Common Mistakes in CNC Machining: Have You Faced These Before?

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 refers to the deviation between the actual geometric parameters (including dimensions, shapes, and relative positions) of a part after machining and its ideal geometric parameters. Machining accuracy, on the other hand, refers to how well the actual geometric parameters of a part match its ideal geometric parameters.

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

Main Causes of Machining Errors

1.Manufacturing Errors of Machine Tools

Manufacturing errors of machine tools include spindle rotation error, guide rail error, and transmission chain error. Spindle rotation error refers to the deviation of the actual spindle axis from the average axis at different time points, 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 the cutting process, wear inevitably occurs on any cutting tool, leading to changes in the dimensions and shapes of workpieces. The impact of cutting tool geometric errors on machining errors varies with the type of tool: for fixed-size tools, manufacturing errors directly affect the machining accuracy of workpieces; whereas for general tools (such as turning tools), manufacturing errors do not directly affect machining errors.

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 of the process system under load mainly stem from insufficient stiffness of workpieces, cutting tools, and machine tool components. If the workpiece has low stiffness, it may deform under cutting forces, affecting machining accuracy.

While the stiffness of tools like outer circle turning tools is usually sufficient, the stiffness of small-diameter boring bars can significantly affect hole machining accuracy. Machine tool component stiffness is influenced by factors such as contact deformation at mating surfaces, frictional forces, low stiffness parts, and clearances, typically 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 inevitable. Since adjustments cannot be perfectly precise, adjustment errors occur. The positional accuracy of workpieces and tools on the machine tool is ensured by adjusting the machine tool, tools, fixtures, or workpieces. When the original accuracies of the machine tool, tools, fixtures, and workpiece blanks meet the process requirements but dynamic factors are not considered, adjustment errors play a decisive role in machining errors.

8.Measurement Error

During or after machining, measurement errors occur due to measurement methods, measuring instrument accuracy, and both objective and subjective factors affecting measurement 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 understanding and identifying these nine errors—machine tool manufacturing errors, tool geometry errors, fixture geometry errors, positioning errors, process-induced deformation errors, thermal deformation errors, adjustment errors, measurement errors, and internal stress—we can take corresponding preventive and corrective measures to enhance machining accuracy and production efficiency.