USEFULNESS OF MODAL ANALYSIS FOR EVALUATION OF MILLING PROCESS STABILITY

The paper presents evaluation of modal analysis usefulness for determination of milling process stability. In the first phase of the study experimental modal analysis was performed and using CutPro 9.5 software, stability lobes were generated. In the next step, machining tests were carried out. The last stage of the experiment involved verification of modal analysis usefulness for evaluation of milling process stability based on surface roughness measurements. Conducted research allowed to state that modal analysis can be a useful tool for determining milling process stability.


INTRODUCTION
Machining is one of the fastest growing methods of manufacturing.Effective type of cutting is milling where it is necessary to determine the appropriate process conditions, ensuring its stable course.The stability of process is significant mainly in the context of obtaining maximum machining capability as well as avoidance of surface quality deterioration, resulting from vibrations of both the tool and the workpiece (particularly during cutting of thin-walled elements).Additionally, they generate accelerated wear of cutting tools and high-frequency noise (Bojanowski & Pawłowski, 2011;Budak, Tunc, Alan & Ozguven, 2012;Campa, de Lacalle & Celaya, 2011;Sun & Xiong, 2017).
Milling process stability is understood as a condition wherein self-excited vibrations do not occur or emerging oscillation is extinguished.This phenomenon is very complex and it depends on many factors.The most important of them are presented in Fig. 1 (Kilic & Altintas, 2016;Maamar, Bouzgarrou, Gagnol & Fathallah, 2017;Płodzień & Żyłka, 2013).Recently a dynamic development of issues related to machining stability has been observed.Currently, the appropriate definition of process condition, ensuring its stable course and avoidance vibrations is possible by using specialized software (e.g.: ShopPro, CutPro) that allows the selection of optimal technological parameters (Płodzień & Żyłka, 2013;Totis, Albertelli, Sortino & Monno, 2014).
The most widely used stability forecasting method of OUPN system (machineclamping-workpiece-tool) is to determine stability lobes for settled process conditions.Stability lobes present the dependence of rotation speed n usually in function of depth of cut ap (or width of cut ae) defining stable and unstable areas.Experimental modal analysis is used for obtaining these curves (Totis et al., 2014;Totis, Albertelli, Torta, Sortino & Monno, 2017;Zhang, Xiong, Ding, Feng & Xiong, 2012).
The issue with OUPN system during milling proceeds mainly from the intermittent work of mill, causing the periodic tool deflection from the workpiece.As a result of this, corrugated structure is formed on work surface after the pass of cutting tool.To sum up, if frequency of entering blade is not equal to frequency of natural vibrations, phase displacement is formed between the tool and the workpiece.Consequently, the effect is a variable value of width of cut layer h, resulting in various cutting forces and generating negative self-excited vibrations (Ahmadi & Ismail, 2012;Ciurana & Quintana, 2011;Lehrich & Lis, 2014;Twardowski, 2006;Zhang et al., 2012).

METHODOLOGY
The aim of the study was to evaluate modal analysis usefulness for determination of milling process stability.In the first stage of the study, modal analysis was carried out and based on stability lobes, technological parameters corresponding to stable and unstable ranges of tool work were read.On the grounds of the obtained results, machining tests were made.Then, the verification of modal analysis usefulness for determination of milling process stability was performed, based on surface roughness measurements.
The study was conducted on vertical machining centre AVIA VMC 800 HS with application of set for modal analysis (Fig. 2), where main elements were: modal hammer with force measurement sensor, accelerometer, data acquisition module and a computer with CutPro 9.5 software, among others.Aluminum alloy EN AW-2024 T351 (chemical symbol: EN AW-AlCu4Mg1) that is widely applied in industry, especially aerospace, was used for the study.Its chemical composition and selected properties are presented in Two solid end mills destined for aluminium alloys machining were used in the experiment:  Seger (PUN-12-00-24/50-100-3AL),  SGS Solid Carbide Tools (44748).
Mill of Seger (PUN-12-00-24/50-100-3AL) that was made on special order is shown in Fig. 1, and Table 2 presents its technical specifications.The cuboidal samples were machined during milling.They were mounted in a vice as is presented in Fig. 5.

Fig. 5. Method of sample fixing
Surface quality was adopted as a criterion for evaluation of milling process stability.For this purpose, parameter Ra was measured by using a contact profilometer -Hommel Tester T1000 (Fig. 6).

Fig. 6. Hommel Tester T1000 profilometer
Measurements of surface roughness were repeated five times for each tested tool.

RESULTS
Based on the collected signals and defined parameters, stability lobes were generated by means of CutPro software.These charts were used for the determination of stable and unstable areas.
The obtained graphs were modified by adding the values of technological parameters for machining tests:  stable workgreen colour,  unstable workred colour.
The obtained stability lobes for mill of Seger is presented in Fig. 7 and Fig. 8 for mill of SGS.The range of stable tool work corresponds to the areas that are located under stability lobes.Based on graphs, the existence of liminal depth of cut ap is noted below where theoretically machining is stable, regardless of cutting speed vc.The characteristic increase of width range of stable and unstable areas with growth of rotation speed n is also visible.It facilitates the selection of machining parameters (rotation speed n), to ensure that they are located in the range of stable or unstable machining, therefore, the highest values of rotation speed n were selected for the verification.Additionally, the points corresponding to the central positions of both stable and unstable areas were taken.
Another phase of study was the verification of modal analysis usefulness for determination of milling process stability based on surface roughness measurements.The values of Ra surface roughness parameter for examined mills and technological parameters corresponding to stable and unstable machining are shown in Fig. 9.With regard to graph (Fig. 9) the green columns present the obtained roughness values which according to the results of modal analysis corresponding to machining parameters ensure stable work.Unstable conditions were marked with red colour.The higher values of Ra parameter were obtained for unstable conditions of machining for both mills.

CONCLUSION
The conducted analysis allowed to formulate following conclusions:  On graphs generated from CutPro software, the existence of liminal depth of cut ap has been noted, where theoretically machining is stable, regardless of rotation speed n.
 The characteristic increase of width range of stable and unstable areas with growth of rotation speed n has been also visible. The higher values of Ra parameter were obtained for unstable conditions of machining for both cases. The performed verification allows to state that experimental modal analysis using CutPro software can be a useful tool for evaluation of milling process stability. The results of modal analysis help to determine machining ensuring required machining quality.

Fig. 9 .
Fig. 9.The values of Ra surface roughness parameter for two tested mills and technological parameters corresponding to stable and unstable machining