Basic tooth profile of common thread
The basic thread profile of common threads is the fundamental standard for thread design and processing. It specifies the thread geometry and dimensional parameters and is a key element in ensuring thread interchangeability and connection performance. Whether it’s bolt-nut connections in mechanical manufacturing or sealing threads in pipes, adherence to a unified basic thread profile standard is essential to ensure accurate fit between threaded parts produced by different manufacturers. The basic thread profile of a common thread is an isosceles triangle. Commonly used internationally are the metric common thread (flank angle 60°) and the imperial Whitworth thread (flank angle 55°). The metric common thread is widely used in my country’s machinery industry due to its standardized dimensions and simplified calculations. Each parameter of the basic thread profile, such as major diameter, minor diameter, pitch diameter, pitch, thread angle, crest height, and root height, has strict definitions and numerical requirements. These parameters directly affect the strength, sealing, and screwability of the thread.
The major diameter, minor diameter, and pitch diameter are the three key diameter parameters of a common thread profile. The major diameter ( d or D ) refers to the maximum diameter of the thread, i.e., the crest diameter for external threads or the root diameter for internal threads. It is the nominal diameter of the thread and is used to identify the thread size (e.g., M10 represents a metric common thread with a major diameter of 10mm ). The minor diameter ( d₁ or D₁ ) is the minimum diameter of the thread, corresponding to the root diameter for external threads or the crest diameter for internal threads. Its size must ensure sufficient root strength and is typically 0.85-0.9 times the major diameter (e.g., the minor diameter of an M10 thread is approximately 8.376mm ). The pitch diameter ( d₂ or D₂ ) 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. The pitch diameter is a key indicator of thread accuracy and directly affects the fit of the thread. Its dimensional deviation must be strictly controlled within a standard range (for example, the pitch diameter tolerance for a 6g-grade external thread is -0.026mm to -0.121mm). During manufacturing, the pitch diameter is typically measured using a thread gauge or a three-dimensional coordinate measuring machine to ensure it meets design requirements.
Pitch, profile angle, and tooth height are key parameters that determine the basic profile of common threads. Pitch (P) is the axial distance between corresponding points on the mid-diameter line of two adjacent threads. It determines the lead of the thread (for single-start threads, the lead is equal to the pitch). The size of the pitch directly affects the thread’s load-bearing capacity and engagement efficiency: large-pitch threads (such as P3mm) have high load-bearing capacity but require fewer turns, making them suitable for heavy-duty connections; small-pitch threads (such as P1mm) require more turns and offer high precision, making them suitable for precision instruments. The profile angle (α) is the angle between the two sides of the thread profile. The profile angle for metric common threads is 60°, while that for imperial Whitworth threads is 55°. The accuracy of the profile angle must be verified using a dedicated template or image measuring instrument. The error is typically controlled within ±15′, otherwise the thread fit will be too loose or too tight. The tooth height ( h ) is the vertical distance from the top to the bottom of the tooth, and is composed of the tooth top height ( h₁ ) and the tooth bottom clearance ( c ). The tooth top height is 0.5413P and the tooth bottom clearance is 0.125P , so the total tooth height h=0.6495P (for example , a P2mm thread has a tooth height of approximately 1.299mm ). Insufficient tooth height will reduce thread strength, while excessive height will increase processing difficulty.
The clearance and chamfer design of the basic thread profile of common threads significantly impacts thread performance. To ensure smooth thread engagement and avoid interference between the crest and root, the basic thread profile specifies the clearance between the crest and root: both the crest of external threads and the root of internal threads are arc-shaped, with a radius of r = 0.1443P. This design reduces stress concentration and prevents damage during thread assembly. A chamfer (typically 45° or 60°) is typically required at the thread end. The chamfer diameter is slightly smaller than the major diameter of the thread and is 0.5-1 pitch length. This serves to guide thread engagement and prevent end thread fracture. For load-bearing parts such as bolts, the chamfer also reduces stress concentration at the end and improves fatigue strength. For example, the end chamfer of an M12 bolt is approximately 11mm in diameter and 1.5mm in length, ensuring ease of assembly without affecting the effective length of the thread.
The standardization and application of the basic profile of common threads must be tailored to actual working conditions. China’s national standard GB/T 192-2003 provides detailed specifications for the basic profile of metric common threads, covering pitch series (coarse and fine), diameter range (1-600mm), and tolerance grades (3-9). Designers must select appropriate parameters based on the specific application scenario: coarse threads (such as M20×2.5) are used for applications requiring high connection strength; fine threads (such as M20×1.5) are used for applications requiring frequent disassembly or connecting thin-walled parts. During machining, the tool profile must perfectly match the basic profile. For example, the cutting edge angle of a thread turning tool must strictly be 60°, and the tool tip radius must meet the requirement of 0.1443P. Failure to do so will result in distortion of the machined thread profile, affecting fit accuracy. By strictly controlling the basic thread profile parameters, an automotive parts manufacturer has achieved a 20% improvement in bolt-nut fit accuracy, significantly improving connection reliability and reducing the failure rate to below 0.1%.
