1、 Selection principle:

1. Ensure the accuracy of measurements. The performance indicators of measuring instruments are the main basis for selecting measuring instruments, with indication error, indication variability, and return error as the main performance indicators.

2. Processing method, batch and quantity selection of measuring instruments. Batch production mainly involves specialized measuring tools, gauges, and specialized instruments. High efficiency mechanized and automated specialized measuring instruments are selected for mass production.

3. Select measuring instruments based on the structure, characteristics, size, shape, weight, material, rigidity, and surface roughness of the parts.

4. Select measuring instruments based on the state and conditions of the parts. In modern machine manufacturing industry, production automation requires measurement automation. Dynamic measurement is more complex than static measurement.

2、 Selection method:

1. Select according to inspection standards, and select measuring instruments for final acceptance of parts according to the international standard GB3177-1997 (Inspection of Smooth Workpiece Dimensions).

According to the different tolerance sizes of the parts, a safety margin A is specified.

Based on the type, accuracy, and improper adjustment factors of the measuring tool, there exists an uncertainty, and its tolerance is determined by μ 1 represents, μ Approximately 0.9A Based on the uncertainty of various measuring tools and instruments μ X, μ X ＜ μ 1.

2. Select according to the accuracy coefficient of the measurement method. The allowable error size of the measurement method mainly depends on the accuracy requirements of the measured part, that is, the tolerance size of the measured parameters. The ratio of the uncertainty of the measurement method to the tolerance of the measured part is called the accuracy coefficient K of the measurement method. The smaller the K, the smaller the measurement method error used. The K value ranges from 1/3 to 1/10, and in general, it can be taken as 1/5.