In modern manufacturing, Computer Numerical Control (CNC) technology is widely utilized for its efficiency and precision. However, even with its advantages, various errors can still occur during CNC machining processes, affecting product quality and machining accuracy. Understanding these errors and implementing effective prevention and correction measures is crucial for enhancing production efficiency. This article will introduce nine common errors in CNC machining, helping you identify and address these challenges.

What is machining error?

Machining error is the difference between a part’s actual geometric parameters (such as dimensions, shape, and position) and its ideal design. Machining accuracy refers to how closely the actual parameters match the ideal ones.

The smaller the error, the higher the accuracy, indicating better precision. Therefore, machining error and accuracy are two ways of describing the same concept, with error size directly reflecting the level of accuracy.

Main Causes of Machining Errors

1.Manufacturing Errors of Machine Tools

Manufacturing errors in machine tools include spindle rotation errors, guide rail errors, and transmission chain errors. Spindle rotation error is the deviation of the actual spindle axis from the average axis at different times, directly affecting the accuracy of the machined workpiece.

Guide rails determine the positional relationship and motion reference of machine tool components. Errors in their manufacturing, wear, and installation quality can lead to guide rail errors. Transmission chain errors arise from manufacturing and assembly errors of transmission chain components, as well as wear during use.

2.Geometric Errors of Cutting Tools

During cutting, wear occurs on cutting tools, altering the dimensions and shapes of workpieces. The impact of tool geometric errors on machining accuracy depends on the tool type: for fixed-size tools, manufacturing errors directly affect machining accuracy; for general tools (e.g., turning tools), they do not have a direct impact.

3.Geometric Errors of Fixtures

Fixtures are used to position workpieces correctly relative to cutting tools and machine tools. Therefore, geometric errors in fixtures have a significant impact on machining errors (especially positional errors).

4.Positioning Errors

Positioning errors stem from discrepancies in reference points and elements during machine operations. Workpiece geometric features serve as key reference points; deviations from design specs result in non-coincident errors. Positioning elements, comprising workpiece and fixture components, may induce inaccuracies due to manufacturing defects or excessive clearances, leading to workpiece displacement.

5.Errors from Process System Deformation

Errors caused by deformation in the process system under load mainly result from insufficient stiffness in workpieces, cutting tools, and machine tool components. Low stiffness in the workpiece can cause deformation under cutting forces, affecting machining accuracy.

While tools like outer circle turning tools generally have sufficient stiffness, small-diameter boring bars can significantly impact hole machining accuracy. Machine tool component stiffness is affected by factors such as contact deformation, friction, low-stiffness parts, and clearances, often requiring experimental determination.

6.Errors Due to Thermal Deformation

Thermal deformation of the process system has a significant impact on machining errors, especially in precision and large-scale machining. Errors induced by thermal deformation can sometimes account for up to 50% of the total errors in workpieces.

7.Adjustment Error

In every machining operation, adjustments to the process system are necessary, leading to adjustment errors due to imperfect precision. The positional accuracy of workpieces and tools is maintained by adjusting the machine tool, tools, fixtures, or workpieces. When the original accuracies meet process requirements but dynamic factors are ignored, adjustment errors become a major contributor to machining errors.

8.Measurement Error

Measurement errors during or after machining arise from measurement methods, instrument accuracy, and both objective and subjective factors affecting precision.

9.Internal Stress

Stress that exists within a part without external forces is referred to as internal stress. Once internal stress forms in a workpiece, it places the metal in an unstable, high-energy state, naturally seeking to convert to a stable, low-energy state. This transformation often leads to deformation and loss of the original machining accuracy of the workpiece.

Conclusion

Errors in CNC machining are critical factors influencing processing accuracy and quality. By identifying these nine errors—machine tool manufacturing, tool geometry, fixture geometry, positioning, process-induced deformation, thermal deformation, adjustment, measurement, and internal stress—we can implement preventive and corrective measures to improve machining accuracy and production efficiency.