Proceedings of SerbiaTrib '13

edge without chamfer and smaller radius, versus thecutting length corresponds to a trapezoidal forcepattern at significantly lower entry impact durationof 0.036 ms.Considering these facts and the results exhibitedin Figure 4b, the chamfered coated cutting edgescan withstand to fatigue failure approximately atwo and half times higher entry impact forceamplitude. In this way, at the same stress level, thefilm failure of a chamfered cutting edge may appearin up milling after a longer cutting time comparedto an insert without chamfer. The temperaturedeveloped close to the transient region of thecutting edge between flank and rake amounts toabout 200 o C at a cutting speed of 200 m/min andchip tool contact time up to roughly 15 ms [4].Thus, in this cutting edge region, the crystallinestructure of the investigated TiAlN film remainsstable, no diffusion or oxidation takes place and thefilm fatigue, which can be investigated by theimpact test, is the prevailing factor.Figure 4. a) Triangular and trapezoidal impact forcesignals b) Effect of impact signal and entry impactdurations on the critical force amplitudeAll applied triangular force signals withdurations (FSD) of 10 ms, 20 ms and 35 ms and thetrapezoidal ones of 20 ms and 40 ms, which arepresented at the upper Figure 4a part, had aconstant signal growth time t e of 5 ms (entry impactduration t e ). In contrast, the displayed force signalsat the bottom of Figure 4a possess different entryimpact durations t e from about 0.5 ms up to 15 ms.These force signals are created by the piezoelectricactuator and measured by the piezoelectric forcetransducer.The effect of the force pattern on the criticalforce amplitude, which induces coating fatiguefailure after one million impacts, is monitored inFigure 4b. According to these results, the criticalfatigue force amplitude remains practicallyinvariable versus the force signal duration atconstant t e . On the other hand, t e affectssignificantly the film fatigue behaviour, as it isexhibited in the same diagram. An increase of theimpact entry duration t e from 0.05 ms up to 15 msresults in a significant critical fatigue impact forceamplitude augmentation from about 60 daN up to220 daN respectively. The cutting load signal, i.e.the stress course versus the cutting length, when achamfered cutting edge is used, resembles to atriangular force signal at entry impact duration of3.6 ms [3]. Moreover, the stress course on a cutting4. FLANK WEAR DEVELOPMENT VERSUSTHE CUTTING EDGE ENTRY IMPACTDURATIONThe contact conditions at the tool entry into thematerial in milling are pivotal for the tool wear [1,2, 4, 7, 8]. The impact load on the cutting edge atthe tool entrance into the workpiece materialdepends on the milling kinematic (up or down,peripheral or face), since these factors affect thedeveloped chip geometry and thus the stress fieldsof the coating versus the tool rotation. The entryimpact duration corresponds to the cutting time, upto the development of the maximum equivalentstress in the coating.For describing the effect of the entry impactduration on the tool wear in milling with coatedtools, the accumulated tool life is introduced. Thelatter parameter refers to a flank wear land widthVB of 0.15 mm. This parameter can be calculatedconsidering the undeformed chip length l cu , thecutting speed v and the attained number of cutsNC 0.15 up to the same VB according to the equationshown in the upper part of Figure 5a. In Figure 5aand 5b characteristic examples concerning theeffect of the entry impact duration on the tool lifeare exhibited. These examples refer to peripheraland face milling of different undeformed chiplengths. Further examples in milling at variousconditions, kinematics and materials are presentedin [3, 9, 10, 11]. As it can be observed in Figure 5a,at an undeformed chip length of roughly 80 mm, asimilar tool wear evolution in up and down, face orperipheral milling develops, leading to almost thesame accumulative tool life.13 th International Conference on Tribology – Serbiatrib’13 15