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35TOOL WEARMaximum wear definition is crucial for determining tool life and cutting time after which a tool cannot be used.Fig. 4.11 shows wear development in normal cutting conditions. Several stages can be associated with a wear development process:I %u2013 initial wear that features rapid wear developmentII %u2013 uniform increase in wear until the critical value of a deterioration is reachedIII - sharp increase in wear resulting in failure.The critical value of a deterioration, which characterizes maximum wear, corresponds to the tool life. A few factors have an impact on tool life, and among them the most important are cutting conditions and machinability of a workpiece material.Fig. 4.11. tool deteriorationmaximum wear parameterI II IIIT cuttingtimeStages of tool wear developmentThere are direct and indirect methods for measuring and inspection of tool wear.Direct methods use microscopes, coordinate measuring machines, digital image processing etc.Indirect methods are based on analyzing end machining results (such as surface finish or accuracy) and operation characteristics (power consumption, cutting force, vibration etc.). In addition, these methods also include visual inspection; appearing noise; checking a cutting edge by a fingernail, which, despite safety requirements, is still common and even total tool failure.Fig. 4.12.Checking tool wear by use of computerized microscope in ISCAR%u2019s Technical CenterWear Detection by %u2026 CoatingIn multi-layer coating of indexable inserts, the top coating layer, in addition to the layer function in wear protection, can play one more important role %u2013 wear detection. Modern coating technologies utilize a colored wear-indicator top coating layer. During cutting operations, distinct visible trails remain on the layer, and this greatly contributes to early wear detection.