Are your previous statements about machining accuracy correct?

As we all know, machining accuracy refers to the degree to which the actual size, shape and position of the machined part surface conform to the ideal geometric parameters required by the drawing. Therefore, when we have a demand for precision machining, our first reaction is to find a precision machining equipment, and our inventory of precision machining equipment comes from parameters. In fact, for the definition of this precision, the standards of each country are different.

Let's take a strict look at the accuracy of those things!
Accuracy: refers to the closeness between the measured results and the true values. High measurement accuracy means that the system error is small. At this time, the average value of the measured data deviates from the true value less, but the data is scattered, that is, the size of the accidental error is not clear.

Precision: refers to the reproducibility and consistency between the results obtained by repeated determination with the same kind of standby sample. It is possible that the precision is high, but the accuracy is inaccurate. For example, the three results measured with a length of 1mm are 1.051mm, 1.053 and 1.052 respectively. Although their precision is high, they are inaccurate.

Accuracy indicates the correctness of the measurement results, precision indicates the repeatability and reproducibility of the measurement results, and precision is the prerequisite for accuracy.
In a promotional article on CNC machine tools, the "positioning accuracy" of machine tool A is 0.004mm, while in the sample of another manufacturer, the "positioning accuracy" of similar machine tool B is 0.006mm. From these data, you will naturally think that the accuracy of machine tool A is higher than that of machine tool B.

However, in fact, it is very likely that the accuracy of machine tool B is higher than that of machine tool A. The problem is how to define the accuracy of machine tool A and B respectively. Therefore, when we talk about the "precision" of CNC machine tools, we must make clear the definition and calculation method of standards and indicators.

1、 Definition of precision:
Generally speaking, accuracy refers to the ability of the machine tool to position the tool tip to the program target point. However, there are many ways to measure this positioning capability. More importantly, different countries have different regulations.
Japanese machine tool manufacturers: JISB6201 or JISB6336 or JISB6338 standards are usually used when calibrating "precision". JISB6201 is generally used for general machine tools and general CNC machine tools, JISB6336 is generally used for machining centers, and JISB6338 is generally used for vertical machining centers.

European machine tool manufacturers, especially German manufacturers, generally adopt VDI/DGQ3441 standard.
American machine tool manufacturers: generally adopt the NMTBA (National Machine Tool Builder's Assn) standard (this standard is derived from a study of the American Machine Tool Manufacturing Association, issued in 1968, and modified later).
When calibrating the accuracy of a CNC machine tool, it is very necessary to mark the standards used together. The JIS standard is adopted, and its data is significantly smaller than that of the NMTBA standard in the United States or the VDI standard in Germany.
The same indicator has different meanings

It is often confused that the same indicator name represents different meanings in different precision standards, while different indicator names have the same meaning. The above four standards, except JIS standard, are all calculated through mathematical statistics after multiple rounds of measurement of multiple target points on the CNC axis of the machine tool. The key differences are:

1. Number of target points
2. Number of measurement rounds
3. Approach the target point from one way or two ways (this point is particularly important)
4. Calculation method of precision index and other indexes
This is a description of the key differences between the four standards. As expected, one day all machine tool manufacturers will follow the ISO standard. Therefore, the ISO standard is selected as the benchmark. The four standards are compared in the following table. In this paper, only linear accuracy is involved, because the calculation principle of rotation accuracy is basically consistent with it.

2、 Temperature influence on accuracy: thermal stability
Steel: 100 x 30 x 20 mm
Change of size when the temperature drops from 25 ℃ to 20 ℃: at 25 ℃, the size is larger by 6 μ m. When the temperature drops to 20 ℃, the size is only 0.12 larger μ m. This is a thermally stable process. Even if the temperature drops rapidly, it still needs a continuous time to maintain the accuracy. The larger the object, the more time it takes to restore the accuracy stability when the temperature changes.
The recommended values of temperature to be maintained for high-precision machining are shown in the table below. If high-precision machining is performed, it is very important not to take temperature changes lightly!