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Energy consumption characterization in precision hard machining using CBN cutting tools

In this paper, the contribution of tool wear to the energy balance was determined for precision hard turning using chamfered CBN cutting tools. The tool nose wear VBC and the corresponding changes of component forces Fc, Ff and Fp resulting from tool wear evolution were continuously measured during wear tests. Based on the cutting mechanics, specific cutting and ploughing energies were calculated for a number of tool wear states. In particular, changes of energy balance due to tool wear under variable feed rate, depth of cut and tool nose radius were discussed. A distinction between material removal conditions resulting from precision cutting and grinding at a very low uncut chip thickness is considered.


Introduction
Hard machining has been established a leading machining technology for various machine components made of hardened steels, such as geared shafts, bearing and hydraulic components, which replaces or assists grinding operations [1,2]. Predominantly, scientific and engineering issues of hard turning cover such problems as cutting mechanics, chip formation, tool wear, surface integrity and part accuracy [3][4][5]. Unfortunately, energy balance in hard machining resulting from the specific action of the cutting edge has not been investigated in a satisfactory manner, but it seems to be an important objective of hard machining research. Moreover, more work is needed to optimize energy usage in metal cutting besides economic objectives because it is a process with large energy consumption and low energy efficiency [6,7]. In case of hard machining, the energy consumption increases distinctly due to extreme high hardness of the material machined and high negative rake angle of the CBN cutting tool used. In addition, tool wear should evidently influence friction and intensify ploughing action of the cutting edge. In general, hard machining is distinguished by dominating radial (passive) force in comparison to conventional turning for which the radial force F r =(0.3-0.5) F c . Consequently, the radial force cannot be neglected in characterizing static and dynamic behaviour of the machining system. This rule is also valid for tool wear evolution, especially when machining with tools of large nose radius of 800 and 1200 μm [8]. In particular [4,9], CBN tools with large negative chamfer angles cause the increase in the passive force and more intensive ploughing action which produces higher friction and wear and reduces tool life. Similarly, tool nose radius affects uncut chip geometry that increases ploughing forces in the hard turning process [10]. In this study, energy consumed in hard turning of 16MnCrS5 (AISI 5115) hardened steel with worn CBN tools under the variable feed rate, the depth of cut and tool nose radius was determined based on measurements of the component forces F c , F f and F p during tool wear [8]. The obtained values of the specific cutting energy corresponding to the three process variables were mapped in the diagram showing its dependence on the undeformed chip thickness [11].
2 Mechanics of the machining process 2.1 Geometry of cross-sectional area of cut In finish hard turning, the depth of cut (a p ) is generally smaller than the nose radius (r ε ) and the cross-sectional area of cut has a comma shape as shown in Fig. 1a. Consequently, the cutting ratio b/h is higher than 1. The cross-sectional area for such a case of cutting is described by the effective contact length l k and the average undeformed chip thickness (UCT) h m . In addition, the effective tool edge angle κ re is defined when replacing a curvilinear cutting edge being a part of the nose by an equivalent straight cutting edge (ECE) shown in Fig. 1b.
Geometrical quantities (the cutting tool angle κ re , the contact length l k , the average UCT h m , the area of cut A nc ) are expressed by a set of equations (1.1-1.4), as follows:

Components of the resultant cutting force and total energy
Measurements of three components of the resultant cutting force were performed in the xyz coordinate system as shown in Fig. 1a. Specific cutting e c and ploughing e p energies are calculated based on the equivalent cutting edge of the length l k as

Experimental details
This study concerns external cylindrical turning of a casehardened 16MnCrS5 (AISI 5115) steel with the average micro-hardness of 850-800 HV 0.05 performed on a CNC lathe Gildemeister CTX 520 linear. Both chemical composition and physical properties of the workpiece material are specified in Tables 1 and 2, respectively. CBN cutting tools, grade WBN 560 by CeramTec, with 56 % CBN content and an average grain size of 3 μm were used. The effective rake angle was γ ne =−24°, and the chamfer angle was γ fe =−30°. The clearance angle α n =6°and the inclination angle λ s =−6°. Furthermore, the cutting edge radius was kept constant at r β =8 μm.  Table 3. Accordingly, the resolution of the resultant cutting force F into three components F z , F y and F x shown in Fig. 1a is different depending on the cutting arrangement. The resultant force components were measured using a three-component piezoelectric dynamometer Kistler-model 9121. The measured signals were processed with a sampling rate of f= 1 kHz and a low-pass filter with a cut-off frequency of f c = 300 Hz [8]. Wear tests were conducted to achieve the limited value of VB C ≈200 μm [8]. Using the methodology described in Sect. 2, the changes of specific energies during all nine sets of wear tests were determined. As mentioned in Sect. 1, the own research findings concerning tool wear coincide, in trends, to the literature reports. In general, the main tool wear mechanisms involved in CBN hard turning are caused by abrasion, adhesion and diffusion depending on the thermal loading, binder phase and chemical stability of CBN tools [13]. However, mechanical wear dominates at low cutting speed and thermal wear at high cutting speed. As a result, the representative tool wear indicator measured in the tool wear tests was VB C . Also, tool wear patterns are similar to those reported in the literature related to finish CBN machining when abrasive wear of tool nose covers the flank (mainly) and the rake (chamfer) faces. In case of low content CBN tools (56 % CBN phase) used, the dominant wear patterns observed create tool nose wear with characteristic ellipsoidal shape as shown in Fig. 2. 3 Results and discussion 3.1 Changes of cutting forces with tool wear Figure 3 shows an exemplary case of force evolutions recorded during tool wear keeping the feed rate at 100 μm, the depth of cut at 100 μm and the tool corner radius at 800 μm. For this set of variable machining parameters, the UCT is equal to about 25 μm (see Table 1) and represents its medium value. It should be noted that the ploughing action and the accompanying spring-back effect and severe sliding friction are intensified when hard machining with very low UCT value, as, for example, equal to 2.5 μm (ten times lower than previously) when the smallest feed of 10 μm was applied. The toolworkpiece reaction is that the passive force reaches the highest value of about 130 N (see course #3 vs. #2 in Fig. 4) for fresh tool, and its value rises distinctly during tool wear. In addition, for a such specific cutting arrangement, the ratio of h/r β is about 0.3. In comparison, according to Kragelski's formula, chip formation under dry cutting conditions occurs when h/r β =0.1-0.2. Moreover, for the maximum UCT value of about 64 μm (r ε =100 μm, f=100 μm, a p =100 μm), the h/r β =8, which means that the ploughing action is distinctly reduced (see course #1 in Fig. 4c). It is evident in Fig. 3 that the passive force F p is the dominant force component and its value increases mostly during tool wear. For instance, the F p force rises from about 80 N for fresh cutting tools to about 130 N for worn tools. However, the rise of F c force is more intensive than feed force F f .
In general, according to graphs presented in Fig. 4, the highest rise of the F p force was observed for the maximum feed rate of 200 μm (Fig. 4a), the maximum corner radius of 1200 μm (Fig. 4b) and the minimum depth of cut of 10 μm (Fig. 4c). An excessive rise of the F c and F f forces during tool wear was observed for the lowest corner radius of 100 μm [8].
The tool wear is a random phenomenon in nature, especially when tool wear test is very long (about 90 min in this study). Such tool wear behaviour can results in an unpredictable change of measured VBc indicator which rather occurs in the second part of tool wear test. As a result, tool wear increment in the time unit can change for course #1 as in Fig. 4b. In this case study with the smallest tool nose radius of 100 μm, the natural change due to wear is its increase, so a new tool with higher nose radius is created. Finally, tool wear can increase more intensively.

Changes of cutting and ploughing energies with tool wear
The basic goal of this study was to characterize the evolution of tool wear in terms of the specific cutting energy (e c ) and the ploughing energy (e p ) consumed. The specific cutting energy  represents the energy required to remove the unit volume of material depending on the value of the cutting force F c , whereas the specific ploughing energy represents the friction losses resulting from the action of the passive force F p . Figure 5 shows the changes in energy balance during tool wear for variable feed rate, depth of cut and tool nose radius, respectively. It should be noted in Fig. 5b that the highest value of specific ploughing energy e p ≈ 90 GJ/m 3 was determined for the lowest feed f=10 μm and the lowest depth of cut a p =10 μm at the end of tool wear test.
Changes of the components of specific energy and total specific energy as functions of the feed rate, nose radius and the depth of cut are presented in Fig. 5a-c, respectively. It was revealed that tool wear contributes mostly to the changes of specific ploughing energy rather than the specific cutting energy (Fig. 5b vs. Fig. 5a).
The general note is that in hard machining, the energy consumed for ploughing action of the tool over the hard surface predominantly overestimates the cutting energy (typically, the ratio e c /e p is lower than 1 and ranges between 0.5 and 0.8).
However, two specific cases can be distinguished in Figs. 4b and 5a: one when the ratio e c /e p oscillates around 1 for tools with the smallest nose radius of 100 μm and the second when the e c /e p decreases down to the minimum value of about 0.26 when the lowest depth of cut of 10 μm was applied. For the latter case, the specific ploughing energy is about four times higher than the specific cutting energy (88 vs. 23.3 GJ/m 3 ). As a result, the total specific energy determined as the sum of the e c and e p increases rapidly during tool wear for the minimum feed of 10 μm up to about 160 GJ/m 3 as shown in Fig. 5a. Less intensive increase of the e t value is observed for the lowest depth of cut of 10 μm for which the maximum e t value approaches 110 GJ/m 3 . For other cases presented in Fig. 5c, the e t value does not exceed 30 GJ/m 3 . This energy amount is characteristic for steel grinding with extremely low UCT of 2 μm or lower [11]. Figure 6 shows the map which compares specific cutting and total energies for different hard CBN turning operations and their evolution during tool wear. These are loge t -logh m graphs which highlight how the UCT influences the values of e c and e t for different machining parameters and tool corner radius. It should be noted that two data sets denoted by symbols 4 and 5 and 6 and 7 correspond to the tool corner radius of 800 μm  . After [13,14] with modifications (h m =24.7 μm) and 400 μm (h m =34.6 μm), respectively. On the other hand, the maximum UCT of 63.7 μm corresponds to the minimum corner radius of r ε =100 μm (bar #9). In Fig. 6a, a line corresponding to both grinding and conventional cutting operations of alloy steels is marked in order to compare energy consumptions in hard and conventional (soft) turning operations. The boundary between grinding and cutting operations was selected at the UCT of 20 μm [11,14]. It is interesting that for medium UCT values ranging from 20 to 50 μm, which corresponds to precision cutting [4], the values of the specific cutting energy related to the beginning of tool wear are lower than for conventional turning operations. However, an intensive ploughing action in hard machining causes that the total specific energy which aggregates both cutting and friction interactions exceeds substantially the specific energy in conventional machining, as depicted in Fig. 6b.

Mapping of energy balance for different chip geometry
As discussed in Sect. 3.2 due to extremely small UCT in the range of several microns, the values of energy consumed when machining with the lowest feed rate of 10 μm and the lowest depth of cut of 10 μm (Fig. 5a) are typical for finish grinding of alloy steels, as presented in [11]. In these cases, the minimum UCT is equal to 2.5 and 7.9 μm, respectively (Fig. 6). It is interesting to note in Fig. 6 that a visible increase of e t during tool wear is also obtained when the tool corner is equal r ε =100 μm which corresponds to the maximum value of h min of about 64 μm (bar #9).

Conclusions
The following conclusions can be drawn based on the obtained results and their analysis: -In CBN hard turning, the ploughing energy is generally higher than cutting energy. Only the e c /e p ratio is about 1 in the case of using tools with a very low nose radius. -The cutting energy for lower feed rate and depth of cut is in the range characteristic for grinding with extremely low UCT of about 2 μm. -The cutting energy for higher feed rate and depth of cut is in the range characteristic for conventional turning of carbon and alloy steels with the UCT higher than 20 μm. -The ratio of uncut chip thickness to the cutting edge radius (h/r β ) describes the intensity of the ploughing action and can be used to distinguish between hard machining and conventional machining operations. In particular, the (h/r β ) ratio of about 0.3 was determined for the minimum h of 2.5 μm, which is close to the boundary between ploughing and cutting actions. -The balance between the specific and ploughing energies (e c /e p ) can be used to optimize hard machining operation. -In order to minimize the energy consumption, finishing hard turning operations should not be performed with extremely low feed rates and depths of cut and larger tool nose radii.
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