Experimental Research On Status Identification Of Eccentricity And Wear For Four-Flute End Mills Based On Spectrum Of Milling Force

In milling process, the tool status will affect the processing quality of the workpiece.Real-time monitoring on tool status can detect and predict earlier failure of tool, and thenappropriate solution can be applied to reduce economic losses. Eccentric, wear, breakage,deflection, chatter etc. are some composition factors of the tool status, there existsphenomenon of interfere with each other within signal characteristics of these factors. Beforejudging the status of the tool, it is necessary to distinguish and identify in advance thesevarious status factors.Cutting force, and spindle current, motor power, etc. that associate with it, are commonlyused as tool condition monitoring signals. Wear is a common failure mode of tool in process,and eccentricity is an inherent phenomenon of tool, their impacts on cutting force and itsassociated signals are similar. Four-flute end mills is a commonly used peripheral mills, ingeometry, the intersection angle between its two adjacent teeth is90o, thus there may existorthogonal phenomenon among cutting forces of every tooth. Based on this characteristic, inthis thesis, four-flute end mills was taken as studying object, its different impacts on cuttingforce, and approaches of identifying its eccentric distance and wear degree are studied.There is only eccentric phenomenon on the tool before engaging cutting, only afterengaging cutting does wear appear. According to geometry of the tool with eccentricity andwear, mathematical expressions of cutting force for four-flute end mills under onlyeccentricity, both eccentricity and wear were deduced.The cutting force under status of no wear was decomposed into nominal component forceof ideal tool and eccentric component force induced by eccentricity, the nominal force is aperiodic signal taking tooth frequency as basic frequency, while the eccentric force is aperiodic signal taking spindle frequency as basic frequency. Based on these mathematicalexpressions, their trigonometric function expansions of Fourier transform were analyzed, andthe amplitudes of even order harmonics for eccentric cutting force wear found to be zero, i.e.,the first harmonic and third harmonic of spindle frequency for resultant cutting force are inrelation with eccentricity, wile are independent of cutting parameters. And the second harmonic is independent of both eccentricity and cutting parameters, the fourth harmonic is inrelation with cutting parameters, while is independent of eccentricity. Moreover, it was alsofound that under the status with no wear, the amplitude of the first harmonic (that is,component force in spindle frequency) is in relation with axial cutting depth, eccentricdistance and immersion angle, and that of the fourth harmonic (that is, component force intooth frequency) is in relation with axial cutting depth, feed per tooth and immersion angle,the eccentric distance can be simply estimated using their ratio.With Matlab software, the simulative cutting force under statuses of neither eccentricitynor wear, only eccentricity, both eccentricity and wear are taken to simulate force spectrums.The simulative results showed that: the first harmonic component of spindle frequency wasmainly induced by eccentricity, and was in relation with wear too, but had nothing to do withcutting parameters, in the case of multi teeth wearing or greater degree of wear on single tooth,wear would have greater impact on the amplitude of this harmonic. The second harmoniccomponent was in relation only with wear, and has nothing to do with cutting parameters andeccentricity. The fourth harmonic component was closely relevant to cutting parameters, butwas not relevant to eccentricity, and was little relevant to wear.These simulation results agree with above theoretical analysis conclusions. The amplitudevariation of first harmonic for cutting force can be used to identify eccentric degree of tool, asthe amplitude ratio of the first harmonic and the fourth harmonic excludes the impacts ofcutting parameters, it can be used to estimate eccentric distance of tool. Also as the amplituderatio of the second harmonic and the fourth harmonic excludes the impacts of both cuttingparameters and eccentricity, it can be used as index of identifying the severe degree of toolwear.Based on the conclusions in theory and simulation analyses, milling and force measuringexperiments were done. Various four-flute end mills and cutting parameters were used to cutmaterials#45steel and T10steel, the major components in direction of feed or cutting widthwere applied substituting the resultant forces, its amplitude spectrums were obtained withalgorithm of FFT, and the identification approaches deduced in section of theory andsimulation analyses was used to estimate eccentric distance and analyze degree of wear. Theexperimental results showed that: amplitudes of the fourth harmonics were approximately proportional to the values of cutting parameters, which indicates that the amplitude of thefourth harmonic is approximately in proportional relationship with the cutting parameters. Inthe original cutting test, the amplitudes of second harmonics were much smaller than those ofthe other harmonics, and was nearly zero, which indicates that under the case of minor wear,the second harmonic of cutting force is near zero. In early cutting tests, the estimated valuesof eccentric distance varied little, which indicate that this estimation approach of eccentricdistance present in this research is effective. In early cutting tests, generally the amplituderatios of the second harmonic to the fourth harmonic tended to increase following the cuttingtest number, under the case with high spindle speed as4560r/min and6330r/min, theamplitude ratios of the second harmonic to the fourth harmonic varied greatly, because toolwill wear more seriously in the case of high spindle speed, these results indicates that theapproach of identifying severe degree of tool wear present in this research exist effectiveness.This research reveals the various impacts of eccentricity, wear and cutting parameters on1~4order harmonic of force for four-flute end mills, and presents a new method forestimating eccentric distance and identifying wear degree, the research conclusions can beused in status monitoring in process of four-flute end mills.