CNC machining accuracy details and solution
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CNC machining accuracy details and solution

Precision

Refers to the reproducibility, consistency between the results obtained from repeated measurements using the same alternate sample. It is possible to have a high degree of precision, but the accuracy is inaccurate. For example, using a length of 1mm to determine the three results obtained were 1.051mm, 1.053, 1.052, although their precision is high, but is inaccurate.

Accuracy indicates the correctness of the measurement results, and precision indicates the repeatability and reproducibility of the measurement results, and precision is a prerequisite for accuracy.

Definition of precision

Generally speaking, accuracy is the ability of a machine tool to position the tool tip point to the programmed target point. However, there are many ways to measure this positioning capability, and more importantly, different countries have different regulations.

JISB6201 is generally used for general-purpose machines and general CNC machines, JISB6336 is generally used for machining centers, and JISB6338 is generally used for vertical machining centers. machining center.

When calibrating the accuracy of a CNC machine tool, it is very necessary to mark the standard it uses together. JIS standard, the data than with the U.S. NMTBA standard or the German VDI standard is significantly smaller.

The same indicators, different meanings

It is often confusing that the same indicator name represents different meanings in different accuracy standards, while different indicator names have the same meaning. The above four standards, in addition to JIS standards, are all in the CNC axis of the machine tool on multiple target points for multiple rounds of measurement, through the mathematical statistics calculated, the key difference is: the number of target points

Number of measurement rounds

From the one-way or two-way approach to the target point (this point is particularly important)

Calculation method of accuracy index and other indexes

This is a description of the key differences between the four standards, and as one would expect, one day all machine tool manufacturers will follow the ISO standard uniformly. Therefore, the ISO standard is chosen here as the benchmark. The following table compares the 4 standards, and this article only deals with linear accuracy, since the calculation principle of rotary accuracy is basically the same.

The main causes of machining errors

1、Spindle rotation error. Spindle rotation error refers to the actual rotation axis of the spindle at each instant relative to its average rotation axis of change. The main reasons for the spindle radial slewing error are: the coaxiality error of several journals of the spindle, various errors of the bearing itself, the coaxiality error between the bearings, spindle deflection, etc.

2, guide error. Guide rail is a machine tool to determine the relative position of the machine components of the benchmark, but also the benchmark of the machine tool movement. Uneven wear and installation quality of the guide rail, is also an important factor causing the error of the guide rail.

3, the transmission chain error. Transmission error of the transmission chain is the error of the relative motion between the first and last two transmission elements in the transmission chain of the inner contact. Transmission error is caused by the manufacturing and assembly errors of each component link in the transmission chain and wear and tear in the process of use.

4, the geometric error of the tool. Any tool in the cutting process, it is inevitable to produce wear and tear, and the resulting change in the size and shape of the workpiece.

5, positioning errors. First, the benchmark does not overlap error. In the parts diagram used to determine the size of a surface, location based on the benchmark called the design benchmark. In the process diagram used to determine the size and location of the processed surface of the process based on the benchmark called the process benchmark. In the machine tool for processing the workpiece, you need to select a number of geometric elements on the workpiece as the positioning reference for processing, if the positioning reference selected does not overlap with the design reference, it will produce the benchmark does not overlap error. Second, the positioning vice manufacturing inaccuracy error.

6, the process system deformation of the error generated by the force. First, the workpiece stiffness. Process system if the workpiece stiffness is relatively low compared to the machine tool, tool, fixture, under the action of cutting force, the workpiece due to insufficient stiffness and deformation caused by the impact on the machining accuracy is relatively large. Second, the tool stiffness. External turning tool stiffness in the direction of the processing surface normal is large, its deformation can be negligible. Boring smaller diameter bore, tool bar stiffness is very poor, tool bar deformation on the hole processing accuracy has a great impact. Third, the stiffness of the machine tool components. Machine parts by many parts, machine parts stiffness so far there is no suitable simple calculation method, or mainly with experimental methods to determine the stiffness of machine parts.

7, the process system caused by thermal deformation of the error. Process system heat deformation on the impact of machining accuracy is relatively large, especially in precision machining and large parts processing, processing errors caused by heat deformation can sometimes account for 50% of the total error of the workpiece.

8, adjustment error. In each process of machining, the process system should always be adjusted in one way or another. Because the adjustment is not absolutely accurate, thus generating adjustment errors. In the process system, the workpiece, tool in the machine tool mutual position accuracy, is through the adjustment of the machine tool, tool, fixture or workpiece, etc. to ensure. When the machine tool, tool, fixture and workpiece blanks, such as the original accuracy of the process requirements and do not take into account the dynamic factors, the impact of the adjustment error, the processing accuracy plays a decisive role.

9, measurement error. Parts in the processing or measurement after processing, due to the measurement method, gauge accuracy, as well as the workpiece and subjective and objective factors have a direct impact on the measurement accuracy.

Process measures to improve machining accuracy

1, reduce the original error

Improve the geometric accuracy of the machine tool used for parts processing, improve the accuracy of fixtures, gauges and tools themselves, control the process system force, heat deformation, tool wear, deformation caused by internal stress, measurement errors, etc. are directly reduce the original error. In order to improve the machining accuracy, need to produce machining errors of the original error analysis, according to the different circumstances of the main original error caused by machining errors to take different measures to solve. For the processing of precision parts should be used to improve the geometric accuracy of precision machine tools, rigidity and control of processing heat deformation; parts with forming surface processing, it is mainly how to reduce the forming tool shape error and tool installation error. This method is a basic method widely used in production. It is to identify the main factors that produce machining errors, and then try to eliminate or reduce these factors. For example, the slender shaft turning, now using a large walking tool reverse turning method, basically eliminates the bending deformation caused by the axial cutting force. If supplemented with a spring top, the effect of thermal elongation caused by thermal deformation can be further eliminated.

2、Compensation of the original error

Error compensation method, is to artificially create a new error, to offset the original process system in the original error. When the original error is negative when the artificial error to take a positive value, and vice versa, to take a negative value, and try to make the two equal in size; or the use of an original error to offset another original error, but also try to make the two equal in size and opposite direction, so as to reduce the processing error, improve the processing accuracy of the purpose.

3、Transfer the original error

Error transfer method is essentially transferring the geometric error, force deformation and thermal deformation of the process system. Error transfer method of many examples. Such as when the machine tool accuracy can not meet the requirements of parts processing, often not just improve the machine precision, but from the process or fixture to find ways to create conditions to make the geometric error of the machine tool to transfer to aspects that do not affect the machining accuracy. Such as grinding spindle taper hole to ensure its coaxiality with the journal, not by the machine tool spindle rotation accuracy to ensure, but by the fixture to ensure. When the machine tool spindle and the workpiece with a floating linkage, the original error of the machine tool spindle is transferred away.

4、Equalization of the original error

In processing, due to the existence of the blank or the previous process error, often resulting in the processing error of the process, or due to changes in the material properties of the workpiece, or the process of the previous process changes (such as the refinement of the blank, the original cutting process cancellation), causing a large change in the original error. To solve this problem, it is best to use the method of grouping to adjust the mean error. The essence of this approach is to divide the original error into n groups according to their size, each group of blank error range is reduced to the original 1/n, and then adjust the processing according to each group respectively.

5、Equalize the original error

For shafts and holes with high precision requirements, the grinding process is often used. The grinding tool itself is not required to have high precision, but it can do relative movement with the workpiece in the process of micro-cutting on the workpiece, the high point is gradually ground off (of course, the mold is also part of the workpiece grinding), and finally the workpiece to achieve a high degree of accuracy. This process of friction and wear between surfaces is the process of continuous reduction of errors, which is the error equalization method. Its essence is to use closely related surfaces to compare with each other, check each other to find out the differences from the comparison, and then make mutual correction or mutual benchmark processing, so that the error of the surface being processed by the workpiece is constantly reduced and homogenized. In the production, many precision reference parts (such as flat, straightedge, etc.) are processed using the error equalization method.

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