Introduction
The positioning accuracy of CNC lathe refers to the position accuracy that can be achieved by the movement of each coordinate axis of the CNC lathe under the control of NC equipment. The positioning accuracy of CNC lathe can also be understood as the movement accuracy of CNC lathe.
Generally, CNC lathes use manual feeding, and their positioning accuracy mainly depends on the reading error, while the positioning accuracy of CNC machine tools mainly depends on the CNC system and mechanical transmission errors, and the positioning accuracy of CNC machine tools depends on CNC program instructions. Under the control of CNC equipment, the movement of each moving part of the CNC lathe is completed under the control of CNC equipment. The accuracy of each moving part under the control of program instructions directly reflects the accuracy of the processed parts. Therefore, positioning accuracy is an important detection content.
01-Linear motion positioning accuracy detection
Generally speaking, when CNC lathes and workbenches are unloaded, the linear motion positioning accuracy is high. The inspection of CNC machine tools should be based on national standards and the provisions of the International Organization for Standardization (ISO standards), and laser measurement should be used as the standard. If a laser interferometer is not used, ordinary users can also use a standard scale with an optical reading microscope for comparative measurement. However, the accuracy of the measuring instrument is 1~2 levels higher than that of the instrument being measured.
The ISO standard stipulates that the average error of each positioning point is calculated from five measurement data, and the positioning point error band is composed of the error-3 error band.
02-Repeatability detection of linear motion
The instrument used for testing is the same as the one used for testing positioning accuracy. The usual detection method is to measure at the midpoint of each coordinate travel and any three positions at both ends. Each position is positioned by rapid movement, and the positioning is repeated 7 times under the same conditions. The value of the stop position is measured and the maximum difference in readings is calculated. The positive and negative signs are used as half of the maximum difference in the three position differences as the repeated positioning accuracy of the coordinate, which is the most basic indicator for measuring the stability of axis motion accuracy.
03-Accuracy backtest of the origin of linear motion
The original feedback accuracy is actually the repeatability accuracy of a specific point on the coordinate axis, and its detection method is exactly the same as the repeatability accuracy.
04-Reverse Error Detection in Linear Motion
The reverse error of linear motion, also known as the loss of momentum, refers to the reverse dead zone of the coordinate axis to the driving components on the transmission chain (such as servo motors, servo hydraulic motors and stepper motors, etc.), as well as the comprehensive reflection of the reverse clearance and elastic deformation errors of each motion transmission pair. The larger the measurement error, the lower the positioning accuracy and repeatability.
The method for detecting the inversion error is to move a certain distance forward or reverse in advance during the stroke of the measured coordinate axis, and give a certain movement indication value to the stop position in the same direction based on the stop position, so that it moves a certain distance first, and then moves the distance in the opposite direction to measure the difference between the stop position and the reference position.
The three positions close to the middle point of the stroke and the two ends are measured multiple times (usually 7 times) to obtain the average value of each position, which is used as the maximum error value.
05-Rotary table position accuracy detection
The measurement has standard turntable, angle polyhedron, circular grating, parallel light tube (collimator), etc., which can be selected according to the actual situation. The range is that the workbench rotates one angle in the forward (or reverse) direction, stops, locks, and locates. Taking this position as the reference, the workbench is then quickly rotated in the same direction, and the position is locked every 30 times for measurement. The maximum value of the difference between the actual rotation angle and the theoretical value (command value) at each positioning point is the graduation error.
If it is a CNC rotary table, a positioning target should be determined every 30 meters, and rapid positioning should be performed 7 times in both positive and negative directions every 30 meters. The difference between the actual position and the target position is the position deviation. Then, the difference between the average position and the target position is calculated according to the method specified in GB10931-89 “Method for evaluating positioning accuracy of CNC machine tools”, that is, the maximum value of the difference between the average position and the standard position, and the difference between the sum of the differences between the average position and the standard position, that is, the positioning accuracy error of the CNC rotary table.
Based on the actual use needs and the characteristics of dry-type transformers, several right-angle points such as 0, 90, 180, and 270 are measured in detail, and the accuracy of these points is required to be one level higher than that of other angular positions.
06-Repeatable indexing accuracy test of rotary table
The test method is to select 3 positions at random within one week on the rotary table, repeat the positioning, and test them in forward and reverse rotation. In all readings, the maximum value of the difference between the theoretical value and the corresponding position is the indexing accuracy. If it is a CNC rotary table, take every 30 times as a measurement point, perform 5 rapid positioning of each target position from both the forward and reverse directions, measure the difference between the actual position and the target position, that is, the positioning error, and then calculate the standard error according to the method specified in GB10931-89. The maximum value of the standard error of each measurement point is 6 times of it, which is the repeated indexing accuracy of the CNC rotary table.
07-Turntable origin return accuracy detection
The measurement method is to reset the origin from 7 arbitrary positions respectively, determine its stop position, and use the maximum difference read as the accuracy of the origin reset.
It should be pointed out that the current detection of positioning accuracy is carried out under the condition of rapid positioning. For some CNC machine tools with poor feed system performance, different positioning accuracy values will be obtained by using different feed speeds for positioning. In addition, the test results of positioning accuracy are related to the ambient temperature and the working state of the coordinate axis. Therefore, most CNC machine tools currently use a semi-closed loop system, and the position detection elements are mostly installed on the drive motor. It is not surprising that an error of 0.01~0.02mm is generated within a 1-meter stroke. Due to the error caused by thermal elongation, some CNC lathes use pre-stretching (pre-tightening) methods to reduce its impact.
The repeatability of each coordinate axis is the most basic accuracy indicator, which reflects the stability of the motion accuracy of the coordinate axis. It is unimaginable that a CNC lathe with lower accuracy can be used stably in production.
Today, as the functions of CNC systems become increasingly rich, the system can compensate for the motion accuracy of each coordinate point, such as pitch accumulation error, chamfer error, etc. Only random errors cannot be compensated, and the repeatability accuracy reflects the comprehensive random error of the feed transmission mechanism, which cannot be compensated by the CNC system. When the error is found, the feed transmission chain can only be fine-tuned. Therefore, if you can choose a CNC lathe, it is best to choose a CNC lathe with higher repeatability accuracy.
