In machining, error is a common phenomenon, but within the allowable range of error, it belongs to qualified workpiece. But in order to ensure the quality of workpiece and reduce the error as much as possible, we must find out the cause of error. At present, in the mechanical processing, the main reasons for the error of workpiece size are machine tool itself error, improper tool use error, fixture error, measuring tool error, processing technology error and so on. Therefore, we must formulate different solutions for different errors, in order to minimize the error and improve the quality of the workpiece 。

1. The main reason of machining error

1.1 geometric error of machine tool the machine tool is to process the metal blank into mechanical parts through cutting, welding, extrusion, forging and other ways. At present, the production of semi-finished products and finished products are processed and manufactured by lathe. Therefore, it can be said that machining through machine tool is one of the main reasons for mechanical error. The machine tool uses a fixture to fix the workpiece, and then uses the high-speed rotating cutter to carry out various cutting work, so the working accuracy of the machine tool directly affects the dimensional accuracy of the machining machine parts. The working accuracy of the machine tool mainly includes the rotation accuracy of the spindle, the precision of the guide rail and the accuracy of the transmission chain.

The spindle of machine tool is the benchmark of cutting tool or clamping workpiece. At the same time, the spindle can also transmit the power and motion to all the workpieces and tools to be processed. Therefore, the error of spindle rotation will directly lead to and affect the accuracy of the whole machined parts. The rotation error of a spindle usually refers to the change of the real rotation axis of the spindle relative to its average rotation axis at each moment. It is usually divided into three forms of axial movement, angular swing and radial circular runout. The main causes of the radial rotation error of the main shaft are: the various errors of the bearing itself, the coaxiality error between the bearings, the coaxiality error of several sections of the main shaft journal, and the winding degree of the main shaft. However, their influence on the radial rotation accuracy of the spindle is generally changed with the specific processing method. Bearing bearing end face and spindle shoulder end face to spindle rotation axis, it is easy to produce axial movement, so it will produce verticality error. The machining error of spindle rotation error caused by different processing methods is also different. When machining inner hole and outer circle by lathe, the main shaft radial rotation error can also cause the cylindricity and roundness error of the workpiece, but generally it will not affect the end face of the workpiece. The axial rotation error of the spindle usually has little effect on the machining of inner hole and outer circle, but it has a significant influence on the flatness and perpendicularity of the end face. When turning thread, the spindle rotation error can also lead to the corresponding periodic error of the lead of the processed thread.

1.2 the error caused by tool, fixture and positioning datum will inevitably lead to wear of any tool in 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, rational selection of tool geometric parameters and cutting parameters, correct use of coolant, etc., can minimize the size wear of the tool. If necessary, compensation device can be used to automatically compensate tool size wear. It is impossible for the positioning elements on the fixture to be absolutely accurate according to the basic dimensions. Their actual dimensions (or positions) are allowed to vary within the respective specified tolerances. However, if the positioning error changes beyond the tolerance range, there will be a gap between the workpiece and the fixture, and a large position change will be produced in the machining process, which will lead to the error of the machined parts.

1.3 the error caused by the force and deformation of the process system in the machining process is mainly determined by the workpiece stiffness, tool stiffness and machine tool component stiffness. If the rigidity of the workpiece is lower than that of the machine tool, tool and fixture, the machining accuracy of the mechanical parts will be affected due to the insufficient rigidity of the workpiece during the high-speed rotation and cutting of the spindle; If the cylindrical turning tool is used for machining mechanical parts, the deformation caused by its stiffness can be ignored. However, when machining the inner hole with small diameter, the tool stiffness is poor, which is easy to produce deformation in the process of high-speed operation, affecting the machining size of the inner hole; There are many complex parts in the machine tool, so the stiffness of various parts has a certain impact on the machining error of mechanical parts.

2. effective measures to improve the error in machining

2.1 the method of directly reducing or eliminating errors can improve the accuracy of machine tools, clamps, measuring tools and tools used in workpiece processing, and control the stress and heat deformation of process system. In order to improve the machining accuracy, the method should be implemented according to the specific situation after finding out the main factors that cause the machining error. For the processing of precision parts, the precision of the machine tool used should be improved as much as possible, and the deformation during the processing should be controlled. The processing of low rigidity parts mainly reduces the stress deformation of the workpiece, and the processing of the forming surface parts, mainly reducing the shape error of the forming tool and the installation error of the props.

2.2 the compensation or offset error method is a new error caused by human to offset the original error in the original process system. The error offset method is to use the original error to offset another error, and try to make the two equal and reverse. There is no essential difference between the two methods, and the production system is called error compensation.

2.3 the original error of the process system can be transferred to the direction without affecting the machining accuracy or the non important direction of the error under certain conditions. In this way, the processing accuracy can be obtained without reducing the original error. By means of error transfer, high precision parts can be processed by general precision machine tools. For example, the position accuracy of the boring machine, the accuracy of the position of the hole system and the dimension accuracy of the spatial bureau are guaranteed by the precision of boring die and boring bar. If radial connection transmission is adopted between boring bar and machine spindle, the machine error is independent of machining precision. In the assembly of machining, some precision problems involve the interrelationship between parts, which is quite complex. If the precision of parts is simply improved to meet the design requirements, sometimes it is not only difficult, but also impossible to set.

3. in conclusion, too large error can not only affect the machining accuracy and quality of the workpiece, but also cause the scrap of the workpiece. However, for the machining, the error is unavoidable. Therefore, it is necessary to find out the specific causes of the error and formulate different solutions according to different reasons, so that the error can be effectively controlled.