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SURFACE ROUGHNESS AND CUTTING FORCES IN TURNING OF TOOL STEEL WITHMIXED CERAMIC AND CUBIC BORON NITRIDE CUTTING TOOLS

Bekir YalçınDepartment of Manufacturing Engineering, Technology Faculty, Süleyman Demirel University Isparta, Turkey

E-mail: [email protected]

Received September 2014, Accepted March 2015No. 14-CSME-84, E.I.C. Accession 3744

ABSTRACTTool steel has been widely used, especially to manufacture forming dies and molds by machining processes.Generally, cubic boron nitride (CBN) and ceramic tools are recommended for finish machining a specificsteel. This study contributes to filling the research gap for the selection of low- content CBN tools or mixedceramic tools for turning of hard tool steel. The turning tests were conducted to determine the performanceof CBN and the mixed ceramic tools in turning soft (HRC22) and hard (HRC52) H13 tool steel with differentcutting speeds, feed rates and depths of cut. ANOVA was used to determine the interaction of the cuttingparameters on the surface roughness and cutting forces obtained from turning tests. The results indicate thatthe surface roughness in hard turning was lower with the CBN tool than with the ceramic tool. On the otherhand, the cutting forces in turning with the ceramic tool were lower. Acceptable regular chip formationincreases with the cutting speed for each tool.

Keywords: hard finish turning; CBN; ceramic tool; chip.

RUGOSITÉ DE LA SURFACE ET FORCES DE COUPE DANS LE TOURNAGEDE L’ACIER À OUTILS AVEC DES OUTILS DE COUPE EN CÉRAMIQUE COMPOSITE

OU NITRURE DE BORE CUBIQUE

RÉSUMÉLes outils en acier sont utilisés de façon courante, spécialement pour manufacturer des matrices de formageet moules par procédé d’usinage. En général, le nitrure de bore cubique (CBN) et les outils de céramique sontrecommandés pour les opérations de finition sur un acier spécifique. Cette étude apporte une contributiondans la recherche pour la sélection d’outils CBN à faible teneur ou céramique composite pour le tournaged’outils en acier dur. Des tests de tournage sont conduits pour déterminer la performance du CBN et desoutils en céramique composite dans le tournage d’acier plus doux (HRC2) et d’acier plus dur (HRC52) H13avec des forces différentes de coupe, de vitesses d’avance et de profondeur de coupe. ANOVA a été utiliséepour déterminer l’interaction des paramètres de coupe sur la rugosité de la surface et les forces de coupeobtenues pendant les tests de tournage. Les résultats indiquent que la rugosité de la surface dans le tournaged’acier dur était inférieure avec l’outil en CBN qu’avec l’outil en céramique composite. D’autre part, lesforces de coupe dans le tournage avec un outil en céramique étaient plus basses. La formation de copeauxacceptable augmente avec la vitesse de coupe pour chaque outil.

Mots-clés : tournage dur de finition; CBN; outil en céramique; copeaux.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 2, 2015 323

1. INTRODUCTION

Tool steel is a very large group of complex alloys that have evolved for many diverse hot and cold formingapplications [1]. AISI H hot-worked tool steel is used for applications such as hot forging, extrusion, andmetal die casting dies due to its high resistance to wear and softening during exposure to hot workingoperations. Chromium-based AISI H13 hot-worked tool steel is widely used in hot working operation. Thissteel has good wear resistance, high hardenability, high strength, and high toughness. This material alsowithstands high working temperature between 315 and 600◦C [2].

The machining of tool steel has great importance in die and mold manufacturing. Ceramics and CubicBoron Nidride (CBN) tools are the best tool materials for finishing hardened tool steel due to their highhot hardness and wear resistance. CBN tools are some of the hardest known after diamond, and have ahigher hardness than ceramic tools at both low and high temperatures. Therefore, CBN tools and someheat-resistant coated carbide tools have widely been used for milling difficult-to-machine materials, such ashardened steel or die steel [3–5]. The main characteristics of CBN are its grain size, percentage of CBN,and binder type. In general, CBN is made with varied CBN contents and some additives. There are twocategories of CBN tools. One contains CBN grains at a volume fraction of about 0.9 with metallic binders(e.g. cobalt). These are referred to as high CBN content tools. The volume fraction of the other type isabout 0.5 to 0.7 and uses ceramic binders (e.g. titanium nitride TiN, titanium carbide TiC). This type isreferred to as low CBN content tools [6]. The wear resistance of the tool increases with the CBN content[6, 7].

Ceramic also has good properties for use in hardened steel turning, such as hot hardness, wear resistance,and greater chemical stability than CBN. The ceramics currently used in metal cutting are based on eitheralumina or silicon nitride and composite ceramic additives such as ZrO, TiC, TiN, or SiC whiskers [7].However, the fracture and thermal shock resistance of alumina tools can be increased by these additives.One investigation [8] indicated that mixed Al2O3 ceramic and zirconium toughened Al2O3 ceramic toolsboth exhibited good performance in machining hardened EN 24 steel (45 HRC). However, a ceramic tool isunsuitable for hardened steel turning of interrupted surfaces [2, 9].

Previous investigations determined the performance of cemented carbide cutting tools in the machiningof tool steels. For example, Xiong et al. [10] studied the turning of AISI H13 hardened tool steel with WC-5TiC-10Co ultrafine cemented carbides. Axinte and Dewes [11] studied the effect of cutting parameters onsurface integrity during high-speed milling of hardened AISI H13 tool steel with solid carbide. A numericalsimulation of AISI H13 tool steel was made by Umbrello et al. [12], who developed a hardness-based flowstress and fracture model. Ghani et al. [13] carried out experimental work to investigate the performance ofa P10 TiN coated carbide insert when finishing milling of hardened AISI H13 steel. Özel [14] conducted anumerical simulation on the influence of edge preparation in a cubic boron nitride cutting tool using finiteelement simulation. Camuscu and Aslan [15] worked on high-speed end milling of AISI D3 cold-workedtool steel hardened to 35 HRC with a coated carbide tool, Al2O3-based ceramic, and CBN tools. Theyreported that the mixed Al2O3 ceramic tool was suitable for the application.

Nowadays, finishing operations are still performed with carbide tools with low cutting speeds (around40 m/min, with tool-life around 30 min). Some investigations about the performance of CBN and ceramictools in high-speed machining have been conducted. For example, Aouici et al. [16] investigated the ef-fects of cutting speed, feed rate, workpiece hardness, and depth of cut on the surface roughness and cuttingforce components in the hard turning of AISI H11 steel with CBN. They reported that the best surfaceroughness was achieved at a lower feed rate and the highest cutting speed. Farhat [17] studied the wearmechanism of a CBN cutting tool during high-speed machining of mold steel. He achieved significant im-provement in tool wear resistance of CBN in comparison to WC tools during high-speed cutting of P20 moldsteel.

324 Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 2, 2015

Fig. 1. Experimental layout and measured cutting forces components.

In the present work an experiment was done to compare the performance of CBN and mixed ceramictools with respect to surface roughness, cutting forces, and chip formations during finish turning of soft(HRC22) and hardened (HRC52) H13 steel. The experiments are designed as full factorial at three differentcutting speeds (Vc), feed rates ( f ), and depth of cut (a). The turning tests were conducted in dry cuttingconditions. Cutting forces and surface roughness were recorded during the experiments. The varianceanalysis (ANOVA) was used to determine the interaction of the cutting parameters with surface roughnessand cutting forces in finish turning. Also, chip formations were examined by evaluating the tool type andcutting conditions.

2. EXPERIMENTAL PROCEDURE

The experimental work was divided into two series. The main aim of the first experiments was the de-termination of finish turning of hardened (HRC52) H13 tool steel. These experiments were performed byfinish turning with three different cutting speeds (Vc), feed rates ( f ), and depths of cut (a) using CBN andceramic tools (Table 1). The finish turning experiment conditions were designed using the Taguchi method.The second series of experiments was carried out to investigate the effect of workpiece hardness and cut-ting parameters on the performance of CBN and ceramic tools during finish turning of soft (HRC22) H13steel. These experiments were carried out through finish turning operation with varying cutting speeds, feedrates, and depths of cut. All cutting operations in hard turning and soft turning were conducted with dryconditions.

2.1. Workpiece and Tool MaterialsTurning experiments were performed in dry conditions using an ALEX ANL-75 CNC lathe with a spindlepower of 15 kw, as given in Fig. 1. The workpiece material was AISI H13 hot-worked tool steel withthe following chemical composition: 0.42% C; 1.2% Si; 0.5% Mn; 0.025% P; 0.005% S; 5.5% Cr; 1.5%Mo; 1.15% V. The workpiece material was conventionally hardened (HRC52) with vacuum controlled heattreatments. Then, the first turning tests were performed through straight finish turning with hardened testmaterial. In the second series of finish turning experiments, the soft AISI H13 (HRC 22) was tested in thesame conditions as the hard finish turning tests. This second series experiments were done to observe thesensitivity of the cutting forces, surface roughness, and chip formation tendency with different workpiecehardness.


Table 1. L18 Taguchi experimental design, cutting forces and surface roughness obtained from finish turning.

The turning test was performed using straight turning on a round bar with 100-mm diameter and220-mm length. A coated CBN7015 grade tool and ceramic 650 grade tool from Sandvik Companywere used. CBN 7015 is a low-content material coated with TiN film, and the insert ISO designa-tion is CNGA120408S01018A 7015. CBN7015 contains 50% CBN with fine grain size in a uniqueceramic binder. The CBN7015 cutting insert has a wiper edge with a honing radius of 0.05 mm.The face land length is 0.1 mm, the face land angle is 18◦, and the nose radius is 0.8 mm. Ce-ramic 650 grade is based on Al2O3 and mixed with Zr (uncoated). The mixed ceramic insert ISOdesignation is CNGA120412T01020WG 650. These were clamped onto a tool holder (ISO designa-tion DCLNL 2525M 12). The combination of the insert and the tool holder resulted in a negativerake angle γ = −6◦, clearance angle α = −6◦, and cutting edge angle χ = 95◦. A Kistler 9257 forcedynamometer was used to measure cutting forces, which was connected to Kistler charge amplifiers(type 5070A11100) using a high impedance cable. Surface criteria measurements (arithmetic averageof roughness, Ra) for each cutting condition were measured by a Hommel Werke T 500 roughnessme-ter. The measurements were repeated three times, and the results were averages for a given machiningpass.

3. RESULT AND DISCUSSION

3.1. Results of the Cutting Forces and Surface Roughness in Hard Material TurningThe finish turning tests were performed to determine the effect of workpiece hardness, cutting speed, feedrate, and depth of cut (DOC) variations on cutting forces and surface roughness. Table 1 shows that themain cutting force Fc is usually highest, followed by the passive force and the feed force in hard finishturning with the CBN tool. According to the measured cutting forces in Table 1, the maximum cutting forcecomponents and surface roughness with the CBN tool (Fc, Fp, Ff , and Ra) are 864.1 N, 400.4 N, 411.7 N,and 3.44 microns, respectively. These results were obtained with a cutting speed of 150 m/min, feed rateof 0.35 mm/rev, and DOC of 0.8 mm (5th experiment). The highest cutting forces and surface roughnesswith the mixed ceramic tool were respectively obtained as 490.5 N, 296.5 N, 286.3 N, and 4.2 microns.These were obtained with a cutting speed of 150 m/min, feed rate of 0.35 mm/rev, and DOC of 0.5 mm (2nd


Table 2. ANOVA table for surface roughness (Ra) in hard material finish turning.

experiment). While the mixed ceramic tool produced a higher surface roughness, the cutting forces with themixed ceramic tool were lower than those of the CBN tool.

It is known that the best surface roughness can be provided by decreasing the feed rate and DOC whileincreasing of the cutting speed. For instance, the lowest surface roughness value (0.3 microns) was achievedwith a cutting speed of 350 m/min, feed rate of 0.05 mm/rev, and DOC of 0.5 mm (8th experiment) with theCBN tool. Also, a similar surface roughness value (0.27 microns) was obtained with CBN in hard turningwith a cutting speed of 150 m/min, feed rate of 0.05 mm/rev, and DOC of 0.2 mm (2nd experiment). Butthe hard turning operation conducted with the 2nd experiment leads the a longer cutting period than with the8th experiment. So, the low depth of cut and feed rate and high cutting velocity in the 8th experiment canbe preferable for hard finish turning of AISI H13 steel with CBN tool, considering the low machining time,low cutting forces (Fc: 116.7 N, Fp: 145.7 N, Ff : 109.9N), and low surface roughness (0.3 microns).

The mixed ceramic tool provided a surface roughness of 0.35 microns at low cutting speed with the CBNtool in hard turning. Table 1 shows that the lowest cutting forces with the ceramic tool are as follows: Fc:116.7 N, Fp: 145.7 N, Ff : 109.9 N. A surface roughness of 0.35 microns was achieved with a cutting speedof 250 m/min, feed rate of 0.05 mm/rev, and DOC of 0.5 mm (1st experiment). Therefore, when comparingthe surface roughness value, the CBN tool can be preferable for finish turning of HRC 52 H13 steel dueto the high removal rate and high cutting speed. With the CBN tool, the maximum main force Fc is 2.25times higher than the passive force and 2.09 times higher than the feed force. The feed rate was found tobe the most influential factor on surface roughness. Table 2 and Fig. 2 show that the tool material and feedrate are significant parameters for the surface roughness in hard finish turning. Also, CBN tools exhibitedbetter surface roughness than the mixed ceramic tool in hard turning, as shown in Fig. 2. Bouacha et al.[18] reported that the surface roughness is highly affected by feed rate. Augusto et al. [9] investigated theturning of interrupted and continuous hardened steel surfaces using ceramic and CBN tools. They found thatCBN tools had much better performance with respect to both tool life and surface roughness than ceramictools. It is know that values of probability less than 0.05 indicate model terms are significant in ANOVAanalyses. In this study, the feed rate was found to be the dominant effective parameter on surface roughnesswith probability of 0.0001, the tool type is secondary affecting parameter with probability of 0.0036 in hard


Fig. 2. The effect of cutting parameters on the surface roughness in hard finish turning.

turning. Surface roughnesses in hard tuning with the CBN tool are lower than those of the mixed ceramictool. This state can be seen in Table 2 and Fig. 2.

The interaction of the cutting parameters on cutting forces in hard finish turning is given in Table 3. Thefeed rate and depth of cut have a significant effect on feed forces. Similarly, the feed rate and DOC are thesignificant parameters for the main force, feed force and passive force. The feed rate and DOC has the sameinfluence with 0.0001 probability value on the main force and feed force in soft and hard finishing. Thefeed force, DOC and tool type were found to be effective parameters on the passive force in soft and hardH13 finishing. But, feed force has the highest influence with 0.0001 probability value, DOC is a secondaryeffective parameter with 0.0059 probability value, while the effect of tool type is less than feed rate andDOC with probability value of 0.0188 on passive force. Hence, the passive forces with CBN are higherthan those of mixed ceramic tool due to values of probability less than 0.05. The cutting speed was foundto negligibly affect parameters on the cutting forces components. The measured cutting force values withthe ceramic tool are lower than those of with the CBN tool. This result can be seen in Fig. 3. Accordingto the figure, Fc forces are largest, followed by Fp forces and Ff forces. The effect of feed rate on Fc wasdrastic with increased depth of cut. As can be seen in Fig. 3a, the feed rate has the highest effect on themain cutting force when the depth of cut is increased from 0.2 to 0.8 mm. Similarly, Ff forces increaseddramatically when increasing the depth of cut and the feed rate. This result is shown in Fig. 3b. The passiveforces increase with the feed rate and depth of cut. The effect of feed rate with DOC of 0.2 mm is lowerthan that of DOC of 0.8 mm. In addition, increasing the cutting speed has a slightly decreasing influenceon Fc forces. Chen [19] reported about a decreasing trend in cutting forces with increase in cutting speed inmedium hard material turning with CBN tool. As can be seen in Fig. 3(d), the main cutting forces with theCBN tool are a little higher than those of the mixed ceramic tool. A similar tendency is shown in Fig. 3e inthe comparison of tool type in hard turning.

When comparing the surface roughness and cutting force components using CBN and ceramic toolsduring continuous hard tuning, some similar trends were obtained in this study. As can be seen in Figs. 3(d)and 3(e), Fc forces with ceramic tool are a slightly lower than those of CBN tool. Diniz et al. [20] andDiniz and Oliveira [21] reported that CBN performs better than ceramic in terms of surface roughness. Luoet al. [3] investigated the cutting forces and temperatures in hard tuning of AISI 4340 steel with ceramicand CBN tools. The cutting force components were lower than those of CBN tools. Bosheh and Mativenga[22] examined the white layer formation in hard turning of H13 tool steel at high cutting speeds using CBNtooling. They reported that surface roughness is also less favorable when using CBN than with the mixedceramic.

Ghani et al. [13] emphasized that high mechanical impact caused by high feed rate and depth of cutinitiates early crack formation on the cutting edge. Bartarya and Choudhury [23] experimentally worked on


Table 3. ANOVA tables (partial sum of squares) for Fc (a), Ff (b), Fp (c) in hard and soft material finish turning.


Fig. 3. The effect of cutting parameters on the cutting forces in hard finish turning.

the effect of cutting parameters on cutting force and surface roughness during finish hard turning AISI52100grade steel. They showed that depth of cut was the most influential parameter affecting the three cuttingforces, followed by the feed. In this study, while the feed rate was found to be the most influential parameteron surface roughness, cutting force components were dominantly influenced by feed rate together with depthof cut. In addition, a slightly decreasing trend in cutting forces was observed with an increase in cuttingspeed to 350 m/min in hard turning. The reason can be interpreted as the chip flow and cutting movement onthe tool-chip contact region during hard finish turning is made easier by increasing cutting speed. Lalwaniet al. [24] showed that the feed rate and depth of cut were the most significant factors affecting cutting forcein hard finish turning of MDN250 (HRC50) steel using a coated ceramic tool.

3.2. Results of Cutting Forces and Surface Roughness in Soft Material TurningMixed ceramic and CBN tools were evaluated for finish turning of soft (HRC22) AISI H 13 hot-workedtool steel. The effect of workpiece material hardness, cutting parameters, and type of tool material on thecutting force components and surface roughness are interpreted in this section. As shown in Table 3, themaximum cutting forces and surface roughness with the CBN tool were obtained with a cutting speed of150 m/min, feed rate of 0.35 mm, and DOC of 0.8 mm (5th experiment). The maximum forces and surfaceroughness were as follows: Fc: 776.2 N, Fp: 342.8 N, Ff : 294.6 N, and Ra: 3.53 microns. The lowestcutting forces and surface roughness were the following: Fc: 107 N, Fp: 93.52 N, Ff : 69.13 N, and Ra:0.63 microns (8th experiment) with a cutting speed of 350 m/min, feed rate of 0.05 mm/rev, and DOC of


Fig. 4. Comparison the main cutting forces in finish tuning of hard (a) and soft (b) material with CBN and ceramictool.

0.5 mm. The minimum cutting forces and surface roughness were as follows: Fc: 93.84 N, Fp: 66.79 N,Ff : 63.37 N, and Ra: 0.36 microns in the first experiment with Vc: 250 m/min; f : 0.05 mm/rev; and a:0.5 mm using the mixed ceramic tool. The maximum cutting forces and surface roughness Fc, Fp, Ff , andRa were respectively obtained as 436.7 N, 281.4 N, 183.3 N, and 4.55 microns with a cutting speed of 150m/min, feed rate of 0.35 mm/rev, and DOC of 0.5 mm. An increase in feed rate and depth of cut caused anincrease in cutting force components in soft material finish turning. As can be seen in Table 1, an increasein workpiece hardness leads to increased cutting forces. For instance, when evaluating the highest cuttingforces (5th experiment) in both hard turning and soft material turning with CBN, there was approximately a9.87% decreasing trend in the main force, 14.5% in the passive force, and 28.4% in the feed force observedwith increasing the workpiece hardness from HRC 55 to HRC 22. This result can be seen in Figs. 4 and 5.

According to Figs. 5, 3(d) and 3(e), cutting forces during finish tuning of hard and soft AISI H13 steelwith the CBN tool are slightly higher than those of the ceramic tool. As can be seen in Fig. 5(b), while thecutting tools exhibited a similar trend with increasing the feed rate in soft material finishing, low increasingtrend in Fp with CBN tool was obtained with increasing the feed rate in hard turning (Fig. 5a). Therefore,cutting forces with the ceramic tool are lower than those of CBN due to easy flow of chips on the rake.ANOVA tables (partial sum of squares) about Fc, Ff , and Fp in soft material finish turning are given inTable 3, and similar trends to hard finish turning were obtained. Figure 6 shows the comparison surfaceroughness in soft and hard material turning with CBN and ceramic tools.

According to Figs. 6 (a, b), the surface roughness in finish turning of hard H13 steel with each tool islower than in soft H13 finish turning. Experimental results showed a decreasing trend in surface roughnesswhen increasing the workpiece hardness from HRC 22 to HRC 55. Also, the CBN tool exhibited bettersurface quality than the mixed ceramic tool in this study. As can be seen in Fig. 6, surface roughnesseswith the mixed ceramic tool are higher than those of CBN tool. Aslan [25] obtained the highest volume ofmetal removal with a CBN tool. Similarly, the CBN tool provided better surface roughness than the ceramictool at high cutting velocity in this study. When evaluating surface roughness in soft material, feed rate andtool material were determined as influential factors on surface roughness. But, the feed rate was found tobe the most significant factor with probability value of 0.0001 on surface roughness with in soft materialfinishing. Also, the tool type is secondary effective parameter with 0.00501 due to values of probability lessthan 0.05. This result is given in Table 4. Namely, the surface roughnesses with CBN tool are lower thanthose of ceramic tool. Also, surface roughnesses in hard H13 finishing with CBN tool are lower than thoseof ceramic tool.


Fig. 5. The cutting forces in finish tuning of hard (a) and soft (b) material with CBN and ceramic tool.

Fig. 6. The comparison surface roughness in hard (a) and soft (b) material turning with CBN and ceramic tool.


Table 4. ANOVA table for surface roughness (Ra) in soft material finish turning.

According to Table 4, the most significant parameter was found to be feed rate on the surface roughnessof soft material finishing with CBN and mixed ceramic tools. It is know that values of probability less than0.05 indicate model terms are significant in ANOVA analyses. Also, the surface roughness is less using CBNtool than with mixed ceramic tool. This state can be seen in Fig. 6. So, tool type in hard and soft materialfinishing was found to be the secondary affecting parameter with probability values of 0.0036 and 0.00501given in Tables 2 and 4, respectively. It can be interpreted that the effect of tool material in hard turning is alittle higher than that of soft turning. The tool material probability value of 0.0036 in hard material turningis smaller than that (0.00501) of soft material turning. The CBN tool is more effective on surface roughnessin hard turning than soft material turning.

3.3. Chip FormationsIn this study, the CBN tools showed much better performance with respect to surface roughness than ceramictools. Also, the cutting forces with the mixed ceramic tool are lower. Klocke [26] classified chip formationaccording to ISO 3685-1977 (E) to evaluate machining with respect of chip formations. The standard givesinformation about acceptable chip types in machining. According to ISO 3685-1977 (E), while the mixedtype chip is not acceptable for the best cutting condition, the spiral, comma, and helical chip formationsare preferred to provide the best cutting condition. Some chip formations for both best surface roughnessand worst surface roughness are given in Fig. 7 for both soft and hard finish turning, unlike other studies.The short comma chip type was obtained in the best hard finish turning condition of the 8th experimentwith CBN. Also, the mixed type chip in the worst hard turning of the CBN tool was observed in the 8thexperiment. The chips in hard turning operations occurred as shot comma type (acceptable type) in thebest finish turning condition, while the chips occurred as mixed chip type (unsatisfactory) in the worst finishturning condition for both CBN (5th experiment) and ceramic tools (2nd experiment). In addition, the helicalchip type occurred in the best cutting condition for each tool in soft finish turning tests, but the mixed chipsoccurred in the worst finish turning condition for each tool. These chip types can be seen in Fig. 7.

The best finish turning condition of soft and hard H13 steel for CBN and ceramic tools are in the 8thand 1st experiments, respectively. Also, the worst conditions are in the 5th and 2nd experiments, respec-


Fig. 7. Chip formations in the best and worst finish turning condition for each tool.

tively. Chip formations supported the surface roughness values in soft and hard finish turning with CBNand ceramic tools. The more acceptable chip formations corresponded to higher surface quality in the finishturning operation, whereas higher feed rate and passive force led to more surface roughness and more mixedchip formations. The tendency of comma chip formation increases with increasing the cutting speed in hardfinish turning. Also, a helical chip tendency was found with increasing the cutting speed in finish turning ofsoft H13 steel. Even if the cutting speed has a negligible effect on the cutting forces, the cutting speed infinish turning contributed to the acceptable (regular) chip formation.

4. CONCLUSION

Generally, CBN and ceramic tools are recommended for finish machining of hard materials. This studywas conducted to contribute to research about the selection of CBN tools or mixed ceramic tools for finishturning of hard tool steel. The effects of workpiece material hardness, cutting parameters, and tool typeon the surface roughness, cutting forces, and chip formations were investigated. Some of the experimentalresults were as follows:

• Firstly, the surface roughness values with the ceramic tool were higher than those of the CBN tool.Also, the surface roughnesses of hard AISI H13 steel with each tool were lower than those of softAISI H13 steel. So, the CBN tool exhibited much better performance than the mixed ceramic tool inhard turning. It is known that the surface roughness increases with increasing feed rate and decreaseswith an increase in workpiece hardness.

• The cutting force components with the CBN tool were higher than those of the mixed ceramic tool.Cutting forces increase with increasing feed rate, depth of cut and workpiece hardness. Low feed rateand depth of cut were beneficial for minimizing the machining force [27]. A similar tendency aboutthe cutting force component was given by Aneiro et al. [28]. On the other hand, cutting speed wasfound to have a negligible effect on the cutting force in this study.


• The main force is 2.25 times higher than the passive force and 2.09 times higher than the feed forcein hard turning with CBN tools. The main forces are at least 1.65 times higher than the passive forceand 1.7 times lower than the feed force with ceramic tools. The main force in finish turning of softH13 steel with the CBN tool was found to be 2.26 times higher than the passive force and 2.63 timeshigher than the feed force. The main force was 2.06 times higher than the passive force and 2.38 timeshigher than the feed force in soft H13 turning with the ceramic tool.

• Parameters R2, R2Adj, and R2Pred are quite high in this study. R2Pred=0.7787, which is in goodagreement with R2Adj [29, 30]. Therefore, the actual squares obtained are highly consistent withR2Pred (Tables 2-4). ANOVA results showed that feed rate has the most significant effect on thesurface roughness. Feed rate and depth of cut have the most significant influence on cutting forcecomponents (Table 3 and Fig. 3). Also, tool type was found to be the secondary influential factor onsurface roughness with probability values of 0.0036 in hard turning due to values of probability lessthan 0.05. In addition, quite a few decreasing trends on cutting forces and surface roughness wereobserved with increasing the cutting speed to 350 m/min in this study.

• Short comma chip types (acceptable and regular) were obtained when increasing the cutting speed(especially at 350 m/min) and decreasing the feed rate and depth of cut. Low cutting speed changedthe chip types to unsatisfactory or irregularly mixed. Also, short helical chip types (acceptable orregular) were obtained in finish turning of soft AISI H13 steel with high cutting speed.

ACKNOWLEDGEMENT

This work was produced from the machining test results of CBN and Ceramic Tools supported by Unitof Scientific Research Projects of Süleyman Demirel University in Turkey, which the author gratefullyacknowledges.

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