Talking about how to improve the precision of CNC machine tools

With the rapid development of China's economy, CNC machine tools have been widely used in the manufacture of machinery as a new generation of machine tools. The rapid development of precision machining technology and the continuous improvement of parts machining accuracy have also raised the precision of CNC machine tools. Requirements. Although the user is very concerned about the positional accuracy of the machine when purchasing CNC machine tools, especially the positioning accuracy and repeat positioning accuracy of each axis. But what about the accuracy of these CNC machines in use? A large number of statistics show that more than 65.7% of new machine tools do not meet their technical specifications during installation; 90% of the CNC machines in use are in an inaccurate working state. Therefore, it is necessary to monitor the working state of the machine tool and perform frequent tests on the accuracy of the machine tool in order to find and solve problems in time and improve the machining accuracy of the parts.

At present, the inspection of the positional accuracy of CNC machine tools usually adopts the international standard ISO230-2 or the national standard GB10931-89. The same machine tool, because of the different standards used, the positional accuracy is not the same, so when choosing the accuracy index of CNC machine tools, also pay attention to the standards it uses. The position standard of CNC machine tools usually refers to the reverse deviation and positioning accuracy of each CNC axis.

The determination and compensation of the two is a necessary way to improve the processing accuracy.

First, the reverse deviation

On the CNC machine tool, due to the error of the reverse dead zone of the drive components (such as servo motor, servo hydraulic motor and stepper motor) and the backlash of each mechanical motion transmission pair in each coordinate axis feed chain, Causes each coordinate axis to form a reverse deviation when it is converted from forward motion to reverse motion, and is also commonly referred to as backlash or loss. For CNC machine tools with semi-closed-loop servo system, the existence of reverse deviation will affect the positioning accuracy and repeat positioning accuracy of the machine tool, thus affecting the machining accuracy of the product. For example, in the G01 cutting motion, the reverse deviation will affect the accuracy of the interpolation motion. If the deviation is too large, it will cause the situation that the circle is not round enough and not square enough. In the G00 rapid positioning motion, the reverse deviation affects the machine tool. The positioning accuracy is such that the positional accuracy between the holes during drilling, drilling and the like is reduced. At the same time, as the equipment's running time increases, the reverse deviation will increase with the gradual increase of the motion pair clearance due to wear. Therefore, it is necessary to periodically measure and compensate the reverse deviation of the machine axes.

Determination of reverse deviation

The method of measuring the reverse deviation: in the stroke of the measured coordinate axis, moving a distance forward or backward in advance and using the stop position as a reference, and then giving a certain movement command value in the same direction to move a distance, Then move the same distance in the opposite direction and measure the difference between the stop position and the reference position. The measurement is performed a plurality of times (generally seven times) at three points near the midpoint and both ends of the stroke, and the average value at each position is obtained, and the maximum value among the obtained average values ​​is the reverse deviation measurement value. Be sure to move a distance before measuring, otherwise you will not get the correct reverse deviation value.

When measuring the reverse deviation of a linear motion axis, the measurement tool usually uses a dial gauge or a dial indicator. If possible, a dual-frequency laser interferometer can be used for measurement. When using a dial gauge or a dial indicator for measurement, it should be noted that the table and the pole do not extend too high and too long, because the cantilever is long and the table is easily moved by the measurement, resulting in inaccurate counting. The compensation value is not true. If the measurement is implemented by programming, the measurement process can be made more convenient and accurate.

For example, to measure the reverse deviation of the X-axis on a three-axis vertical machine, first press the watch against the cylindrical surface of the spindle and then run the following procedure to measure:

N10 G91 G01 X50 F1000; workbench right shift

N20 X-50; workbench shifts to the left to eliminate drive clearance

N30 G04 X5; pause for observation

N40 Z50; Z axis lift keeps off

N50 X-50: Workbench shifts to the left

N60 X50: Workbench right shift reset

N70 Z-50: Z-axis reset

N80 G04 X5: Pause for observation

N90 M99;

It should be noted that the results measured at different operating speeds of the workbench will vary. In general, the measured value of the low speed is larger than that of the high speed, especially when the machine shaft load and the motion resistance are large. When moving at low speed, the table moves at a lower speed, and it is not easy to overshoot (over the "back gap"), so the measured value is large; at high speed, due to the high speed of the table, overshoot is likely to occur. The measured value is too small.

The method of measuring the amount of reverse deviation of the rotary motion axis is the same as that of the linear axis, except that the instrument used for the detection is different.

Compensation for reverse deviation

Domestic CNC machine tools, positioning accuracy is quite a few > 0.02mm, but there is no compensation function. For this type of machine tool, in some cases, one-way positioning can be realized by programming method, and the backlash can be cleared. When the mechanical part is unchanged, as long as the low-speed one-way positioning reaches the interpolation starting point, then the interpolation processing is started. . When the interpolation feed is reversed, the backlash value is officially interpolated to improve the accuracy of the interpolation process, and the tolerance of the part can be basically guaranteed.

For other types of CNC machine tools, there are usually several addresses in the memory of the numerical control device, which are used to store the backlash value of each axis. When an axis of the machine tool is commanded to change the direction of motion, the numerical control device automatically reads the backlash value of the axis, compensates and corrects the coordinate displacement command value, so that the machine tool is accurately positioned at the command position, eliminating or subtracting The adverse effect of small reverse deviation on machine accuracy.

In general, only a single backlash compensation value can be used in the CNC system. In order to balance the high and low speed motion accuracy, in addition to being mechanically better, only the reverse deviation value measured during fast motion can be used. Since the compensation value is input, it is difficult to balance, and both the rapid positioning accuracy and the interpolation accuracy at the time of cutting are taken into consideration.

For CNC systems such as FANUC0i and FANUC18i, there are two types of backlash compensation for fast motion (G00) and low speed cutting feed motion (G01). According to the different feeding modes, the CNC system automatically selects different compensation values ​​to complete higher precision machining.

The G01 cutting feed motion measured backlash value A input parameter NO11851 (G01 test speed can be determined according to the commonly used cutting feed rate and machine characteristics), the G00 measured backlash value B input parameter NO11852 It should be noted that if the CNC system is to perform the specified backlash compensation, the fourth bit (RBK) of parameter number 1800 should be set to 1; if RBK is set to 0, the specified inverse will not be executed. Compensate for the gap. G02, G03, JOG and G01 use the same compensation value.

Second, the positioning accuracy

The positioning accuracy of CNC machine tools refers to the positional accuracy that can be achieved by the movement of the moving parts of the machine tool under the control of the numerical control system. It is an important precision of the CNC machine tool which is different from the ordinary machine tool. It is the same as the geometric precision of the machine tool. Accuracy has an important impact, especially on the pitch error in pore processing. A CNC machine tool can judge its machining accuracy from the positioning accuracy it can achieve. Therefore, detecting and compensating the positioning accuracy of CNC machine tools is the necessary way to ensure the processing quality.

Determination of positioning accuracy

At present, the dual-frequency laser interferometer is used for machine tool detection and processing analysis. The laser interferometry principle is used to measure the laser real-time wavelength, so the test accuracy is improved and the applicable range is enhanced. The detection method is as follows:

Installing a dual-frequency laser interferometer;

Install an optical measuring device on the axis of the machine tool to be measured; adjust the laser head so that the measuring axis is parallel or parallel with the machine axis of movement, that is, the optical path is pre-adjusted and collimated; and the measurement parameters are input after the laser is warmed up;

The machine tool is measured according to the specified measuring procedure;

Data processing and result output.

Compensation for positioning accuracy

If the positioning error of the CNC machine tool exceeds the error tolerance range, the machine tool must be compensated for errors. The common method is to calculate the pitch error compensation table and manually input the CNC system of the machine tool to eliminate the positioning error. Since the three-axis or four-axis compensation points of the CNC machine machine may have hundreds of thousands of points, manual compensation takes more time and is easy. An error occurred.

Now the computer is connected with the machine tool CNC controller through the RS232 interface. The automatic calibration software written by VB is used to control the laser interferometer to work synchronously with the CNC machine tool to realize the automatic detection of the positioning accuracy of the CNC machine tool and the automatic pitch error compensation. The compensation method is as follows: :

Back up existing compensation parameters in the CNC control system;

The machine tool CNC program for point-by-point positioning accuracy measurement is generated by the computer and transmitted to the CNC system; the positioning error of each point is automatically measured;

A new set of compensation parameters is generated according to the specified compensation point and transmitted to the CNC system, and the pitch automatic compensation is completed; repeat c. Perform accuracy verification.

According to the accuracy of each axis of the CNC machine tool, the pitch error automatic compensation function and the backlash compensation function are used to reasonably select the compensation points of each axis to achieve the best precision state of the CNC machine tool, and the efficiency of detecting the positioning accuracy of the machine tool is greatly improved. .

Positioning accuracy is an important indicator of CNC machine tools. Although the machine with high precision and small error can be selected as much as possible during the user's purchase, the longer the equipment is put into use, the more the equipment wears, causing the positioning error of the machine tool to become larger and larger, which is fatal to the processed and produced parts. Impact. Using the above method to accurately measure and compensate the reverse deviation and positioning accuracy of each coordinate axis of the machine tool, it can reduce or eliminate the adverse effect of reverse deviation on the accuracy of the machine tool, improve the positioning accuracy of the machine tool, and make the machine tool optimal. Accuracy status to ensure the quality of the part.