| The cutting forces generated during the cutting process affect the quality of the workpiece and the tool wear.Real-time and accurate cutting force monitoring is of great significance for studying the cutting mechanism and improving the processing quality.A mathematical model for cutting force measurement tool system embedded with thin film strain sensor,which was based on the measurement theory of cutting force for thin film strain sensor embedded in tool holder,was established in this paper.According to the theory of tool strength and stiffness,the principle of mutual coordination between deformation and sensitivity of multilayer composite thin film strain sensors embedded in tool holder,several typical cutting force measuring tool systems embedded thin film sensors in tool holder with optimized structure were innovatively designed,which was used for on-line monitoring and measuring of uniaxial or multi-axial cutting force.On this basis,a cutting force measuring tool system embedded with Ni Cr thin film strain sensor was mainly studied.The thin film sensor embedded in the tool holder with optimized structure was a analyzed by Finite Element Simulation.The preparation technology of functional layer films in the composite structure of sensor was studied.The resistance strain coefficient and static uniaxial loading test of the thin film sensor specimen were carried out,and the corresponding relationship between the static output voltage and the load of the thin film sensor was determined.The feasibility and correctness of the Ni Cr thin film strain sensor embedded in the tool holder with optimized structure for measuring the main cutting force were verified and determined.The main research results and innovations were as follows:(1)Based on the measurement theory of cutting force for thin film strain sensor embedded in tool holder,the principle of tool strength and stiffness,the requirement of mutual coordination between deformation and sensitivity of multilayer composite thin film strain sensors embedded in tool holder,several typical cutting force measuring tool systems embedded thin film sensors in tool holder with optimized structure were innovatively designed,which were mainly composed of thin film sensors,tool holder with optimized structure,cutting insert,compression components,and signal transmission & processing system.On this basis,a cutting force measuring tool system embedded with Ni Cr thin film strain sensor was mainly studied.The tool structure was optimized with grooving,drilling to form stepped sidewalls in the appropriate position.The thin film sensor was consisting of a 45 steel substrate,a Ni Cr thin film strain grid and Si3N4 insulating layers.Ni Cr thin film strain grid is linked for Wheatstone bridge.The research showed that under the same conditions,the output voltage of the optimized cutting tool system was more than 1 times higher than that of the original one.(2)Based on the shear lag effect of strain transfer in the alloy film sensor,the strain transfer function of the alloy film sensor was established.The effects of the layout and size of the film resistance grid on substrate surface on the strain lag coefficient and measurement accuracy of the film strain sensor were investigated.The influence of the structure size and the arrangement of the substrate surface for resistance grid on the shear lag coefficient and the measuring accuracy of the thin film sensor was studied.A method for calculating the resistivity of thin films with different thickness based on the roughness model of substrate and film surface was proposed,the theoretical formula of resistivity of corresponding thin film resistance grid was established.The influence of roughness of substrate and film surface on resistivity of film resistance grid was investigated.It provided a certain theoretical basis for the reasonable design of the layout and structure size of the film resistance grid on substrate surface,and ensured the better strain measurement performance of the sensor.(3)Taking the thin film sensor embedded in the optimized tool as the research object,the finite element analysis model of cutting force measuring tool system embedded with nickel-chromium thin film strain sensor was established,and its mechanical simulation analysis was carried out.The stress distribution of the resistance grid on the substrate surface was studied,which was affected by the thickness,width,spacing and length of the resistance grid.According this,the size of the resistance grid was optimized.The relationship between the output voltage of the thin film sensor and the uniaxial or multi-axial cutting force was researched.When the multi-directional force was applied on tool nose of the analysis model at the same time,the initial layout of the thin film resistance grid on the substrate surface could not eliminate the measurement interference error caused by x and y directional force on the z directional force.In order to reduce and eliminate those errors,two optimal arrangement methods of thin film resistance grid on the substrate surface were put forward.,The finally optimized arrangement method could eliminate the measurement interference error caused by x direction force to z direction,the measurement interference error caused by y direction force to z direction force accounted for 9.84%.,The effect of reducing the interference error of z direction force measurement is achieved through the above optimization.The theoretical relationship between the output voltage of the thin film sensor and the z force is obtained.(4)The preparation method of each layer of thin film sensor was studied and compared.Si3N4 thin film was prepared by RF(radio frequency)sputtering deposition method according to the previous experimental work of our research group.The Ni Cr thin film resistance grid was prepared on the surface of the 45-steel substrate by a process step of developing reverse glue,sputtering deposition of Ni Cr film and stripping reverse glue.The prepared Ni Cr thin film resistance grid pattern was observed by laser microscope,the thickness of the Si3N4 film and the Ni Cr film was measured.The cause of the defect of the film resistance gate was studied.It is found that the error of film thickness deviating from theoretical value was larger,ranging from 20% ~ 27.75%,but in general,Si3N4 film was above 2400 nm and Ni Cr film is above 700 nm.(5)The width of the Ni Cr thin film resistance grid was measured by an ultra-depth microscope.The width and length error of the resistance grid were within 3%,the width of the resistance gate is slightly larger than that of the electrode,ranging from 4.2% to 5.3%.The resistivity of the Ni Cr thin film was 0.984×10-6?·m measured by RTS-9 four-probe test,and the measurement result was smaller than the normal value with an error of 1.06%.According to the results of resistance measurement,the fabrication process of the sensor was optimized,so that the ratio between the measured value and the designed value was ranging from 2 and 4,which basically meet the design requirements.(6)Force Calibration and test of Ni Cr thin film strain sensor embedded in tool holder with optimizing structure for cutting force measurement were carried out.The mechanical properties and related parameters of the Ni Cr Thin film strain sensor were studied.The resistance strain coefficient of thin film sensor was obtained by uniaxial tensile test,which was 1.53.By the test of static loading on tool nose,the static force sensitivity of thin film strain sensor was calibrated with 0.012 m V/N;Compared with the theoretical value,the maximum error of calibrated test result was 21.44%.Basing on the z direction force measurement by KISTLER-9272 dynamometer and comparing the output voltages of two kinds of thin film strain sensor embedded in tool holder with optimized and initial structure,the results demonstrate that the output voltage of the optimized tool system is 0.68 times higher than that of the original one. |
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