Analysis of the stability domain of machine tool system

Among the precision components of phased array radar, there are many shell-like members with deep cavity thin walls and small rounded corners. This kind of housing part is the core functional unit of phased array radar, and various high frequency microwave circuits and microelectronic devices are installed inside. The surface roughness and the flatness of the bottom surface directly affect the grounding and cooling effect of the built-in electronic components. The uniformity of the external dimensions of the housing parts directly affects the interchangeability of the housing components. Therefore, there is a high demand for dimensional accuracy, positional accuracy and surface roughness of such deep cavity housing parts.

In the machining of such parts, a lot of slender tools, that is, large aspect ratio tools, are used, and the elongated large length-to-diameter tool has poor rigidity. In the process of cutting with a large aspect ratio tool, if the machining parameters are not properly selected, the deviation of the part size caused by the tool and the cutting vibration of the machined surface caused by the cutting flutter will occur, which will inevitably reduce the dimensional accuracy of the part. And surface quality. In conventional production, the machining parameters of the machining are mainly dependent on experience and the trial cutting of the parts. In the early stage of processing precision deep-cavity thin-walled shell parts, due to improper selection of processing parameters, serious cutting vibration lines were generated on the inner side of deep-cavity thin-walled parts. After repeated trial-cutting to obtain new cutting parameters, qualified parts could be processed. However, the processing stability is poor, and the parts processing pass rate and processing efficiency are low. These prompted us to consider advanced scientific methods to obtain optimized CNC machining cutting process parameters. Using advanced cutting process dynamics simulation optimization technology, through the creation of mathematical models, engineering test analysis and simulation, through the dynamic testing of the relevant cutting machine tools, as well as dynamic simulation of precision deep cavity thin-walled shell parts. On the basis of a small number of experimental verifications, the cutting stability domain and optimized cutting parameters of parts such as precision deep cavity shells at different processing stages are quickly obtained. Avoid the chattering of the tool during the machining process, significantly improve the surface quality, product qualification rate and processing efficiency of the part.

1 The composition of the numerical control machining process parameter dynamics simulation optimization system is based on the dynamics simulation. The composition of the CNC machining process parameter optimization system is as shown. It mainly includes: (1) Establishing a system experimental modal analysis test platform, and performing modal parameter identification experiments. Test the dynamic characteristics of the cutting machine tool; (2) Establish the simulation analysis model of the different machining stages of the part and analyze its corresponding dynamic characteristics; (3) Comprehensively determine the length of the different tools by means of the "milling process dynamics simulation optimization system" Ratio, the cutting stability domain of the machining system and the optimized cutting parameters for different machining stages. Its function is: reasonable selection of cutting parameters, effectively suppressing the chattering phenomenon of the slender tool, and improving the processing quality and processing efficiency of the parts.

2 Simulation of dynamic characteristics of deep cavity thin-walled parts The modal parameters of the part system play an important role in determining the flutter stability domain of the entire cutting system (machine tool, tool? part system). Considering that the natural frequency of the part will change with the geometry of the part, the simulation analysis model of the different processing stages of the precision deep cavity shell part is established, and the CAE software such as MSC/NASTRAN /MARC is used in the actual clamping method. Constraints are carried out, and the finite element modal simulation analysis is used to obtain the dynamic characteristics of the workpiece at each stage and the modal parameters of the workpiece system to provide basic data for the calculation of the cutting stability domain.

As shown in the thin-walled deep cavity part, the rigidity of the part is very good at the initial stage of the part processing, that is, the natural frequency of the part is high. For this part, at the end of the processing, the part is driven to become thinner, and the rigidity of the part is weakened, which may cause chattering of the part during the machining process. Therefore, it is important to consider the rigidity of the workpiece when it is processed to the final stage. The semi-finished parts of the parts in the later stage of processing are as shown. At this time, the part has a small amount of machining allowance, and the clamping method is adopted by pressing the platen. After analysis by MSC software, the first 10 modes are as shown. The machining at this stage is to mill the machining allowance to obtain the final part.

From the modal analysis results, the first two natural frequencies are lower than 1 000 Hz, which may cause chatter at high speed cutting. From the vibration mode cloud diagram, there is a more obvious local mode, which is related to the partial compression of the constraint mode, which makes the vibration of some parts of the workpiece larger. In actual machining, consider increasing the contact area between the pressure plate and the workpiece. Limit local modalities to some extent. When determining the processing flutter stability domain and optimizing the cutting parameters, the dynamic characteristics of the part need to be considered.

3 cutting machine dynamics testing test cutting machine dynamics testing can be divided into software and hardware two major parts. Among them, the hardware part is mainly used for vibration mode experiments on machine tools.

The fundamental purpose of vibration modal analysis is to find the dynamic relationship between excitation and response, and to establish a model that reflects the inherent dynamics of the system itself. In order to accurately simulate the vibration of the tool during the milling process, it is necessary to obtain an accurate prediction model or frequency response function φ( jω) that reflects the tool response. In the process of cutting flutter stability domain analysis, the transfer function analysis of the processing system is the basis of the stability domain analysis. The analysis of the dynamic characteristics of the machine tool system is mainly for the vibration mode of the "machine tool? tool" system, and comprehensive analysis is carried out on this basis.

For the "machine tool? tool" system, considering the complexity of the machine tool and the tool system, it is not suitable for obtaining the modal parameters by means of simulation analysis. Therefore, the hammer impact test was carried out on the cutter tip portion of the "machine tool? tool" system. The frequency response experimental data is obtained by the modal parameter identification method, and the dynamic model parameters such as the natural frequency ωn, damping ratio ξ, and stiffness coefficient k of the actual system are calculated, thereby obtaining the transfer function reflecting the actual “machine tool?” system. The calculation of the stability domain provides the necessary conditions. The modal analysis results are shown and the system is subjected to flutter stability domain analysis.

4 Machine tool? Tool system flutter stability domain analysis using the "milling process dynamics simulation optimization system" software independently developed by Beijing University of Aeronautics and Astronautics, through the comparative analysis of the vibration stability domain simulation results of several machine tools, you can find the tool overhang The change of quantity seriously affects the dynamic performance of the whole machine tool system. When the overhanging amount of the tool of the same diameter and material is different, the minimum limit depth of the flutter stability domain varies greatly, and the range of the spindle speed corresponding to each stable flap is also different.

On the same machine tool, the cutter with the same diameter and material, the minimum limit depth of the flutter stability domain will decrease with the increase of the overhang amount; the main stable flap of the flutter stability domain will follow the overhang amount. The increase from the high speed segment to the low speed segment is more pronounced when the amount of overhang is large. When the tool overhang reaches a certain value, the dynamic performance of the "machine tool" system also changes.

5Experimental verification for the problem of cutting vibration marks in a radar thin-walled deep cavity shell part (cavity depth 52 mm, cavity wall thickness 1. 2 mm), analysis of part dynamics, machining machine and tool dynamics test On the basis of the simulation analysis of the vibration system stability domain of the processing system, the machine tool and the tool are: Machine model: M IKRON HSM700, tool (development and application of compound tool) Model: FRA ISA, cylindrical spiral end mill Material: Carbide, diameter = 10 mm, number of teeth = 2, overhang = 61. 6 mm. The simulation results are shown.

From the simulation of the machine tool? Tool flutter stability domain map analysis, as shown in the optimized process parameters: spindle speed N = 22 000 ~ 23 000 r / min; feed rate V f = 4 500 (mm / min); The cutting depth A p ≤ 1. 5 mm; the radial depth of cut A e ≤ 5 mm. When processing the above-mentioned precision deep cavity thin-walled shell parts in the previous stage, the process parameters selected by trial cutting: spindle speed N = 16 000 r /min; feed rate V f = 8 000 mm / min; axial depth of cut A p = 0. 8 ~ 1 mm; radial depth of cut A e = 0. 1 ~ 0. 6 mm. It falls on the edge of the simulation map's secondary stable domain. The stability domain analysis map clearly explains why the machining performance is unstable when machining the precision deep cavity thin-walled housing parts, and the side walls of the parts are prone to chattering.

The machining is verified according to the preferred stability domain provided by the stability domain analysis map, when the parameter is locked at the spindle speed N = 22 000 r/min; the feed rate V f = 4 500 mm / min; the axial depth of cut A p = 1. 40 mm; radial depth of cut A e = 4. 00 mm. Not only improves the processing quality stability of the precision deep cavity thin-walled shell parts, but also improves the processing qualification of parts to over 95%. Moreover, it has changed the long-term high-speed milling that must adopt the concept of high speed and less cutting. When the spindle speed is increased, the cutting amount of the tool is increased, and the side edge of the tool is fully utilized for machining, which can significantly improve the cutting efficiency.

The flutter stability domain map shown shows the two sets of processing data before and after the kinetic simulation. The left set of data is the data we obtained through trial cutting according to experience, and the right two sets of data are tested with reference to the flutter stability domain map. Cut the data. From the acquisition of cutting parameters and test results, the CNC machining cutting parameters obtained through the dynamic simulation approach are faster and more optimized. It not only ensures the processing quality of precision deep cavity parts, but also improves the processing efficiency of parts.

6 conclusion

In order to quickly determine the reasonable precision machining parameters of deep cavity thin-walled parts, the numerical simulation machining process parameter optimization method of dynamic simulation is used to test the specific machining machine and different long diameters by establishing the experimental modal analysis test platform of the milling process dynamics system. Based on the dynamic characteristics of the tool, the "Cutter Process Dynamics Simulation Optimization System" is applied to obtain different tool characteristics for specific machine tools, machining system cutting stability fields and optimized cutting parameters for different machining stages.

In the numerical control machining of a radar precision deep cavity shell and other parts, the optimization process of the NC machining process parameters by dynamic simulation improves the removal efficiency of the part material, effectively suppresses the cutting flutter phenomenon of the slender tool, and solves the precision deep. The problem of cutting vibration is generated in the sidewall of the thin-walled shell part during milling. The product qualification rate is over 95%, the processing efficiency is increased by 30%, and the optimization cutting parameter acquisition time is 1/3 of the conventional trial cutting.