Detailed Explanation of the Outer Diameter, Middle Diameter and Bottom Diameter of External Threads
author: YITECH
2025-02-21
Threads are a common connection method in mechanical engineering and are widely used in various equipment and structures. As a form of threaded connection, the geometric parameters of external threads are crucial to the reliability and performance of the connection. This article will discuss in detail the concepts, calculation methods and applications of the outer diameter, middle diameter and bottom diameter of external threads in threaded connections.
1. Basic concepts of threads
A thread is a continuous spiral formed along the surface of a cylinder or cone, used to achieve a tight connection between two or more parts. The main geometric parameters of a thread include the outer diameter (major diameter), middle diameter and minor diameter (bottom diameter), as well as the pitch.
Outer diameter (major diameter):
The maximum diameter of a thread, the diameter of an imaginary cylinder that coincides with the crest of an external thread or the bottom of an internal thread. When representing a thread, the nominal diameter usually refers to the outer diameter.
Middle diameter:
The diameter of an imaginary cylinder with equal widths of the protrusion and groove on the tooth profile, which is approximately the average diameter of the thread. The middle diameter is the diameter that determines the geometric parameters and matching properties of the thread.
Minor diameter (bottom diameter):
The minimum diameter of the thread, the diameter of the imaginary cylinder that coincides with the bottom of the external thread or the top of the internal thread. Pitch: the axial distance between two corresponding points on the median line of adjacent teeth.
2. External diameter of external thread
The external diameter of the external thread is its nominal diameter, which is also the maximum diameter of the thread. In the marking of the external thread, the nominal diameter is usually directly marked, such as M10 for an external thread with a nominal diameter of 10mm.
The calculation of the major diameter of the external thread and the tolerance control are crucial to the reliability and accuracy of the threaded connection. According to the national standard GB/T 197/196, the calculation of the major diameter of the external thread needs to consider its tolerance grade. Taking the M8 external thread as an example, the upper limit of the 6h grade major diameter is φ8, and the lower limit is φ8-0.212=φ7.788. The upper limit of the 6g grade major diameter is φ8-0.028=φ7.972, and the lower limit is φ8-0.028-0.212=φ7.76.
3. The middle diameter of external thread
The middle diameter of external thread is an important parameter in its geometric dimensions, which directly affects the matching properties and strength of the thread. The calculation of the middle diameter is based on the major diameter and pitch of the thread. The specific formula is: d2=d-P×0.6495, where d is the major diameter of the thread and P is the pitch.
Taking the external thread M8 as an example, the middle diameter is calculated as: 8-1.25×0.6495≈7.188mm. The tolerance control of the middle diameter is equally important. According to national standards, the upper limit of the tolerance of the 6h grade middle diameter is the basic size, and the lower limit is the basic size minus the corresponding tolerance. In addition to the tolerance, the 6g grade middle diameter also needs to consider the basic deviation.
4. The bottom diameter of the external thread
The bottom diameter of the external thread is its minor diameter and the minimum diameter of the thread. The calculation formula of the bottom diameter is: d1=d-P×1.0825, where d is the major diameter of the thread and P is the pitch.
Taking the M30×2 thread as an example, its bottom diameter is calculated as: 30-2×1.0825=27.835mm. The size of the bottom diameter has a certain influence on the strength and sealing of the threaded connection, so it needs to be strictly controlled during processing and testing.
5. Thread processing and testing
Thread processing is an important process in mechanical manufacturing. Common processing methods include turning, milling and rolling. Among them, turning is one of the most commonly used thread processing methods. When turning threads, parameters such as cutting speed, feed rate and tool shape need to be strictly controlled to ensure the geometric dimensions and surface quality of the threads.
Thread detection is a key link in ensuring the quality of threaded connections. Commonly used detection methods include thread micrometer measurement, three-needle measurement and single-needle measurement. Among them, the three-needle measurement method has a higher measurement accuracy and is suitable for the measurement of the middle diameter of triangular threads, trapezoidal threads and serrated threads.
6. Application of threaded connections
External threads are widely used in various equipment and structures, such as bolts, nuts, pipe joints, etc. In threaded connections, the matching quality of external threads and internal threads directly affects the reliability and performance of the connection. Therefore, when designing and manufacturing threaded connectors, it is necessary to strictly control the geometric dimensions and tolerance grades of the threads.
In addition, for threaded connectors that need to withstand large loads or special working conditions, strength verification and fatigue life analysis are also required to ensure their safety and reliability during use.
DongGuan Yi Tai Electronic Technologies Co.,Ltd could make all threads for your products, any custom service needs please do not hesitate to contact us.
DongGuan YiTai Electronic Technologies Co.,Ltd
DongGuan YiTai Electronic Technologies Co.,Ltd is a manufacturing service provider located in Dongguan, China.
YiTai specialized in CNC turning machining and sheet metal fabrication mainly. As a member of Hung Mou Group, we focus on the overseas marketing development. And based on our parent company’s manufacturing capability and resources, YiTai also expended machining services such as die casting, injection molding, aluminum profile extrusion, 3D printing, which are committed to providing customers with one-stop purchasing services and experience.
CNC MACHINING , CNC MILLING , CNC TURNING , SHEETMETAL , FASTENER , OTHERS
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