In the usual precision machining process, parts are often unqualified due to some precision problems. Today we will talk about the precision of precision machining.

1. Overview

1.1 Machining accuracy and machining error

Machining accuracy refers to the degree of conformity between the actual parameters (size, shape and position) of the parts after machining and the ideal parameters. The actual machining cannot be completely consistent with the ideal part, and there will always be deviations of different sizes. The degree of deviation of the actual parameters of the parts after machining from the ideal parameters is called machining error. There is no way to avoid this. You can only find ways to make the actual parameters of the machining accuracy close to the ideal parameters.

1.2 Original error

The precision machining process system (referred to as the process system) composed of CNC machine tools, fixtures, cutting tools and workpieces will have various errors. These errors will vary in various specific working conditions. The method (or enlargement or reduction) is reflected as the machining error of the workpiece. The original errors of the process system mainly include geometric errors of the process system, positioning errors, processing errors caused by the force deformation of the process system, processing errors caused by the thermal deformation of the process system, deformation caused by the redistribution of the internal stress of the workpiece, and principle errors, adjustments Error, measurement error, etc.

1.3 Methods of studying machining accuracy

Method analysis calculation method and statistical analysis method to study the precision of precision machining. The error can be reduced as much as possible.

CNC precision machining

2. Process system assembly error

2.1 Geometric errors caused by CNC machine tools

The forming movement of the cutting tool relative to the workpiece in processing is generally completed by the CNC machine tool. Therefore, the machining accuracy of the workpiece depends to a large extent on the accuracy of the CNC machine tool. CNC machine tool manufacturing errors have a greater impact on the machining accuracy of the workpiece: spindle rotation error, guide rail error and transmission chain error. The wear and tear of CNC machine tools will reduce the accuracy of machine tools. Therefore, choosing a good CNC machine tool is also a way to reduce the error. Generally speaking, some CNC machine tools produced in our country are far less than foreign CNC machine tools. In contrast, domestic CNC machine tools have machining errors. Larger. Changzhou Xingongyi Company introduced Japan’s Tsugami CNC machine tool, which not only has high precision but also high work efficiency.

2.1.1 Spindle rotation error

The spindle of a CNC machine tool is the benchmark for clamping a workpiece or tool, and transmits motion and power to the workpiece or tool. The spindle rotation error will directly affect the accuracy of the processed workpiece. So it is necessary to buy a good spindle.

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 decomposed into three basic forms: radial circular runout, axial runout and angular swing.
The main reasons for the radial rotation error of the main shaft are: the coaxiality error of several sections of the main shaft journal, various errors of the bearing itself, the coaxiality error between the bearings, and the rotation of the main shaft. However, their influence on the radial rotation accuracy of the spindle varies with the processing method.

The main reason for the axial movement is the perpendicularity error of the spindle shoulder end face and the bearing bearing end facing the spindle rotation axis. Different machining methods have different machining errors caused by spindle rotation errors. When machining the outer circle and inner hole on the lathe, the radial rotation error of the spindle can cause the roundness and cylindricity errors 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 outer circle and inner hole, but it has a greater influence on the perpendicularity and flatness of the processed end surface. When threading, the spindle rotation error can cause periodic errors in the lead of the machined thread.

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 pre-tighten the rolling bearings, etc., can improve the rotation accuracy of the machine tool spindle.

2.1.2 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 also the benchmark for machine tool movement. The accuracy requirements of lathe guide rails mainly have the following three aspects: straightness in the horizontal plane; straightness in the vertical plane; parallelism (twist) of the 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 that cause the guide rail error. Rail wear is one of the main reasons for the decline in machine tool accuracy.

2.1.3 Transmission chain error

Transmission chain error refers to the error of the relative movement between the transmission elements at both ends of the transmission chain. It is generally measured by the angle error of the end element of the transmission chain.

2.2 The geometric error of the tool

In the cutting process of any tool, it is inevitable to produce wear, and thus cause the size and shape of the workpiece to change. Correct selection of tool materials and selection of new wear-resistant tool materials, reasonable selection of tool geometric parameters and cutting quantities, correct sharpening of tools, and correct use of coolants and cutting fluids can effectively reduce tool size wear. If conditions permit, the compensation device can also be used to automatically compensate the tool size and wear. I believe that if you want to make high-precision parts, this device still needs to be installed.

3. Positioning error

3.1 Benchmark misalignment error

The datum used to determine the size and position of a certain surface on the part drawing is called the design datum. The reference used to determine the size and position of the processed surface of this process on the process drawing is called process reference. In general, the process reference should coincide with the design reference. When processing a workpiece on a machine tool, several geometric elements on the workpiece must be selected as the positioning reference (or measurement reference) during processing. If the selected positioning reference (or measurement reference) does not coincide with the design reference, a reference error Coincidence error. The reference non-coincidence error is equal to the variation of the positioning reference relative to the design reference in the process dimension direction. When doing this process, the drawings are still drawn by the masters in the factory, which is much more reliable.

3.2 Inaccurate manufacturing error of positioning pair

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 made absolutely accurate according to the basic dimensions, and their actual dimensions (or positions) are allowed to vary within the specified tolerances. At the same time, the positioning reference surface on the workpiece will also have manufacturing errors. The workpiece positioning surface and the fixture positioning element together form a positioning pair. The maximum position change of the workpiece caused by the inaccurate manufacturing of the positioning pair and the matching gap between the positioning pairs is called the inaccurate manufacturing error of the positioning pair.

4. Errors caused by force and deformation of the process system

4.1 Workpiece stiffness

In the process system, if the rigidity 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 due to insufficient rigidity will have a greater impact on the machining accuracy, and the maximum deformation can be determined by material mechanics Relevant formula estimation.

4.2 Tool stiffness

The rigidity of the external turning tool in the normal (y) direction of the machined surface is very large, and its deformation can be ignored. Boring inner holes with a small diameter, the rigidity of the tool bar is very poor, and the force and deformation of the tool bar have a great influence on the accuracy of hole processing. The deformation of the tool bar can also be estimated according to the relevant formulas of material mechanics.

4.3 Rigidity of machine parts

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, experimental methods are mainly used to determine the stiffness of machine tool components. There is no linear relationship between deformation and load, loading curve and unloading curve do not coincide, and unloading curve lags behind the loading curve. The enclosed area between the two curves is the energy lost in the loading and unloading cycles, which is consumed by frictional work and contact deformation work; after the first unloading, the deformation is not restored to the starting point of the first loading. This shows that there is residual deformation. After multiple loading and unloading, the starting point of the loading curve coincides with the end of the unloading curve, and the residual deformation is gradually reduced to zero; the actual stiffness of the machine tool components is much smaller than we estimated by the entity.

5. Errors caused by thermal deformation of the process system

The thermal deformation of the process system has a relatively large impact on the machining accuracy, especially in the precision machining and large parts processing, the machining error caused by the thermal deformation can sometimes account for 40% to 70% of the total error of the workpiece. Machine tools, tools, and workpieces are affected by various heat sources, and the temperature will gradually rise. At the same time, they also radiate heat to surrounding materials and spaces through various heat transfer methods. When the heat input per unit time is equal to the heat output, the process system has reached a thermal equilibrium state. It is also very important to choose a good heat treatment. If you do not have the technology in this area, you must outsource processing. When outsourcing processing, you must find a suitable heat treatment plant.

6. Conclusion: Ways to improve machining accuracy

“Reduce the original error; transfer the original error; share the original error equally; average the original error; error compensation” is the best summary of how to reduce the error and improve the accuracy.