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Advanced accuracy detection method of five-axis CNC machine tools

Dec 26, 2019

Traditional testing methods and tools are only used for small and medium-sized CNC machine tools with low accuracy. Most large-scale CNC machine tools in domestic factories are imported, and they use high-precision laser interferometers and ballbars. Today, judging the accuracy of machine tools through a series of direct inspections is still the method of choice for many machine tool manufacturers and users. The measurement uncertainty of the single detection method is very small, and the results are highly reliable. The more mature detection methods for linear axes are as follows:

Laser interferometer to measure positioning error

 five-axis CNC

Positioning accuracy of linear interferometer measured by laser interferometer

Use a laser interferometer to measure the positioning error as shown. Since the rotary axis of some machine tools is installed below the work surface, the movement accuracy of the rotary axis of the rotary axis can only be measured and improved when the machine tool is assembled. The single detection method can only be achieved: using a laser interferometer and a mirror group to measure the angular positioning accuracy of the rotary axis.

1.Cue stick method

The cue bar method is a method recommended in IOS 230-1. In 1982, J BBRYAN at the Lawrence Livermore National Laboratory in the United States, first developed a cue bar & cue bar for fast detection of CNC machine tool motion errors. It consists of a precision metal ball and a retractable connecting rod. A grating ruler for detecting displacement is inlaid in the middle of the connecting rod. When measuring as shown in the figure, one ball is fixed on the workbench through a magnetic steel seat, and the other Mounted on the spindle by the same device.

five-axis CNC

Ballbars can simultaneously measure the accuracy of circular movements under the state of two-axis linkage. The new ballbar equipment can measure the accuracy of circular trajectories on the XY, YZ, and XZ planes after one installation. Parameters such as verticality, repeatability, clearance, servo gain ratio matching of each axis, servo performance, and screw periodicity error can be reflected in the radius change of the circular motion profile. Generally, the measurement period does not exceed 1h,

Ballbar is a one-dimensional displacement measurement. When measuring multi-axis machine tools, complex linkage schemes need to be set up. The information that can be obtained by one-dimensional measurement is limited. From the perspective of three-dimensional measurement in space, many key error source information will be missed. The installation and adjustment of the ballbar instrument is still a work that requires higher technology and is difficult to achieve. Automatic and efficient machine accuracy inspection.

2.Plane grating method

As shown in the figure, a flat grating with a diameter of 140mm and a high-precision orthogonal grating is engraved on the workbench. A reading grating is installed on the spindle end. Within the effective working range of the flat grating, the workbench and the The relative motion of the main shaft is regular circular motion, linear motion or irregular complex curved motion.You can directly read the signal of good motion trajectory through the read head at the main shaft end and subsequent circuits, and the reading resolution can reach 5nm.

 five-axis CNC

The plane orthogonal grating method can detect the trajectory errors of the machine's linear compound trajectory movement (a combination of linear trajectory and circular trajectory), and calculate the 21 motion errors of the three linear axes of the CNC machine tool. In addition to the higher price of the instrument, it is the preferred method for today's field motion accuracy diagnosis. But the plane grating also has some limitations, because its detection is limited to the plane. It is not possible to evaluate the spatial accuracy of the five-axis machine tool multi-axis linkage (especially the linkage including the rotary axis).

3.R-Test method

The Rest measuring device was invented by Switzerland's S WEIKERT and WKNAPP in 2004 [Cs7, specifically for error measurement of five-axis CNC machine tools. The rest device consists of a high-precision standard ball (ceramic or steel) and a probe head equipped with 3 (or 4) displacement sensors. The plane of the sensor probe and the surface of the sphere are in contact with each other, and the standard ball and the detection head are respectively mounted on the main shaft end or the workbench, as shown in FIG. 5. When the machine tool performs multi-axis linkage, the standard ball moves with the spindle and the detection system moves with the table. The 5-axis machine tool can ensure that the relative position of the ball center and the base space remains unchanged during multi-axis linkage, that is, the tool center point] and the space of the workpiece The relative position does not change. Due to the geometric error of the machine tool, the relative positions of the sphere and the probe head are slightly offset, and the displacement in the three directions of the space is measured by the sensor. The measurement results of 3 (or 4) displacement sensors are converted into spatial X, YZ, and direction offsets. In addition, ZARCAR-BASHI and others have proposed a measuring device Capball, which uses a non-contact sensor. Japan's Soichi IBARKI and others conducted a more in-depth study of the R-test system and proposed the use of non-contact measurement. ISO TC30 / SC2 is already discussing revenue R-test. IBS Precision Engineering and similar R-test devices already in commercial use by FIDIA. Switzerland's WKNAPP and others are dedicated to R-test and Dn7 testing with precision ball discs. A precision ball disc is mounted on the CNC machine tool's workbench, and then the R-test probe is installed on the spindle end. Rest measures the ball and disc in a specific Kawaii order.

Compared to laser interferometers, planar gratings and ballbars, R-test has the advantages of 3D displacement detection. It can obtain a lot of information in one measurement, can analyze most error sources of five-axis machine tools, and rest can meet the standards. Balls or precision ball disks, etc., have great potential to realize complex motion detection, and are the hotspots in the research of machine tool accuracy today.

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