and their calculation formulas are in the appendix of B18.6.4.

For example, we need to use 1/4 "-20 pan head thread to form self-tapping threads to assemble a flat plate 0.25" thick and 1 0.21 "thick. the maximum length of the screw tail end is 0.175" and the screw length tolerance is 0 0.03 ". then the screw length should be 0.25 0.21 0.175 0.03=0.665". therefore, a 3/4 "long screw should be used. At this time, the length exposed to the other end after combination is 0.75-0.21-0.25=0.290 ", and self-tapping screws are generally suitable for 1/8" length increase.

4 Material, heat treatment, final treatment and mechanical performance requirements: Self-tapping screws can be made of carbon steel, stainless steel, copper or aluminum. However, carbon steel self-tapping screws account for the vast majority, even more than 95%. Self-tapping screws made of materials other than steel have mechanical properties (hardness and torsion strength) stipulated in JIS B1055 Annex 2. The rest have no national standards or other recognized standards. The mechanical properties are generally determined by the agreement between the purchaser and the manufacturer. Therefore, the problems discussed in this paper are almost focused on carbon steel materials.

4.1 material: Carbon steel self-tapping screws are made of low carbon steel. When required by customers, medium carbon steel manufacturers are also used. However, most of them use C1018-C1022 materials (this is because the carbon content should be above 0.15% in heat treatment). Most of the raw material wire is subjected to the process of stretching line → annealing or spheroidizing → fine stretching line to meet its forming conditions. The specified material composition is as follows:
Inch 1 self-tapping screw material chemical composition
Self-tapping screw specification analysis 2 component limit, by weight%
carbon manganese
#4 or smaller furnace analysis 0.13 - 0.25 0.60 - 1.65
Product inspection 0.11 - 0.27 0.57 - 1.71
#5 to 1/2 inch furnace analysis 0.15 - 0.25 0.70 - 1.65
Product Inspection 0.13 - 0.27 0.64 - 1.71
1. Metric screws have no special requirements and restrictions on their chemical composition.
2. In-furnace analysis only provides order information. Product inspection is to consider the segregation of carbide in the screw core.

4.2 heat treatment: Self-tapping screws need to be carburized to obtain a very hard surface for thread forming or cutting. After carburizing and tempering heat treatment, the mechanical properties required in the specification are:

Surface hardness: Generally, the surface of self-tapping screws should have a hardness of more than 45 HRC after carburizing, so that they can be tapped into the iron plate. The surface hardness and core hardness of the drill tail screw are a little higher than those of the general self-tapping screw (J78 requires the surface hardness to be 50 ~ 56HRC), because the drill tail screw has one more drilling operation. For routine testing or quick inspection purposes, the surface hardness can be checked using HR 15N ,knoop or microgram microhardness. The choice of these methods depends on the size of the testable area of the article. The surface of the product should be slightly treated before testing the hardness value. If the hardness value is lower than the specified value, refer to the following method: use a 500g load Knoop or 300g microgram micro hardness machine to read the hardness value 0.002 inches below the surface of the product. if the full carburizing depth is 0.004 inches or less, you can use a 100g load to test 0.001 inches below the surface. When measuring surface hardness and carburized depth, to ensure that the embedded material is properly supported, the reading will be taken from the longitudinal profile measurement from the center to the range beyond the centerline. The sample should be properly supported in the embedding material. When measuring the outer diameter of the cut-off surface of the sample on the microscope during the test, at least 95% of the outer diameter of the original sample should be required.

Carburizing depth: carburizing depth is very important, carburizing is too shallow, screws can not be assembled correctly, carburizing is too deep, the center of the torque and ductility will be affected. Generally, the carburizing depth of self-tapping screws shall be measured at the midpoint of the tooth mountain at the midpoint of the screw cut-off surface (at least 95% of the outer diameter of the original sample). ISO 2702 stipulates that screws less than ST 3.9 can be measured at the tooth valley to the center.

Hardness of the core after tempering: The hardness of the core should be measured at the full tooth base diameter (parallel) on the screw cut-off surface near the tail end from the tooth base diameter to the middle point of the radius.

Microstructure: When examined by metallographic method, there should be no banded ferriferous iron between the surface and the heart. The appearance of fertilizer iron indicates incomplete heat treatment (insufficient heating temperature; Or the quenching speed is too slow to cause the fertilizer iron to precipitate in the grain boundary first). For those with strength requirements, their characteristics may be reduced.

Hardness Requirements of Metric Triangle Tooth Self-Tapping Screws after Heat Treatment Unit: mm
Request item Shakusize SAE J1237 DIN 7500
Surface hardness All dimensions 45 HRC Min. 450 HV Min(1)
Heart hardness All sizes 28~38 HRC 240~370 HV(2)
Carburizing depth M2 ~ M2.5 0.05~0.18 0.04~0.12
M3 ~ M3.5 0.05 ~ 0.18
M4 ~ M5 0.10 ~ 0.23 0.10 ~ 0.25
M6 ~ M8 0.13 ~ 0.28 0.15 ~ 0.28
M10 0.15 ~ 0.32
M12 ---
(1) Use HV 0.3 load measurement test. (2) Use HV 5 load measurement test. (3) SAE J1237

Hardness requirements of metric wide tooth tapping screws after heat treatment Unit: mm
Request Item ISO 2702 DIN 7504 DIN 267/12 JIS B1055
Surface hardness All dimensions 450 Min(1) 560 Min(1) 450 Min(1) 450 Min(1) 450 Min(1)
Heart hardness All sizes 270~390 (3) 270~425 (2) 270~390 (1) 270~390
Carburizing depth ST2.2 ~ 2.6 0.04~0.10 --- 0.04~0.10 0.04~0.10
ST2.9 ~ 3.5 0.05 ~ 0.18 0.05 ~ 0.18 0.05 ~ 0.18 0.05 ~ 0.18
ST3.9 ~ 5.5 0.10 ~ 0.23 0.10 ~ 0.23 0.10 ~ 0.23 0.10 ~ 0.23
ST6.3 ~ 8.0 0.15 ~ 0.28 0.15 ~ 0.28 0.15 ~ 0.28 0.15 ~ 0.28
(1) Use HV 0.3 load measurement test. (2) Use HV 0.5 load measurement test. (3) ST3.9 (includes) HV 5 load measurement test
ST4.2 (including) More use HV 10 load measurement test

Hardness requirements for inch self-tapping screws after heat treatment
Request item Shakusize SAE J78 SAE J81 SAE J933
Surface Hardness All Sizes 50~56 HRC 45 HRC Min. 45 HRC Min.
Heart Hardness All Sizes 32~40 HRC 28~38 HRC 28~38 HRC(2)
Full carburizing depth (1) # 4 to #6 0.002-0.007"
#8 to #12 0.004-0.009"
1/4 or more 0.005-0.011"
(1) The test position of full carburizing depth shall be located at the midpoint from the upper tooth mountain to the tooth bottom of the thread ventral. When using a small hard load of 500g
When the test reading value of the degree machine is converted to the HRC value, the maximum carburizing depth shall not exceed 42 HRC.
(2) The hardness should not exceed the maximum value, and preferably not greater than Rockwell C36 to ensure that it will not be damaged during assembly and use.

4.3 surface treatment: most self-tapping screws are coated with zinc or zinc phosphate. Cadmium plating is rarely used because of its high cost and toxicity. If appearance is important, you can choose nickel or chromium plating. Surface treatment will produce two problems: one is that surface treatment will not only increase the size (such as the reduction of holes, etc.) and different surface treatments will affect the relationship between torque, tightening degree and strength, so different regulations shall be made according to zinc plating, cadmium plating or phosphate treatment when doing the test. When testing the applicable tapping hole diameter, the screws used must be of the same surface treatment. The second is that high hardness, small size self-tapping screws often fail due to hydrogen embrittlement. Self-tapping screws must not be carburized in order to penetrate into the iron plate. High hardness and high carbon content will be produced after carburizing treatment, and hydrogen embrittlement will occur when electroplating or pickling treatment is done in this condition. The methods recommended in the specification to eliminate hydrogen embrittlement are:
A). Baking within 1 hour after plating or pickling treatment.
B). Temperature 375~425 oF(190~220 oC).
C). Time to 4 small hours.
Hydrogen embrittlement test: The hydrogen embrittlement test is mainly to test whether hydrogen remains in the screw structure of the self-tapping screw and whether it will cause embrittlement of the screw (usually embrittlement does not occur at once and only appears after a certain stress is applied). Electroplated or coated screws should be installed in the test iron plate and steel Pinghua company specified in Table 3, and standard Pinghua company should be used for the lower bearing surface of the protruding head screw head, oval screws use a matching chamfered spacer steel sheet. The thickness of the flat or spacer on the lower surface of the head should be consistent with the maximum invalid tooth length of the screw. For half-tooth long screws, the shank can be supported by a longer cold-rolled steel spacer or Pinghua to ensure that the full tooth part can be combined and maintained in the thickness of the test iron plate. The screw should be locked to 80% of the maximum breaking torque. The breaking torque is determined by the average value of locking 5 screws until they are broken. After continuing to keep the locking for 24 hours, loosen the screw and then turn it to The original locking torque, the screw should have no obvious failure.

Note: SAE J81 and J1237 have specified locking in tapped internal threads and specified locking torque values.

5 The functionality of self-tapping screws: The functionality of self-tapping screws includes ductility (Ductility); Tapping test (Drive Tese); Tapping torque (Drive Torque) and attack speed test (Drive Speed), etc. as follows:
Ductility (Ductility): Self-tapping screws after carburizing treatment → quenching → tempering, in order to avoid excessive brittleness caused by improper heat treatment, it is usually necessary to test its ductility (toughness) status. The test method is to place the screw sample into a hardened test seat at a certain angle, or other suitable equipment. The cavity of the test seat shall be 0.020 - 0.040 inches (0.5 - 1.0mm) larger than the nominal diameter of the screw according to ASME B18.6.4. Apply axial pressure to the top of the screw head until the lower bearing surface of the head and the screw axis show a permanent bend at a certain angle. This test can be repeated with the head until it is permanently bent. Rejected if head is separated

Ductility test angle specified in each specification
F.I.P.
1000 SAE
J78 SAE
J81 ASME
B18.6.4 SAE
J1237 he
B1055
10O 5O 7O 10O 10O 10O ~ 15O

Tapping test (Drive Tese): Tapping test is the most important feature of self-tapping screws. When self-tapping screws cannot be tapped into the combined iron plate, they will lose their main function. Therefore, each specification requires an entry test. B18.6.4 specifies that tail types AB, B, BP, C, D, F, G and T (except BF,BT and BG) shall be tested for assembly. During the test, the screw must be screwed into a steel plate with a specified hardness, thickness and pre-drilled holes. During the screwing, the thread must have no visual deformation and no torque limit during the test. As for the screws of triangular teeth (see SAEJ78 , J1237 , FIP 1000 , DIN 7500), in terms of assembly test, it is not only required that the threads should be deformed invisibles during screwing, but also the upper torque limit during assembly test.

Testing Iron Plate Thickness and Aperture Size of Self-Tapping Screws of English Specifications
Self-tapping (wide teeth) mechanical teeth
Type A Type B Specifications Type D,F,G,T Type TT
(Triangular teeth)
Specification Plate Thickness Aperture Specification Plate Thickness Aperture Plate Thickness Aperture Plate Thickness Aperture
#2-32 0.048 0.076 #2-32 0.048 0.076 #2-56 0.078 0.073 0.125 0.075
#3-28 0.048 0.081 #3-28 0.048 0.081 #3-48 0.094 0.081 0.125 0.087
#4-24 0.048 0.086 #4-24 0.048 0.086 #4-40 0.109 0.096 0.125 0.098
#5-20 0.048 0.107 #5-20 0.048 0.107 #5-40 0.109 0.101 0.125 0.110
#6-18 0.075 0.116 #6-20 0.075 0.116 #6-32 0.140 0.120 0.125 0.120
#7-16 0.075 0.129 #7-19 0.075 0.129 … … … … …
#8-15 0.075 0.136 #8-18 0.075 0.136 #8-32 0.140 0.147 0.188 0.147
#9-14 0.075 0.149 … … … … … … …
#10-12 0.125 0.159 #10-16 0.125 0.159 #10-24 0.188 0.173 0.188 0.166
… … … … … … #10-32 0.188 0.177 0.188 0.172
#12-11 0.125 0.188 #12-14 0.125 0.188 #12-24 0.188 0.199 … …
#14-10 0.125 0.217 … … … … … … …
… … … 1/4-14 0.1875 0.217 1/4-20 0.250 0.228 0.250 0.219
… … … … … … 1/4-28 0.250 0.234 … …
#16-10 0.1875 0.238 … … … … … … …
#18-9 0.1875 0.261 … … … … … … …
#20-9 0.1875 0.290 … … … … … … …
… … 5/16-12 0.1875 0.272 5/16-18 0.313 0.290 0.312 0.277
… … … … … … 5/16-24 0.313 0.295 … …
… … 3/8-12 0.188 0.328 3/8-16 … … 0.375 0.339
1. The iron plate is made of low carbon steel with a thickness tolerance of ± 0.002 "below 0.23. Hole tolerance ± 0.001"; Hardness 70~85 HRB. For dimensions, refer to ANSI B18.6.4 ; FIP 1000 ; SAE J81 ; J933.
2. The size of the iron plate used by TYPE C is the same as that of TYPE A.
Testing of Metric Size Self-Tapping Screws Iron Plate Thickness and Aperture Size
Self-tapping (wide teeth) Mechanical teeth (triangular teeth)
DIN 267/12 JIS B1055 Standard DIN7500 SAE J1237
Specifications Plate Thickness Aperture Plate Thickness Aperture Plate Thickness Aperture Plate Thickness Aperture
ST2.2 1.3-1.2 1.96-1.91 1.30-1.17 1.955-1.905 M2 2.0 1.8 3.0 1.77
ST2.6 -- 2.235-2.185 M2.5 2.5 2.3 2.25
ST2.9 1.3-1.2 2.47-2.42 2.465-2.415 M3 3.0 2.75 2.70
ST3.3 -- 2.730-2.680 M3.5 3.5 3.2 3.15
ST3.5 2.1-1.9 2.98-2.93 2.06-1.85 2.970-2.920 M4 4.0 3.6 5.0 3.60
ST3.9 3.29-3.24 3.290-3.240 M5 5.0 4.6 4.55
ST4.2 3.49-3.44 3.480-3.430 M6 6.0 5.5 6.0 5.40
ST4.8 3.2-3.1 4.08-4.03 3.23-3.10 4.065-4.015 M8 8.0 7.4 8.0 7.30
ST5.5 4.79-4.74 4.785-4.735 M10 10.0 9.3 10.0 9.20
ST6.3 5.1-4.7 5.53-5.48 5.05-4.67 5.525-5.475 M12 -- -- 12.0 11.00
ST 8.0 6.94-6.89 6.935-6.885
Low-carbon steel with iron plate carbon content below 0.23% in JIS B1055, hardness 70~85 HRB (125~165 HB).
The iron plate of SAE J1237 is made of low carbon steel with hardness of 75~90 HRB. Bore diameter tolerance ± 0.025mm.
DIN 267/12 iron plate hardness of 72~87 HRB. DIN 7500 iron plate thickness and hole diameter tolerance is not specified.
Requirements of each specification for maximum screw-in torque test
Dimensions DIN 7500 SAE J1237 FIP 1000 Dimensions SAE J81 FIP 1000
N.m N.m lb-in lb-in lb-in
Phos. Zinc Phos. Zinc Phos. Zinc Phos. Zinc
M2 × 0.4 0.3 0.4 0.6 4 5 #2-56 4.5 6 4.5 6
M 2.5 × 0.45 0.6 1.0 7 9 #3-48 7.5 9.5 7.5 9.5
M3 × 0.5 1.0 1.3 1.7 11 15 #4-40 9 13 9 13
M 3.5 × 0.6 1.6 1.9 2.4 17 21 #5-40 12 16 12 16
M4 × 0.7 2.4 2.6 3.4 22 30 #6-32 14 20 14 20
M5 × 0.8 4.7 4.8 6.0 42 53 #8-32 25 32 25 25 32
M6 × 1.0 8.0 7.5 9.2 66 81 #10-24 35 52 35 52
M8 × 1.25 20 16 20 142 177 #10-32 -- -- 35 52
M10 × 1.5 39 28 35 248 310 1/4-20 90 120 90 120
M 12 × 1.75 -- 46 55 -- -- 5/16-18 180 240 180 240
3/8-16 240 300 240 300

Torque strength test: All sizes and types of self-tapping screws are required for torque test. During the test, the screws are installed in a blind hole fixing device (at least 2 threads), and the screw head is exposed (at least 2 threads). Use a calibrated torque device with a display accuracy of less than ± 2% to apply torque until the screw breaks (usually the break is located at the screw shank or the junction of the head and neck), and the measured torque must be at least equal to the specified value or higher to be qualified.

Requirements of various specifications for mechanical tooth torsion strength of metric self-tapping screws
Dimensions DIN 7500 SAE J1237 FIP 1000
TY. F, T triangle tooth
N.m min N.m min lb-in min lb-in min
M2 × 0.4 0.5 0.7 4 6
M2.5 × 0.45 1.0 1.2 10 11
M3 × 0.5 1.5 2.2 18 19
M 3.5 × 0.6 2.3 3.5 27 31
M4 × 0.7 3.4 5.2 41 46
M5 × 0.8 7.1 10.5 83 93
M6 × 1.0 12 17.7 142 157
M8 × 1.25 29 43.0 354 380
M10 × 1.5 59 87.0 -- 770
M12 × 1.75 -- 152.0 -- -- --

Requirements for Torsional Strength of Self-tapping Teeth (Wide Teeth) of Metric Self-tapping Screws in Each Specification
尺寸 ISO 2702 JIS B1055 DIN 7504 DIN 267/12
N. M. M.
ST 2.2 0.45 -- 0.45
ST 2.6 0.90 --
ST 2.9 1.50 1.50
ST 3.3 2.00 --
ST 3.5 2.70 2.80
ST 3.9 3.40 3.40
ST 4.2 4.40 4.50
ST 4.8 6.30 6.50
ST 5.5 10.00 10
ST 6.3 13.60 14
ST 8.0 30.50 -- 31

4.3 SAE J81 :
Mixed with ANSI/ASME B18.6.4 and SAE J933 requirements for thread forming self-tapping screws, SAE J81 includes material selection, heat treatment requirements, carburizing depth, surface and core hardness. Generally speaking, it is roughly the same as J933. It also includes drying requirements after plating. Assembly and torque testing require the same requirements. Also added tensile strength, tightening load, guaranteed torque, ductility and hydrogen embrittlement tests. But the tensile strength requirement is 135,000 psi.

Hex and hex head (with sufficient length) screws need to be tested for vertical tensile strength. In addition, it is necessary to measure the tightening load to ensure the torque. When measuring the tightening load, the screws should be assembled on the same steel plate as the assembly test and tightened to the specified torque value. The torque value should slightly exceed the lower limit specified for the torque test. The screw should not break under this torque.

All thread forming screws should be tested for ductility by bending the screw head at an angle of 7 degrees to the shaft. At the same time, the electroplated screws shall be tested for hydrogen embrittlement. The screws shall be screwed into the preset holes according to the specified torque and kept for 24 hours. The screws shall be qualified (this value is approximately equal to 75% of the lower limit specified in the torque test).
The last definition is that J81 recognizes

4.4 SAE J78 :

4.5 FIP 1000 :

5 screw selection and application:

The force that allows the screw to be assembled-called the Drive Torque-and to form or tap the mating thread, if the force continues, the screw will close, tighten, and continue to exceed-called the limit Torque (Utimate Torque )-the screw will break, twist, thread tear (slip) or self-combine the workpiece to come out.

Before using all self-tapping screws, the most important question is: "What is the correct tightening torque? How much torque allows the screw to tighten without causing damage to the screw or the bonded workpiece?". It is very obvious that the tightening torque must be between the screwing-in torque and the limit torque. And there is no reason to recommend using the middle of the two. There are several factors will affect the rotation torque and the limit torque, the more important factors are the screw type, size, combined with the workpiece composition and hardness, combined with the workpiece thickness and pre-drilling preparation method, of which the most controversial is the pre-drilling fit The size of the screw, interestingly, other factors will affect the size of the pre-drilling. Therefore, pre-drilling is the final and most important factor in the design. If the pre-drilled hole is too large, the screw can be easily screwed in, but it will destroy the integrity of the bond and cause the screw to come out when the torque limit is not reached. If the pre-drilled hole is too small, the screw-in torque will increase and even risk of twisting. Even if the screw is tight and tight, too small a difference between the screw-in torque and the limit torque may cause the combination to fail.

The torque problem caused by various factors affecting pre-drilling should be determined according to the screwing-in torque and the limit torque. "What is the tightening torque?"

5.1 screw type:

Usually, we can easily decide whether to use thread forming, thread cutting, thread rolling or self-tapping screws from the use of environmental factors, such:
-Is it necessary to bear the external load after the screw is combined?
-What is the material of the joint? Steel? Cast iron? Aluminum? Plastic? Wood or other?
-What is the thickness of the bond?
-Screws for through holes or blind holes?
-If used for through holes, is the cleaning factor behind the hole important?

-Is tapping debris or drilling debris acceptable?
-Pre-holes are machined by drilling? Cutting? Beating or forging?
-Is the screw assembly manual? Semi-automatic or fully automatic?
-Does it need to be disassembled after combination?
-Is the environment corrosive?
-What is the cost of screws? What is the cost of assembly?
Self-tapping screws with the same name have different outer diameters for wide threads and mechanical threads. Generally speaking, the size of the preparation holes is also different. Usually, we only need two sets of preparation holes of different sizes, one set is suitable for thread forming and thread cutting self-tapping screws of mechanical threads, and the other set is suitable for thread forming and thread cutting self-tapping screws of wide threads. The preparation holes for thread rolling and thread forming screws are also suitable for thread cutting screws, because the torque required for cutting is less than the torque required for forming, all things being equal.

5.2 screw size:

In the combination of construction, good design usually recommends that if the combination of bolt and nut fails due to excessive force applied in the combination operation or excessive stress load in the application situation, the bolt should be broken and the thread should not be torn. The same concept applies to the assembly of self-tapping screws. Especially for use in situations where there is a stress load. If the combination fails, the screw should be broken rather than broken or pulled out.

The function of resisting thread damage and pulling out is provided by material strength and thread shear stress area. In other words, it is controlled by screw size, acting thread length and mating thread overlap depth. The acting length of thread is equal to the thickness of the combined workpiece. The depth of the thread overlap is determined by the diameter of the preparatory hole, and the size of the screw, another determinant, determines the torque and tensile strength.

At any combined thickness, increasing the screw size will increase the ratio of the tensile stress load area to the thread shear stress load area, often causing the thread to tear or pull out. If the size of the screw is reduced, the ratio will decrease, but the screwing torque will increase and the screw will break. Therefore, how to balance the tensile strength to resist thread tearing or pulling out depends on the size of the screw and the thickness of the workpiece used.

In the steel British and American unified threaded fasteners, we usually find that the thread binding length is approximately equal to double the fastener diameter D to prevent thread tearing, which also looks reasonable for thread cutting screws. Because the minimum diameter of the pre-drilled hole is the same as the minimum diameter of the nut, the screw size is equal to or less than the material thickness to prevent thread tearing. Practically, due to the low resistance of thread cutting screws, they can be used on materials with a diameter of 1.5 times or more. The thread is rolled into a screw due to greater resistance, so it is recommended to use a material thickness of 1.1 times the diameter. For thread forming screws, due to the use of wide threads and high torque, it is difficult to decide what size screws to use to prevent thread tearing or pulling out. Fortunately, most thread forming screws are rarely used in high-load situations. Therefore, it is a reasonable choice to prevent thread tearing or pulling out to 1.3 times the diameter with the material thickness.

Let's sum up with an example: if we use a material with a thickness of 0.125 ", then we first consider #6 thread rolling screw (1.1x.125 =.137 ), #5 thread cutting screw (1x.125 =.125 ), #8 thread forming screw. In case of thread tearing or pulling out, the thickness of the material used for #6 thread rolling screws shall not be less than 0.125 "( .138/1.1 ), #6 thread cutting screw 0.138" (.138/1 ), and #6 thread forming screw 0.106 "( .138/1.3 ).

The above recommendations are based on iron plates. The use of other materials should be adjusted for the effect of the material on shear. Most importantly, the above recommendations assume that the pre-drilled holes are of standard size.

5.3 material hardness and thickness:

Obviously, the greater the hardness or the greater the thickness of the composite material, the greater the torque required. Therefore, the pre-drilling also needs to be increased, but the adjustment of the pre-drilling size limit can only be adjusted within a relatively narrow range.

Usually, the appropriate hole diameter is about the midpoint of the screw thread to keep the thread overlapping during drilling, and the hole diameter limit cannot be more than 75% of the thread height. For small screws, there are some possibilities for adjustment. For example, the thread height of #8 wide thread is only 0.022 ". If the hole diameter is only half the height of the thread, the thread overlap height of the thread cutting screw into the assembly is only 0.011", however, thread forming and thread rolling self-tapping screws will have a higher thread overlap due to extrusion. When the aperture is 75% of the thread height, the thread overlap of thread cutting screws into components is only 0.005 ", while thread forming and thread rolling self-tapping screws are not too much. This means that the adjustment range of aperture is only 0.010".

For harder or thicker materials, choose larger screws and reduce the screw-in torque rather than increase the hole diameter to allow smaller screws to be assembled.