The results of the measurement of cutting force components during machining are presented, the resulting cutting forces and machined surfaces are compared and the influence of cutting parameters is assessed.
5.1 Components of cutting force in machining
Figure 10 illustrates the directly measured cutting force components Fx, Fy, Fz for the machined material AlZn5.5MgCu. The sampling frequency of the dynamometer was 5000 Hz. The total cutting force F was calculated from these measured data, the values of which are then exported using Matlab.
As shown in Figure 10, the measurement itself captures the entire machining process from the first tool-workpiece contact to the completion of chip cutting. For this reason, only a slice of the measurement record that presents the chip machining itself was analysed.
In the case of the measurement of the cutting force components for the machined materials 90MnCrV8 and 42CrMo4, the procedure was identical.
5.2 Comparison of the resulting cutting forces
Based on the obtained cutting force data from the machining process, a graphical comparison of the resulting cutting force in the machining process for each material and cutting speed was created. The results of the experiment are shown in Figure 11.
The achieved cutting forces at the individual tool rotation speeds for machining AlZn5.5MgCu were in the low range up to 50 N. When the rotation speed of the monolithic tool was increased, the cutting force decreased up to 35 N at a tool rotation speed of 500 m⋅min−1.
The mean cutting force values in the individual machining experiments for 42CrMo4 were very similar. The visible difference of the individual tool rotation speed settings is in the variance of the measured force values. For a tool rotation speed of 300 m⋅min−1, it ranges from 35 N to 77 N. This variance of values is the highest of the experimental measurements performed.
When machining the hardest material, which was 90MnCrV8, the values of the total cutting force were in the range of 90 N ± 5 % at a tool cutting speed of 500 m⋅min−1. Higher values of the total cutting force and also a dispersion of values were observed when machining at lower tool rotation speeds. Based on these data, we can assume that the monolithic tool can be used for machining materials with enhanced mechanical properties under properly specified cutting conditions.
5.3 Comparison of the machined surface
In the image of the enlarged surface of AlZn5.5MgCu, square and hexagonal tool marks can be seen after machining under the given conditions. These marks and their character are determined by the resulting direction of the main cutting motion. The surface formed at a cutting speed of 500 m⋅min−1 was different from the other cutting speeds and had a hexagonal structure. The hexagonal features are distributed over the entire surface of the machined material and can be formed by the worn part of the tool. During rotary turning of 42CrMo4 at a cutting speed of 100 m⋅min−1, the surface showed diamond-shaped machining marks, in which elliptical traces of the circular cutting edge of the tool appeared on the colour map. The resulting cutting motion vector gives the overall orientation direction of these traces. For a steel workpiece machined with a rotary tool at a cutting speed of 300 m⋅min−1, the tool marks have a striped character with fine rectangular shapes. The direction recorded by the colour map shows the vector of the resulting cutting motion. In the images of the surface machined at a cutting speed of 500 m⋅min−1, the layered tool marks of the rotary tool can be seen after high-speed machining. At the high cutting speed of 500 m⋅min−1, the shape of the machining marks is different for steel compared to the other two lower cutting speeds. This effect was evident in the machining of AlZn5.5MgCu. For the 90MnCrV8 material machined at a cutting speed of 100 m⋅min−1, rectangular strip formations appear on the surface, the direction of which is also determined by the vector of the resulting cutting motion. The width of the strip is defined by the knife geometry and the feed rate. Images of the machined surface at a cutting speed of 300 m⋅min−1 show finely textured rectangular layers. Their direction is determined by the resulting main cutting motion vector. The structure is finer but similar for identical parameters of 42CrMo4 steel.
From the recorded surfaces (Figure 12) and their roughness (Figure 13), it can be concluded that the monolithic knife and the overall technology is suitable for the implementation of high-speed machining. The machined surfaces generally have relatively low roughness values using the given parameters. By improving the overall technological system of a given process (such as increasing the frequency of rotation of the workpiece), the roughness values could be improved for the technology used.
5.4 Influence of cutting machining parameters
In order to determine the cutting parameters, a pilot experiment was carried out to obtain basic data on the cutting force in the forced rotation machining process. The cutting parameters used were chosen within the ranges shown in Table 6 due to the system and machine stiffness. In this experiment, the effect of the rotation direction of the tool with respect to the workpiece was also investigated, where negative rotation speeds indicate a counterclockwise orientation, and positive speeds present a clockwise rotation orientation.
Table 6
Selected cutting parameters with respect to the system and machine stiffness
vw (m.min-1)
|
vT (m.min-1)
|
ap (mm)
|
f (mm)
|
100 - 200
|
50 - 200
|
0.25 – 0.75
|
0.05 – 0.15
|
The effects of the factors vw, vt, f, and ap were analysed by the response surface method. The aim was to intensify the individual parameters and their dependence on the magnitude of the cutting force F. After the experiments were completed, the experimental data were subjected to statistical processing (Table 7).
The analysis of variance (Table 7) of the total cutting force F shows that the most significant effect is achieved by the depth of cut ap, which directly influences the size of the material removed during machining. According to the linear model, the influence of ap size on cutting force F reaches 24.55%. The workpiece rotation speed vw has the smallest influence on the F size of cutting force (0.20%). Also, the feed size f cannot be considered statistically significant as its influence on the change of cutting force F reaches the value of 0.64%. The accuracy of the mathematical model reaches 90.60% with a prediction coefficient of 85.09%. From the above data, it is possible to construct the response surface equation F (1):
F = 583 - 0.09 vw + 0.088 vt - 1715 ap - 1710 f - 0.00103 vw2 + 0.005289 vt2 + 2187 ap2 + 10450 f2 + 0.00007 vw . vt - 0.297 vt . ap - 2.27 vt . f (1)
Table 7. Analysis of variance output for total cutting force F
A graphical interaction (Fig. 14) of the selected cutting parameters was constructed from the experimentally obtained data and the subsequent mathematical model. When the graphical representation is used, the influence of the two cutting parameters on the total cutting force F can be clearly observed. One of the investigated parameters was the tool rotation vt. For identical tool rotation speed and different rotation directions, the magnitude of the total cutting force is affected by 50N. For the parameter ap = 0.75 (mm), there was an increase in the cutting force of 100N when the direction of rotation was changed. From the above, it can be seen that the direction of rotation of the tool itself has an effect on the cutting force.
With a properly selected tool rotation, it would be possible to apply cutting parameters (ap, f) to increase machining productivity compared to productivity in the opposite direction of tool rotation.