In design, the selection of producttolerance levels is to determine the manufacturing accuracy of dimensions. Dueto the relationship between dimensional accuracy, processing difficulty,processing cost, and workpiece quality, it is necessary to correctly handle therelationship between usage requirements, processing technology, and cost whenselecting tolerance grades. In the high-precision region, there is a slightimprovement in machining accuracy, but the machining cost increases sharply. Atthe same time, the feasibility of measurement should also be considered. So,the selection of high tolerance grades should be particularly cautious.

The basic principle for selecting tolerancelevels in product design is to choose lower tolerance levels as much aspossible while meeting the usage requirements. Mainly consider the followingaspects:

1. In commonly used size ranges, for lowprecision holes and shafts, choosing the same tolerance level is called the"principle of process equivalence". For higher precision levelfittings, hole machining is generally more difficult, so choosing a hole with atolerance level one level lower than the shaft is recommended;

2. For block gauges, gauges, fittingdimensions, particularly precise parts, non fitting dimensions, and rawmaterials, different tolerance levels should be corresponded according togeneral principles;

3. Different processing methods, such asgrinding, turning, boring, pulling, milling, planing, inserting, drilling,casting, etc., correspond to different tolerance levels;

4. The precision requirements for matchingbetween the cooperating components;

5. The tolerance level for small clearancesshould be higher, while the tolerance level for large clearances can be lower;

6. In non benchmark mating types, theprecision requirements for all parts are not high, and their tolerance levelscan differ by 2-3 levels;

7. For the case where the tolerance levelof the matching size is between 5 and 12 levels, it can be selected byreferring to different application tables.

Understanding the methods for determiningtolerance levels in design is particularly meaningful for us to gain a deeperunderstanding of drawing requirements and product applications, and to betterdetermine measurement methods and principles. The image measuring instrument isa non-contact measurement method that can achieve three-dimensional geometricdimensions. In the three-dimensional coordinate system, products are placedaccording to the fitting relationship and reference position, and a coordinatesystem is established. During the manufacturing process of the fitting partsand after the completion of the finished product, they are decomposed andfinally measured to complete the qualification judgment of the dimensions.