Development of cutting force components in high-speed cutting on turning centre

The article deals with the investigation of high-speed cutting. The influence of cutting speed on the development of individual cutting force components in turning was determined. Cutting tests were carried out on turning centre during the machining of C45 medium carbon steel material. The cutting tool material was cubic boron nitride. Cutting speed was selected with respect to the high-speed cutting (HSC) based on previous studies. In the experiment, cutting force components were recorded. From the results, a decreasing trend in the cutting force values was observed from the cutting speed higher than 1100 m.min-1. It can be caused by changing of metallurgical and mechanical properties in the cutting zone because the overall cutting resistance during HSC was reduced. Chips changed colour to orange during the machining due to temperature changes. No great benefit was observed when high-speed cutting for turning medium carbon steel material. The inefficiency, in this case, is caused by the high price and low tool life of the CBN cutting inserts as well as the cutting force that decreases slowly after the initial increase with a rapidly increasing cutting speed.


Introduction
High-speed cutting (HSC) differs in many respects from conventional processing, not only because of the high cutting speed of practice.Most work materials showed significant changes in the cutting mechanism beyond certain cut speed values.The condition of HSC for different materials varies depending on intrinsic properties such as microstructure, alloy composition, strength, hardness, heat treatment or thermal conductivity.HSC is different from conventional machining in many scientific areas such as cutting mechanisms, cutting force development, machine surface, chip formation mechanisms, heat generation and their spread [1].
In addition, HSC has found that it generates a significant amount of heat.In fact, the temperatures in the primary shear area are high enough to soften the cutting material, reducing the cutting force components.In conventional processing, the cutting force is higher than in HSC processing.The surface integrity and quality of the processed surface differ significantly from conventional machining due to the low cutting depth and the high cutting speed, which eliminates the formation of the built-up edge, the edges without burrs, the components without stress and the extremely good surface finish.While in conventional processing, there was moderate surface integrity and quality due to high cut depth, moderate cutting speed, heat-affected zone.One significant distinction lies in the heat transfer within high-speed cutting, where approximately 75 to 80 percent of the heat is conducted through the chip, 10 percent is dissipated into the cutting tool, and the remainder is transferred to the workpiece.In contrast, traditional machining involves a significant absorption of heat by both the cutting tools and workpieces.Moreover, the two processes exhibit marked differences in chip morphology, with high-speed cutting resulting in segmented chips and conventional cutting producing continuous chip formation [2,3].
The materials used for cutting tools must fulfil two critical criteria: they must exhibit high resistance to plastic deformation and fracture, in addition to being chemically stable, heat-resistant, and possessing an optimal balance of hardness and strength values [4].Consequently, materials such as cubic boron nitride (CBN) or polycrystalline diamond (PCD), known for their exceptional hardness and stability, are frequently employed in HSC, depending on the type of workpiece material.
The research presented in [5] primarily focuses on studying the tool wear characteristics of CBN at varying cutting speeds during the dry turning of Inconel 718.This investigation involved a combination of experiments and Finite Element Method (FEM) simulations.The study systematically analyzed the tool wear mechanisms affecting both the rake and rear faces at different cutting speeds.Furthermore, it explored the impact of tool wear on the cutting temperature of the CBN tool and the distribution of stress within the surface layer of the machined Inconel 718 material, employing FEM simulations.
In study [6], a finite element model was created to simulate high-speed machining of the Ti-6Al-4V titanium alloy.The outcomes of this finite element model demonstrated its ability to accurately predict chip formation and cutting forces across various cutting conditions.Additionally, it provided in-depth insights into the high-speed cutting process that cannot be easily obtained through actual machining.To further validate the model, a cutting experiment was conducted.
The research detailed in [7] explores the impact of cutting parameters on the assessment of a production sustainability index, which encompasses factors such as energy consumption, tool cost, and surface roughness during high-speed machining of grey cast iron.The study revealed that a minimal depth of cut (ap = 0.1 mm) yields optimal performance for all the mentioned assessments, although achieving the best performance involves a trade-off between cutting speed and feed rate.
In the paper described in [8], an investigation into the wear mechanisms of PCBN tools with various geometries, cutting edge preparations, and compositions was conducted during high-speed cutting of EN-GJL-250 gray cast iron.This study delves into the evaluation of tool wear mechanisms and their implications.
Article [9] reports the findings of an experimental investigation into the mechanical strength of aging Inconel 718 during high-speed cutting, using both coated and non-coated polycrystalline cubic boron nitride (PCBN) tools.The evaluation included parameters such as cutting forces, tool lifespan, tool wear, and the quality of the machined surface.The results revealed that the benefits of coating on PCBN tools had a limited impact on cutting speeds.At speeds exceeding 300 m.min 1 , the coating did not provide any significant advantage in terms of tool lifespan.It was observed that tool lifespan was highly sensitive to cutting speed, decreasing by 25 % as the speed increased from 250 m.min 1 to 350 m.min 1 .Chemical and abrasive wear mechanisms played a dominant role in tool wear.The study in [10] leads to the following conclusion for high-speed machining of Inconel 718: achieving a surface roughness parameter (Ra) below 0.5 µm is possible when the cutting speed is around 500 m/min and a modified cutting edge is employed, even though high tool wear is anticipated in both tested edge configurations.
In paper [11], the research delved into the effects of cutting parameters, tool material, and tool wear on cutting forces, cutting temperature, and tool lifespan during high-speed milling of TC21, a damagetolerant titanium alloy.The study found that milling parameters, tool material, and tool wear significantly influenced cutting force, cutting temperature, and tool lifespan.The results indicated that the cutting speed should be kept below 250 m.min 1 , and the cutting insert should be replaced when the flank wear reaches 0.15 mm to minimize cutting edge temperature.
Article [12] examined the performance of sialon ceramic solid end mills in machining the GH4099 nickel-based superalloy.Optimal cutting performance was achieved when the sialon ceramic end mills had helix, rake, and relief angles of 35°, 15°, and 12°, respectively.A 35° helix angle led to the lowest wear and superior surface quality, with reduced cutting force and temperature.While increasing the rake angle initially decreased flank wear and improved surface quality, it started to increase after reaching a certain value.A rake angle of -15° resulted in the lowest cutting force and temperature with high-quality machined surfaces.A similar effect was observed with an increase in the relief angle.A 12° relief angle produced lower cutting force and temperature with superior surface quality.
The investigation in paper [13] focused on tool wear and variations in cutting forces during highspeed cutting of Ti-6Al-4V titanium alloy using uncoated cemented tungsten carbide inserts under dry conditions.It was observed that the cutting force component in the negative Y-direction was dominant and exhibited higher magnitudes than the components in the X and Z-directions.Additionally, a positive correlation was found between the cutting force component Fy and tool wear, which could serve as a useful indicator for monitoring the high-speed machining process.
High-speed machining is a crucial aspect of advanced manufacturing technology [14,15].While it has been successful in machining aluminum alloys, its application has been increasingly extended to casting machining, especially in the production of stamping dies, hardened alloy steel machining, and the manufacture of forging, stamping dies, and injection moulds.The primary goal is to reduce hardness and lead times while improving dimensional accuracy and surface quality.The cost-effective implementation of high-speed cutting relies on advancements in machine tools and tool materials [16].In summary, high-speed turning and milling operations offer several advantages, such as reduced machining time, lower cutting forces, reduced tool wear in some cases, and enhanced surface finishing [17].The choice of cutting speed values for high-speed cutting of various materials is illustrated in Figure 1, with values ranging from 600 m.min 1 to 2300 m.min 1 generally accepted for high-speed machining of steel [18].
As mentioned above, HSC has achieved success in machining the aluminum alloys what is wellknown.In this article, the influence of cutting speed on the development of individual cutting force components in turning of C45 steel material was determined.

Materials and Methods
In the experiment, the influence of cutting speed on the development of individual cutting force components in turning was investigated.Cutting speed values were selected to reach the high-speed cutting (HSC) process.

. Selected cutting tool and workpiece material
The CNGA 120408 turning inserts (Figure 2) and DCLNL 2525M12 toolholder (Figure 3) were selected for this research.Tool producer of turning inserts and toolholder was Sandvik Coromant.Tested cutting tool material was cubic boron nitride (CBN).The toolholder was cut due to reduce the effect of tool overhang on cutting process.The selected workpiece material was medium carbon steel material of DIN ISO C45 (AISI 1045) grade in this paper.Chemical composition is shown in Table 1.We selected a round bar with diameter of ø 125 mm.

. Cutting Tests
The aim of cutting tests to determine the influence of cutting speed on the development of cutting force components.For cutting tests, the DMG CTX alpha 500 turning centre was used.Workplace of turning centre with experimental setup can be seen in Figure 4.No coolant was used in the experiment.The shape of workpiece was round rod where external diameter of round rod was 130 mm.The workpiece was clamped into the three jaws chuck where claw jaws was used for good stiffness of clamping.Cutting tests were performed by longitudinal turning.The length of machined surface was 28 mm.In this experiment, a Kistler 9257B static dynamometer was used to measure cutting force components.In the experiment, 11 trials were performed.From cutting parameters, cutting speed was varied while feed and depth of cut was kept constant because the experiment is just focused on effect of cutting speed.Cutting speed values were selected due to research of authors [18,19].The values of depth of cut and feed were selected according to recommendation of tool producer.The cutting parameters used in the experiment are shown in Table 2.

Results and Discussion
As mentioned in previous section, 11 trials were performed where cutting force components were measured during the longitudinal turning in order to determine the influence of cutting speed on the development of individual cutting force components in turning.
From the theory of machining, it is well known fact that cutting force increases with increasing cutting speed [20,21] during the stable machining condition.This theory can be applied to conventional machining.However, when the high values of cutting speed are reached, metallurgical and mechanical properties in the cutting zone will be change.Therefore, the overall cutting resistance during HSC is reduced [19].Machining processes during the turning with cutting speed values of 900 m.min 1 (Figure 5a) and 1300 m.min 1 (Figure 5b) are recorded in Figure 5. Chips changed colour to orange during the cutting process with cutting speed value of 1300 m.min 1 due to high temperature.The too high temperatures in the primary cutting zone cause softening of the machined material and thus a reduction in cutting (tangential) force what is shown in graph of dependence of cutting force components on cutting speed (Figure 6).Root Mean Square (RMS) of cutting force components values was evaluated and compared after measurement.In this case it is the most appropriate way of evaluation due to the orientations of the cutting force components.The dynamometer has its own coordinate system, so these force components (Fx, Fy and Fz) must be approximated to the standard measurement of cutting force compoentns (Fc, Fp, Ff).Therefore, some of the force components measured by the dynamometer have a negative value.For this reason, when evaluating RMS, cutting force components have positive values.
As can be seen from the graph (Figure 6) increasing cutting force with increasing cutting speed was recorded for cutting speed value up to 1100 m.min 1 .If the cutting speed exceed the value of 1100 m.min 1 (marked by red arrow), cutting force Fc (tangential) will decrease.The same phenomenon was observed for resultant force R. Force R is the resultant force of all cutting force components.If the cutting speed exceed the value of 1100 m.min 1 (marked by yellow arrow) resultant force R will decrease.It can be caused by changing of metallurgical and mechanical properties in the cutting zone because this trend was observed for each subsequent higher value of the cutting speed.This phenomenon is related to HSC.Although the radial and axial forces also decrease as seen from the graph it is not entirely obvious when the character of these components starts to change to a downward trend.This is related to the very fast wear of the cutting tool.From the theory of machining for conventional cutting, it is known that Fc > Fp > Ff [22] and it is also true for high-speed cutting as seen from the graph (Figure 6).

Cutting force components:
F ccutting (tangential) force F ppassive (radial) force F ffeed (axial) force Recording of the measurement of the cutting force components is shown in graphs in Figure 7.As mentioned above, the cutting force components (absolute values) initially started to increase (Figure 7a), but after cutting speed reached the value of 1300 m.min 1 , the absolute values of cutting forces components started to decrease.The wear of the cutting tool is so high, that it has resulted in the workpiece oscillation, which results in a large dispersion of the measured cutting force components, when the cutting speed reaches 1300 m.min 1 (Figure 7b).It is evident that with increasing cutting speed, the tool wear also increases, which leads to a shortened tool life.This is particularly true at higher cutting conditions, where the cutting insert experiences elevated pressure and temperature, resulting in accelerated wear [23].For steel, cutting speeds ranging from 600 m.min 1 to 2300 m.min 1 are typically accepted for high-speed cutting [18].The lower limit of the cutting speed in the paper was determined to be 1100 m.min 1 , as this is where the cutting (tangential) force started to decrease.Also, the colour of the chip started to change colour due to the increasing temperature.The shape of the chip was not continuous but segmented, that is characteristic of HSC [2,3].
Even though the tool life was not evaluated in this paper, it should be added that the tool life of the cutting insert from deployment to its removal was only a few seconds.After approximately 30 seconds the cutting insert was removed due to catastrophic tool failure as seen in the picture (Figure 8).Although the wear of the cutting tool was high, the cutting force components continued to decrease.It follows that the influence of the HSC machining process, where the cutting force decreases, was more significant than the influence of the cutting tool wear and its increased cutting force components.

Conclusion
In the article, the influence of cutting speed on the development of cutting force components was determined during the high-speed cutting of C45 medium carbon steel material.In the cutting tests, the DMG CTX alpha 500 turning centre was used.Cutting tests were performed by longitudinal turning and a Kistler 9257B static dynamometer was used to measure cutting force components.Cutting speed was varied while feed and depth of cut was kept constant because the experiment is just focused on the effect of cutting speed for HSC.
From the experiment, following points can be concluded that were performed:  Decreasing trend in the cutting (tangential) force was observed when the cutting speed value of 1100 m.min 1 was exceeded. The decreasing trend in the passive (radial) force and feed (axial) force was also observed, but it is not clear at which cutting speed value this trend started. The highest component was the cutting (tangential) force, followed by the passive (radial) force and then the feed (axial) force.This matches the theory of machining. The influence of the HSC process is greater than the influence of the tool wear on the cutting force components.
However, these points can be only applied to C45 steel material because HSC is based on the type of machined workpiece material.In HSC, various areas of cutting speed values are used for various workpiece materials.No great benefit was observed when high-speed cutting for medium carbon steel material.The inefficiency, in this case, is caused by the high price and low tool life of the CBN cutting inserts as well as the cutting force that decreases slowly after the initial increase with a rapidly increasing cutting speed.But HSC could be used for machining difficult-to-cut materials where HSC can have a better potential.
Further research will be focused on the influence of cutting edge microgeometry on tool wear, cutting force components, and surface roughness when HSC.

Figure 1 .
Figure 1.High speed machining/cutting range for various types of materials [18].

Figure 4 .
Figure 4. Workplace of turning centre with workpiece in the cutting tests.

Figure 5 .
Figure 5. Machining process during the machining with cutting speed value of a) vc = 900 m.min 1 and b) vc = 1300 m.min 1 .

Figure 6 .
Figure 6.The influence of cutting speed on cutting force components.

Figure 8 .
Figure 8. Catastrophic tool failure of CBN turning insert.

Table 1 .
Chemical composition of C45 machined material.

Table 2 .
Cutting parameters used in the experiment.