The influence of lathe geometric errors and work errors on machining accuracy makes the actual geometric parameters (size, shape, position) of the parts after processing 

not consistent with the ideal geometric parameters, affecting the processing quality. Therefore, in machining, errors are inevitable, but the errors must be within the allowable 

range. Through error analysis, the basic laws of its changes can be grasped, so that corresponding measures can be taken to reduce processing errors and improve processing

accuracy.

01. Classification of errors

①【Random error】

Random error is an error in which the absolute value and sign cannot change predeterminedly when the same quantity is measured multiple times under the same conditions.

There are many factors that produce random errors, and most of these factors are accidental and unstable.

②【System error】

Systematic error refers to the error in which the size and sign of the error remain unchanged or change according to certain rules when the same measurement value is 

measured multiple times under the same conditions. That is, the former case is a fixed value system error, and the latter case is a straightening system error.

③【Coarse error】

Gross errors are errors that exceed those expected under specified conditions.

03. The main reasons for errors in lathe processing

[1. Positioning error]

One is the datum mismatch error. The datum used to determine the size and position of a certain surface on the part drawing is called the design datum. The datum used to 

determine the size and position of the processed surface in this process on the process diagram is called the process datum. When processing a workpiece on a machine tool, 

several geometric elements on the workpiece must be selected as the positioning datum during processing. If the selected positioning datum does not coincide with the design datum, datum misalignment errors will occur.

The second is the manufacturing inaccuracy error of the positioning vice. The components on the fixture cannot be manufactured absolutely accurately according to the basic 

dimensions, and their actual size (or position) is allowed to vary within the tolerance range specified respectively. The workpiece positioning surface and the fixturpositioning 

element together form a positioning pair. Due to the inaccurate manufacturing of the positioning pair and the matching gap between the positioning pairs, the maximum 

position variation of the workpiece is caused, which is called the positioning pair manufacturing inaccuracy error.

[2. Geometric error of tool]

Any tool will inevitably wear during the cutting process, which will cause changes in the size and shape of the workpiece. Correct selection of tool materials and new 

wear-resistant tool materials, reasonable selection of tool geometric parameters and cutting dosage, correct use of coolant, etc. can minimize tool and dimensional wear. 

If necessary, a compensation device can be used to compensate for tool dimensional wear.

[3. Lathe spindle rotation error]

The spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at an instant. The main reasons for the radial 

rotation error of the spindle are: the coaxiality error of several sections of the spindle journal, the error of the bearing itself, the coaxiality error between the bearings, the 

spindle winding, etc. Appropriately improve the manufacturing accuracy of the spindle and box body, select high-precision bearings, improve the assembly accuracy of the 

spindle components, balance the high-speed spindle components, and preload the rolling bearings, etc. Both can improve the rotation accuracy of the machine tool spindle.

【4. Adjustment error】

In every process of machining, the process system must be adjusted in one way or another. Since the adjustment cannot be absolutely accurate, adjustment errors occur. 

In the process system, the mutual position accuracy of the workpiece and the tool on the machine tool is ensured by adjusting the machine tool, tool, fixture or workpiece.

[5. Transmission chain error]

The transmission error of the transmission chain refers to the relative motion error between the first and last two wheel transmission elements in the internal transmission 

chain. Transmission errors are errors caused by manufacturing and assembly errors of various components in the transmission chain, as well as wear and tear during use.

[6. Errors caused by thermal deformation of the process system]

Thermal deformation of the process system has a greater impact on machining accuracy. Especially in precision machining and large-piece processing, the machining error 

caused by thermal deformation can sometimes account for 50% of the total error of the workpiece. Machine tools, cutting tools and workpieces are affected by various heat 

sources, and the temperature will gradually increase. At the same time, they also dissipate heat to the surrounding materials and space through various heat transfer methods.

[7. Guide rail error]

The guide rail is the benchmark for determining the relative positional relationship of various machine tool components on the machine tool, and is also the benchmark for 

machine tool movement. The accuracy requirements of lathe guide rails mainly include the following three aspects: straightness in the horizontal plane; straightness in the 

vertical plane; and parallelism of the front and rear guide rails. In addition to the manufacturing errors of the guide rails themselves, uneven wear and installation quality of 

the guide rails are also causes of guide rail errors.

【8. Measurement error】

When parts are measured during or after processing, the accuracy is directly measured due to the measurement method, measuring tool accuracy, and workpiece and 

subjective and objective factors.

【9. Personnel error】

Personnel errors are errors caused by the measurement personnel's supervisory factors and the operator's technical level.

[10. Errors caused by force deformation of the process system]

One is the workpiece stiffness. In the process system, if the stiffness of the workpiece is relatively low compared to the machine tool, tool, and fixture, under the action of 

cutting force, the deformation of the workpiece caused by insufficient stiffness will have a greater impact on the machining accuracy.

The second is the stiffness of the tool. The rigidity of the cylindrical turning tool in the normal (y) direction of the processing surface is very large, and its deformation is 

negligible. When turning an inner hole with a small diameter, the stiffness of the tool holder is very poor, and the tool holder deforms greatly due to force, which has a 

great impact on the accuracy of the machined hole.

The third is the stiffness of machine tool components.

02. Preventive measures to reduce errors

1 Measurement method. Measurement method refers to the combination of measuring instruments and measurement conditions used in measurement. Before 

measurement, the best measurement method should be determined based on the characteristics of the object being measured.

2 Measurement accuracy. Measurement accuracy refers to the degree of consistency between the measurement results and the true value. The accuracy of the effective value 

of the measurement result is determined by the measurement accuracy.

3 Reduce the original error. Improving the geometric accuracy used in parts processing, improving the accuracy of measuring tools, fixtures and tools themselves, and 

controlling the force, thermal deformation, tool wear, deformation caused by internal stress, measurement errors of the process system are all direct reductions of the original 

error.

4 Transfer the original error. The essence of this method is to transfer the original error from the error-sensitive direction to the error-insensitive direction. Shift the original 

error to a non-sensitive direction.

5 Differentiate the original error. In order to improve the machining accuracy of a batch of parts, the method of dividing some original errors can be adopted. For the surfaces

of parts that require high machining accuracy, the method of gradually homogenizing the original errors during the continuous trial cutting process can also be adopted.

6 Average the original error. The process of continuously reducing and averaging the original errors of the processed surface through processing. The principle of 

homogenization is to find out the differences between closely related workpieces or tool surfaces through mutual comparison and inspection, and then perform mutual 

correction or benchmark processing.

7 Error compensation method. For some original errors in the process system, error compensation methods can be adopted to control their impact on part processing errors. 

This method artificially creates a new original error to compensate or offset the original error inherent in the original process system, thereby reducing processing errors and 

improving processing accuracy.

8 Error cancellation method. Use an original original error to remove part or all of the original original error or another original error.

In short, in lathe processing, errors are inevitable. As long as the causes of the errors are analyzed in detail, the tools, measuring tools, fixtures, measurement methods and 

related handles are adjusted to the best, and the processing errors are minimized, so as to improve the lathe. Processing accuracy, improve production efficiency, and process 

parts that meet drawing requirements.