CNC machine tool precision testing items and common tools

1 Introduction

For every factory, buying a CNC machine tool is a considerable investment. In order to make the invested equipment truly play a backbone role in production, ensure that qualified parts are processed, and recover the cost as soon as possible, it is crucial.

Experience shows that more than 80% of machine tools must be debugged on site during installation to meet their technical indicators. Therefore, when accepting new machine tools, they must be calibrated so that the machine tools can meet their technical indicators and expected quality and efficiency as soon as they are installed.

In addition, experience also shows that 80% of the machine tools that have been put into production are in an abnormal super-performance working state after being used for a period of time, and even exceed their potential bearing capacity. Therefore, new machine tools usually need to be calibrated again after half a year of use, and then they can be calibrated once a year. Regularly detecting machine tool errors and timely correcting pitch, backlash, etc. can effectively improve the accuracy of machine tools in production and use, improve the quality of parts processing, avoid waste, and greatly improve the utilization rate of machine tools. In short, timely revealing machine tool problems can avoid loss of machine tool accuracy and destructive use of machine tools.

With the further promotion and application of CNC technology, more and more CNC machine tools use their own probe systems to detect the size of workpieces and tools and perform digital profiling. It should be noted that the realization of the above functions is closely related to the accuracy of the machine tool itself. If the machine tool accuracy is not calibrated regularly, it is impossible to accurately complete the above work.

Renishaw ML10 laser interferometer linear displacement measurement software can provide data analysis according to the following standards: BS4656 British three-measurement machine standard; BS3800 British machine tool standard; ISO 230-2 international standard; VDI/DGQ 3441 German Engineers Association Machine Tool Standard; VDI 2617 German Engineers Association Three-Measurement Machine Standard; NMTBA American Machine Tool Association Standard; GB10931-89 Chinese National Standard; ASME B89.1.12M American Society of Mechanical Engineers Standard; ASME B5.54 American Society of Mechanical Engineers Standard; E60-099 French Standard; JISB2330 Japanese National Standard.

2 Advanced technology of Renishaw UK

Renishaw UK is a global multinational company specializing in the design and manufacture of high-precision testing instruments and equipment. Its main products are three-dimensional coordinate measuring machines and probes for CNC machine tools, laser interferometers, ballbars, etc. It provides the machinery manufacturing industry with a series of quality assurance methods for pre-sequence (laser interferometer and ballbar), in-sequence (workpiece probes and tool setting probes for CNC machine tools) and post-sequence (probes and configurations for three-dimensional measuring machines). All of its technologies and products are aimed at ensuring the accuracy of CNC machine tools, improving the performance of CNC machine tools, and improving the efficiency of CNC machine tools. It can ensure and improve the processing accuracy and quality of the working machines of CNC machine tool manufacturers and expand the market for finished products.

2.1ML10 laser interferometer

Renishaw ML10 laser interferometer provides a high-precision instrument for machine tool calibration. It has high accuracy of ±1.1PPM (at 0-40℃), large measuring range (linear length 40m, optional 80m), fast measuring speed (60m/min), high resolution (0.001μm), and good portability. Since Renishaw laser interferometer has automatic linear error compensation function, it can easily restore machine tool accuracy and is more popular with users!

In order to make everyone further understand the unique advantages of ML10 laser interferometer in detecting the accuracy of CNC machine tools, the following focuses on the application of ML10 laser interferometer in accuracy detection.

(1) Geometric accuracy detection can be used to detect straightness, verticality, pitch and yaw, flatness, parallelism, etc.

(2) Position accuracy detection and its automatic compensation can detect the positioning accuracy, repeat positioning accuracy, micro displacement accuracy, etc. of CNC machine tools. The Renishaw ML10 laser interferometer can not only automatically measure the machine's errors, but also automatically compensate for its linear errors through the RS232 interface, which saves a lot of time compared to the usual compensation methods and avoids operator errors caused by manual calculations and manual CNC input. At the same time, the maximum number of compensation points on the measured axis can be selected to achieve the best accuracy of the machine tool. In addition, the operator does not need to have knowledge of machine tool parameters and compensation methods.

At present, the compensation software available includes Fanuc, Siemens 800 series, UNM, Mazak, Mitsubishi, Cincinnati Acramatic, Heidenhain, Bosch, Allen-Bradley. [page]

(3) Detection of CNC rotary table indexing accuracy and its automatic compensation. Now, the ML10 laser interferometer plus the RX10 rotary table reference can also be used for automatic measurement of rotary axes. It can perform fully automatic measurement of any angle position and any angle interval with an accuracy of ±1. The new international standard has recommended the use of this new technology. Compared with the traditional method of using autocollimator and polyhedron, it not only saves a lot of measurement time, but also obtains a complete rotary axis accuracy curve, knows every detail of its accuracy, and gives statistical results processed according to relevant standards.

(4) Detection of dual-axis positioning accuracy and its automatic compensation Renishaw's dual laser interferometer system can synchronously measure the positioning accuracy of a certain axial movement of a large gantry mobile CNC machine tool driven by dual servos, and can also automatically compensate for the linear errors of the two axes through the RS232 interface.

(5) Dynamic performance detection of CNC machine tools Using RENISHAW dynamic characteristics measurement and evaluation software, laser interferometers can be used to perform machine tool vibration testing and analysis (FFT), dynamic characteristics analysis of ball screws, response characteristics analysis of servo drive systems, dynamic characteristics (low-speed creep) analysis of guide rails, etc.

2.2 QC10 ballbar

In the accuracy detection of CNC machine tools, QC10 ballbar and ML10 laser interferometer are two complementary instruments, neither of which can be missing. The ML10 laser interferometer focuses on detecting the various precisions of machine tools; while the QC10 ballbar is mainly used to determine the reasons for the loss of precision of machine tools and to diagnose machine tool failures. However, compared with the ML10 laser interferometer, the QC10 ballbar is not yet known by the majority of users. Therefore, the following will focus on the principle, function and application of the QC10 ballbar in detection.

2.2.1 What is a ballbar?

Renishaw's QC10 ballbar is a tool for rapid detection of two-axis linkage precision of CNC machine tools and machine tool fault analysis. It consists of a high-precision displacement sensor installed in a retractable fiber rod. The sensor includes two coils and a movable inner rod. Its working principle is similar to that of a displacement sensor using LVDT technology. When its length changes, the inner rod moves into the coil, the inductance changes, and the detection circuit converts the inductance signal into a displacement signal with a resolution of 0.1μm, which is transmitted to the PC through an interface. Its accuracy is ±0.5μm (at 20℃) detected by the laser interferometer.

When the machine tool moves in a circle with the ballbar length as the radius according to the predetermined program, the QC10 sensor detects the change in the direction of the machine tool's movement radius. Renishaw's QC10 analysis software can quickly separate the machine tool's straightness, verticality, repeatability, backlash, axis ratios and servo performance from the change in radius.

2.2.2 Main functions

(1) Rapid calibration of machine tool accuracy By using the ballbar to detect the machine tool at different feed rates, the operator can select a feed rate that meets the accuracy requirements of the workpiece for processing, thereby avoiding the generation of scrap.

(2) Rapid diagnosis and analysis of machine tool faults and problems The ballbar can quickly identify and analyze machine tool problems. It can mainly check the reverse difference, screw backlash difference, servo gain mismatch, verticality error, screw period error and other performance. For example, after a machine tool crashes, the ballbar can be used to detect and quickly tell the operator the accuracy status of the machine tool and whether it can continue to be used. The method of using the ballbar to detect machine tool accuracy has been specified in the ISO standard.

(3) Convenient maintenance of machine tools. The ballbar can tell the user how the machine tool accuracy changes, which can alert maintenance engineers to machine tool problems and perform preventive maintenance to avoid major failures.

(4) Shorten the development cycle of new machine tools. The ballbar can be used to analyze the influence of the selection of machine tool lubrication system, servo system, bearing pair, etc. on the precision performance of the machine tool. In this way, the original design can be changed according to the test results, thus shortening the development cycle of new machine tools.

(5) Convenient machine tool acceptance test For machine tool manufacturers, ballbars can be used to quickly conduct factory inspections of machine tools to check whether their precision meets the design requirements. Ballbars are now recommended for use by international machine tool inspection standards such as ISO230 and ANSI B5.54. For machine tool users, ballbars can be used to conduct machine tool acceptance tests to replace NAS test piece cutting, or test pieces can be cut after the machine tool has been inspected with a ballbar.

2.2.3 Working precision inspection

The QC10 ballbar is an instrument that can quickly (10 to 15 minutes), conveniently and economically inspect the two-axis linkage performance of CNC machine tools. It can be used to replace NAS test piece cutting for working precision.