Professional analysis of machining accuracy

  • Machining accuracy is the actual size, shape and position of the machined part surface in accordance with the ideal geometric parameters required by the drawing! The ideal geometric parameter, for size, is the average size; For shape, it is absolute circle, cylinder, plane, cone and straight line; For mutual position, it is absolutely parallel, vertical, coaxial, symmetrical, etc.


    1. Dimensional accuracy

    After processing, the actual size of the part is consistent with the tolerance zone center of the part size.


    1. Shape accuracy

    It refers to the conformity degree between the actual geometry of the machined part surface and the ideal geometry.


    1. Position accuracy

    It refers to the actual position accuracy difference between the relevant surfaces of parts after machining.


    1. Interrelationship

    Generally, when designing machine parts and specifying the cnc machining services accuracy of parts, attention should be paid to controlling the shape error within the position tolerance, and the position error should be less than the dimensional tolerance. That is to say, the shape accuracy of precision parts or important surfaces should be higher than the position accuracy, and the position accuracy should be higher than the size accuracy.


    The deviation between the actual and ideal geometric parameters of a part is called machining error. The machining error reflects the machining accuracy. The larger the error is, the lower the machining accuracy is, and the smaller the error is, the higher the machining accuracy is.


    Machining accuracy is mainly used for the degree of production. Machining accuracy and machining error are terms to evaluate the geometric parameters of machined surface. Machining accuracy is measured by tolerance grade. The smaller the grade value is, the higher the accuracy is; Machining error is expressed by numerical value. The larger the numerical value is, the larger the error is. High machining accuracy means small machining error, and vice versa.


    There are 20 tolerance grades from IT01, it0, it1, it2, it3 to it18. If IT01 indicates, the machining accuracy of the part is the highest, if it18 indicates, the machining accuracy of the part is the lowest. Generally speaking, it7 and it8 are medium machining accuracy.


    The actual parameters obtained by any cnc machining method will not be absolutely accurate. From the function of the part, as long as the machining error is within the tolerance range required by the part drawing, it is considered that the machining accuracy is guaranteed.


    Difference between accuracy and precision:


    1. Accuracy

    It refers to the close degree between the measured result and the real value. High accuracy of measurement means that the system error is small, and the average value of measurement data deviates less from the true value, but the data is scattered, that is, the size of accidental error is not clear. Accuracy means the correctness of measurement results, precision means the repeatability and reproducibility of measurement results, precision is the prerequisite of accuracy.


    1. Precision

    It refers to the reproducibility and consistency between the results obtained by repeated determination with the same spare sample. It is possible that the precision is high, but the precision is not accurate. For example, the three results obtained by measuring the length of 1 mm are 1.051 mm, 1.053 and 1.052 respectively. Although their precision is high, they are not accurate.