Machining error refers to degree of deviation between actual geometric parameters (geometric size, geometric shape and mutual position) of part after machining and ideal geometric parameters.
Degree of agreement between actual geometric parameters and ideal geometric parameters after part is machined is machining accuracy. The smaller machining error, the higher degree of conformity and the higher machining accuracy.
Machining accuracy and machining error are two formulations of a problem. Therefore, size of machining error reflects level of machining accuracy. Main reasons for machining errors are as follows:

Manufacturing error of machine tool mainly includes spindle rotation error, guide rail error and transmission chain error.
Spindle rotation error refers to variation of actual rotation axis of spindle relative to its average rotation axis at each instant, which will directly affect accuracy of workpiece to be processed. Main reasons for spindle rotation error are coaxiality error of spindle, error of bearing itself, coaxiality error between bearings, and rotation of spindle. Guide rail is benchmark for determining relative positional relationship of each machine tool component on machine tool, and it is also benchmark for machine tool movement.
Manufacturing error of guide rail itself, uneven wear of guide rail and installation quality are important factors that cause guide rail error. Transmission chain error refers to relative motion error between transmission elements at beginning and end of transmission chain. It is caused by manufacturing and assembly errors of each component in transmission chain, as well as wear during use.

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

Function of fixture is to make workpiece equivalent to tool and machine tool to have correct position, so geometric error of fixture has a great influence on machining error (especially position error).

Positioning error mainly includes reference misalignment error and inaccurate manufacturing error of positioning pair. When processing workpiece on machine tool, several geometric elements on workpiece must be selected as positioning datum during processing. datum) does not coincide, datum misalignment error will occur.
Workpiece positioning surface and fixture positioning element together form positioning pair. Maximum position variation of workpiece caused by inaccurate manufacturing of positioning pair and matching gap between positioning pairs is called manufacturing inaccuracy error of positioning pair. Inaccurate manufacturing error of positioning pair will only occur when adjustment method is used for processing, and will not occur in trial cutting method.

 

Workpiece stiffness: If workpiece stiffness in process system is relatively low compared to machine tools, tools, and fixtures, under action of cutting force, deformation of workpiece due to insufficient stiffness will have a greater impact on machining errors.
Tool rigidity: Rigidity of cylindrical turning tool in normal (y) direction of machined surface is very large, and its deformation can be ignored. When boring an inner hole with a small diameter, rigidity of tool bar is very poor, and force deformation of tool bar has a great influence on machining accuracy of hole.
Rigidity of machine tool parts: Machine tool parts are composed of many parts, and there is no suitable simple calculation method for rigidity of machine tool parts so far. At present, experimental methods are mainly used to determine rigidity of machine tool parts. Factors affecting rigidity of machine tool components include influence of contact deformation of joint surface, influence of friction, influence of low-rigidity parts, and influence of clearance.

Thermal deformation of process system has a great influence on machining error, especially in precision machining and large-scale machining, machining error caused by thermal deformation can sometimes account for 50% of the total workpiece error.

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

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

Stress that exists inside part without external force is called internal stress. Once internal stress is generated on workpiece, 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 that workpiece loses its original machining accuracy. .