Critical Analysis of Preheating Effects on CNC Machining Tolerances

Individuals working in the processing industry often exhibit a strong determination not to concede defeat when it comes to achieving precision, treating μm-level processing accuracy as if it were easily attainable. However, high-precision processing is a demanding technical field that requires a deep understanding of various factors. Many people are unaware of the impact temperature has on precision, making it unreasonable to discuss precision without this knowledge. In this article, we will provide a more comprehensive overview of this topic.

 

PART 1 Basic common sense: the impact of temperature changes on materials

As we know, materials generally exhibit characteristics of thermal expansion and contraction. During precision processing, the issue of temperature should not be overlooked. Temperature differences can be considered the “enemy” of precision. If we fail to take this crucial factor seriously, how can we achieve precision? After all, most machines are made of steel and cast iron, which can change shape and length due to both ambient temperature and the heat generated by the machine itself.

The specific amount of deformation a material undergoes due to thermal expansion and contraction depends on its inherent properties and the range of temperature change. Below is a table showing the expansion coefficients of steel and copper. For example, steel has a linear expansion rate of 12 micrometers per meter of length for every 1°C change in temperature.
The expansion coefficient of steel is shown in the figure below:

 

For example: if the workpiece length is 200mm and the temperature changes by 10℃, its expansion value is 0.02mm.

The expansion coefficient of copper is shown in the figure below:

 

For example: When the electrode length is 200mm and the temperature changes by 10°C, the expansion value is 0.05mm.

 

PART 2 Temperature causes detection errors

If the workpiece and the instruments or gauges used for detection are made of different materials, and the detection is not performed at the standard temperature of 20°C, the deviation from this standard temperature can significantly contribute to detection errors.

 

For example, when conducting inspections, if a 100mm long steel gauge is heated by the warmth of a person’s hand, its temperature may rise by 4°C, resulting in a length change of 4.6μm.

Moreover, when measuring high-precision CNC machinery parts, it is essential to use measurement methods that are equally precise. If the accuracy of the measuring instrument or equipment is not high, achieving high-precision measurements becomes impossible.

 

PART 3 Important processing concept: maintaining thermal stability

Consider a steel part measuring 100x30x20 mm. When the temperature decreases from 25°C to 20°C, its size changes: at 25°C, the size is 6 μm larger, while at 20°C, the size is only 0.12 μm larger. This phenomenon is part of the thermal stabilization process. Even when the temperature drops rapidly, it takes time for the part to maintain stable accuracy. Generally, larger objects require a longer duration to achieve stable accuracy after a temperature change.

 

Some factories that lack experience in precision machining often blame equipment accuracy for issues with unstable precision. However, experienced factories recognize the importance of maintaining a stable thermal environment for both the ambient temperature and the machine tools. They understand that even if a machine tool is highly precise, consistent machining precision can only be ensured in a stable temperature environment and under balanced thermal conditions.

 

Maintaining thermal stability is a crucial concept in precision machining that requires a thorough understanding. Some individuals may be uncertain about whether the ideal temperature should be set at 20°C or 23°C. The key factor, however, is to ensure the stability of the target temperature value. Ideally, the temperature is typically set at 20°C. However, in practical workshop environments, the temperature is often controlled between 22°C and 23°C, provided that fluctuations are strictly monitored.

 

PART 4 ​​Correctly understand machining accuracy and analysis

Machining accuracy is typically categorized into two aspects: precision and accuracy. The following figure provides a clearer understanding of this distinction.

 

Precision
Precision refers to the reproducibility and consistency of results obtained when the same sample is measured multiple times. It’s important to note that high precision does not necessarily indicate high accuracy. For example, if three measurements of a standard length of 1 mm yield results of 1.051 mm, 1.053 mm, and 1.052 mm, the precision of these measurements is high; however, they are not accurate.

 

Accuracy
Accuracy refers to how close a measurement result is to the true value. When a measurement has high accuracy, it indicates that the system error is small, meaning the average value of the measured data closely aligns with the true value. However, this assessment does not clarify the variability in the data, specifically the extent of random errors.

 

Relationship between precision, accuracy and temperature
Precision and accuracy are typically closely related to temperature. If the precision of the processed parts is high but accuracy is insufficient, it may indicate that temperature fluctuations in the workshop are minimal, but the deviation from the standard temperature is significant. Conversely, if the parts are highly accurate but lack precision, it is likely that temperature fluctuations in the workshop are large, leading to considerable variability in accuracy. Lastly, if the parts are neither precise nor accurate, it suggests that the workshop’s temperature significantly deviates from standard temperature and control requirements.

 

PART 5 Forgotten Machine Preheating

When using precision CNC machine tools for precision metal machining in factories, you may have encountered a common issue: the first product machined each morning often lacks satisfactory accuracy. Similarly, when machining begins after a long break, the accuracy of the first batch of parts can be unstable, increasing the likelihood of errors, especially regarding positional accuracy.

The machine tool can only ensure stable machining accuracy in a controlled temperature environment and a thermal equilibrium state. Therefore, preheating the machine tool is a fundamental practice for precision machining. The machining accuracy can vary significantly after prolonged periods of inactivity. This variation occurs because the temperature of the spindle and each moving axis stabilizes at a certain level once the machine has been running for a period. As machining time increases, the thermal accuracy of the CNC machine tool eventually stabilizes, highlighting the importance of preheating the spindle and moving parts before starting work.

Unfortunately, many factories overlook the “warm-up” phase for machine tools, either ignoring it or being completely unaware of its necessity. If a machine has been idle for more than a few days, it is advisable to preheat it for over 30 minutes before beginning high-precision machining. If the machine has only been idle for a few hours, a preheating period of 5 to 10 minutes will suffice. During preheating, the machine tool should be engaged in repetitive movements along the machining axis. Ideally, multi-axis movement should be employed, such as moving the XYZ axis diagonally from the lower left to the upper right corner of the coordinate system several times.

In practice, you can write a macro program for the machine tool to automate the preheating process by performing these repeated movements. Once the machine tool is fully preheated, it can be put into high-precision machining production, ensuring stable and consistent machining accuracy.

 

 

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