Tools can fail via various mechanisms. These mechanisms are complicated because of the number of factors affecting the cutting operations. According to Creese (1999), there are five different mechanisms, classified into two categories: primary and secondary.
The primary failure mechanisms are:
1. Flank wear
a. Rough cuts, VB = 0.76 mm [VB defined in Figure 3.1] b. Finish cuts, VB = 0.38 mm
2. Crater wear
The secondary (subsequent) failure mechanisms are:
3. Oxidation
4. Breakage (shock, fatigue)
5. Chipping of the tool (chatter, vibrations) 6. Plastic deformation
Tool wear is most often associated with flank wear (VB), which can be defined as the loss of tool material from the tool flanks during cutting. It is not always uniform along the major and minor cutting edges of the tool, and it occurs along the flank or relief face below the cutting edge. It becomes progressively deeper (more in-depth) as the tool wears, as shown in Figure 3.1. Tool wear is a complex phenomenon occurring in different metal cutting processes and is an event inherent in any cutting process. It can happen gradually by adhesion, abrasion and diffusion or may be subject to very rapid catastrophic mechanisms. Flank wear most commonly forms by the friction between the flank face of the tool against the newly machined surface of the workpiece, which leads to the loss of the cutting edge. Flank wear affects the dimensional accuracy and surface finish quality. Increasing the cutting speed leads to decreasing of adhesion wear and slightly oxidation wear, while all other types of wear increase. (Vučina et al. 2013).
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From the process point of view, flank wear is the most important wear mechanism that needs to be controlled and it is easier to measure. The mechanism of the material loss is better understood for most machining situations. It was often used as a criterion of tool life since it can be described using the Taylor tool life equation. According to the temperature distribution on the tool face, flank wear is mainly dominated by abrasive wear due to the change in the metallurgy properties of the workpiece material (Xie et
al. 2005). In the other word, during the cutting oprations, the cutter will effect the
workpiece, in particular, the temperature of the workpiece will rise and therefore, the metallurgy properties of the workpiece will change.
Figure 3.1 The wear in end milling cutter (ISO8688-2 2016) VB2
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Figure 3.2 shows the generally accepted relationship between flank wear and tool life. The behaviour of the tool wear curves is roughly divided into three regions (Koren et
al. 1991). Nonlinear at the initial stages (running in), linear at intermediate steps
(stationary) refers to a linear function of time and the slope is affected by work material as well as cutting conditions and nonlinear at the final stages (severe) when the flank wear is considerable before the tool breaks/fails completely. In the third state, the flank wear is substantial, and the cutter will wear much faster than to the other phases. Intensive vibration, higher cutting forces and raised temperature will have been induced in the latter phase. It is, therefore, highly recommended that the cutter is monitored more carefully to avoid tool breakage that arises at the end of this stage.
Each tool wear curve can be considered as;
Stage Ι: when the initial contact between the new cutter and the workpiece happens, the sharp cutting edge wears rapidly. It is relatively short and occurs within the first few minutes of tool use. In this phase, a high rate of initial wear results from the small contact area associated with the sharp cutting edge and with high contact pressure. These contribute to the breakdown or rounding off of the cutting edge. The initial wear value is usually given as VB=0.05-0.1mm.
Stage Π: in this stage, the cutting edge was rounding thus, this leads to improve the micro-roughness. The wear rate is proportional to the cutting time and is relatively constant. Tool wear will normally occur over a prolonged period at a minimal rate. Stage Ш: in this stage, the flank wear rate is increasing rapidly. This leads to increasing cutting force and temperature, and then the tool loses its cutting ability.
When the wear rises to a critical value, the component surface roughness will be increased, mainly when chipping occurs. The cutting force and temperature will increase rapidly due to increasing friction in the tool-workpiece and tool-chip interfaces. The flank wear will affect and change the shape of the component produced. Practically, this region of wear should be avoided. The stages of wear combined with a variety of
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wear modes make wear prediction difficult. This problem requires a systematic approach.
Despite the changes of the cutting conditions (for example cutting speed, V1, V2, V3, and V4), the general shape of the flank wear curve remains the same as shown in Figure 3.2. However, changes do affect the tool life, i.e. the gradient of the curve, especially the straight (linear) section. Cutting speed, feed rate and depth of cut are important cutting parameters in relation to tool wear. Tool wear is affected by many factors represented in fishbone diagram shown in Figure 3.3.
Contact between the cutting tool and the removed material chip can produce the most extreme conditions that apply only to the actual cutting area (Li 2012). This wear will change the tool geometry, which in turn will influence the cutting force, the power being consumed, the component surface finish and they can have profound effects on the overall quality of the machined workpiece and the dynamic stability of the process (Alamin 1996) (Jindal 2012). It is therefore crucial and necessary to understand tool wear in cutting operations in order to plan tool changes and avoid failure-related costs.
Figure 3.2 Flank wear as a function of cutting time (Black and Kohser 2017)
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The analysis of the information is challenging because the tribological behaviour of tool wear is not clearly defined and requires expertise for the interpretation of data. Nevertheless, wear analysis is recognised as a valuable source of information when managing machine performance.