The principle of selecting a tolerance level in CNC Machining Parts

Tolerance level refers to the level that determines the accuracy of the size. The national standard is divided into 20 levels, from IT01, IT0, IT1, IT2 to IT18. The larger the number, the lower the tolerance level (processing accuracy), and the allowable range of variation (tolerance) The greater the value, the less difficult it is to process.

The essence of selecting the tolerance level is to correctly resolve the contradiction between the use requirements of the machine parts and the manufacturing process and cost. The principle of selecting a tolerance level is to select a lower tolerance level as much as possible on the premise of meeting the requirements for the use of the part. Accuracy requirements should be consistent with the possibility of production, that is, reasonable processing technology, assembly technology and existing equipment should be used. However, if necessary, methods of improving equipment accuracy and improving technology must be adopted to ensure the accuracy of the product. It is extremely important to select the appropriate tolerance level for the fit size. Because in many cases, it will determine the working performance, service life and reliability of the matching part, and at the same time determine the manufacturing cost and production efficiency of the part.


(1) The selection of tolerance level should first ensure the use requirements.


(2) When selecting the tolerance level, it is necessary to meet both the design requirements and the possibility and economy of the process. Choosing the best machining accuracy is a very complicated technical and economic problem. It must consider not only the processing cost, but also the increased assembly costs due to the improvement of processing accuracy, and the impact of accuracy on product performance and economic indicators (reliability, life, fuel consumption, etc.).


(3) The tolerance level is stipulated so as to ensure both the accuracy of the machine and the interchangeability of parts and components, and the economics of the manufacturing machine. That is to say, as long as the low accuracy can guarantee the function and accuracy of the machine, do not demand the accuracy of parts too high. That will increase the manufacturing cost. Specifically, the tolerance level of the machine should be determined according to the type of the machine and the use of a certain part. Tolerances include dimensional tolerances and fitting tolerances. When using it, refer to the recommended tolerance levels of various parts listed in the mechanical design manual for flexible application.

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