Nine major errors in cnc machining; how many do you know?

This article analyzes the nine major errors of machining. Avoid these errors, and you can go further in CNC machining service.

Error-prone situations in CNC machining

CNC machining error refers to the degree of deviation between the actual geometric parameters (geometric size, geometric shape, and mutual position) of the part after machining and the ideal geometric parameters.

The degree of agreement between the actual and ideal geometric parameters after the part is machined the machining accuracy. The smaller the machining error, the higher the degree of conformity and the higher the machining accuracy.

CNC machining accuracy and machining error are two formulations of a problem. Therefore, the machining error's size reflects the machining accuracy level. The main reasons for machining errors are as follows:

1. Manufacturing errors of machine tools

The manufacturing error of the machine tool mainly includes the spindle rotation error, the guide rail error, and the transmission chain error.

Spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at each instant, which will directly affect the accuracy of the workpiece to be processed. The main reasons for the spindle rotation error are:

The coaxiality error of the spindle.

The error of the bearing itself.

The coaxiality error between the bearings.

The rotation of the spindle.

The guide rail is the benchmark for determining the relative positional relationship of each machine tool component on the machine tool and is also the benchmark for the machine tool movement.

The guide rail's manufacturing error, the guide rail's uneven wear, and the installation quality are the important factors that cause the guide rail error. Transmission chain error refers to the relative motion error between the transmission elements at the beginning and end of the transmission chain. It is caused by manufacturing and assembly errors of each component in the transmission chain and worn during use.

2. The geometric error of the tool

Any tool will inevitably wear during the cutting process, which will cause changes in the size and shape of the workpiece. The influence of geometric tool error on machining error varies with the type of tool: when a fixed-size tool is used for machining, the manufacturing error of the tool will directly affect the machining accuracy of the workpiece; for general tools (such as turning tools, etc.), its manufacturing error It has no direct effect on machining errors.

3. The geometric error of the fixture

The fixture's function is to make the workpiece equivalent to the tool and the machine tool to have the correct position, so the geometric error of the fixture greatly influences the machining error (especially the position error).

4. Positioning error

The positioning error mainly includes the reference misalignment error and the inaccurate manufacturing error of the positioning pair. When processing the workpiece on the machine tool, several geometric elements on the workpiece must be selected as the positioning datum during processing. datum) does not coincide, the datum misalignment error will occur.

The workpiece positioning surface and the fixture positioning element form the positioning pair. The maximum position variation of the workpiece caused by the inaccurate manufacturing of the positioning pair and the matching gap between the positioning pairs is called the manufacturing inaccuracy error of the positioning pair. The inaccurate manufacturing error of the positioning pair will only occur when the adjustment method is used for processing and will not occur in the trial cutting method.

5. Error caused by force deformation of process system

Workpiece stiffness: If the workpiece stiffness in the processing system is relatively low compared to machine tools, tools, and fixtures, under the action of cutting force, the deformation of the workpiece due to insufficient stiffness will have a greater impact on machining errors.

Tool rigidity: The rigidity of the cylindrical turning tool in the machined surface's normal (y) direction is very large, and its deformation can be ignored. When boring an inner hole with a small diameter, the rigidity of the toolbar is very poor, and the toolbar's force deformation greatly influences the hole's machining accuracy.

The rigidity of machine tool components: Machine tool components are composed of many parts. There is no suitable simple calculation method for the stiffness of machine tool components. Experimental methods mainly determine the stiffness of machine tool components. The factors affecting the rigidity of machine tool components include the influence of contact deformation of the joint surface, the influence of friction, the influence of low-rigidity parts, and the influence of clearance.

6. Errors caused by thermal deformation of the processing system

The processing system's thermal deformation greatly influences the machining error, especially in precision and large-scale machining; the machining error caused by thermal deformation can sometimes account for 50% of the total workpiece error.

7. Adjustment error

In each CNC machining process, there is always one way or another adjustment to the processing system. Since the adjustment cannot be accurate, an adjustment error occurs. In the processing system, the mutual positional accuracy of the workpiece and the tool on the machine tool is guaranteed by adjusting the machine tool, tool, fixture, or workpiece. When the original precision of machine tools, tools, fixtures, and workpiece blanks all meet the technical requirements without considering dynamic factors, the adjustment error plays a decisive role in the machining error.

8. Measurement error

When the part is measured during or after processing, the measurement accuracy is directly affected by the measurement method, the accuracy of the measuring tool and the workpiece, and subjective and objective factors.

9. Internal stress

The stress that exists inside the part without external force is called internal stress. Once internal stress is generated on the workpiece, the workpiece metal will be in an unstable state of high energy level. It will instinctively transform to a stable state of low energy level, accompanied by deformation, so the workpiece loses its original machining accuracy...

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