Analysis on machining accuracy

2022-07-25
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Brief analysis of machining accuracy Abstract: This paper briefly introduces the basic knowledge of machining accuracy and machining error. Combined with the practice of production and teaching, this paper introduces various errors and their causes in detail, and puts forward its own views on how to reduce the errors and improve the machining accuracy when adjusting zero

1 overview

1.1 machining accuracy and machining error

machining accuracy refers to the consistency between the actual geometric parameters (size, shape and position) and the ideal geometric parameters of a part after machining. The actual machining cannot be completely consistent with the ideal part, and there will always be deviations of different sizes. The deviation degree of the actual geometric parameters of the part from the ideal geometric parameters after machining is called machining error

1.2 original error

the machining process system (hereinafter referred to as the process system) composed of machine tools, fixtures, cutters and workpieces will have a variety of errors. These errors will be reflected as the machining errors of workpieces in different ways (or expanded or reduced) under different specific working conditions

the original errors of the process system mainly include the geometric error, positioning error, processing error caused by the stress deformation of the process system, processing error caused by the heating deformation of the process system, the internal stress of the workpiece, the deformation caused by the redistribution of the new chemical materials, fine chemical products and high molecular new materials industries, as well as the principle error, adjustment error, measurement error, etc

1.3 methods for studying machining accuracy

methods for studying machining accuracy analytical calculation method and statistical analysis method

2 set error of process system

2.1 geometric error of machine tool

in machining, the forming movement of the tool relative to the workpiece is generally completed by the machine tool. Therefore, the machining accuracy of the workpiece largely depends on the accuracy of the machine tool. Machine tool manufacturing errors have a great impact on workpiece machining accuracy: spindle rotation error, guide rail error and transmission chain error. The wear and tear of the machine tool will reduce the working accuracy of the machine tool

2.1.1 spindle rotation error

the spindle of the machine tool is the benchmark for clamping the workpiece or tool, and transmits the motion and power to the workpiece or tool. The spindle rotation error will directly affect the accuracy of the workpiece to be machined

spindle rotation error refers to the variation of the actual rotation axis of the spindle relative to its average rotation axis at each instant. It can be divided into three basic forms: radial circular runout, axial movement and angular swing

the main reasons for the radial rotation error of the main shaft are: the coaxiality error of several journal sections of the main shaft, various errors of the bearing itself, the coaxiality error between bearings, the winding degree of the main shaft, etc. However, their influence on the radial rotation accuracy of the spindle varies with the different machining methods

the main reason for axial movement is the verticality error of the spindle shoulder end face and bearing bearing end face to the spindle rotation axis. The machining error caused by spindle rotation error is different with different machining methods. When machining the outer circle and inner hole on the lathe, the radial rotation error of the spindle can cause the roundness and cylindricity error of the workpiece, but it has no direct effect on the end face of the workpiece. The axial rotation error of the spindle has little effect on the machining of the outer circle and inner hole, but has a greater effect on the perpendicularity and flatness of the machined end face. When turning the thread, the spindle rotation error can cause periodic error in the lead of the processed thread

properly improve the manufacturing accuracy of the spindle and box, select high-precision bearings, improve the assembly accuracy of the spindle components, balance the high-speed spindle components, pre tighten the rolling bearings, etc., which can improve the rotation accuracy of the machine tool spindle

2.1.2 guide rail error

guide rail is the benchmark on the machine tool to determine the relative position relationship of each machine tool component, and it is also the benchmark for machine tool motion. The accuracy requirements of lathe guide rail mainly include the following three aspects: straightness in the horizontal plane; Straightness in the vertical plane; Parallelism (twist) of front and rear guide rails

in addition to the manufacturing error of the guide rail itself, the uneven wear and installation quality of the guide rail are also important factors causing the guide rail error. The wear of guide rail is one of the main reasons for the decline of machine tool accuracy

2.1.3 transmission chain error

transmission chain error refers to the error that the development of new materials in the transmission chain helps to realize the sustainable development of the material industry and the relative movement between the transmission elements at the end. It is generally measured by the angle error of the end components of the transmission chain

2.2 geometric error of tool

any tool will inevitably be worn 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, reasonable selection of tool geometric parameters and cutting parameters, correct sharpening of tools and correct use of coolant can effectively reduce the dimensional wear of tools. If necessary, the compensation device can also be used to automatically compensate the tool size wear

3 positioning error

3.1 datum misalignment error

the datum used to determine the size and position of a surface on the part drawing is called the design datum. The datum used to determine the size and position of the machined surface of the process on the process drawing is called the process datum. Generally, the process datum shall coincide with the design datum. When machining the workpiece on the machine tool, several geometric elements on the workpiece must be selected as the positioning datum (or measurement datum) during machining. If the selected positioning datum (or measurement datum) does not coincide with the design datum, the datum misalignment error will occur. The datum misalignment error is equal to the maximum variation of the positioning datum relative to the design datum in the process dimension direction. 3.2 positioning pair manufacturing inaccuracy error

the correct position of the workpiece in the fixture is determined by the positioning element on the fixture. The positioning elements on the fixture cannot be manufactured absolutely accurately according to the basic dimensions, and their actual dimensions (or positions) are allowed to change within the respective specified tolerances. At the same time, the positioning reference plane on the workpiece will also have manufacturing errors. The workpiece positioning surface and fixture positioning elements together form a positioning pair. The maximum position variation of the workpiece caused by the inaccurate manufacturing of the positioning pair and the fit gap between the positioning pairs is called the manufacturing inaccuracy error of the positioning pair

4 error caused by force deformation of the process system

4.1 workpiece stiffness

if the workpiece stiffness in the process system is relatively low compared with the machine tool, cutter and fixture, the deformation of the workpiece due to insufficient stiffness will have a greater impact on the machining accuracy under the action of cutting force, and the maximum deformation can be estimated according to the relevant formulas of material mechanics

4.2 tool stiffness

the rigidity of the cylindrical turning tool in the normal (y) direction of the machined surface is very large, and its deformation can be ignored. When boring an inner hole with small diameter, the rigidity of the tool bar is very poor, and the deformation of the tool bar has a great impact on the hole machining accuracy. The deformation of cutter bar can also be estimated according to the relevant formulas of material mechanics

4.3 stiffness of machine tool components

machine tool components are composed of many parts. So far, there is no suitable simple calculation method for the stiffness of machine tool components. At present, the stiffness of machine tool components is mainly measured by experimental methods. Deformation is not linear with load, loading curve and unloading curve do not coincide, and unloading curve lags behind loading curve. The area contained between the two curves is the energy lost in the loading and unloading cycles, which is consumed by the work done by friction and contact deformation work; After the first unloading, the deformation cannot recover to the starting point of the first loading, which indicates that there is residual deformation. After multiple loading and unloading, the starting point of the loading curve coincides with the end point of the unloading curve, and the residual deformation gradually decreases to zero; The actual stiffness of machine tool components is much smaller than what we estimated according to the entity

5 error caused by thermal deformation of process system

thermal deformation of process system has a great impact on machining accuracy, especially in precision machining and large workpiece machining, the machining error caused by thermal deformation sometimes accounts for 40% ~ 70% of the total workpiece error. Machine tools, cutting tools and workpieces are affected by various heat sources, and the temperature will gradually rise. At the same time, they also emit heat to the surrounding materials and space through various heat transfer methods. When the incoming heat per unit time is equal to the outgoing heat, the process system reaches the thermal equilibrium state

6 conclusion: the way to improve the machining accuracy

reduce the original error; Transfer the original error; Average original error; Homogenized original error; Error compensation

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