Effect of increased mechanical strength on the machinability of graphite cast irons in face milling

Due to the low cost and high vibration damping capacity, gray cast irons are commonly used in machine tool bases, in addition to applications with noise restrictions, such as engine blocks, housings, and brakes. The matrix’s graphite, sulfides, and ferrite/pearlite ratio are some of the most important parameters governing the machinability of the gray cast irons. This work aims to evaluate the machinability of high-resistance gray cast irons of the FC 300 grade, in two versions, with the addition of molybdenum (FC 300 (Mo)) and with refined graphite and addition of molybdenum (FC 300 (Mo+RG)), for use in cylinder heads and engines blocks, compared to materials that have been used for this purpose, gray cast iron FC 250 and the compacted graphite cast iron FV 450. The materials were characterized according to the cementite interlayer spacing and microhardness of the perlite matrix, the number of eutectic cells, and the distribution of manganese sulfide inclusions, with those characterizations being correlated with the machinability results. The face milling process was chosen for the tests with uncoated cemented carbide tools without cutting fluids; the cutting speed and feed rate varied in addition to the type of cast iron. Analysis of tool life, wear mechanisms, and machined surfaces’ quality (Ra roughness parameter) were the output variables considered. Using harder materials such as FC 300 (Mo) or FC 300 (Mo+RG) for the engine would bring benefits such as increased durability, reduced weight, and improved efficiency, but it has a cost in machining. These harder materials have a shorter tool life compared to the commonly used gray cast iron FC 250, but the difference in performance is less pronounced under more extreme cutting conditions, such as high cutting speed and feed rate. When analyzing the surface roughness, FC 250 produced a smoother surface at low cutting speeds but performed worse than FC 300 (Mo) and FC 300 (Mo+RG) at higher cutting speeds.


Introduction
Stricter regulations on gas emissions have led car manufacturers to explore different vehicle engine options [1]. Currently, to improve performance in the manufacture of cylinder blocks and cylinder heads of high-powered diesel engines, as well as other castings, a certain number of alloying elements are generally added to gray cast iron [2]. Cast iron grade FC 300 materials are widely used to manufacture various industrial products. The reasons are good castability, machinability, and abrasion resistance, and its vibrationdampening capacity is much higher than that of light steels [3][4][5]. Given this, the mechanical resistance of cast iron, that is, their ability to withstand external stresses without causing In memoriam of Wilson Luiz Guesser.
* Leonardo Rosa Ribeiro da Silva leorrs@ufu.br them to cause plastic deformations, and consequently their grades are conditioned to their final structure obtained [6,7]. In addition to cast iron, engine blocks can be made of aluminum alloys. Despite the weight reduction resulting in lower fuel consumption, increased resistance to corrosion, and higher thermal exchange capability of the aluminum alloys, cast iron blocks are more commonly used due to their higher mechanical resistance, vibration absorption, and lower cost [8,9]. Another option would be an aluminum block with cast iron sleeves [10]. In cast iron, the properties depend on the shape and amount of graphite and the amount of ferrite and/or perlite in the metallic matrix, in which the resistance is increased with higher levels of perlite and the reduction of the interlayer spacing of the perlite [6,11,12]. The usual mechanism for increasing the tensile strength and hardness of gray cast irons is the addition of alloying elements, such as chromium (Cr), molybdenum (Mo), tin (Sn), and copper (Cu). Additional amounts of copper and tin (pearlized elements) promote the refining of the pearlite or reduction of interlayer spacing, which results in increased strength. On the other hand, chromium and molybdenum form carbides, and, like Sn and Cu, Mo can also be used as a perlite refiner [13,14]. To achieve a minimum tensile strength of 300 MPa, the carbon content is also reduced from 3.2 to 3.0%, which results in smaller graphite lamellae, thus reducing the risk of starting and crack propagation. This way, a 10 to 20% increase in mechanical properties is achieved from this [15,16]. The addition of alloying elements depends on the elemental composition and manufacturing method to provide the desired mechanical properties. Therefore, reducing or eliminating Cu use by technical measures without reducing the mechanical properties and processability in mass production will bring some economic and technological benefits [17,18]. Adding molybdenum (Mo) tends to decrease the length of the graphite flakes and the spacing between the pearlite layers, despite the decrease in size of pearlite because Mo strengthens the matrix, which improves the tensile strength both at room temperature and at high temperatures [19]. Other materials can still be used as alloying elements, such as niobium (Nb) which increases the mechanical strength but tends to increase the coefficient of friction [20].
The study on machinability, cutting strategies, and application technology of gray cast iron is today a priority in large companies and research institutions worldwide, becoming an effective guarantee for high-quality products [21]. Gray cast iron has become a popular material among other cast metals, being widely applied in modern industrial production, mainly for its low cost (20 to 40% less than steel), in addition to having a wide variety of mechanical properties achievable, as good castability, machining property convenient, and good wear resistance [22,23]. The microstructure of gray cast iron is characterized by graphite lamellas dispersed in the ferrous matrix. The casting practice can influence the nucleation and growth of graphite flakes so that the size and type improve the desired properties. The quantity and size of graphite, morphology, and distribution of these lamellae are critical in determining the mechanical behavior of gray cast iron [24].
According to Guesser [14], the machinability of gray cast irons increases as you move towards higher strength grades due to the increased abrasiveness with the increase in the amount of perlite in the matrix and due to the decrease in lubricating action and consequent reduction in ease chip breaking with a decrease in the amount of graphite. Another way to evaluate the machinability of cast iron is by combining the hardness test with an evaluation of microstructure due to some microconstituents that adversely affect machinability (ASM). Thus, it is emphasized that the microstructure plays a crucial role in altering the mechanical properties of any material [18,25]. Controlling the microstructure, optimizing the process parameters, and adding alloy elements are highly necessary [26]. The main constituent elements of cast iron are mainly carbon, phosphorus, and silicon, among others. The presence of silicon and phosphorus determines the solubility of carbon in molten metal [27].
Silicon is one of the essential elements in producing gray cast iron because it is a stabilizing element of graphite. Thus, it promotes graphite development at the site of iron carbides. It is verified from the experience that the Si content of around 3% restricts the formation of iron carbide because no carbon is left in the chemical form [28].
Adding nickel refines the pearlite structure and graphite gray cast iron, improving strength and hardness to balance the differences in thick sections. High hardness, like that of white cast iron bars, produced by adding sodium chloride salt to gray cast iron, is recommended for wear resistance [29].
As for molybdenum, this is a carbide-forming element used to strengthen and harden iron because of the transformation of austenite into fine perlite and bainite, generally added in gray cast iron to refine perlite [30,31]. Molybdenum is not a promoter of perlite; however, it is usually added as a ferromolybdenum containing 60 to 70% (Mo) [32,33]. Copper and molybdenum in gray cast iron guarantee greater hardness and tensile strength than the common pearlitic types of gray cast iron [17].
The gray cast irons of FC 300 grade possess good mechanical properties, where combinations of multiple structures with low silicon content, fine graphite, and perlite make them stronger and increase their resistance [34,35]. The combination of microstructure with graphite reinforcement and excellent mechanical strength has led the cast iron alloys of the FC 300 grade to be used in a wide range of industrial applications, such as automotive parts, internal combustion engines, piping systems, and construction parts [36].
However, compared with conventional FC 250, the FC 300 grade is considered a lower-cut material. The problems encountered during the machining of the FC 300 grade are caused by changes in the microstructure formation due to the addition of alloying elements [34,36]. This is an issue to be considered in industrial environments because it becomes difficult to predict the actual tool life and to specify the most suitable cutting conditions for a given cast alloy.
The integrity of the surface is directly related to the quality achieved in the final machining, which strongly affects the product's performance. Among the factors that can influence the quality of the product's surface are the cutting speed, the feed rate, the depth of cut, the geometry of the tool, the wear of the tool, and the properties of the part [37]. New technologies such as cyber-physical systems, artificial intelligence, augmented reality, extensive data analysis, the Internet of Things, and the Industrial Internet of Things should be integrated to tackle these challenges [38,39]. Simulation allows experimentation and validation of the product, process, system design, and configuration and is considered an essential set of tools and information intelligence methods for successfully implementing digital manufacturing [40]. Lin et al. [41] studied the optimization of machining a nitrogen-containing gray cast iron alloy to obtain machining parameters. Using an integrated optimization model based on genetic algorithm and hierarchical analysis (AHP), the results show that cutting depth has the most significant effect on cutting force, and cutting speed has the most significant effect on cutting temperature and surface roughness. The optimization improved the Ra roughness by 23.53%.
In practice, machining operations can only fully develop the potential of machine tools and cutting tools after optimizing the cutting parameters to obtain the best work efficiency and tool life [42,43].
The present study aims to evaluate the influence of the microstructure on the machinability of gray high-strength cast iron, of the FC 300 grade, for use in diesel engine blocks and heads. For this purpose, two versions of FC 300 Gy cast iron were produced, one with graphite refining (FC 300 (RG) ) and the other, which in addition to graphite refining, has molybdenum addition (FC 300 (Mo+RG) ). The FC 250 Gy cast iron and vermicular iron FV 450 were also investigated for comparison purposes. These materials were machined in the front milling, with carbide tools, with comparisons of the tool life and the surface roughness of the part, considering in the analysis and discussions of the results the microstructural characteristics of the materials.
In Section 2, the materials used, procedures, and design of experiments are characterized. In Section 3, the results are presented, discussed, and compared with other relevant works in the literature. In Section 4, the main conclusions and proposals for future work are shown.

Material characterization
The materials tested through the face milling process were the high-strength gray cast iron FC 300 (Mo) , which has molybdenum as an alloying element, the FC 300 (Mo+RG) , which also has molybdenum as an alloying element, as well as graphite refinement, the gray cast iron FC 250, and the compressed graphite cast iron FV 450. The last two will be investigated to serve as references in the comparisons.
The micrographs of the materials were obtained using a scanning electron microscope (SEM, Hitachi TM 3000) as well as the micrographs showing 100% pearlitic matrix in both, obtained by optical microscopy (Olympus microscope), this being a technique used to view a sample close up with the magnification of a lens with visible light. Figure 1 shows the distribution of the graphites of the investigated materials. The graphite morphology was evaluated using scanning electron microscopy (SEM) and the pearlitic matrix using optical microscopy after etching with Nital (3%). The chemical composition, mechanical properties, and characteristics of the matrix and graphite of the materials were made available by the manufacturer [12] and detailed in Tables 1 and 2.
The metallographic analysis allowed us to observe the appearance of graphite in the three gray irons as thin and uniform flakes with random orientation, typical of form I and type A [13]. In the compacted graphite cast iron, the shapes observed as graphite III and VI correspond, respectively, to the form of worms and nodules, as shown in Fig. 1d. For all the materials, this information about the metallography of gray cast iron is crucial for analyzing the machining results since the number of eutectic cells reflects directly on the volume and size of the graphites. A larger number of cells tend to have a thinner structure with a smaller graphite size, which reduces the crack propagation condition, making machining difficult. Table 3 shows the Brinell hardness of the materials, the Vickers microhardness of the pearlite, and the mechanical strength of all the tested materials.
To determine the amount of manganese sulfide inclusions (MnS/mm 2 ), for each cast iron sample, ten images at a magnification of 500 × were used, and for area evaluation, 150 particles of manganese sulfide were randomly selected from the images. Figure 2a shows examples of the measured particle areas.
The cementite interlayer spacing of the perlite was determined by etching the samples with 3% Nital reagent. Twenty-five images were taken at a magnification of 20,000 × by scanning electron microscopy of different regions, with 25 points being selected at random to measure the intersections of the cementite lamellae according to the methodology established by Vander Voort and Roosz [14].

Material samples, cutting tools, and milling tests
The work materials were supplied by Tupy S.A. [12] in the form of rectangular bars whose dimensions were approximately 400 mm long, 240 mm wide, and 40 mm thick. Before the face milling tests, the specimens had their surfaces premachined. This process was employed to remove residues from the sand-casting process and the chill zone, as both can increase the tool wear, masking the influence of the input parameters. The cutting tools used in this research were uncoated cemented carbide inserts manufactured by Walter Tools, ISO code-LNHU130608R-L55T Tiger-tec® Silver, grade WKP 25S. These inserts have a double-sided tangential rhomboid geometry suitable for finishing. These tools were mounted on a 90° angle cutter head, BLAXX model, code F5141.B27.080.Z10.12, with a diameter of 80 mm and capacity for ten inserts, also manufactured by Walter Tools. Figure 3 and Table 4 show the details of the tools used in the experiments. Only one insert was mounted in the milling cutters for the trials, which according to Richetti et al. [15] does not compromise the comparative results.
The tool life and surface roughness (Ra) tests followed a factorial design of experiments (DOE) of mixed factorial, with two quantitative variables (cutting speed v c ; feed rate f) with two levels each and a qualitative variable (the workpiece material) with four levels. To guarantee statistical reliability, all tests were repeated twice, totaling three tests in each condition (test and two replicates), resulting in 48 trials. A reliability index of 95% and a significance level of 5% were used for all experiments. The high ( +) and low ( −) levels of the variables are shown in Table 5. In a predominantly concordant milling cutting, the axial depth of cut (ap) and the radial depth of cut (ae) remained constant at 1 mm and 60 mm, respectively.
The levels of the input variables were defined to provide a tool life suitable for the carbide inserts so that they were not too long (at low levels), consuming much time and material, nor too short (at high levels), offering little information about the test. As the qualitative variable (material of the workpiece) has 4 levels, instead of 8 tests, 16 were necessary, totaling 48 tests with repetitions.
The tests were carried out under dry conditions, in a vertical CNC machining center Discovery model 760, with main spindle power of 11 kW and a maximum rotation of 10,000 RPM, from the manufacturer ROMI Bridgeport. For the tool life tests, maximum flank wear (VB Bmax ) equal to 0.40 mm was used as the end-of-tool life criterion. At the end of each run, the tool wear and surface roughness of the workpiece were measured.   To measure the surface roughness parameters (Ra), a portable roughness meter was used, model SJ201 P/M from MITUTOYO. This instrument has a needle probe with a 2 μm diamond tip radius and 0.1 μm resolution. A 0.8 mm cut-off filter was used, following the recommendation of the ISO 4288 standard [44].

Distribution of MnS particles and pearlite interlayer spacing
The results of the average amount of MnS inclusions/mm 2 and the average area of these inclusions in the gray cast iron are shown in Fig. 4. The vermicular cast iron, in turn, does not present MnS inclusions in its microstructure since sulfur is reduced in these materials. Figure 4a shows slight differences in the averages of these particles. The cast iron with graphite refining FC 300 (Mo+RG) has, on average, 418 MnS/mm 2 particles, in which 27% of the inclusions measured in the sample have areas between 11 and 20 μm 2 . On the other hand, 21% of the particles have areas between 6 and 10 μm 2 , and less than 6 μm 2 represents about 13% of total inclusions, as shown in Fig. 4b. The FC 250 presented 391 MnS/ mm 2 particles, with 33% of inclusions having areas between 11 and 20 μm 2 , 20% of the particles having areas between 6 and 10 μm 2 , and 7% of inclusions having areas between 1 and 5 μm 2 . The FC 300 (Mo) has, on average, 351 MnS/mm 2 particles that are present in the material as follows: 25% of the analyzed particles have areas between 11 and 20 μm 2 , 24% of the inclusions have areas between 6 and 10 μm 2 , and only 5% of the MnS particles have areas smaller than 6 μm 2 . To check if there are statistically significant differences in the number of inclusions per square millimeter among the three materials, an analysis of variance (ANOVA) was carried out regarding the average amount of MnS/mm 2 particles. Table 6 shows the results of the analysis of variance for the average amount of MnS particles/mm 2 , while Table 7 shows the results of the analysis of the distribution of the areas of the MnS particles.
The data were obtained using a 95% confidence interval and a 5% significance level using Matlab® software. The  analysis of variance (ANOVA) was also used for the cementite interlayer spacing (μm) of the perlite and the size of the MnS particle areas of the materials used in the research. The results of the pearlite interlayer spacing of the tested materials are shown in Fig. 5. Note that, in terms of average values, the FC 300 (Mo+RG) has the smallest spacing between layers, an average of 0.29 μm. Thus, this factor may have influenced the hardness of this material, which was greater than the FC 300 (Mo) and FC 250 ( Table 3). The variance (ANOVA) analysis was used to confirm a significant difference between the material's interlayer spacing (μm). Table 8 shows the results of the analysis of variance for interlayer spacing (μm). It can indicate that, for gray cast iron with molybdenum and graphite refining, there will be an increase in tensile strength and abrasiveness, thus lowering the machinability. Figure 6 illustrates the average results of the tool lives of the evaluated materials for the cutting conditions shown in Table 5. The average results were calculated from three tests (one test and two replicas) in each cutting condition experimented with. For this, the tool life represents the cutting time taken strictly when the maximum flank wear reached the value of 0.4 mm (VB Bmax = 0.4 mm), the end-of-tool life criterion used, which is the same methodology used by da Silva et al. [18]. Based on the results, it is observed that the FC 250 presented the best machinability among the analyzed materials, the explanation being that its mechanical properties are inferior compared to the other analyzed materials, as shown in Table 3. To show statistical differences between all materials, an analysis of variance (ANOVA) with 95% reliability was performed according to the values illustrated in Fig. 6, with the result in Table 9. Based on these results, it is observed that all input variables significantly influenced the process, with the cutting speed the variable with the most significant influence and the type of material the variable with the lowest. Figure 7 illustrates the individual effects of each input variable indicated in Table 9 about the machining time. Figure 7a shows a clear relationship between the increase in mechanical properties and the reduction of machinability,   [5] in the drilling process of similar materials. Figure 7b illustrates that the increase in cutting speed resulted in a decrease in machinability, which, according to da Silva et al. [5] and França et al. [45], is explained for graphitic cast irons by the increase in forces and temperatures at the cutting interface due to the increase in cutting speed. Figure 7c indicates that the increase in feed resulted in a decrease in machinability, which can be explained by the greater volume of material removed by tool interaction resulting in greater forces on the cutting edge [46]. Figure 8 illustrate the combined effects of the material, cutting speed, and feed-on material machinability. This analysis allows us to infer that the most significant differences in machinability between the materials are observed for the lowest speed (v c = 230 m/min), with the tool life of the materials being similar for the most severe cutting conditions (v c = 350 m/min, f = 0.2 mm/rev), corroborating the fact that the cutting parameters have a much more significant effect on machinability than the materials evaluated as indicated by the analysis of variance in Table 9. This graph also shows that the FC 250 has machinability considerably superior to the other materials for the lower cutting conditions, confirming the challenge of machining high-strength graphitic cast irons, particularly at higher cutting speeds.

Tool life and wear mechanisms
Through an approach similar to that of da Silva et al. [5], the average machinability (based on the tool lives) values for each material about the parameters indicated in Table 5 were linearly correlated with Table 3 and Fig. 5. Among the variables analyzed, only the macrohardness of the materials was found to have a significant correlation with the machinability of the process, despite the microhardness of the matrix showing a linear correlation of almost 70%, Fig. 9a.
The more significant correlation of hardness between UTS and interlayer spacing, Fig. 9b, is because hardness is the property that most correlates with the dynamic shear strength of the material for the high shear rates observed in machining [46]. The more significant correlation of Brinell macrohardness, Fig. 9d, about the Vickers microhardness, Fig. 9c, of the matrix can be explained by the greater measurement area of the macrohardness, which also takes into account the influence of graphite on the stress/strain behavior of the material, which according to Collini et al. [22] is a defining characteristic of the mechanical behavior of graphitic cast irons.
At the end-of-tool life tests, the cutting tools were taken to a scanning electron microscope (SEM) to analyze the wear mechanisms of the tools. The predominance of adhesion mechanisms (attrition), microabrasion, and chipping  It is observed that the flank wear is more pronounced in the most severe conditions, but the wear mechanisms are practically unchanged, concluding that they are only accelerated with higher cutting and feed speeds. When machining compacted graphite cast iron (FV 450) and gray cast iron of a greater resistance (FC 300 (Mo+RG) and FC 300 (Mo) ), the presence of micro/macrochipping is always observed, while in machining the less resistant gray cast iron (FC 250) chipping does not appear. When machining the FC 250, the photos in Figs. 10d and 11d show the predominance of the microabrasive mechanism. However, considering that in this material, there is no evidence of the presence of precipitates of very high hardness, the microabrasion must have originated in the particles of the tool itself, lost by attrition (adhesion), leading to the conclusion of the occurrence of this wear mechanism as well [23].
It is observed that the lowest flank wear among all the tests occurred in the cast iron alloy FC 250. According to da Silva et al. [5], this is due to the high carbon content associated with the lower number of eutectic cells, a higher percentage of broad graphite lamellae, and the lower hardness   Figure 12 illustrates the average roughness (Ra) of the machined surfaces of each material for each cutting condition. Due to the difficulty in finding correlations between machining parameters and the resulting surface roughness, an analysis of variance (ANOVA) was performed for the results illustrated in Fig. 12, with this analysis being illustrated in Table 10. The analysis of variance indicated that for a 95% confidence interval, the variation of the input parameters has no statistical effect on Ra.

Analysis of roughness of the machined surfaces
According to Guesser et al. [25], the graphite shape significantly impacts the surface finish when machining graphite cast irons. This is due to its stress-concentrating effect dictating the shear mode of the material in the cutting zone and its ejection from the die through the phenomenon known as open grain, directly impacting the roughness of the machined surface. Based on these facts, a new analysis of variance was carried out and illustrated in Table 11, excluding the compacted graphite cast iron, since the presence of vermicular and nodular graphites in it makes the tribosystem at the cutting interface completely different [5].
The analysis of variance illustrated in Table 11 indicated that among the input variables analyzed, only the cutting speed presented, for a 95% confidence interval, statistical significance about the roughness of the machined surface. This effect can best be seen in Fig. 13, which illustrates that higher cutting speeds resulted in lower roughness values. The possible explanation for this phenomenon is the fact that higher shear rates due to the higher cutting speed decrease the shear of the material in the primary and secondary shear planes [26], in addition to making graphite ejection difficult in the open-grain phenomenon by decreasing the time that the material remains undergoing plastic deformation with the increase in the shearing rate [27].
In Fig. 14, correlations were drawn between the mechanical properties of the material and the average roughness for all the cut variables evaluated, in a similar way done for the tool life, Fig. 9, but now, only for the gray cast irons. Linear correlations above 70% were observed only for the macroscopic mechanical properties that consider the matrix and graphite, i.e., the tensile strength and Brinell macrohardness. Since these properties consider the stress concentration of the graphitic phase and its interactions with the material matrix, the overlap of the effects of the phenomena caused by the presence of this second phase on the surface integrity of the material is evident, i.e., the ejection of the graphite (open grain) and the shear in the cutting zone.

Conclusions
This paper studied the tool life, wear mechanisms, and surface roughness in the face milling process of four different graphitic cast irons, for two levels of cutting speed and feed rate. The output variables were compared with the material's mechanical properties, allowing a better understanding of the results. The investigation allowed the following conclusions to be drawn: • The FC 250 Gy cast iron outperformed the other materials in the tool life. This performance, however, was more pronounced for the lower cutting speed and feed rate tested. • The compacted graphite cast iron FV 450 presented the poorest machinability considering the tool life, however with results close to the high strength gray cast irons (FC 300 (Mo) and FC 300 (Mo+RG) ) at the more aggressive cutting conditions. • The Brinell hardness was the material property that better correlated with the tool life, as it evaluates both the matrix and the graphitic phase of the materials. • Adhesion (attrition), microabrasion, and microchipping were the prevailing wear mechanisms for all machining conditions tested. The flank wear was more pronounced at the more aggressive cutting conditions. • The input variables did not show statistical significance on the surface roughness of the evaluated materials. However, when the FV 450 was taken out of considera-tion, the cutting speed presented statistical significance for the surface parameter Ra. • Both the UTS and the Brinell macrohardness are correlated with the surface roughness of the machined surface, as those parameters represent the properties of the matrix and the graphitic phase, which are linked with the shear behavior of the materials. • Substituting the FC 250 Gy cast iron by a harder material is advantageous for the engine, increasing durability, reducing weight, and improving efficiency; however, it has a cost. The use of harder materials (FC 300 (Mo) and FC 300 (Mo+RG) ) reduces tool life in comparison to the commonly used gray cast iron FC 250 but is superior to compact graphite cast iron FV 450. Under more severe cutting conditions (high cutting speed and feed rate), the differences reduce. When analyzing surface roughness, FC 250 showed lower roughness at low cutting speeds but was worse than FC 300 (Mo) and FC 300 (Mo+RG) at higher cutting speeds. • Research never ends, chiefly when experimental works are considered. It is advisable as future work proposals the alteration of the machining parameters, exploring them further, as well as the application of minimum quantity lubrication (MQL) or wet machining, trying new material, coating, and geometry of the tool is another unquestionable valuable attempt; the measurement of the machining force, cutting power, and temperature would add knowledge to the machining of this