General knowledge of fasteners

1. The parameters that need to be marked to describe the fasteners are: product name (standard), specification, material, strength grade, and surface treatment. Such as: DIN912, M4-0.7×8, SCM435, 12.9 grade, black.

1. Product name (standard): Note: For screws without standard, which are non-standard parts, drawings are required.

For example, DIN912, the Chinese name is: hexagon socket head screw, which is the product name.

However, the most accurate way is to call the standard, because GB70 is also the same product name; but the two standards are inconsistent in size in many places.

The most influential standards in the world are: German Standard (DIN), International Standard (ISO), Chinese National Standard (GB), American Standard (ANSI), Japanese Standard (JIS).

2. Specifications, the general name of the screw is the tooth pattern name diameter * screw length.

The diameter of tooth pattern is commonly used in metric system: M2, M3, M4, M5, M6, M8, M10, M12, etc.;

Commonly used American systems are: 4#-40, 6#-32, 8#-32, 10#-24, 1/4-20, 5/16-18, 3/8-16, 1/2-13, etc.

The length of the screw refers to the effective length of the embedded object. For example, the total length of countersunk head screws is loaded, half the length of the head is added to the half countersunk head screws, and the length of the cylinder head screw does not include the head size. like:

For specifications, it is best to add pitch for the full name. For example, M4-0.7×8, the outer diameter of the 4 fingers is 4mm, 0.7 means the distance between the two crests is 0.7mm, and the 8 means the effective length of the embedded object is 8mm.

In order to simplify, do not write the pitch, we default the coarse tooth to the standard tooth, because it is the most common; so there is no need to mark it. This is only available in the metric system, and the tooth pitch is still required for American products.

Here we will focus on the specifications of American screws. For example, 6#-32*3/8, 6# is the outer diameter of the tooth, which is close to 3.5mm; 32 is 32 threads per inch of thread length (equivalent to the thread of a metric screw). Distance); 3/8 is the length of the screw (specifically the same as metric screws).

Two formulas to remember here: tooth outer diameter A#=(Ax0.013+0.06)x25.4(mm), 1 inch=25.4mm.

Among them, 2#=2.2mm, 4#=2.9mm, 6#=3.5mm, 8#4.2mm, 10#=4.8mm are data to be memorized. The number of teeth corresponding to the screws of each specification must also be memorized: 2#-56, 4#-40, 6#-32, 8#-32, 10#-24, 1/4-20, 5/16-18 , 3/8-16, 1/2-13 (American standard teeth).

Note: American-made UNC teeth are standard teeth, and UNF is fine teeth. We default coarse teeth to standard teeth.

3. Material: The most common materials are: carbon steel, stainless steel, stainless iron, copper, aluminum, etc.

Carbon steel is divided into low carbon steel (such as C1008/C1010/C1015/C1018/C1022), medium carbon steel (such as C1035), high carbon steel (C1045/C1050), alloy steel (SCM435/10B21/40Cr).

Generally, C1008 is made of ordinary grade products, such as grade 4.8 screws and ordinary grade nuts; C1015 is generally used for eye screws; C1018 is generally used for machine screws, of course, it is also used for self-tapping screws; C1022 is generally used for self-tapping screws ; C1035 hits 8.8 grade screws; C1045/10B21/40Cr hits 10.9 grade screws; 40Cr/SCM435 hits 12.9 grade screws.

Stainless steel has SS302/SS304/SS316 as the most common. Of course, a large number of SS201 products are popular now, and even products with lower nickel content, we call non-authentic stainless steel products; the appearance looks similar to stainless steel, but the anti-corrosion performance is much different.

4. Strength grade: The strength grade mainly refers to carbon steel fasteners.

The common strength grades of carbon steel screws are: 4.8, 5.8, 6.8, 8.8, 10.9, and 12.9. The nuts are correspondingly: level 4, level 6, level 8, level 10, and level 12.

Generally, screws below grade 8.8 are called ordinary screws, while screws above grade 8.8 (including grade 8.8) are high-strength screws. The difference is that high-strength screws need to undergo quenching and tempering heat treatment.

5. Surface treatment: Surface treatment is mainly to increase the anti-corrosion performance, but also partly takes into account the color, so it is mainly for carbon steel products, which generally require surface treatment.

Common surface treatments are: blackening, galvanizing, copper plating, nickel plating, chrome plating, silver plating, gold plating, dacromet, hot dip galvanizing, etc.;

There are many types of zinc plating, including blue and white zinc, blue zinc, white zinc, yellow zinc, black zinc, green zinc, etc., and they are also divided into environmental protection and non-environmental protection. Each type of plating has a variety of coating thicknesses to meet different requirements. The effect of the salt spray test.

Basic knowledge of fastener products (functions)

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1. In terms of function:

1. Screw torque requirements: the torque of the hexagon socket screw is relatively larger, the torque of the hexagon socket screw is smaller, and the torque of the cross recess is smaller (so this type of screw is generally an ordinary grade screw).

2. Assemble the hexagonal bolts generally with adjustable wrenches, torx wrenches and open-end wrenches. The assembly efficiency of the adjustable wrench is low, but it is very versatile. It is suitable for various head specifications of the hexagonal screws; the torx wrench is the most efficient, but it is not suitable for some occasions. A torx wrench has only 2 heads, and more CNC programming knowledge is concerned. The WeChat public account (NC programming teaching) can only be used on the hex head screws of two specifications; the open-end wrench is similar to the torx wrench, but it can be used with a longer sleeve. The smaller the size of the external hexagonal screw, the higher the corners and corners of the external hexagonal bolt, otherwise the head will easily slip when subjected to (wrench) force. In order to save materials, Wenzhou people invented the outer hexagonal cavity. The single weight of the socketed hexagonal screw is light, the thickness of the head is thin, and the force is easy to slip, and the head may also be unscrewed.

3. The hexagon socket screws are assembled with a hexagon wrench. This has high requirements for the inner hexagonal hole. The larger the hole, the wrench slips, and the smaller one cannot be inserted. The smaller the size of the inner hexagon, the higher the requirement for the hole. For some large-size hexagon socket screws, as long as one of the opposite sides of the hexagon socket is qualified, it can be assembled normally with a wrench; for some small-size hexagon socket screws, such as the M2 hexagon socket screw, the wrench can be inserted into it with a little force. The hexagon will slip, so a slightly larger opposite side is not qualified. Therefore, M2, M2.5, and M3 hexagon socket screws (especially tightening products) are easy to slip during the assembly process.

4. The Phillips screw is assembled with a screwdriver and does not require too much force, so the strength is only 4.8 grades. Occasionally, some screws that require high strength can be carburized and heat treated.

5. In the supporting use of the product, we generally recommend that the grade of the screw is one level higher than that of the nut, which has the most economic benefits. For example, a grade 8.8 screw is matched with a grade 4 nut; so when you change it next time, you only need to change the nut.

2. Heat treatment:

Heat treatment is mainly for carbon steel screws, mainly quenching and tempering heat treatment and carburizing heat treatment, to meet the requirements of different environments for the strength of screws.

1. Quenched and tempered heat treatment: products with strength grades of 8.8 and above are all quenched and tempered heat treated products. The characteristic of this heat treatment is that the hardness is relatively uniform inside and outside.

When the same material is heat treated, the higher the hardness, the worse the toughness. Therefore, it is necessary to have a safe match. While meeting the hardness, the toughness is also guaranteed.

2. Carburizing heat treatment: Self-tapping screws basically need this treatment, which is characterized by a very hard surface and a soft core; because it needs to be tapped into a hard iron plate.

The risk of self-tapping nails is high. For example, self-tapping nails often encounter broken ends. For more CNC programming knowledge, pay attention to the WeChat official account (CNC programming teaching). The possible reasons are: ①Hydrogen embrittlement; ②The hardness is too high or too low Incurred twisting; ③The cross groove is too deep; ④The head is too thin; ⑤There is no R angle at the head-neck joint, which leads to stress concentration; ⑥Improper operation; etc.

Three, hydrogen embrittlement risk:

1. Under normal circumstances, products with hardness >32HRC may have hydrogen embrittlement risk in electroplating. Therefore, all products above 10.9 (including 10.9) and products that have undergone carburizing heat treatment (self-tapping nails) will have hydrogen embrittlement risks when they are electroplated.

2. Hydrogen embrittlement means that when the product is electroplated, H+ enters the metal to form bubbles, causing the screw to continue on the spot during use, but delayed fracture occurs within 24 hours.

3. Products with hydrogen embrittlement risk need to be sent to a dehydrogenation furnace within 4 hours after electroplating, and stored at 200 degrees Celsius for about 8 hours; this is called dehydrogenation treatment.

4. Hydrogen embrittlement treatment can greatly reduce the risk of hydrogen embrittlement, but it cannot be completely avoided. Therefore, it is strictly forbidden to use electroplated products when 100% of the risk of hydrogen embrittlement is guaranteed; instead, use dacromet, sandblasting and other surface treatment processes .

Four, fastener performance and processing technology development direction:

1. High strength while ensuring toughness;

2. The weight can be reduced under the same size;

3. The volume can be reduced when the mechanical strength can be guaranteed;

4. The toughness can be strengthened when the strength is guaranteed;

5. When the appearance is satisfied, the anti-corrosion ability can be enhanced;

6. Improved accuracy limit;


Post time: Aug-02-2021