About tolerance and measurement precision.

Based on a certain reference value, the difference between the maximum size and the minimum size that allows an error is called "tolerance". For example, if a cylinder with a length of 50 mm is manufactured, if it is specified as a qualified product within an error of ±0.1 mm, it can be said that "the tolerance is ±0.1 mm". Although the drawing specifies a length of 50 mm, the actual manufactured product is not exactly 50 mm. High-precision processing equipment will produce extremely small errors such as 49.997 mm and 50.025 mm. In addition, in the measurement to confirm whether the manufactured product is of the specified length (50 mm), variable factors may also cause deviations in the measured value. Material expansion and contraction caused by changes in temperature and humidity, and deformation caused by measuring instrument pressure are all deformation factors. As described above, in the manufacturing process and the inspection process, it is necessary to take into account the error from the design value. If errors are not allowed, it can be said that most of the manufactured products are unqualified. Based on the design value, on the premise of not causing deviations in each process, set the allowable error range (the range that can maintain quality), which is the role of tolerance.