Basic dimensions of common threads
Common threads are the most widely used thread type in mechanical manufacturing. The standardization of their basic dimensions provides a crucial guarantee for the interchangeability and versatility of parts. The basic dimensions of common threads primarily include major diameter, minor diameter, pitch diameter, pitch, and thread angle. These interrelated parameters collectively determine the thread’s fit and load-bearing capacity. Understanding the definition, calculation methods, and standard specifications of common thread basic dimensions is fundamental to thread design, processing, and inspection, and is crucial for ensuring the assembly precision and performance of mechanical products.
The major diameter is the nominal diameter of a common thread, referring to the maximum diameter of the thread. For external threads, this is the crest diameter, and for internal threads, this is the root diameter. It’s denoted by the symbols “d” (external) or “D” (internal). The major diameter is a key parameter of a thread and is used to identify the thread size. For example, M10 indicates a common thread with a major diameter of 10mm. National standards specify a series of nominal diameters for common threads, ranging from 1mm to 100mm. Commonly used nominal diameters include 6mm, 8mm, 10mm, 12mm, and 16mm. Standard series should be preferred to ensure thread interchangeability. The tolerance grade for the major diameter is determined by the thread’s precision requirements. Coarse threads are generally 6g (external) or 6H (internal), while fine threads may adopt higher tolerance grades, such as 5g6g or 4H5H.
The minor diameter is the minimum diameter of a thread. It’s the root diameter for external threads and the crest diameter for internal threads, denoted by the symbol “d₁” (for external threads) or “D₁” (for internal threads). The size of the minor diameter directly affects the strength of the thread. A smaller minor diameter reduces the thread’s load-bearing capacity, while a larger minor diameter results in a loose fit. The formula for calculating the minor diameter is: d₁ = d – 1.0825P for external threads and D₁ = D – 1.0825P for internal threads , where P is the thread pitch. For example, for an M10×1.5 standard thread, the external minor diameter is d₁ = 10 – 1.0825×1.5 , which is ≈ 8.376 mm , and the internal minor diameter is D₁ = 10 – 1.0825×1.5, which is ≈ 8.376 mm. The tolerance grade of the minor diameter matches the major diameter. Generally, when the minor diameter tolerance of the external thread is 6g, the corresponding actual size should fluctuate within a certain range to ensure the screwability and strength of the thread.
The pitch diameter is the diameter of an imaginary cylinder whose generatrix passes through the point where the groove and projection on the thread profile are equal in width. It is represented by the symbol “d₂” (for external threads) or “D₂” (for internal threads). The pitch diameter is a key parameter that determines the quality of thread fit, and its dimensional accuracy directly affects the thread’s tightness and torque transmission capacity. The formula for calculating the pitch diameter is: d₂ for external threads = d – 0.6495P ; for internal threads, D₂ = D – 0.6495P . For example, for a standard M10×1.5 thread, the pitch diameter d₂ = 10 – 0.6495×1.5 , which is ≈ 9.025mm . The tolerance grade of the mean diameter is the main indicator of thread accuracy. Ordinary threads are divided into precision grade ( grade 4 ), medium grade (grade 6) and rough grade (grade 8). The commonly used medium grade 6 is suitable for general use, precision grade 4 is used for occasions with higher requirements, and rough grade 8 is used for occasions with lower requirements.
Pitch is the axial distance between corresponding points on the mid-diameter line of two adjacent threads, denoted by the symbol “P” and measured in mm. Ordinary threads are divided into coarse and fine threads. Coarse threads have a larger pitch and, for a given nominal diameter, have only one pitch. For example, M10 coarse threads have a pitch of 1.5mm. Fine threads have a smaller pitch and can have multiple pitches for the same nominal diameter. For example, M10 fine threads have pitches of 1.25mm, 1mm, and 0.75mm. The pitch affects the thread’s self-locking properties and transmission efficiency. A larger pitch reduces self-locking performance but increases transmission efficiency. A smaller pitch improves self-locking performance but increases CNC machining difficulty. National standards specify pitch series corresponding to different nominal diameters. The appropriate pitch should be selected during design based on the thread’s intended use and strength requirements.
The flank angle is the angle between adjacent flanks on a thread profile. The standard flank angle for common threads is 60°, denoted by the symbol “α.” The flank angle affects the stress response and CNC machining difficulty of the thread. A flank angle of 60° provides good strength and symmetry, making it easier to machine and measure. Flank angle errors affect thread fit accuracy. Generally, the flank angle error is required to be within ±30°, while high-precision threads must be within ±10°. Furthermore, common threads have fillets on both the crest and root to reduce stress concentration and improve fatigue strength. The crest radius is typically 0.1443P, and the root radius is typically 0.1082P.
The marking of basic dimensions for common threads should comply with national standards. A complete thread marking includes the nominal diameter, pitch, tolerance zone designation, and engagement length designation. For example, “M10×1.5 – 6g – L” indicates a common thread with a nominal diameter of 10mm, a pitch of 1.5mm, an external tolerance zone of 6g, and a long engagement length; “M12 – 6H” indicates a common thread with a nominal diameter of 12mm, a coarse pitch (1.75mm), an internal tolerance zone of 6H, and a medium engagement length. Engagement lengths are categorized as short (S), medium (N), and long (L). Generally, the engagement length designation is omitted; the medium engagement length is assumed by default. Correctly marking the basic dimensions of common threads clearly expresses the specifications and precision requirements of the thread, providing a clear basis for processing and inspection.
