Threaded fittings are everywhere, and there are many kinds and standards. For inexperienced people, the appearance and performance of threads are almost the same, so it is impossible to identify them accurately. Master the necessary thread structure and terminology, so as to communicate with manufacturing partners efficiently to get what they need. What are the types of threads? How to choose thread correctly? What do the m, g, Tr and b of thread label mean? What are the differences between GB, GB Wyeth and GB thread? Let’s take a look together ~Actually, it’s not just this reason, Hub bolt Its own advantages are also obvious, and it is normal for the market to perform well. https://www.chinatianlong.com/
1. A brief history of thread
As early as the 4th century BC, naturalists in ancient Greece learned the basic spiral form, and applied it to pressing wine and oil. Until today, we still use archimedean spiral to get water.
Fig. 1 archimedean spiral water intake device still in use today.
By the 14th century, hand-made threads and screws appeared. During the industrial revolution, it was more necessary than ever to connect mechanical objects with bolts.
These early nuts, bolts and screws were made by hand or simple lathe, and they could not be interchanged and used universally in different industries or even between different companies in the same industry.
Figure 2 Hand-made threads 200 years ago vs modern machine-made threads
With the continuous progress of industrial technology, countries have formulated their own unified standards and international common standards in order to improve efficiency. At present, there are mainly two kinds of thread measurement standards, English and metric. The latter is adopted in China, and metric mm is used as the measurement mark unit of thread.
2. Thread definition
Thread refers to a continuous convex spiral ridge made by winding a cylinder or cone in a spiral form. The former is called straight thread and the latter is called conical thread. This spiral structure converts rotary motion into linear motion to realize linear moving of objects.
Figure 3 Straight thread and taper thread
Threads are fastened or driven by the connection of internal and external threads. The thread on the outer surface of the workpiece is external thread, such as common bolts or screws; Inside the workpiece is internal thread, which is generally on the parts to be assembled.
3. Five elements of thread
A complete description of thread should include at least five elements, namely, tooth type+nominal diameter+number of threads+pitch (or lead)+rotation direction. Among them, tooth type is the key factor to determine the final use of thread.
dentoform
Tooth type refers to the geometric shape of thread, and different tooth types determine whether the thread is used for connection or transmission. At present, the common tooth types are mainly triangular, trapezoidal, serrated and rectangular thread types.
Triangular thread is mainly used for connection, and the other three tooth types are used for power transmission.
Fig. 4 Comparison of four common types of thread teeth.
Triangular threads are mainly common threads with code M and pipe threads with code G.. M-thread is the most commonly used connecting thread, which is divided into coarse thread and fine thread, and fine thread is mainly used in small or thin-walled parts. Pipe threads are used to connect water pipes, gas pipes and other pipelines.
Trapezoidal thread code Tr is used for power transmission of screw rods of various machine tools. The serrated thread code B can only transmit power in one direction. Fig. 5 is a classification of threads by use and tooth type.
Figure 5 Classification of threads
diameter
Indicates that the nominal diameter is used for thread size, and the nominal diameter (metric unit) is used for all threads except the internal diameter (inch) of the pipe thread.
External thread and internal thread are represented by uppercase and lowercase letters “D” and “D” respectively. For example, the large, medium and small diameters of external threads are represented by D, d1 and d2; Internal threads are denoted by d, D1 and D2.
Large diameter refers to the imaginary cylinder diameter tangent to the top of external thread or the bottom of internal thread; The minor diameter is the imaginary cylinder diameter tangent to the bottom of external thread or the top of internal thread; The pitch diameter is the most important parameter to realize the fit and strength of threads, and it is an imaginary cylindrical diameter whose tooth width on the pitch line is consistent with the width of adjacent tooth slots.
Fig. 6 Large, Medium and Small Diameters of External Thread and Internal Thread
line number
The number of thread lines refers to the number of spiral lines when forming threads, which can be divided into single line and multi-line. Single thread refers to the thread formed along one helix, and multi-thread refers to the thread formed along two or more helices.
Figure 7 Single and Double Thread
Pitch (lead)
The axial distance p between two corresponding points on the pitch line of two adjacent teeth on the thread is called the pitch; On the same thread, the axial distance Ph between two corresponding points on the pitch line of two adjacent teeth is called lead.
Figure 8 Pitch and Lead
Fig. 9 Rotation direction of thread
4. Common standards of threads in various countries
The prevailing standards of threads in various countries are mainly two types of thread measurement and identification standards in metric units and inch units.
Metric or international standard thread
Metric thread was first adopted in central France from 1898 to 1908, and then widely popularized in various countries. GB metric thread standard was adopted in China.
Metric thread code M is suitable for connecting workpieces with a diameter of 0.25mm to 300mm. The thread angle is 60, the tooth top is flat, which is easy to turn, and the tooth bottom is circular, which can increase the thread strength. Metric threads are divided into coarse teeth and fine teeth.
Figure 10 Metric Thread Standard
British standard Wyeth thread
Originated in Britain, joseph whitworth, a British citizen, invented this thread in 1841, hence the name. The thread angle of this kind of thread is 55, and the top and bottom of the tooth are arc-shaped, also known as B.S.W thread.
Fig. 11 British standard Wyeth thread standard
British standard fine thread
In shape, this kind of thread is similar to B.S.W thread, and the thread angle is 55, but the number of threads per inch is more, that is, the thread thickness is thinner, which makes the grip stronger, and it is used for parts that need high strength or bear large vibration.
Fig. 12 British standard fine pitch thread standard
American standard thread
The crest and bottom of American standard thread are both flat, with good strength and stress resistance. The thread angle is 60, and the specification is expressed by several teeth per inch. It is divided into three grades: coarse teeth (NC), fine teeth (NF) and extra fine teeth (NEF).
Fig. 13 American standard thread standard
Unified standard thread
Developed by the United States, Britain and Canada, it is a commonly used English thread at present. Compared with American standard thread, the thread angle is 60, and the specification is also expressed by the number of teeth per inch, which is also divided into three levels: coarse thread (UNC), fine thread (UNF) and extra fine thread (UNEF).
Fig. 14 unified thread standards of the United States, Britain and Canada.
German DIN round thread standard
It is a standard thread set by DIN in Germany. The thread is round and suitable for the connection of light bulbs and rubber tubes. The thread code is Rd.
Fig. 15 Wyeth thread standard
Trapezoidal Tr thread
Trapezoidal thread, also known as Acme thread, is a kind of special transmission thread that can be adjusted by nut after wear. When the metric thread standard is adopted, the thread angle is 30, and the thread angle when the British thread standard is adopted is 29.
Fig. 16 Wyeth thread standard
5. National Metric Thread Identification and Examples
China’s GB standard stipulates that the unit for marking ordinary threads is millimeters, with the first letter being the thread code, the second digit representing the nominal diameter (major diameter) of the thread, and the subsequent symbols representing the tolerance code, the screwing length code and the screwing direction code in turn. Among them, the fine pitch must be marked, and the coarse pitch can be omitted.
Fig. 17 5-part labeling contents of common thread
Remarks on the codes of each part: the characteristic code indicates the thread type, for example, m indicates the common thread; The dimension code indicates the nominal diameter 〜 pitch, for example, 8×1 indicates that the nominal diameter is 8 and the pitch is 1; The code of the tolerance zone consists of the tolerance grade (number) and the basic deviation (external thread is represented by lowercase letters and internal thread is represented by uppercase letters); The code of screwing length is denoted by codes L, N and S, respectively. When the thread is of medium screwing length, code N is not marked. The rotation direction code is LH for left-handed threads, and LH for right-handed threads.
Example interpretation
Example 1: What thread does M20x1.5LH-5G6g-s represent?
Example 2: What kind of thread does B36x14 (p7)-7h-L refer to?
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Example 3: How to express the left-handed single-thread fine-tooth common thread with a nominal diameter of 20 and a pitch of 1.5?
6. Custom thread processing method
Threads can be customized by three methods: NC turning, three-axis linkage milling and tapping and threading.
Method 1 NC turning
CNC turning with turning tool has a simple tool structure, which is the most commonly used customized machining method for single piece and small batch of threaded workpieces.
Figure 18 Custom Machining of Turning Thread
Method 2 NC milling
Generally, disc milling cutter or comb milling cutter is used for milling. Disc milling cutter is mainly used for milling trapezoidal external threads on screw rods, worms and other workpieces, while comb milling cutter is used for milling internal and external common threads or tapered threads.
Fig. 19 Custom Machining of NC Milling Thread
Method 3 Tapping and threading
Tapping is to screw the tap into the bottom hole drilled in the workpiece in advance with a certain torque, so as to process it into the required internal thread.
Figure 20 Tapping
Sleeve thread is a molding method of cutting external thread on bar workpiece with die.
Fig. 21 Set of silk
On the one hand, the accuracy of tapping and threading depends on the accuracy of tap or die, on the other hand, it depends on the processing experience. Many non-standard small-diameter internal threads can only be processed by tap. Tapping and threading can be done by hand, or by lathe, drilling machine, tapping machine or threading machine.
