The primary cause is errors in CNC precision machining.

Machining accuracy in CNC refers to the degree of consistency between the actual geometric parameters (dimensions, shapes, and positions) of a machined part and the ideal geometric parameters. Errors are inevitable but must be kept within permissible limits. Through error analysis, we can grasp the fundamental laws of variation, take corresponding measures to reduce machining errors, and improve machining accuracy. The causes of errors can be broadly summarized as follows:

1. Spindle rotation errors
Spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at any given moment. The main causes of radial spindle rotation errors include: coaxiality errors of the spindle journal sections, various errors in the bearings themselves, coaxiality errors between bearings, spindle deflection, etc.

2. Guideway errors
Guideways serve as the reference for determining the relative positional relationships among various machine tool components and also as the motion reference for the machine tool. Uneven wear of the guideways and poor installation quality are also significant factors contributing to guideway errors.

3. Transmission chain errors
Transmission errors in the drive chain refer to the relative motion errors between the transmission elements at the head and tail ends of the chain. These errors result from manufacturing and assembly inaccuracies and wear of the components in the transmission chain.

4. Tool geometric errors
During any cutting process, tool wear is inevitable, which leads to changes in the workpiece's dimensions and shape.

5. Positioning errors

• Inconsistent datum errors: The datum used to determine the size and position of a surface is called the design datum. The reference used to determine the size and position of the machined surface is called the process reference. When machining a workpiece, geometric elements on the workpiece must be selected as positioning datums during the process. If the selected positioning datum does not match the design datum, a datum misalignment error occurs.

• Inaccuracy of positioning pair manufacturing errors: Errors in the manufacture of the positioning elements themselves also contribute.

6. Errors caused by force and deformation in the process system

• Workpiece stiffness: In the process system, if the workpiece stiffness is lower than that of the machine tool, cutting tool, and fixture, the workpiece deformation due to insufficient stiffness under cutting forces will significantly affect machining accuracy.

• Tool stiffness: For external turning tools, the stiffness in the normal direction to the machined surface is high, so deformation can be neglected. For boring small-diameter holes, the boring bar has poor stiffness, and the stress and deformation of the bar greatly affect hole machining accuracy.

• Machine tool component stiffness: Machine tool components are assembled from many parts. To date, there is no simple and suitable calculation method for component stiffness; it is mainly determined through experimental methods.

7. Errors caused by thermal deformation of the process system
Thermal deformation of the process system has a substantial effect on machining accuracy, especially in precision machining and large-part processing. Machining errors caused by thermal deformation can sometimes account for up to 50% of the total workpiece error.

8. Adjustment errors
The process system must be adjusted in one way or another. Since adjustment cannot be perfectly accurate, adjustment errors inevitably occur. In the process system, the mutual positional accuracy between the workpiece and the tool on the machine tool is ensured by adjusting the machine tool, tool, fixture, or workpiece. When the initial accuracy of the machine tool, tool, fixture, and workpiece blank meets the process requirements, the influence of adjustment errors will play a decisive role in machining accuracy.

9. Measurement errors
When measuring parts during or after machining, the measurement accuracy is directly affected by various factors, leading to measurement errors.