In mechanical manufacturing, the geometric accuracy of parts has a crucial impact on assembly performance and functional use. Geometric tolerance, as an important part of geometric tolerance, mainly includes two major categories: shape tolerance and position tolerance, which are used to control the allowable deviation of the geometric elements of parts relative to the ideal shape or position. In order to ensure that the parts meet the design requirements, accurate measurement must be carried out. This article will briefly introduce several common geometric tolerances and their measurement methods.



1. Shape tolerance and measurement methods



Shape tolerance refers to the limitation of shape errors of a single geometric element of the part, mainly including straightness, flatness, circularity, and cylindricity.



1. Straightness

Straightness refers to the degree of deviation between the actual straight line and the ideal straight line. Common measurement methods include using a blade, a flat ruler with a feeler gauge for detection, or using an optical collimator for high-precision measurement.



2. Flatness

Flatness represents the maximum deviation between the actual surface and the ideal plane. It is usually measured by the three-point method or four-point method, where a micrometer (or a percentage gauge) is used to measure the height difference of each point on a flat plate, and then the flatness error is calculated through data processing.



3. Circularity

Circularity is used to control whether the cross-section of the part is close to the ideal circular shape. The common method is to use a roundness gauge, rotate the measured part, and record its contour changes with a probe to analyze the circularity error.



4. Cylindricity

Cylindricity comprehensively controls the roundness, straightness, and elemental line parallelism of the cylindrical surface. Due to its complexity, it usually requires the use of coordinate measuring machines (CMM) or multi-axis roundness gauges for comprehensive measurement.



2. Positional tolerance and measurement methods



Positional tolerance refers to the relative position error between two or more geometric elements, including parallelism, perpendicularity, coaxiality, symmetry, and positional tolerance.



1. Parallelism and Perpendicularity

These two tolerances are used to control the directional relationship between the measured element and the datum element. It is often adopted to fix a micrometer on the measured surface and move it relative to the datum surface for measurement, reading the maximum fluctuation value.



2. Coaxiality

coaxiality represents the degree of overlap between two axes. Special measurement equipment such as coordinate measuring machines or the use of a heart axis with a micrometer for rotation measurement is required during measurement.



3. Symmetry

Symmetry controls the symmetry of the measured element relative to the datum plane. The common method is to establish a datum plane and then measure the deviation values on both sides of the symmetry plane.



4. Positional tolerance

Positional tolerance is the ideal position accuracy of the measured element relative to the datum. It is usually measured by coordinate measuring machines for precise coordinate measurement and compared with the theoretical value.



Application of modern measurement technology



With the continuous improvement of manufacturing accuracy, traditional manual measurement methods are no longer able to meet high-precision requirements. Modern measurement technologies such as laser interferometers, optical measurement systems, coordinate measuring machines (CMM), and digital image processing technology have been widely applied in the measurement of geometric tolerances. These devices not only improve measurement efficiency but also greatly enhance the accuracy and repeatability of measurement results.



Conclusion



Accurate measurement of geometric tolerances is of great significance for ensuring product quality and improving assembly accuracy. Choosing appropriate measurement tools and methods, combined with modern measurement technology, can effectively improve measurement accuracy and efficiency, providing reliable protection for precision manufacturing. Therefore, mastering the principles and methods of geometric tolerance measurement has become one of the essential skills for engineering and technical personnel.