Common thread measurement method
As the most commonly used thread type in mechanical connections, the measurement method for common threads is directly related to the reliability and interchangeability of threaded connections. Common thread measurement primarily involves testing dimensional accuracy, form and position accuracy, and surface quality. Dimensional accuracy is the core, involving key parameters such as pitch diameter, top diameter, and pitch. Depending on the measurement principle and accuracy requirements, common thread measurement methods can be divided into two categories: comprehensive measurement and single-item measurement. Each method has its own focus and is suitable for different production scenarios and accuracy requirements.
The comprehensive measurement method uses thread gauges to comprehensively inspect various thread parameters. Whether the gauge can be screwed in smoothly determines the thread’s conformity. This efficient and convenient measurement method is widely used for rapid inspection in mass production. Common thread gauges include plug gauges and ring gauges. Plug gauges are used to inspect internal threads, while ring gauges are used for external threads. Each gauge type is further divided into a go gauge and a stop gauge. The go gauge simulates the maximum physical boundary to ensure thread fit and ensure smooth screwing into the workpiece. The stop gauge simulates the minimum physical boundary to limit dimensional deviation, ensuring that the thread cannot be screwed in or that the screwing depth does not exceed a specified value (usually 2-3 threads). The advantages of the comprehensive measurement method are simplicity and efficiency, and it can simultaneously inspect multiple thread parameters such as pitch diameter, pitch, and thread angle. However, its disadvantage is that it cannot obtain specific dimensional values and can only determine thread conformity. This method is suitable for applications where precision is not a priority.
The single-item measurement method uses special measuring tools or instruments to measure each parameter of the thread individually, which can obtain specific dimensional values and is suitable for high-precision thread inspection and quality analysis. When measuring the pitch, a pitch gauge, tool microscope or three-dimensional coordinate measuring machine can be used. The pitch gauge compares the degree of fit with the thread profile and is simple to operate but has low accuracy. The tool microscope uses an optical system to magnify the thread image and measure the distance between the corresponding points of two adjacent teeth, with an accuracy of up to 0.001mm. The three-dimensional coordinate measuring machine scans the thread surface with a probe and can accurately measure the actual value and cumulative error of the pitch, with an accuracy of up to micron level. When measuring the thread angle, a tool microscope or projector is mainly used to obtain the thread angle value by measuring the angle between the two sides of the thread profile. The standard thread angle of ordinary threads is 60°, and the allowable error is ±30′. When measuring the mean diameter, you can use a thread micrometer, three-needle measurement method or tool microscope. The three-needle measurement method is a precise method for measuring the mean diameter of external threads. By placing three measuring needles of equal diameter into the thread groove, the distance outside the measuring needles is measured with a micrometer, and the mean diameter value is calculated using a formula with an accuracy of up to 0.002mm.
In actual measurement, the appropriate measurement method must be selected based on the thread’s accuracy level and production batch. For ordinary threads with accuracy level 6 or below, a comprehensive measurement method can meet the requirements, such as the inspection of standard parts such as bolts and nuts. For precision threads with accuracy level 5 or above, such as machine tool lead screws and threaded connections in precision instruments, a single measurement method must be used for comprehensive inspection to ensure that all parameters meet design requirements. In addition, the measurement environment also has a significant impact on measurement accuracy. Thread measurements should be performed in a constant temperature environment (20±2°C) to avoid thermal deformation of the workpiece and measuring tools caused by temperature changes. Before measurement, the thread surface and measuring tools must be cleaned to remove impurities such as oil, burrs, etc. to prevent them from affecting the measurement results. For high-precision measurements, the measuring tools must also be calibrated to ensure their accuracy meets the requirements.
Data analysis and processing after thread measurement are crucial for ensuring measurement reliability. With the comprehensive measurement method, if the go gauge fails to screw in or the stop gauge screws in too deeply, it indicates an out-of-tolerance condition. The cause must be analyzed and repair measures implemented. With the individual measurement method, the measurement results must be compared with the tolerance requirements in the design drawings to determine whether each parameter is within the permitted range. For example, the pitch diameter tolerance of common threads varies depending on the precision grade and nominal diameter. The pitch diameter tolerance of a Class 6 precision M10×1.5 external thread is 0.224mm. Any measurement result outside this range is considered unqualified. If the deviation is minor, unqualified threads can be repaired through grinding or other methods; however, if the deviation is significant, the thread must be scrapped. Through scientific and rational measurement methods and rigorous data analysis, the processing quality of common threads can be effectively guaranteed, ensuring the reliability and interchangeability of threaded connections.
