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Threaded fittings are everywhere, and there are many types and standards. For inexperienced people, the appearance and performance of threads are almost the same, making it difficult to accurately identify them. Master the necessary thread structure and terminology to effectively communicate with manufacturing partners and obtain the required information. What are the types of threads? How to choose threads correctly? What do the M, G, Tr, and B markings on threads represent? What are the differences between Chinese standard, British standard Wyeth, and American standard threads? Let's take a look together~
As early as the 4th century BC, ancient Greek naturalists learned the basic spiral form and applied it to extracting wine and oil. Even today, we still use the Archimedean spiral to fetch water.
▲ The Archimedes spiral water intake device still in use today
In the 14th century, handmade threads and screws appeared. During the Industrial Revolution, it was more necessary than ever to use bolts to connect mechanical objects.
These early nuts, bolts, and screws were made by hand or using simple lathes, and they were not interchangeable or interchangeable between different industries or even companies within the same industry.
▲ Handcrafted threads 200 years ago vs. modern mechanism threads
With the continuous advancement of industrial technology, countries have developed their own unified standards and internationally recognized standards in order to improve efficiency. At present, there are mainly two types of thread measurement standards: imperial and metric. China adopts the latter, using metric mm as the measurement identification unit for threads.
A thread refers to a continuous raised spiral shaped ridge that is wound in a spiral form around a cylinder or cone. The former is called a straight thread, and the latter is called a tapered thread. This spiral structure converts rotational motion into linear motion to achieve linear movement of objects.
▲ Straight thread and tapered thread
Threads serve the purpose of fastening or transmission through the connection of internal and external threads. The thread on the outer surface of the workpiece is an external thread, such as commonly seen bolts or screws; The internal thread of the workpiece is usually on the parts that need to be assembled.
3. Thread 5 major elements
A complete description of threads should include at least 5 elements, namely tooth profile+nominal diameter+number of threads+pitch (or lead)+direction of rotation. The tooth profile is a key factor determining the end use of the thread.
◇ Dental shape
Tooth profile refers to the geometric shape of a thread, and different tooth profiles determine whether the thread is used for connection or transmission purposes. At present, there are four common types of thread profiles: triangular, trapezoidal, serrated, and rectangular.
The triangular thread is mainly used for connection, while the other three tooth profiles are used for power transmission.
▲ Comparison of Four Common Types of Threads
Triangle threads are mainly composed of ordinary threads with code M and pipe threads with code G. M thread is a commonly used connecting thread, divided into coarse and fine threads. Fine threads are mainly used on small or thin-walled parts. Pipe threads are used for connecting pipelines such as water pipes and air pipes.
The trapezoidal thread code is Tr, used for power transmission of various machine tool screws. The serrated thread with code B can only transmit power in one direction. Figure 5 shows the classification of threads by purpose and tooth type.
▲ Classification of threads
◇ Diameter
The nominal diameter is used to indicate the thread size. Except for pipe threads, which use the inner diameter (inches) of the pipe as the nominal diameter, all other threads use the larger diameter as the nominal diameter (metric unit).
External threads and internal threads are represented by the uppercase and lowercase letters "d" and "D" respectively to indicate thread diameter. For example, the large, medium, and small diameters of external threads are represented by d, d1, and d2; Internal threads are represented by D, D1, and D2.
The major diameter refers to the imaginary cylindrical diameter that is tangent to the external thread crest or internal thread root; A small diameter is the imaginary cylindrical diameter that is tangent to the external thread root or internal thread crest; The pitch diameter is an important parameter for achieving fit and strength of threads, which is the imaginary cylindrical diameter on the pitch line with the tooth width consistent with the adjacent tooth groove width.
▲Large, medium, and small diameters of external and internal threads
◇ Number of lines
The number of threads refers to the number of spiral threads formed during the formation of a thread, which can be divided into single and multiple threads. Single line thread refers to a thread formed along one spiral line, while multi line thread refers to a thread formed along two or more spiral lines.
▲ Single and double thread
◇ Pitch (lead)
The axial distance P between two adjacent threads on the centerline is called the pitch; The axial distance Ph between two adjacent teeth on the same thread and their corresponding points on the centerline is called the lead.
▲ Pitch and lead
▲ Rotation direction of threads
4. General standards for threads in various countries
The prevailing standards for thread measurement in various countries are mainly based on two types of thread identification standards: metric units and inch units.
◇ Metric or international standard thread
The metric thread was first adopted in central France from 1898 to 1908, and then widely promoted among countries. China adopts the GB metric thread standard.
Metric thread code M, suitable for connecting workpieces with diameters ranging from 0.25mm to 300mm. The thread angle is 60 °, the tooth tip is flat, easy to turn, and the tooth base is arc-shaped, which can increase thread strength. Metric threads are divided into coarse and fine threads.
▲ Metric Thread Standard
◇ British Standard Wyeth Thread
Originating from England, this thread was invented by British citizen Joseph Whitworth in 1841, hence its name. The thread angle of this type of thread is 55 °, and both the tooth top and tooth bottom are circular arcs, also known as B.S.W thread.
▲ British Standard Wyeth Thread Standard
◇British standard fine thread
In terms of shape, this thread is similar to B.S.W thread, with a thread angle of 55 °, but with more threads per inch, resulting in thinner thread thickness and stronger grip. It is used for roots that require high strength or components that need to withstand large vibrations.
▲ British standard fine thread standard
◇American standard thread
The top and bottom of the American standard thread are both flat, with good strength and stress resistance. The thread angle is 60 °, and the specifications are expressed in terms of the number of teeth per inch, divided into three levels: coarse teeth (NC), fine teeth (NF), and ultra-fine teeth (NEF).
▲ American thread standard
◇ unified screw thread
Developed jointly by the United States, the United Kingdom, and Canada, it is currently the commonly used British thread. Compared with American standard threads, the thread angle is 60 °, and the specifications are also expressed in terms of how many teeth per inch, which are also divided into three levels: coarse teeth (UNC), fine teeth (UNF), and ultra-fine teeth (UNEF).
▲ Unified thread standard among the United States, the United Kingdom, and Canada
◇ German DIN round thread standard
It is a standard thread specified by DIN in Germany, with a circular thread suitable for connecting light bulbs and rubber tubes. The thread code is Rd.
▲ German DIN round thread standard
◇ Trapezoidal Tr thread
Trapezoidal thread, also known as Aikem thread, is a special type of transmission thread that can be adjusted with a nut after wear. When using metric thread standards, the thread angle is 30 °, while when using imperial thread standards, the thread angle is 29 °.
▲ Trapezoidal Tr thread standard
5. National standard metric thread identification and examples
According to the GB standard in our country, the unit for marking ordinary threads is millimeters. The first letter is the thread code, the second digit represents the nominal diameter (major diameter) of the thread, and the following symbols represent the tolerance code, screw length code, and screw direction code. Among them, the pitch of fine teeth must be marked, and the pitch of coarse teeth can be omitted.
▲ 5 parts of the annotation content for ordinary threads
Annotations on Part Codes: Feature codes indicate thread types, such as M for regular threads; The size code represents nominal diameter x pitch, for example 8x1 represents nominal diameter of 8 and pitch of 1; The tolerance zone code consists of tolerance grade (number) and basic deviation (lowercase letters for external threads and uppercase letters for internal threads); The code L, N, and S are used to represent long, medium, and short screw lengths, respectively. When the thread has a medium screw length, the code N is not indicated; The rotation code for left-hand threads is labeled LH, while right-hand threads are not labeled.
◇ Example Explanation
Example 1: What thread does M20x1.5LH-5g6g-S represent?
Example 2: What type of thread does B36x14 (P7) -7H-L refer to?
Example 3: How to represent a left-handed single threaded fine tooth ordinary thread with a nominal diameter of 20 and a pitch of 1.5?
6. Thread customization processing method
Threads can be customized through three methods: CNC turning, three-axis linkage milling, and threading.
Method 1: Numerical Control Turning
The use of turning tools for CNC machining has a simple tool structure and is a commonly used custom machining method for single piece and small batch threaded workpieces.
▲ Customized machining of turning threads
Method 2: Numerical Control Milling
Generally, disc milling cutters or comb milling cutters are used for milling. Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screw rods and worm gears, while comb milling cutters are used for milling internal and external ordinary threads or tapered threads.
▲ CNC milling thread customization processing
Method 3: Tapping and threading
Tapping is the process of using a certain torque to thread a tap into a pre drilled bottom hole on a workpiece, in order to create the desired internal thread.
▲ tapping
Thread threading is a forming method that uses die to cut external threads on a bar workpiece.
▲Threading
The accuracy of tapping and threading machining depends on both the precision of the tap or die and the machining experience. Many non-standard small-diameter internal threads can only be machined with a tap. Tapping and threading can be done manually, or by lathe, drilling machine, tapping machine or threading machine.
(Source: One Processing)
