Nine Major Causes of Mechanical Machining Errors

Machining error refers to the degree of deviation between the actual geometric parameters (geometric dimensions, shapes, and mutual positions) of a part after machining and the ideal geometric parameters. The degree to which the actual geometric parameters of a part after machining conform to the ideal geometric parameters is known as machining accuracy.

The smaller the machining error, the higher the degree of conformity, and the higher the machining accuracy. Machining accuracy and machining error are two ways of expressing the same issue. Therefore, the magnitude of machining error reflects the level of machining accuracy.

Main causes of mechanical machining errors:

1.Manufacturing errors of machine tools

Manufacturing errors of machine tools mainly include errors in the spindle rotation, guide rail errors, and transmission chain errors. Spindle rotation error refers to the variation in the actual rotation axis of the spindle at each moment relative to its average rotation axis, which directly affects the accuracy of the workpiece being machined. The main causes of spindle rotation error include the concentricity error of the spindle, the error of the bearing itself, the concentricity error between bearings, and the deflection of the spindle. The guide rail is the benchmark for determining the relative position of various machine tool components and also the benchmark for machine tool movement. The manufacturing error of the guide rail itself, uneven wear of the guide rail, and the quality of installation are important factors causing guide rail errors. Transmission chain error refers to the error in relative motion between the transmission elements at the beginning and end of the transmission chain. It is caused by the manufacturing and assembly errors of the various components in the transmission chain, as well as wear during use.

2.Geometric errors of cutting tools

Any cutting tool will inevitably experience wear during the cutting process, leading to changes in the size and shape of the workpiece. The impact of cutting tool geometric errors on machining error varies with the type of tool: for fixed-dimension cutting tools, the manufacturing error of the tool directly affects the machining accuracy of the workpiece; for general tools (such as lathe tools), their manufacturing error has no direct impact on the machining error.

3.Geometric errors of fixtures

The role of the fixture is to ensure the correct position of the workpiece relative to the cutting tool and machine tool, so the geometric error of the fixture has a significant impact on machining error (especially positional error).

4.Positioning error

Positioning error mainly includes non-coincidence error of benchmarks and inaccuracy error of the manufacturing of positioning pairs. When machining a workpiece on a machine tool, several geometric elements on the workpiece must be selected as the positioning benchmarks for machining. If the chosen positioning benchmarks do not coincide with the design benchmarks (used in the part drawings to determine the size and position of a surface), a non-coincidence error of benchmarks will occur. The positioning surface of the workpiece and the positioning elements of the fixture together form the positioning pair. The maximum positional variation of the workpiece caused by the inaccuracy of the positioning pair manufacturing and the fit clearance between the positioning pairs is called the inaccuracy error of the manufacturing of the positioning pair. This error only occurs during machining with the adjustment method and does not occur during the trial cutting method.

5.Error caused by deformation of the process system due to force

Workpiece stiffness: If the stiffness of the workpiece is relatively low compared to the machine tool, cutting tool, and fixture in the process system, the deformation of the workpiece due to insufficient stiffness under the action of cutting force will have a significant impact on machining error. Tool stiffness: The external cylindrical lathe tool has high stiffness in the normal (y) direction of the machining surface, and its deformation can be neglected. When boring smaller internal holes, the stiffness of the tool holder is poor, and the deformation of the tool holder under force greatly affects the accuracy of the hole machining. Machine tool component stiffness: Machine tool components are composed of many parts; there is no suitable simple calculation method for machine tool component stiffness, and it is currently mainly determined by experimental methods. Factors affecting machine tool component stiffness include the impact of contact deformation of joints, the influence of frictional forces, the impact of low stiffness parts, and the influence of gaps.

6.Error caused by thermal deformation of the process system

Thermal deformation of the process system has a significant impact on machining error, especially in precision machining and large part machining, where the machining error caused by thermal deformation can sometimes account for up to 50% of the total error of the workpiece.

7.Adjustment error

In each process of mechanical machining, some sort of adjustment work must be carried out on the process system. Since adjustment cannot be absolutely accurate, adjustment error occurs. In the process system, the mutual position accuracy of the workpiece and cutting tool on the machine tool is ensured by adjusting the machine tool, cutting tool, fixture, or workpiece, etc. When the original accuracy of the machine tool, cutting tool, fixture, and workpiece blank meets the process requirements and dynamic factors are not considered, adjustment error plays a decisive role in machining error.

8.Measurement error

When measuring parts during or after machining, the measurement method, the accuracy of the measuring tools, as well as the workpiece and subjective and objective factors all directly affect the measurement accuracy.

9.Internal stress

Stress that exists within a part without the action of external forces is called internal stress. Once internal stress occurs in a workpiece, it will cause the metal of the workpiece to be in an unstable state of high energy level. It inherently tends to transform towards a stable state of low energy level, accompanied by deformation, thereby causing the workpiece to lose its original machining precision.