With the rapid development of China's economy, CNC machine tools are used as a new generation of machine tools. They have been widely used in machinery manufacturing. The rapid development of precision machining technology and continuous improvement of parts machining accuracy have raised the precision of CNC machine tools. The request. Although users are choosing CNC machine tools, they all value the position accuracy of the machine tool, especially the positioning accuracy and repeatability of each axis. But what exactly is the accuracy of these CNC machine tools in use? A large number of statistics show that: 65.7% of new machine tools do not meet their technical specifications when they are installed; 90% of the CNC machines in use are in misaligned state. Therefore, it is very necessary to monitor the working status 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 precision of the parts machining.
At present, the accuracy test 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 position accuracy obtained is not the same, so when selecting the precision index of CNC machine tools, it should also pay attention to the standards it adopted. The position standard of CNC machine tools usually refers to the reverse deviation and positioning accuracy of each CNC axis. Measuring and compensating for both are necessary ways to improve machining accuracy.
First, reverse deviation
In the CNC machine tool, due to the existence of errors such as the reverse dead zone of the drive components (eg, servo motors, servo-hydraulic motors, and stepping motors, etc.) on the feed chains of the coordinate axes, the backlash of the mechanical motion transmission pairs, etc. Causes each coordinate axis to form the backward deviation when moving from the forward movement to the reverse movement, usually also called the backlash or the amount of inaction. For CNC machine tools using a semi-closed-loop servo system, the existence of reverse deviation will affect the positioning accuracy and repeatability of the machine tool, which will affect the machining accuracy of the product. If the G01 cutting movement, reverse deviation will affect the accuracy of the interpolation movement, if the deviation is too large will cause the "circle is not round enough, not square" situation; and in the G00 rapid positioning movement, reverse deviation affect the machine The positioning accuracy of the holes makes the position accuracy between holes smaller when drilling, boring, and other holes are processed. At the same time, with the increase of the equipment operation time, the reverse deviation will increase with the gradual increase of the wearer's clearance due to wear. Therefore, it is necessary to periodically measure and compensate the reverse deviation of each coordinate axis of the machine tool.
[Determination of Reverse Deviation]
Reverse bias measurement method: in the travel of the measured coordinate axis, a distance is moved forward or backward in advance and this stop position is used as a reference, and a certain movement command value is given in the same direction to make it move a certain 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 multiple times (usually seven times) at three points near the midpoint of the stroke and at both ends, and the average value at each position is determined, and the maximum value among the obtained average values is the reverse deviation measurement value. Be sure to move a certain distance before measuring, otherwise you cannot get the correct reverse bias value.
When measuring the deviation of the linear motion axis, the measurement tool is usually equipped with a dial gauge or a dial indicator. If conditions permit, a dual-frequency laser interferometer can be used for measurement. When using dial gauges or dial gauges to measure, it should be noted that the table seat and the table rod do not protrude too much too long, because the measurement due to a long cantilever, the table seat is easily moved by force, resulting in inaccurate counts, The compensation value is also not true. If the programming method is used to achieve the measurement, the measurement process can be made more convenient and accurate.
For example, to measure the X-axis misalignment on a three-coordinate vertical machine, first press the table against the cylindrical surface of the spindle, and then run the following program to measure:
N10 G91 G01 X50 F1000; table right
N20 X-50; table left, eliminating drive gap
N30 G04 X5; pause for observation
N40 Z50; Z-axis lift off
N50 X-50: Table left
N60 X50: Table to the right to 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 may vary. Under normal circumstances, the low-speed measured value is higher than high-speed, especially when the machine axis load and movement resistance are large. When the speed is low, the speed of the table is low, and overshoot overshoot (relative “backlash”) is not easy to occur, so the measured value is relatively large; at high speeds, the overshoot overshoot is likely to occur due to the high speed of the table. The measured value is too small.
The method of measuring the amount of backward 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 of reverse bias]
Domestic CNC machine tools have a lot of positioning accuracy > 0.02mm, but they have no compensation function. For this kind of machine tool, in some occasions, one-way positioning can be achieved by programming, and the backlash can be cleared. When the mechanical part is not changed, as long as the low-speed one-way positioning reaches the interpolation starting point, then the interpolation processing is started. . When the reverse feed is reversed in the interpolation, the backlash value can be formally interpolated, which can improve the accuracy of interpolation processing and basically ensure the tolerance requirements of the parts.
For other types of CNC machine tools, there are usually several addresses in the memory of the CNC device, which are used to store the backlash value of each axis. When a certain axis of the machine tool is instructed to change the direction of movement, the numerical control device will automatically read the value of the backlash of the axis and compensate and correct the coordinate displacement command value so that the machine tool can be accurately positioned at the command position and eliminated or reduced. The adverse effect of small reverse bias on machine tool accuracy.
The general numerical control system only has a single backlash compensation value available. In order to take into account the high and low-speed movement accuracy, in addition to doing a better mechanically, only the reverse deviation value measured during rapid movement can be taken as The compensation value is input, so it is difficult to achieve balance, taking into account the rapid positioning accuracy and the interpolation accuracy during cutting.
For numerical control systems such as FANUC0i and FANUC18i, two types of backlash compensation are available for fast motion (G00) and low-speed cutting feed motion (G01). According to different feeding methods, the CNC system automatically selects different compensation values to complete higher-precision machining.
The backlash value A measured by G01 cutting feed motion is input to parameter NO11851 (the test speed of G01 can be determined according to the commonly used cutting feed speed and machine tool characteristics), and the backlash value B measured by G00 is input to parameter NO11852 . It should be noted that if the CNC system performs the specified backlash compensation separately, the fourth bit (RBK) of the parameter number 1800 should be set to 1; if the RBK is set to 0, the respectively specified reverse direction should not be executed. Clearance compensation. G02, G03, JOG and G01 use the same compensation value.
Second, positioning accuracy
The positioning accuracy of CNC machine tools refers to the position accuracy of the measured moving parts of the machine tool under the control of the CNC system. It is an important precision of CNC machine tools that is different from ordinary machine tools. It shares the geometric accuracy of machine tools with the machine tool cutting. Accuracy has an important impact, and it has a decisive influence on the pitch error in the pore processing. A CNC machine can judge its machining accuracy from the positioning accuracy it can achieve. Therefore, the detection and compensation of the positioning accuracy of CNC machine tools is a necessary way to ensure the processing quality.
[Measurement of positioning accuracy]
At present, the use of dual-frequency laser interferometer for machine tool detection and processing analysis, the use of laser interferometry principle, the laser real-time wavelength as a measurement benchmark, so improve the test accuracy and enhance the scope of application. The detection method is as follows:
Installing a dual-frequency laser interferometer;
Install the optical measuring device in the direction of the machine coordinate axis that needs to be measured;
Adjust the laser head so that the measuring axis is collinear or parallel with the axis of movement of the machine tool, that is, the light path is pre-aligned and collimated;
Before laser warm-up, input measurement parameters;
According to the prescribed measurement procedure, the machine tool performs measurements;
Data processing and result output.
[Compensation of positioning accuracy]
If the measured positioning error of the CNC machine tool exceeds the allowable error range, the machine tool must be compensated for the error. 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 tool may have hundreds of thousands points, the manual compensation takes more time and is easier. Error.
Now through the RS232 interface, the computer is connected with the CNC controller of the machine tool, and the automatic calibration software written by VB controls 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 automatic pitch error compensation. The compensation method is as follows: :
Back up the existing compensation parameters in the CNC control system;
Computer-generated CNC machine program for point-to-point positioning accuracy measurement and transmitted to the CNC system;
Automatically measure the positioning error of each point;
A set of new compensation parameters are generated according to the specified compensation point and transmitted to the CNC system. The automatic pitch compensation is completed;
Repeat the accuracy verification.
According to the accuracy status of each axis of CNC machine tools, the automatic compensation of pitch error and the backlash compensation function can be used to reasonably select the compensation points for each axis so that the CNC machine tool achieves the best accuracy and greatly improves the efficiency of detecting the positioning accuracy of the machine tool. .
Positioning accuracy is an important indicator of CNC machine tools. Although the user can choose the machine with high accuracy and small error as much as possible, the longer the equipment is put into use, the more the equipment wears out and the more the positioning error of the machine becomes larger, which is fatal to the processed and produced parts. Impact. The above method is used to accurately measure and compensate the reverse deviation and positioning accuracy of each coordinate axis of the machine tool, which can well 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 in the best Accuracy status, thus ensuring the quality of parts processing.
