We deal with processing every day, and often mention processing accuracy. But when you talk about accuracy, are you really right? Let's take a look at those things about "processing accuracy" today!
The distinction between precision and precision
Accuracy indicates the correctness of the measurement result, precision indicates the repeatability and reproducibility of the measurement result, and precision is a prerequisite for accuracy. The figure below is a good illustration.
Refers to the degree of closeness between the obtained measurement result and the true value. High measurement accuracy means that the system error is small. At this time, the average value of the measured data deviates less from the true value, 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 of repeated measurements using the same kind of spare sample. It is possible that the precision is high, but the precision is not high. For example, the three results obtained by measuring with a length of 1 mm are 1.051 mm, 1.053, and 1.052, respectively. Although their precision is high, they are not accurate.
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Definition of machine tool accuracy
When you are comparing CNC machine tools, if the "positioning accuracy" of the A machine tool factory sample is marked as 0.002mm, and the "positioning accuracy" of the B machine tool factory sample is marked as 0.004mm. Through these two intuitive data, you will naturally think that the machine tool of machine tool plant A has higher precision than machine tool plant B.
However, in fact, it is very likely that the machine tools of Machine Tool Plant B have higher accuracy than Machine Tool Plant A. The problem lies in their accuracy definition standards. Therefore, when we talk about the "precision" 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 position the tool tip to the program target point. However, there are many ways to measure this positioning capability, and more importantly, different countries have different regulations.
European machine tool manufacturers, especially German manufacturers, generally adopt the VDI/DGQ3441 standard.
Japanese machine tool manufacturers:
When calibrating "precision", JISB6201 or JISB6336 or JISB6338 standards are usually used. JISB6201 is generally used for general-purpose machine tools and general CNC machine tools, JISB6336 is generally used for machining centers, and JISB6338 is generally used for vertical machining centers.
U.S. machine tool manufacturers:
The NMTBA standard is usually used (the standard is derived from a study by the American Machine Tool Manufacturers Association, promulgated in 1968 and later modified).
When calibrating the accuracy of a CNC machine tool, it is very necessary to mark out the standards it adopts. Using the Japanese JIS standard, its data is significantly smaller than the German VDI standard or the American NMTBA standard.
The same indicator, different meaning
It is often easy to confuse: the same index name represents different meanings in different accuracy standards, but different index names have the same meaning. The above four standards, except for the 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) The number of target points
2) Measure the number of rounds
3) Approach the target point from one-way or two-way (this point is particularly important)
4) Calculation method of accuracy index and other indexes
This is a description of the key differences between the four standards. As people expect, one day, all machine tool manufacturers will conform to the ISO standard. Therefore, the ISO standard is chosen as the benchmark here. The four standards are compared in the following table. This article only deals with linear accuracy, because the calculation principle of rotation accuracy is basically the same.
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Thermal stability (the influence of temperature on accuracy)
Steel: 100 x 30 x 20 mm
The size change when the temperature drops from 25℃ to 20℃: At 25℃, the size is 6μm larger, when the temperature drops to 20℃, the size is only 0.12μm larger. This is a thermally stable process, even if the temperature drops rapidly, It still takes a sustained period of time to maintain accuracy. The larger the object, the more time it takes to restore the accuracy and stability when the temperature changes.
For high-precision machining, the temperature problem must not be ignored, because the temperature difference is the enemy of accuracy. Specifically, materials will expand with heat and contract with cold. The steel we use expands linearly to a length of 12μm per meter when the temperature changes by 1°C. This is the fact that every machine in every corner of the world is unchanged.
In factories without precision machining experience, when doing precision machining, they often attribute the instability of accuracy to the accuracy of the equipment. For factories with precision machining experience, they all know that this is the most basic common sense, and they will attach great importance to the environmental temperature and the heat balance of the machine tool. They are very clear that even high-precision machine tools can only obtain stable machining accuracy under a stable temperature environment and thermal equilibrium.
