UDK 621.937:669.14.018.8	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 48(3)373(2014)
THE EFFECT OF THE CUTTING SPEED ON THE CUTTING FORCES AND SURFACE FINISH WHEN MILLING CHROMIUM 210 Cr12 STEEL HARDFACINGS WITH UNCOATED CUTTING TOOLS
VPLIV HITROSTI REZANJA NA SILE REZANJA IN KVALITETO POVRŠINE PRI REZKANJU KROMOVIH NAVAROV 210 Cr12 Z REZILNIM ORODJEM BREZ PREVLEKE
Abdullah Altin
Van Vocational School of Higher Education, Yuzuncu Yil University, 65080 Van, Turkey
aaltin@yyu.edu.tr
Prejem rokopisa - received: 2013-06-25; sprejem za objavo - accepted for publication: 2013-07-12
The experimental study presented in this paper aims to select the most suitable cutting and offset parameter combination for a milling process in order to obtain the desired surface roughness value for a machined workpiece of chromium 210 Cr12 steel, in terms of cutting speed, feed rate and depth of cut for the milling process. A series of experiments was performed on chromium material with a cutting width of 50 mm using a round, uncoated, cemented-carbide insert on a engine power 5.5 kW Jhonford VMC550 CNC vertical machining center without any cutting fluid. The experiments were carried out using four different cutting speeds ((70, 90, 110, 130) m/min) at a constant depth of cut (1 mm) and feed rate (0.3 mm/r) and the effects of the cutting speeds on the primary cutting force and the surface roughness were investigated. The cutting force (Fc) and the surface roughness (Ra) decreased with the workpiece material's easy machinability. From the experiments, the highest average primary cutting force was obtained as 658.17 N at a cutting speed of 70 m/min. The lowest average surface roughness was 0.36 ^m, which was obtained at a cutting speed of 70 m/min. The experimental results indicated that the obtained chip form is narrow and short stepped.
Keywords: machinability, uncoated cemented-carbide insert, cutting speed, cutting force, surface roughness
Namen predstavljene študije je izbira najprimernejših parametrov rezanja, hitrosti rezanja, hitrosti podajanja in globine reza pri rezkanju in kombinacije parametrov pri postopku rezkanja za doseganje želene hrapavosti površine obdelovancev iz kromovega jekla 210 Cr12. Izvršena je bila serija preizkusov rezanja širine 50 mm na kromovem materialu z vložki iz karbidne trdine brez prevleke na CNC vertikalnem obdelovalnem centru Jhonford VMC550 moči 5,5 kW in brez hladilne tekočine za rezanje. Preizkusi so bili izvršeni pri štirih hitrostih rezanja ((70, 90, 110, 130) m/min) pri konstantni globini rezanja (1 mm) in hitrosti podajanja (0,3 mm/r). Preiskovan je bil vpliv hitrosti rezanja na primarno silo rezanja in hrapavost površine. Sila rezanja (Fc) in hrapavost površine (Ra) sta se zmanjševali z lažjo obdelovalnostjo obdelovanca. Preizkusi so pokazali, da je povprečje največje sile rezanja 658,17 N pri hitrosti rezanja 70 m/min. Povprečje najmanjše hrapavosti površine 0,36 ^m je bilo doseženo pri hitrosti rezanja 70 m/min. Rezultati preizkusov so pokazali, da so dobljeni ostružki ravni in kratko stopničasti.
Ključne besede: obdelovalnost, vložki iz karbidne trdine brez prevleke, hitrost rezanja, sila rezanja, hrapavost površine
1 INTRODUCTION	milling instead of grinding, thus improving the productivity and reducing the cost.6 Chip formation is one of
High-chromium steel belongs to the group of cor-	the most important aspects of the cutting process, with a
rosion-resistant materials and this leads to its practical	factor such as tool wear being related to the behaviour of
applications.1 High-chromium hardfacing materials are	the workpiece material around the cutting edge.7 The
widely used in industry due to their excellent wear	formation mechanism of the chip depends on the thermal
resistance.2 The wear resistance of these materials was	properties and the metallurgical state of the workpiece
mainly achieved by a high hardness and a high carbide	material, as well as on the dynamics of the machine's
content, and this makes the machining of these hard-	structure and the cutting process.8 The chip-formation
facings extremely difficult.3 There is a widespread need	process of hardfacing and the tool-wear characteristics
for abrassion-resistant materials in industries as diverse	were previously investigated.9 This paper further reports
as mining and food processing.2,4 The productivity of	on a measurement of the cutting forces and the surface
machining operations can be expanded and the quality of	roughness to predict the machining quality of hard-
products can be improved by using higher cutting speeds	facings at different cutting speeds, which is another key
than are traditionally applied. Developments in cutting	factor in the machinability of chromium steel 210 Cr12
tools, work materials and machine tools have resulted in	materials. The cutting forces generated during a machin-
the spread of high-speed cutting technology.5 The deve-	ing operation are mainly influenced by the properties of
lopment of ultra-hard CBN materials has opened up the	the workpiece and the tool material, the machining
possibility to machine these materials by turning or	parameters used and other condition, e.g., the coolant.
Factors such as tool wear, surface quality and chip formation, etc., are all affected by the cutting temperature. In many cases the temperature was a limiting factor for the cutting-tool efficiency.7,8,10 When cutting some very hard materials using ceramic tools, e.g., case-hardened alloy steel and superalloys, sufficient cutting heat is needed to soften the workpiece material.1113 Extensive research work has been done to investigate the machining process of these high-chromium hardfacing mate-rials.14,15
2 MATERIALS AND METHOD
2.1 Experimental Specimens
The workpiece material was chromium steel 210 Cr12 with the chemical composition shown in Table 1. The machining tests were performed by the single-point milling of this material in flat form with dimension of 100 mm X 50 mm x 30 mm. The milling machine had a continuously variable spindle speed of up to 10000 m/min with a maximum power of 5.5 kW. The mechanical properties and Typical EDS analyses of the specimens are given in Table 2 and Figure 1. The morphology of the surface is given in Figure 2.
Table 1: Chemical composition of the workpiece material (chromium steel 210 Cr12), w/%
Tabela 1: Kemijska sestava materiala obdelovanca (kromovo jeklo 210 Cr12), w/%
C
2.(
Si
0.28
Mn
0.39
0.017
S
0.012
Cr
11.48
Cu
0.15
Mo
0.02
Ni
0.31
2.2 Cutting parameters, cutting tool and tool holder
The milling tests were conducted with uncoated cemented-carbide cutting tools. No coolant was used during the tests. The tools were commercial-grade inserts with the geometry RPHX1204MOEN. Four different cutting speeds were selected, i.e., 70 m/min, 90 m/min,
110 m/min and 130 m/min, while the cutting parameters, feed rate and depth of cut were kept fixed at 0.30 mm/r and 1 mm, The cutting speeds were chosen by taking into consideration the recommendations of the manufacturing companies. In this study the purpose was to investigate the surface roughness and the material quality on the main cutting force, taking into account the cutting speed. Surtrasonic 3-P measuring equipment was used for the measurements of the surface roughness. The cutting diameter was 50 mm and the cutting width was 30 mm. The experiments were carried out with a 90° lead-angle milling cutter and only one cutting insert was used in the milling cutter. The used tool geometry was as follows: rake angle 0°, clearance angle 7°.
A regression and variance analysis was applied during the experimental study.
Table 2: Mechanical properties of chromium steel 210 Cr12 Tabela 2: Mehanske lastnosti kromovega jekla 210 Cr12
Temperature of annealing T/°C
800-850
Hardening °C
930-960
Cooling
At stove
Environment
Weather, lubricant or hot bath 400-450 °C
Hardness HB
Max. 250
Hardness, after hardening (HRC)
63-65
2.3 Machine-Tool and Dynamometer type
The milling tests were carried out under orthogonal cutting conditions on a Jhonford VMC550 CNC vertical machining center without any cutting fluid, with a max. power of 5.5 kW and a max. revolution number of 10,000 r/min. During the dry cutting process, a Kistler brand 9257 B-type three-component piezoelectric dynamometer under a tool holder with the appropriate load amplifier is used to measure the three orthogonal cutting forces (Fx, Fy, Fz) acting on the cutting tool in the X, Y, Z directions, data-acquisition software. This allows direct and continuous recording and a simultaneous graphical
Figure 1: Typical EDS analysis of the mechanical workpiece material chromium 210 Crl2 steel
Slika 1: Značilna EDS-analiza obdelovanca iz kromovega jekla 210 Cr12
Figure 2: Morphology of surface of the chromium 210 Cr12 steel (SEM)
Slika 2: Značilnosti površine kromovega jekla 210 Cr12 (SEM)
Figure 3: System architecture: left: schematic system scheme; right: data sampled and recorded, each record is composed by 12 values [(Fxmax,
Fy, Fz), (Fx, Fy max» Fz), (Fx, Fy, Fzmax), x, y, z]
Slika 3: Zgradba sistema: levo: shematski prikaz sistema; desno: zapis zbranih podatkov; vsak zapis sestoji iz 12 vrednosti [(Fxmax, Fy, Fz), (Fx, Fymax, Fz), (Fx, Fy, Fzmax), X, y, z]
visualization of the three orthogonal cutting forces. The technical properties of the dynamometer and the schematic diagram of the experimental setup are given in Table 3 and Figure 3. Reference system (Fx, Fy) fixed to the cutting tool in the rotating dynamometer and milling process under orthogonal cutting conditions are given in Figure 4. Surtrasonic 3-P measuring equipment is used for the measurement of the surface roughness. The measurement processes are carried out with three replications. For measuring the surface roughness on the workpiece during machining, the cut-off sampling lengths are considered as 0.8 mm and 2.5 mm. The ambient temperature is (20 ± 1) °C. The resultant cutting force was calculated to evaluate the machining performance. The reference system (fx, fy) was fixed to the cutting tool, in the rotating dynamometer. In this study, the technical properties of dynamometer and the general specifications of the CNC vertical machining center used in the experiments are given in Tables 3 and 4. The levels of the independent variables are shown in Table 5. The results of the regression and variance analysis in the models are given in Tables 6 and 7.
3 RESULTS AND DISCUSSION 3.1 Cutting forces and surface roughness
In this work the aim was to define the variation of the cutting forces related to the cutting speed and the alteration experimentally in order to investigate the effects of the cutting parameters on the surface roughness in the milling process for the chromium steel material. The experiments were successfully carried out and practical results for the milling process were obtained. The main cutting force changed, depending on the cutting speed and the uncoated material of the cutting tool, and while the depth of cut and the feed rate were constant, the cutting speed was changed in all the expe-
riments.716 The main cutting-force values with respect to the cutting speed are given in Figure 5. The lowest main cutting force of 212 N is observed at a cutting speed of 110 m/min. Figure 5 indicates that increasing the cutting speed decreases the main cutting force, excluding the area between 110 m/min and 130 m/min. The obtained main cutting-force values at the cutting speeds of 70 m/min, 90 m/min, 110 m/min and 130 m/min are 244 N, 236 N, 212 N and 231 N, respectively. The results of Figure 5 show that the cutting speed must be increased in order to reduce the main cutting forces.16 However, in this study, a decrease is observed in the main cutting force between 70 m/min and 110 m/min. It is considered that this case is caused by plastic deformation, flank edge, crater and notch wear, which are formed at the cutting tool because of the high temperatures of the shear area when using uncoated carbide tools that have a low
Figure 4: Reference system (Fx, Fy) fixed to the cutting tool in the rotating dynamometer and milling process under orthogonal cutting conditions
Slika 4: Referen~ni sistem (Fx, Fy), pritrjen na orodje za rezanje v rotacijskem dinamometru in postopek rezkanja v razmerah ortogonal-nega rezanja
density at high cutting speeds. The effect of uncoated carbide inserts was found to be important on the main cutting force, but the effect of the cutting speeds was not important in the analysis of variance.17 The main cutting force increases in spite of increasing the cutting speed at 130 m/min. As a result of an increase of 85.7 % in the cutting speed (130 m/min), while working at low cutting speeds (70 m/min) a decrease of 13.1 % was found in the main cutting force. The high temperature in the flow region and the decreasing contact surface area caused the main cutting force to decrease in comparison to the increased cutting speed. The decrement of the cutting force depends on the material type, the working conditions and the cutting-speed range. It was found that by increasing the cutting speed from 110 m/min to 130 m/ min, the main cutting force increases by 9.4 %. Since the rake angle changes due to breakage of the cutting tool, a decrease of the main cutting force in spite of increasing the cutting speed can be attributed to the tool wear. Tool breakage affects the rake angle negatively, which causes an increase of the main cutting force. High temperature in the flow region and a decrease of the contact area and the chip thickness cause the cutting force to decrease depending on the cutting speed. The material properties, working conditions and cutting speed all affect the cutting-force decrement18. As a result of the experimental data (Figure 4), the main cutting force decrement of 13.1% with an increasing cutting speed of 42.85 % is observed at 110 m/min. The scatter plot between the surface roughness and the cutting speed, as shown in Figure 6, indicated that there is linear relationship between the surface roughness and the cutting speed. The results of Figure 6 show that the average surface roughness increases by 61.1 % with an increasing cutting speed from 70 m/min to 130 m/min. The average surface-roughness values were found to be (0.36, 0.365, 0.425 and 0.58) ^m for cutting speeds of (70, 90, 110, and 130) m/min, respectively. As is widely known, the cutting speed must decrease to improve the average surface roughness.19 According to the round-type insert, the change of the three axes cutting forces and chip for-
Figure 5: Cutting force (Fxmax) values (N) with respect to the cutting speeds at a constant feed rate
Slika 5: Vrednosti sile rezanja (Fxmax) v (N) glede na hitrosti rezanja pri konstantni hitrosti podajanja
mation at V =90 m/min are given in Figure 7. The most remarkable result concluded from Figure 7 is that the main cutting force was decreased in the 110 m/min, the opposite direction of the rake angle, for all cutting speeds, while the cutting force was decreased by increasing the rake angle in a positive direction. This is due to changing the tool/chip contact area. The change of the main cutting force depends on the cutting speed and the uncoating material of the cutting tool. After the prepared test specimens were cut for experimental purposes, they were measured with a three-component piezoelectric dynamometer to obtain the main cutting force. Cemen-ted-carbide tools for the cutting of the chromium steel show a low performance at high cutting speeds.20 In this study, since chromium steel is used, the cemented-car-pide tools showed a weaker performance.9 The results of this experimental study can be summarized as follows: increasing cutting speed by 85.7 %, (70-130 m/min) causes the main cutting force to decrease by 13.1 %, and increasing the cutting speed by 57.1 % causes the main cutting force to decrease by 13.75 %. The minimum main cutting force value of 212 N was obtained at a cutting speed of 110 m/min. The experimental results indicated that the obtained chip form is narrow and short stepped. Chip formation at V = 90 m/min are shown in Figure 8.
Table 3: Technical properties of the dynamometer Tabela 3: Tehnične značilnosti dinamometra
Force interval (Fx, Fy, Fz)
Reaction
Accuracy F
Accuracy Fz
Natural frequency fo(x,y,z)
Working temperature
Capacitance
Insulation resistance at 20 °C
Grounding insulation
Mass
-5 kN ..... 10 kN
< 0.01 N
^ 7.5 pC/N
« 3.5 pC/N
3.5 kHz
0 °C ^.70 °C
220 pF
>1013 Q
>108 Q
7.3 kg
Table 4: General specifications of the CNC vertical machining center used in the experiments
Tabela 4: Osnovne značilnosti uporabljenega CNC vertikalnega obdelovalnega centra
Model
Travel X, Y, Z
Table Dimensions
Tool Changer
SPIRSIN
Phase number
Frequency
Max revolution number
CNC FANUC 0-M YO M. 1998
500 mm x 450 mm x 450 mm
705 mm x 450 mm
18 tools
Divisor with tiltable axis
3
50 Hz
10000 r/min
Figure 6 clearly shows the effect of feed rate, cutting speed and cutting-tool material on the average surface roughness. According to this figure, in order to obtain the smallest surface roughness, it is necessary to use the RPHX1204MOEN cutting tool at low feed rate (0.30 mm/r) and low cutting speed (70 m/min). In addition, in
Figure 6: Surface-roughness values (pm) with respect to the cutting speeds at a constant feed rate
Slika 6: Vrednosti za površinsko hrapavost (pm) glede na hitrost rezanja pri konstantni hitrosti podajanja
order to find out which important parameter effects the surface roughness, a variance analysis was made for this aim. According to the results of the ANOVA in Table 7, the most efficient cutting parameter that affected the surface roughness was found to be the tool material of the uncoated insert (81.24 %), followed by the cutting speed (4.56 %).
Table 5: Level of independent variables Tabela 5: Nivo neodvisnih spremenljivk
Variables	Level of variables			
	Lower	Low	Medium	High
Cutting speed, v/(m/min)	70	90	110	130
Feed, //(mm/r)	0.3	0.3	0.3	0.3
Axial depth, da/mm	1	1	1	1
Table 6: Regression analysis of experiment Tabela 6: Regresijska analiza preizkusov
Regression Analysis: Fx versus V; Vf
*	Vf is highly correlated with other X variables
*	Vf has been removed from the equation
The regression equation is Fx = (870 - 1.09) V
Predictor	Coef	SECoef	T	P
Constant	869.72	18.86	46.12	0.000
V	-1.0862	0.1840	-5.90	0.028
5 = 8.23028 R - Sq = 94.6 % R - Sq(adj) = 91.9 %
Source	DF	SS	MS	F	P
Regression	1	2359.9	2359.9	34.84	0.028
Residual error	2	135.5	67.7		
Total	3	2495.4			
Figure 7: The change of the 3 axis cutting forces for a (1 mm) constant depth of cut and (0.3 mm/r) constant feed rate, dependent on the cutting speed (m/min)
Slika 7: Sprememba 3-osnih sil rezanja pri konstantni globini rezanja (1 mm) in konstantni hitrosti podajanja (0,3 mm/r) v odvisnosti od hitrosti rezanja (m/min)
Figure 8: Chip formation at V = 90 m/min Slika 8: Oblika ostružkov pri V =90 m/min
4 CONCLUSIONS
The machinability of chromium-carbide-based hard-facing appears to be strongly related to its microstruc-tural properties, and in particular to the presence and deformation characteristic of the large carbides. The effect of cemented carbide on the main cutting force is much clearer than the effect of the cutting speed. There is an incremental-decremental relationship between the cutting speed and the main cutting force. The breaking on chip contact surface was affected by the tool rake angle in a negative way. The negative chip angle caused an increase in the main cutting force. An increasing relationship between the cutting speed and the arithmetic average surface roughness as well as between the coating
Table 7: Variance analysis of experiment Tabela 7: Analiza variance preizkusov
Source	Df	Sum of Squares	Mean square	F ratio	Probability	PD
Cutting speeds	3	61906.2	20635.4	0.8436	0.5075	4.56
Uncoated insert	3	1102720.4	367573.5	15.0262	0.0012	81.24
Error	8	195697.0	24462			14.42
Total	14	1357422.4				100
number and the average surface roughness was observed. In the case of the coated tools, the effect of the cutting speed on the surface roughness was much more pronounced than the effect of the different coated cemen-ted-carbide inserts. The experimental results can be used in industry in order to select the most suitable parameter combination in order to achieve the required surface.
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