How to check the accuracy of CNC machine tools
Checking the precision of CNC machine tools is not a matter of conducting a “comprehensive inspection” and then stopping. Instead, it requires first clarifying the inspection approach and then making targeted judgments based on different precision types.
In practical applications, the accuracy inspection of CNC machine tools can usually be approached from four core dimensions:
1. Geometric accuracy inspection
Geometric accuracy reflects the structural accuracy of a machine tool when it is stationary or at low speed, and it is the foundation of all machining accuracy.
The focus is on the straightness, parallelism, and perpendicularity between the axes of the machine tool, as well as the assembly relationship of key components such as the spindle and worktable.
If there are problems with geometric accuracy, even the most advanced CNC system will struggle to produce stable machining results.
2. Positioning accuracy and repeatability check
Positioning accuracy focuses on the deviation between the actual moving position of the machine tool and the commanded position, while repeatability accuracy reflects the consistency of the machine tool in multiple reciprocating movements.
This level of precision directly affects the dimensional accuracy of the parts and is a key indicator that must be controlled in precision machining.
3. Volume accuracy inspection
Volumetric accuracy is a comprehensive evaluation of the overall motion error of a machine tool in three-dimensional space. It is not an error of a single axis or a single direction, but the result of the superposition of multiple errors.
In large machine tools or high-precision, multi-axis machining scenarios, volumetric accuracy is often more valuable for reference than single-axis accuracy.
4. Dynamic accuracy check
Dynamic accuracy focuses on the performance of a machine tool under actual cutting conditions, including stability during feed, acceleration, deceleration, and load changes.
Even if the static test results are qualified, if the machine tool vibrates, has tracking errors, or has contour distortion during processing, it indicates that there are potential problems with dynamic accuracy.
Common testing methods for CNC machine tools
In practical applications, the accuracy inspection of CNC machine tools is not accomplished using only one method, but rather by combining corresponding inspection methods according to different accuracy types. The following are some of the most common and practical inspection approaches used in the field.
1. Geometric accuracy testing methods
Geometric accuracy inspection is mainly used to determine whether the machine tool structure is in the correct state, and is usually performed under non-cutting or low-load conditions.
By inspecting the straightness, parallelism, and perpendicularity of each moving axis, guide rail installation and assembly errors or long-term wear problems can be quickly identified. This type of inspection is a fundamental step in all precision checks.
2. Positioning accuracy testing methods
Positioning accuracy testing focuses on the accuracy of machine tool movement to a specified position.
By comparing the difference between the commanded displacement and the actual displacement, it is possible to determine whether the CNC system, transmission mechanism and compensation parameters are normal, which is suitable for machining scenarios with clear dimensional requirements.
3. Repeatability accuracy testing method
Repeatability testing is used to evaluate the consistency of a machine tool during multiple reciprocating motions.
Even if the positioning error is small, if the repositioning is unstable, it will lead to significant dimensional fluctuations in batch processing. Therefore, this step is usually used in conjunction with positioning accuracy testing.
4. Trial cutting and processing verification methods
Trial cutting test is a method to verify the overall performance of a machine tool by actually machining a test piece.
This method can intuitively reflect the dimensional accuracy, contour accuracy, and surface quality of a machine tool under real cutting conditions, and is an important basis for judging dynamic accuracy and comprehensive machining capabilities.
5. Comprehensive Accuracy Evaluation Method
In high-precision or multi-axis machining scenarios, a single test result is often insufficient to fully reflect the machine tool’s condition.
By comprehensively analyzing geometric accuracy, positioning accuracy, and trial cutting results, it is possible to more accurately determine whether the machine tool meets the current processing requirements.
CNC machine tool precision testing tools
The tools used for CNC machine tool precision testing are essentially designed to meet different precision requirements.
Choosing the right testing tools is more important than blindly pursuing high-precision equipment. The following section categorizes and explains common tools according to their usage scenarios.
1. Basic testing tools
These tools are mainly used for routine inspections and preliminary assessments. They are simple to operate and suitable for quickly confirming the machine tool’s condition on-site. They are commonly used to check geometric accuracy, spindle condition, and table surface condition, and can promptly detect obvious deviations or abnormalities.
2. Medium-precision testing tools
Medium-precision tools are typically used to check positioning accuracy and repeatability, making them suitable for machining scenarios where dimensional consistency is critical. They strike a balance between inspection efficiency and accuracy, and are the primary means of routine precision inspection in most factories.
3. High-precision testing equipment
High-precision testing equipment is mainly used in precision machining or high-standard projects, and can comprehensively reflect the motion and spatial errors of machine tools. These tools are typically used for acceptance testing of critical equipment, accuracy assessment, or confirmation before high-end machining tasks.
4. Process verification tools
Machining verification tools do not directly measure machine tool motion, but rather reflect machine tool accuracy by detecting machining results. They are closer to actual production needs and are often used to verify whether a machine tool meets machining requirements under real cutting conditions.
Reasonable frequency of CNC machine tool accuracy testing
Precision inspection of CNC machine tools is not something that should be done as frequently as possible. Instead, a reasonable inspection cycle should be determined based on the equipment’s condition, machining requirements, and usage intensity. Indiscriminate inspection or prolonged neglect of inspection will both affect machining stability.
1. Testing of new equipment or equipment after installation
After the new machine tool is installed and debugged, a systematic precision check should be performed. The main purpose of this step is to confirm whether the machine tool meets the factory or acceptance standards and to establish benchmark data for subsequent normal production.
2. Periodic inspections under normal production conditions
Under stable production conditions, it is recommended to check the machine tool accuracy at fixed intervals. Periodic inspections can help detect trends in accuracy changes in a timely manner, preventing problems from accumulating and affecting the quality of batch processing.
3. Specialized testing before the commencement of high-precision or critical projects.
When machine tools are used for precision parts or demanding projects, a targeted accuracy check should be performed before machining. This check aims to “confirm availability,” focusing on ensuring that the machine tool’s current condition meets project requirements.
4. Inspection after repair, adjustment, or abnormal conditions.
After a machine tool has undergone component replacement, structural adjustment, or exhibited abnormal machining results, a precision test should be performed again. This step helps determine whether the problem has been effectively resolved and avoids operating the machine while it is malfunctioning.
At last
Precision inspection of CNC machine tools is not a one-time task, but a crucial step throughout the entire lifespan of the equipment. By clearly defining the type of precision required, selecting appropriate testing methods and tools, and establishing a reasonable inspection frequency, it is essential to ensure the machine tool maintains stable and reliable performance during actual machining. Continuous attention to precision not only affects machining quality but also directly impacts production efficiency and product consistency.