The investigated analytical and numerical models were also evaluated in comparison to fractions of heat transferred into the workpiece based on measurements according to the literature review. As the deviations between the utilized models were determined to be small, the results of the improved calculation of Komanduri and Hou’s model represent the simulation approaches in this comparison. Regarding measurement-based results, the early works by Boothroyd were taken into account by calculating the fraction of shear plane heat transferred into the workpiece for the investigated orthogonal cutting processes by means of the provided regression equation in Boothroyd and Knight’s publication [

24]. Furthermore, the recently conducted comprehensive study by Augspurger et al. [

32] is also considered. The provided regression equation by Augspurger et al. solely depends on the thermal number, and does not describe the fraction of shear plane heat, but the fraction of the total cutting energy transferred into the workpiece. Thus, for comparison with simulated data, the regression equation had to be further processed. Initially, the cutting and thrust forces for the experiments conducted in their study were determined based on the force model by Kienzle [

38] and corresponding input data from König and Essel [

39] for the four workpiece materials. According to the approach already successfully realized for the determination of purely model-based input parameters for the thermal analysis, based on the cutting and thrust forces, the shear angle, the total shear power, and the fraction of total shear power in relation to the total cutting power were calculated. The fraction of shear plane heat transferred into the workpiece was then determined by multiplying the results from the regression equation and the reciprocal of the fraction of total shear power in relation to the total cutting power.

Figure 9 shows the results of the regression equation by Boothroyd and Knight, and the further processed data based on the regression equation by Augspurger et al., in comparison to the improved calculation of Komanduri and Hou’s model. Boothroyd and Knight’s results show a similar characteristic compared to the simulation results. As already found in the early work by Boothroyd [

28] when comparing his experiments with Weiner’s equation, the models tend to underestimate the fraction of heat transferred into the workpiece. This is also true, and to a much larger extent for experimental-based results from Augspurger et al. for dimensionless numbers

$\sqrt{{Y}_{L}}$ higher than one. However, while the characteristic of Boothroyd and Knight’s results are similar to the analytical model, the characteristic of these results is different. For the higher fraction of shear plane heat transferred into the workpiece for dimensionless numbers above one, the following reason Boothroyd also gave in his early work seems reasonable. In the modeling approaches a plane heat source at the shear plane is assumed and heat transport into the workpiece can only be realized by conduction. However, due to a cutting edge radius instead of a perfectly sharp tool and strain-rate dependent material properties in the shear zone, in reality, the primary deformation zone extends into the workpiece. Thus, in addition to heat transport by conduction convection into the workpiece also occurs. This effect becomes higher with an increasing thermal number, because convection dominates the heat transport. In correspondence to this effect, in reality, friction at the flank face of the tool cannot be completely neglected, even when wear is controlled. The frictional heat is almost completely induced into the workpiece because it is generated behind the shear zone and conduction into the chip is negligible. Furthermore, this heat will also increase with increased cutting velocity, according to tribology knowledge. In conclusion, for dimensionless numbers above one, the deviation between the measurement and the simulation might be explained by the extension of the shear zone, the existing cutting edge radius, and the friction at the flank face of the tool. For confirmation of this statement, chip formation simulations by Puls et al. [

26] were further processed in the already described way to give information on the fraction of shear plane heat transferred into the workpiece. As indicated in

Figure 9, Puls et al. also varied the cutting edge radius

r_{β} and the flank wear within the simulation approach, and the results clearly support the described influence on the heat partition. Puls et al. did not provide data for low dimensionless numbers, and it cannot be argued whether their chip formation simulation follows the trend from the experimental-based results by Augspurger et al. for low dimensionless numbers. Two reasons might be relevant to explain the deviations for dimensionless numbers below one. First, the calculation of the fraction of heat transferred into the workpiece utilized in the study by Augspurger et al. might be erroneous for these processes. In their approach, the fraction of heat is based on the average temperature calculated from measured temperature fields on a surface by an infrared camera before and after the cut. Considering decreasing dimensionless numbers, these measurements become more and more complicated. This is due to a narrower heated zone under the workpiece surface because of low uncut chip thicknesses, and especially due to higher process times with low cutting velocities. According to Augspurger et al., the experiments were conducted in a way that heat flow in normal direction to the workpiece can be neglected. However, with increasing process time, heat transport by conduction becomes dominant, and may lead to relevant not detected heat losses in this direction. As pointed out by Augspurger et al., measuring the temperature field during the process does not seem to be a reasonable solution, because of the highly challenging measurement of fields with large temperature differences occurring between chip and workpiece. The second reason for lower fractions of heat for low dimensionless numbers might be the processing of the original data to obtain the shear power and the shear angle. It was shown for the own experiments that the utilized models’ accuracies become lower for processes with extreme machining parameters. Thus, experimentally validated input parameters for the processes investigated by Augspurger et al. and also Puls et al. would be highly valuable. This might also be the reason why the regression equation by Boothroyd and Knight show a better agreement to the modeling approaches. In conclusion, research question 4 is to be answered in the affirmative, and the idealized conditions in the utilized models do have a relevant influence on heat partition in comparison to experimental-based results. For heat partition in industry relevant processes, e.g., milling and drilling, investigations have to clarify if these deviations are still relevant for the fraction of shear plane heat transferred into the workpiece.