We deal with machining every day, and we often mention machining accuracy. But, when you say precision, are you really right? Let’s take a look at the “machining accuracy” today!

01 The distinction between accuracy and precision

Accuracy means the correctness of the measurement results, precision means the repeatability and reproducibility of the measurement results, and precision is the prerequisite for accuracy.

Accuracy

Refers to the degree of closeness between the obtained measurement results and the true value. The high measurement accuracy means that the systematic error is small. At this time, the average value of the measurement 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 measurements using the same spare sample. It is possible to have high precision, but the precision is not high. For example, the three results obtained by using a length of 1mm for measurement are 1.051mm, 1.053, and 1.052 respectively. Although they have high precision, they are not accurate.

02 Definition of machine tool accuracy

When you compare CNC machine tools, if the “positioning accuracy” of the sample of A machine tool factory is marked as 0.002mm, and the “positioning accuracy” of the sample of B machine tool factory is marked as 0.004mm. Through these two intuitive data, you will naturally think that the machine tools of A machine tool factory are more accurate than B machine tool factory.

However, in fact, it is very likely that the machine tools of B machine tool factory are more accurate than A machine tool factory. The problem lies in the standard of their precision definition. Therefore, when we talk about the “accuracy” of CNC machine tools, we must clarify the definitions and calculation methods of standards and indicators.

Generally speaking, accuracy refers to the ability of the machine tool to locate the tool nose point to the program target point. However, there are many ways to measure this positioning ability, and more importantly, different countries have different regulations.

European machine tool manufacturers:

European machine tool manufacturers, especially German manufacturers, generally adopt the VDI/DGQ3441 standard.

Japanese machine tool manufacturers:

When calibrating “accuracy”, JISB6201 or JISB6336 or JISB6338 standards are usually used. JISB6201 is generally used for general-purpose machine tools and ordinary CNC machine tools, JISB6336 is generally used for machining centers, and JISB6338 is generally used for vertical machining centers.

American machine tool manufacturers:

The NMTBA standard is usually adopted (the standard originated from a study of the American Machine Tool Builders Association, promulgated in 1968 and later revised).

When calibrating the accuracy of a CNC machine tool, it is very necessary to mark the standard it uses. Using the Japanese JIS standard, the data is significantly smaller than the German VDI standard or the American NMTBA standard.

Same indicator, different meaning. What is often confusing is that the same indicator name has different meanings in different precision standards, but different indicator names have the same meaning. The above four standards, except the JIS standard, are all calculated by 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) Measure the number of rounds
3) Approaching the target point from one-way or two-way (this point is especially important)
4) Calculation method of accuracy index and other indexes

This is a description of the key points of difference between the 4 standards, and as one would expect, one day all machine tool manufacturers will follow the ISO standard uniformly. Therefore, the ISO standard is chosen as the benchmark here. Another: This article only involves linear accuracy, because the calculation principle of rotational accuracy is basically the same.

03 Thermal stability (the influence of temperature on accuracy)

Suppose there is a steel part with the following dimensions: 100 x 30 x 20 mm

The size change when the temperature drops from 25°C to 20°C: at 25°C, the size is 6 μm larger, and when the temperature drops to 20°C, the size is only 0.12 μm larger. This is a thermally stable process, even if the temperature drops rapidly, It still takes a sustained time to maintain accuracy. The larger the object, the more time it takes to stabilize the accuracy when the temperature changes.

For high-precision machining, the temperature problem must not be ignored, because the temperature difference is the enemy of precision. Specifically, materials will expand with heat and contract with cold. The linear expansion of the steel we use will cause a change of 12 μm per meter of length when the temperature changes by 1°C. This is a fact that is constant for every machine in every corner of the world.

Factories without experience in precision machining often attribute the instability of precision to equipment precision problems when doing precision machining. For factories with experience in precision machining, they all know this is the most basic common sense, and they will attach great importance to the thermal balance of the ambient temperature and the machine tool. They are very clear that even high-precision machine tools can only obtain stable machining accuracy in a stable temperature environment and thermal equilibrium state.

Hope the above content can help you!