People who are engaged in machining are unwilling to admit defeat when it comes to precision. Sometimes, some people seem to regard 1 micron processing accuracy as a piece of cake when they talk about it. However, in fact, high-precision machining is a technical topic that needs to be treated rigorously. This article aims to provide everyone with a more comprehensive knowledge of high-precision machining.
01
Basic common sense: the effect of temperature changes on materials
As we all know, materials are affected by thermal expansion and contraction. In precision machining, temperature issues must not be ignored! Temperature difference is the nemesis of accuracy. If we don't pay attention to the key issue of temperature, how can we discuss accuracy in depth? Since most machines are made of steel and cast iron, they change shape and length under the influence of room temperature and the heat generated by the machine itself.
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The degree of thermal expansion and contraction of a material depends on the type of material and the magnitude of the temperature change. The following provides a table of expansion coefficients of steel and copper. Taking steel as an example, its linear expansion will produce a change of 12μm per meter when the temperature changes by 1°C. A deep understanding of this data is critical to ensuring the stability of precision machining.
The expansion coefficient of steel is shown in the figure below:
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Example:
Work piece length: 200 mm
Temperature change: 10℃
Expansion value: 0. 02 mm
The expansion coefficient of copper is shown in the figure below:
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Example:
Electrode length: 200 mm
Temperature change: 10℃
Expansion value: 0.05 mm
02
Detection error caused by temperature
When workpieces, inspection instruments and gauges are made of different materials and are not under standard temperature conditions during inspection, deviations from the standard temperature (20°C) will always be a key factor leading to inspection errors.
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Detection error due to temperature
For example, heating a 100mm long steel block by 4°C, such as the temperature of the palm of your hand, will cause its length to change by 4.6μm.
It is worth noting that when measuring high-precision parts, it is necessary to have higher-precision measuring tools. If the accuracy standard of the measuring instrument or equipment itself is not high, where do high-precision measurement results come from?
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03
Important processing concept: maintaining thermal stability
Steel: 100 x 30 x 20 mm
Changes in size when the temperature drops from 25℃ to 20℃: At 25℃, the size is larger by 6μm. When the temperature drops to 20℃, the size is only larger by 0.12μm. This is a thermally stable process, even if the temperature drops rapidly, A sustained period of time is still required to maintain accuracy. Larger objects require more time to regain accuracy and stability when the temperature changes.
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For factories without precision machining experience, unstable precision is often blamed on the accuracy of the equipment when performing precision machining. On the contrary, factories with precision machining experience know that this is the most basic understanding. They understand that the thermal balance between ambient temperature and machine tools is critical to maintaining stable machining accuracy. These experienced factories clearly understand that even with high-precision machine tools, stable processing accuracy can only be achieved by maintaining a stable temperature environment and thermal balance.
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Maintaining thermal stability is an indispensable and important concept in precision machining. Some people may have doubts about whether the temperature should be maintained at 20°C or 23°C. However, the most critical thing is to ensure that the stability of a target value can be maintained. Although theoretical books usually recommend 20°C, actual workshops often choose between 22-23°C. The focus is on strictly controlling temperature fluctuations.
04
Correct understanding of machining accuracy and analysis
Generally speaking, machining accuracy can be divided into precision and precision. The picture below is a visual illustration.
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Precision
It refers to the reproducibility and consistency between the results obtained by repeated measurements using the same spare sample. It's possible to have high precision, but that doesn't mean the results are accurate. For example, the three results obtained by using a length of 1mm are 1.051mm, 1.053, and 1.052. Although they have high precision, they are inaccurate.
Accuracy
Refers to the closeness between the obtained measurement results and the true value. High measurement accuracy means that the system error is small, when the average value of the measured data deviates less from the true value, but when the data is scattered, that is, the size of the accidental error is unclear.
Relationship between precision, accuracy and temperature
Generally speaking, if the machined parts are more precise but not accurate, it may be because the workshop temperature fluctuates within a small range, but there is a large deviation from the standard temperature. Therefore, the size of the obtained parts is relatively consistent, but there is a large deviation from the target size. On the contrary, if the parts are more accurate but not precise, it may be because the workshop temperature fluctuates significantly relative to the standard temperature, causing the part size to appear discrete. distribution; and if the part is neither precise nor accurate, this may indicate that the shop temperature deviates widely from the standard temperature and fluctuates widely.
05
Forgotten machine tool warm-up
Factories use precision CNC machine tools for high-precision machining. Have you ever had this experience: when the machine is turned on every morning for processing, the machining accuracy of the first piece is often difficult to reach the ideal level; when the machine is turned on after a long holiday to process the first batch of parts, the accuracy is often poor. The risk of failure is particularly prominent during stable, high-precision machining, especially when it comes to maintaining positional accuracy.
Only in a stable temperature environment and thermal balance can machine tools ensure stable processing accuracy. For situations where high-precision machining and production are required immediately after starting up, preheating the machine tool is the most basic precision machining common sense.
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Because the temperature of the spindle and each motion axis of the CNC machine tool will be relatively maintained at a certain fixed level after running for a period of time. At the same time, as the processing time goes by, the thermal accuracy of CNC machine tools gradually becomes stable. Therefore, it is very necessary to preheat the spindle and moving parts before performing high-precision machining.
However, many factories often ignore or do not understand the preparation link of "warm-up exercise" of machine tools. It is recommended that when the machine tool is idle for more than several days, it is recommended to preheat for more than 30 minutes before high-precision machining; if the machine tool is idle for only a few hours, it is also recommended to preheat for 5-10 minutes before high-precision machining.
The preheating process involves the machine tool participating in the repeated movement of the processing axis. It is best to carry out multi-axis linkage. For example, let the XYZ axis move from the lower left corner to the upper right corner of the coordinate system, and repeatedly move diagonally. This process can be achieved by writing a macro program on the machine tool.
After the machine tool has been fully preheated, the machine tool can be put into high-precision processing with full vigor, and you will obtain stable and consistent processing accuracy.
