Research and Application of Computer-Assisted Measurement Technology of Cutting Force-EEWORLD

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The measurement of cutting force is not only conducive to studying cutting mechanism, calculating power consumption, optimizing cutting amount and tool geometric parameters, but more importantly, it can monitor the cutting process through the change of cutting force, reflect the cutting state such as tool wear or breakage, rationality of cutting amount, machine tool failure, chatter, so as to timely control the cutting
process , improve cutting efficiency and reduce the scrap rate of parts. Commonly used cutting dynamometers are resistance strain gauge type and piezoelectric type. Using computers to collect and process cutting force measurement data has been very popular in cutting experiments and production practices. The dynamometer converts the three measured cutting forces into electrical analog signals and converts them into digital signals through A/D converters and inputs them into the computer, which performs various analysis and processing on the collected data. Traditional computer measurement and analysis use general or specially designed interface boards, and the subsequent analysis and processing programs are generally compiled in C/C++ or BASIC in a general software programming environment, which does not fully utilize the powerful functions of the computer, and the communication function and data processing function are difficult to realize. In addition, if the cutting force test software compiled for a certain processing method is applied to another processing method, the program code needs to be greatly changed, which lacks intuitiveness, versatility and flexibility.
　　Virtual instrument is an important technology in the field of computer-aided measurement and control today. It uses computer as a unified hardware platform, equipped with modular hardware interface cards with test and control functions and data exchange, supplemented by software modules with test instrument functions and realistic images, and realizes the functions of traditional measurement and control instruments through unified command and dispatch of system management software. This software-centric system does not have to be restricted by the functions designed by manufacturers like traditional instruments. It can make full use of the computer's super-strong computing, display and connection expansion capabilities to flexibly define powerful instrument functions. Compared with traditional instruments, virtual instruments have obvious advantages in terms of intelligence, processing power, performance-price ratio, and operability.
　　It is very advantageous to use virtual instrument technology to collect and process cutting parameters online and monitor the cutting process. No research in this area has been reported so far. This paper introduces virtual instrument technology into the measurement of cutting force, and designs a virtual instrument that measures three-dimensional cutting force and analyzes and processes cutting force data.
　　2 System Block Diagram
　　Taking turning as an example for experimental research, the block diagram of cutting force measurement experimental research is shown in Figure 1 (the system interface block diagram is also applicable to other processing methods). Test instruments: Resistance strain gauge turning dynamometer is used in conjunction with YD-28 dynamic resistance strain gauge. Resistance strain gauge has the advantages of high sensitivity, instantaneous force measurement, mutual interference of various forces can be eliminated by electric compensation principle, good dynamic characteristics, low price, etc.
　　Universal interface card PCI-1200 is used for data acquisition and A/D conversion. PCI-1200 card is a multi-function data acquisition control card based on 32-bit PCI bus, supporting DMA mode and double buffer mode, ensuring uninterrupted acquisition and storage of real-time signals; it has multiple functions such as supporting unipolar and bipolar analog signal input, providing 16 single-ended/8 differential analog input channels, 2 independent D/A output channels and 24-line TTL digital I/O.
　　Connection method: The turning dynamometer used in this system has a force measurement range of 0~3000N, and the resolution is 5.0N when measuring the cutting force Fx, Fy, and Fz in the three directions of X, Y, and Z. The three lead wires of the dynamometer correspond to the three cutting force components Fx, Fy, and Fz (some dynamometers also have lead wires for measuring torque). The three cutting force signal wires are connected to the three channels of the strain gauge (all strain gauge output signal wires not used for measurement are left hanging); insert the PCI-1200 card into an empty PCI slot on the computer motherboard and connect it to the connector through a 50-core data cable.
　　3 Software Design
　　After configuring the corresponding hardware modules, the main work of virtual instrument design is to compile the corresponding software to complete data acquisition, storage analysis, output and display. National Instruments Corporation of the United States has put forward the loud slogan of "software is instrument". Its innovative product Labview is currently the most successful and widely used virtual instrument development environment (in fact, the concept of virtual instrument was originally proposed when developing LabVIEW). As a program development environment, LabVIEW uses the graphical programming language G, which is different from other languages. The generated program is a block diagram instead of text, and contains a variety of powerful function libraries, with data acquisition, GPIB, serial port control, data analysis, display and storage, and network functions.
　　Virtual instrument software includes three levels: instrument driver, application program and soft panel program.
　　The instrument driver is mainly used to initialize the virtual instrument and set specific parameters and working modes to keep the virtual instrument in normal working state. LabVIEW has equipped the PCI-1200 card with a driver.
　　The following focuses on the design of soft panel program and application program.
　　1. Soft panel
　　The soft panel program is used to provide an interface between the virtual instrument and the user. It can generate a graphical interface similar to the traditional instrument panel on the computer screen to display the measurement results, etc. Users can also simulate various operations of traditional instruments through the switches and buttons on the soft panel, and operate the virtual instrument through the keyboard or mouse. The
　　soft panel of the cutting dynamometer: uses a real-time trend chart (chart) to display the three-way cutting force; continuously extends the new data behind the existing data, and the waveform is continuously pushed forward for display; controls the cutting force scale calibration through three buttons; stores the measured data in the form of a data text file for analysis and processing and waveform playback; sets the button as a storage switch, and controls the storage interval and storage path at the same time.
　　2. Application program
　　The application program is mainly used to analyze and process the data input to the computer. The user defines the function of the virtual instrument by compiling the application program. This is the main part of software design.
　　The software functions include three-dimensional cutting force data acquisition and storage, cutting force waveform display and playback, cutting force calibration, experimental data statistical analysis (error analysis, abnormal data processing, etc.), cutting force empirical formula, cutting power calculation and cutting process evaluation.
　　Data acquisition and storage
　　LabVIEW provides powerful DAQ (Data Acquisition) product software support, and the data acquisition function is easy to implement. Use the AI WaveformScan.vi node in the Analog Input Utilities sub-template in the DAQ template to control the data acquisition of each channel of the PCI-1200 card (mainly control the acquisition card scanning frequency and the number of scans of each channel). The program uses a three-dimensional array to store the sampling values of the three component forces for waveform display. The value of the array element is dynamic and is controlled by a switch button to determine whether to store it.
　　Cutting force calibration
　　The dynamometer needs to be calibrated so that the output reading during force measurement can be converted into force value. The correctness of the calibration will directly affect the reliability of the measurement results. Calibration is divided into static calibration and dynamic calibration.
　　Use a standard force ring to load each force component of the dynamometer separately, read the face value or waveform ordinate value on the virtual instrument soft panel, and obtain the relationship between the loading force and the panel reading, and record the output readings of other force components at the same time. The ordinate scale is automatically marked through the algorithm inside the program, and the scale coefficient of each force component direction is recorded. The sampled value can be automatically converted into a force value during subsequent sampling. When
　　loading, the point of application of the force should be strictly at the position of the tool tip, and the direction of the line of action should be accurate. For the mutual interference of the various force components, software methods can be used to eliminate it: first measure the interference value of each force component (it is a constant within the linear range), and during actual measurement, directly increase or decrease it during data processing and waveform display according to the actual reading.
　　Determination of cutting force empirical formula
　　The single factor experiment method was adopted. The cutting force index formula was established by the least square method. Since the main factors affecting the cutting force are the depth of cut and the feed rate, they are included in the empirical formula, and other minor factors are used as the correction values of the empirical formula. For each cutting force component, the regression line equation in the logarithmic coordinate system is first established, and then converted into the exponential value in the empirical formula.
　　LabVIEW integrates the commonly used mathematical and signal analysis algorithm programs in data acquisition and test analysis, and provides an advanced digital and signal analysis environment. The coefficients and exponents in the formula can be obtained without special programming. After selecting points, statistical analysis and abnormal data processing for the data measured in the test, the linear fit (Linear Fit) and exponential fit (Exponential Fit) nodes on the Curve Fitting (Curve Fitting) sub-template in the Mathematics template are directly used to calculate the coefficient values and exponential values.
　　Cutting power calculation
　　Cutting power P is of great significance for the determination of tool wear or breakage, and is an important indicator for cutting process monitoring. Cutting power refers to the power consumed in the cutting process, which is the sum of the power consumed by the tangential cutting force Fz and the axial cutting force Fx. Since there is no displacement in the cutting depth direction, the radial cutting force Fy does not consume power. The following formula is used to approximate the calculation of Pm: Pm=FmV×10-3kW, where V is the cutting speed, which can be calculated by inputting the workpiece speed and the diameter of the workpiece to be processed at the test site.
　　Determination of cutting state
　　Through the change of cutting force, the cutting process state such as tool wear and breakage, chatter, built-up edge and the rationality of cutting parameters can be determined. The state determination is realized by using the knowledge-based reasoning method (it is required to establish several criteria and databases in advance according to cutting tests or cutting manuals). The following is an example of the determination of tool wear.
　　Tool wear mainly depends on the physical and mechanical properties of tool materials, workpiece materials and cutting conditions. The main causes of normal tool wear are mechanical wear, thermal and chemical wear. Tool wear generally has four types: hard point wear, adhesive wear, diffusion wear and chemical wear. When different tool materials process different workpiece materials under different cutting conditions, the main causes of wear may be one or two of them. Tool wear will directly affect machining efficiency, quality and cost. The tool wear process is divided into the initial wear stage, the normal wear stage and the rapid wear stage. When the tool reaches the rapid wear stage, the tool cannot be used any more (this limit is called the blunting standard). However, in actual production, it is impossible to frequently remove the tool to measure the wear amount to determine whether the tool has been blunted. It can only be determined based on some phenomena in the cutting process (such as changes in cutting force).
　　When tool wear increases, the cutting force acting on the front and rear tool faces also increases. The increase in cutting force, the change in cutting force ratio, the change in dynamic cutting force, etc. can be used to determine chip breakage, changes in built-up edge, or the wear state of the front and rear tool faces and blunt circles of the tool.
　　Since the number of tools and workpiece materials used by each specific manufacturing enterprise is limited, it is possible to conduct several cutting tests in advance to record the three-way cutting force ratio and waveform curve that reaches the blunting standard when a certain tool cuts a certain material, and store the instantaneous value and average value of the cutting force in an array (establish a judgment database). The cutting force waveform curve obtained during actual turning is judged for similarity with the blunting waveform curve stored in the library to obtain a similarity quantitative index. The predetermined similarity threshold is used as the criterion for determining whether the tool is blunt; in general, the waveform curve is straight, so the average value and instantaneous value of the cutting force can also be directly compared to determine the blunt condition of the tool.
　　The cutting test shows that the key to successfully using cutting force to reflect the tool wear state is the waveform curve similarity judgment module and the establishment of the cutting force waveform curve when the tool is blunt.

　　4 Conclusion
　　Although the research and application of computer-aided measurement technology for cutting force is not a new topic, the development of virtual instrument software technology based on LabVIEW makes it more powerful, more convenient to program, and more flexible to use, especially in the processing and graphical display of cutting force data. The cutting force measurement virtual instrument developed in this paper has reached a practical level, and with slight improvements, it can also be applied to the measurement of other cutting parameters (such as cutting temperature). With the in-depth study of virtual instrument software technology, it is believed that it will play a greater role in the monitoring of mechanical processing processes. 文章内容结束

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