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Brief introduction to the main reasons and preventive measures of CNC lathe machining error

Jun 13, 2019

The influence of the geometric error and working error of the lathe on the machining accuracy makes the actual geometric parameters (size, shape and position) of the parts not conform to the ideal geometric parameters, which affects the processing quality. Selecting a quality machine to look at ALLES CNC, therefore, in machining, errors are unavoidable, but the error must be within the allowable range. Through error analysis, grasp the basic laws of its changes, and take corresponding measures to reduce machining errors and improve machining accuracy.

Brief introduction to the main reasons and preventive measures of CNC lathe machining error


First, the classification of errors


1. Random error. The random error is the error that the absolute value and the symbol are unpredictable when the same magnitude is measured multiple times under the same condition. There are many factors that produce random errors, many of which are contingent and unstable. Such as the gap of the measuring mechanism, the change of the friction between the movements, the variation of the measuring force and the fluctuation of the temperature.


2. Systematic error. The systematic error refers to the error that the magnitude and sign of the error remain the same or change according to a certain rule when the same measurement value is repeatedly measured under the same condition. That is, the former case is a fixed value system error, and the latter case is a straightening system error.


3. Gross error. Gross error is the expected error beyond the specified conditions. This kind of error is caused by the subjective negligence of the measurer to cause errors in reading, mistakes, or sudden changes in objective conditions (internal interference, vibration).


Second, the main reason for the error in lathe processing


1. Positioning error.


First, the benchmark does not coincide with the error. The basis on which the surface dimensions and locations are used to determine a surface is called the design basis. The basis on which the size and position after processing the surface to be machined in this process are determined on the process drawing is called the process reference. When machining a workpiece on a machine tool, several geometric elements on the workpiece must be selected as the positioning reference during machining. If the selected positioning reference does not coincide with the design basis, a reference misalignment error will occur.


Second, the positioning sub-manufacturing inaccuracy error. The components on the fixture are not absolutely accurate in their basic dimensions, and their actual dimensions (or positions) are allowed to vary within the specified tolerances. The workpiece positioning surface and the fixture positioning component 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 called an inaccurate error in positioning manufacturing.

2. Geometric error of the tool. Any tool in the cutting process will inevitably wear out, and thus cause changes in the size and shape of the workpiece. Proper selection of tool materials and selection of new wear-resistant tool materials, reasonable selection of tool geometry and cutting amount, proper use of coolant, etc., can minimize tool and size wear. If necessary, the compensation device can also be used to compensate for tool size wear.


3. Turning spindle spindle rotation error. The spindle rotation error refers to the variation of the actual rotation axis of the spindle at a moment relative to its average rotation axis. The main reasons for the spindle radial rotation error are: the coaxiality error of the spindles of the spindle, the error of the bearing itself, the coaxiality error between the bearings, and the spindle winding. Appropriately improve the manufacturing precision of the spindle and the box, select high-precision bearings, improve the assembly accuracy of the spindle components, balance the high-speed spindle components, and pre-tension the rolling bearings. Both can improve the rotation accuracy of the machine tool spindle.

4. Adjust the error. In each process of machining, the process system is always adjusted in this way. Since the adjustment cannot be absolutely accurate, an adjustment error is generated. In the process system, the mutual positional accuracy of the workpiece and the tool on the machine tool is ensured by adjusting the machine tool, tool, fixture or workpiece. When the original precision of machine tools, tools, fixtures and workpiece blanks meets the process requirements without considering dynamic factors, the influence of adjustment errors plays a decisive role in machining accuracy.


5. Transmission chain error. The transmission error of the drive chain refers to the error of the relative motion between the first and last transmission elements in the internal drive chain. Transmission error is the manufacturing and assembly error of each component of the drive chain, as well as the error caused by wear during use.


6. Errors caused by thermal deformation of the process system. The thermal deformation of the process system has a great influence on the machining accuracy. Especially in precision machining and large-piece machining, the machining error caused by thermal deformation can sometimes account for 50% of the total error of the workpiece. Machine tools, tools and workpieces are subjected to various heat sources, and the temperature is gradually increased. At the same time, they also radiate heat to the surrounding materials and spaces through various heat transfer methods.


7, rail error. The guide rail is the reference on the machine tool to determine the relative positional relationship of each machine tool component, and is also the benchmark for machine tool motion. The accuracy requirements of the lathe guide mainly have the following three aspects: straightness in the horizontal plane; straightness in the vertical plane; parallelism of the front and rear rails. In addition to the manufacturing tolerances of the guide rail itself, the uneven wear and installation quality of the guide rail is also an important factor in the guide rail error.


8, measurement error. When parts are measured during or after machining, the accuracy is directly measured due to measurement methods, gage accuracy, and workpiece and subjective and objective factors. Such as: temperature, vibration, dust, etc., where the temperature caused by the measurement error is the largest.

9. Personnel error. The personnel error is the error caused by the supervisor's supervisory factors and the skill level of the operator. The method used by the surveyor for the gage is incorrect, and the measurement caused by factors such as the resolution of the reading value and the ability to adjust the gage are not good.


10. The error caused by the deformation of the process system. One is the rigidity of the workpiece. In the process system, if the workpiece stiffness is relatively low relative to the machine tool, the tool, and the fixture, the deformation caused by the insufficient rigidity of the workpiece under the action of the cutting force has a large influence on the machining accuracy. The second is the rigidity of the tool. The external turning tool has a large rigidity in the normal (y) direction of the machined surface, and its deformation is negligible. When the inner diameter of the hole is small, the rigidity of the shank is very poor, and the arbor is greatly deformed by force, which has a great influence on the precision of the machining hole. The third is the rigidity of the machine tool components. Machine tool components are composed of many parts. There is no suitable simple calculation method for the rigidity of machine tool components. At present, experimental methods are mainly used to determine the rigidity of machine tool components. The deformation is not linear with the load. The loading curve and the unloading curve do not coincide, and the unloading curve lags behind the loading curve.


The area enclosed between the two curved lines is the energy consumed in the loading and unloading cycle. It consumes the work done by the friction and the contact deformation work; after the first unloading, the deformation does not recover from the starting point of the first loading. This 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 is gradually reduced to zero.


Third, the common geometric error and working error phenomenon of lathe


1. The roundness is too bad when turning the workpiece.


2. Taper is produced when the cylindrical part of the car is used.


3. After the finishing of the car, the flatness of the end face of the workpiece is out of tolerance.


4. After the finishing of the car, the end face of the workpiece is round and overrun.


5. When the outer circle is turned, the straightness of the workpiece line is out of tolerance.


6. When drilling, expanding, reaming, the workpiece aperture is enlarged or the hole is flared.


7. The pitch accuracy is too poor when turning the thread.


8. There are chaotic vibration lines on the surface of the car.


9. When the outer car is rounded, there will be regular ripples on the surface.


10. Regular ripples appear on the circumferential surface of the outer circle of the finished car.

1. Measurement method. The measurement method refers to the combination of the measuring instrument and the measurement conditions used in the measurement. Before the measurement, the measuring instruments to be used should be determined according to the characteristics of the object to be tested, such as accuracy, shape, quality, material and quantity, to determine the best measurement method.


2. Measurement accuracy. Measurement accuracy refers to the degree to which the measurement result is consistent with the true value. The measurement error is always inevitable in any measurement process, and the error is large, indicating that the measurement result is far from the true value and the precision is low; on the contrary, the error is small and the precision is high. Therefore accuracy and error are two relative concepts. Due to measurement errors, any measurement can only be an approximation of the true value of the element. The accuracy of the RMS values of the measurement results above is determined by the measurement accuracy.


3. Reduce the original error. Improve the geometric precision used in part processing, improve the accuracy of measuring tools, fixtures and tools themselves, control the force of the process system, deformation by heat, tool wear, deformation caused by internal stress, measurement error, etc., all directly reduce the original error. Selecting the quality machine tool to look at ALLES CNC, in order to improve the machining accuracy, analyze the original error of the machining error, and take different measures to solve the main original error of the machining error according to different situations. For the machining of precision parts, the geometric precision, rigidity and control of the thermal deformation of the precision machine tools should be improved as much as possible. For the parts with the forming surface and the special surface, the shape error of the forming tool and the installation error of the tool are mainly reduced.

4. Transfer the original error. The essence of this method is to shift the original error from the error-sensitive direction to the error-insensitive direction. Transfer the original error to a non-sensitive direction. The extent to which various original errors are reflected in the machining error of the part is directly related to whether it is in the direction of error sensitivity. If you try to transfer it to the non-sensitive direction of the machining error during the machining process, the machining accuracy can be greatly improved. Transfer the original error to other aspects that have no effect on the machining accuracy.


5. Differentiate the original error. In order to improve the processing accuracy of a batch of parts, a method of differentiating some original errors can be adopted. For the surface of the part with high processing precision, it is also possible to adopt a method of gradually homogenizing the original error during the continuous trial cutting process. According to the law of error reflection, the workpiece size of the blank or the upper process is divided into X groups according to the size, and the size range of each set of workpieces is reduced to 1/x. Then, according to the error range of each group, the exact position of the tool relative to the workpiece is adjusted, so that the center of the size dispersion range of each group of workpieces is basically the same, so that the size dispersion range of the whole batch of workpieces is greatly reduced, and the precision is improved.

6. Homogenization original error: the process of continuously reducing and averaging the original error of the surface to be processed by processing. The principle of homogenization is to find out the difference between the closely related workpieces or tool surfaces by mutual comparison and inspection, but to perform mutual correction or reference processing.


7. Error compensation method. For some original errors of the process system, an error compensation method can be adopted to control its influence on the machining error of the part. This method artificially creates a new original error, which compensates or offsets the original error inherent in the original process system, and achieves the purpose of reducing machining errors and improving machining accuracy.


8. Error cancellation method: Use one of the original original errors to remove some or all of the original original error or another original error.



Fifth, the basic requirements of the geometric accuracy of the lathe


The geometric accuracy of the lathe refers to the geometric accuracy of some basic parts of the lathe, the geometric accuracy of the mutual position and the geometric accuracy of the relative motion. Includes the following:


1. Straightness of the guide rail in the vertical plane (longitudinal);


2. The guide rails should be in the same plane (horizontal).


3. Straightness of the saddle moving in the horizontal plane.


4. The parallelism of the movement of the tailstock to the movement of the saddle.


5. The axial movement of the main shaft and the jumping of the support surface of the main shaft shoulder.


6. The radial runout of the spindle centering journal.


7. Radial runout of the spindle axis.


8. Parallelism of the spindle axis to the longitudinal movement of the saddle.


9. The radial runout of the spindle tip.


10. Parallelism of the tailstock sleeve axis to the movement of the saddle.


11. The parallelism of the axis of the cone sleeve of the tailstock sleeve to the movement of the saddle.


12. The height of the top and bottom of the main shaft and the tailstock.


13. The parallelism of the longitudinal movement of the small slide to the axis of the spindle.


14. The verticality of the middle slide on the spindle axis.


15. The axial movement of the screw.


16. The cumulative error of the pitch generated by the screw.

Six, common faults in lathes


1, the brakes are not working. During the stoppage of the lathe, the spindle cannot be stopped quickly, affecting work efficiency and prone to accidents.


2, boring car. That is, during the turning process, the spindle is stopped when the amount of the backing knife is large.


3. The motorized feed stops when the power is turned.


4, the chuck circle is big. The spindle clearance is too large.


5. The spindle temperature is too high. The spindle clearance is small, the friction is increased, the time is long, and the work is full. Excessive frictional heat causes the spindle temperature to be too high.


In short, in the lathe processing, the error is unavoidable, as long as the cause of the error is analyzed in detail, the tools, gauges, fixtures, measuring methods and related handles are adjusted to the best, and the machining error is minimized, so that the lathe can be improved. Processing accuracy, improve production efficiency, and process parts that meet the requirements of the drawings.

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