EXPERIMENTAL STUDY ON SURFACE ROUGHNESS OF FACE MILLED PARTS WITH ROUND INSERT AT VARIOUS FEED RATES

In the present paper, the variation of surface roughness of machined parts during symmetrical face milling is investigated. During this experimental work, the effect of using a milling insert with a round geometry under various feed rate values on the topography of milled parts is examined. For that purpose, both 2D and 3D surface roughness measurements were performed in three planes parallel to the feed direction, with one of the planes being on the symmetrical plane and the other two being at the same distance from it but in opposite sides. The analysis of the experimental results indicated that although surface roughness increases gradually with increase of feed rate, a considerable increase of surface roughness occurs for feed rate values over 0.4 mm/tooth. Moreover, the overall higher surface roughness values were found to be on the symmetrical plane, which was also more affected by the increase of feed rate than the other two planes.


INTRODUCTION
Surface quality of mechanical components is considered very crucial, as it is one of the main indicators of surface integrity and it was shown to affect produced parts' life directly [1]. High surface quality at the last stages of the production can usually be obtained by abrasive processes suitable for finishing, like grinding or polishing, but it is also required to achieve sufficient surface quality from the early stages of manufacturing when turning or milling processes are carried out. In specific, face milling, which is widely used in the industrial environment, is used to create flat surfaces with high dimensional accuracy, appropriate flatness and high surface quality.
Surface roughness is one of the most important aspects of surface quality. For that reason, many scientific works regarding surface roughness during face milling have already been reported in the relevant literature. In the existing experimental studies, it was shown that surface roughness decreases at high cutting speed values [2][3][4], possibly due to more stable conditions. Moreover, improved machining performance was observed at intermediate and high speeds [3,4]; on the contrary, lower values of cutting speed were related to built-up edge creation and chip fracture, leading to deterioration of surface quality [2,3]. The increase of feed rate, axial and radial depth of cut also lead to surface quality proposed a methodology for predicting the 3D topography based on the kinematics of the face milling process, which can be applied to various insert geometries. By using a regularly spaced grid of points along the feed and pick feed direction they were able to sufficiently predict the surface roughness, including runout effects, as a good agreement was found with experimental face milling tests. Felhő et al.
[9] presented a detailed approach for the modeling of surface roughness during face milling with different cutting insert geometries, such as polygonal, round and triangular. The insert geometrical features were modeled, and the technological parameters of the process were taken into account, so that the calculation of the surface topography was possible based on the insert trajectory relative to the workpiece. With this model, sufficient accuracy of the prediction of Ra and Rt was achieved, compared to experimental results. Moreover, Jin et al.
[10] presented a unified prediction model of 3D surface topography in face milling, considering multierror sources, with cutting wipers on the inserts. The model could generate point cloud to simulate the machined surface topography and was established by considering the insert geometry, machining process parameters and toolworkpiece dynamics, and integrating them with several error sources. Svalina et al. [11] employed an alternative approach by correlating 23 machined surface images with their actual roughness statistical measurements.
In addition to the above-mentioned works, few researchers have employed artificial intelligence methods for surface roughness prediction. Pimenov et al. [12] compared various methods such as random forest, multilayer perception, radial-basis functions and regression trees for the prediction of surface roughness by monitoring the tool wear. The analysis of the results showed that random forest and regression tree methods exhibited higher accuracy and could provide essential information about the relation of input and output variables, as well as their critical values. Abbas et al. [13] used ANN with the Edgeworth-Pareto method for a face milling operation in order to optimize the cutting conditions, e.g. cutting speed, depth of cut and feed per tooth for a desired value of Ra. Alharthi et al.
[1] developed an ANN model and regression analysis model to predict surface roughness for different values of spindle speed, depth of cut and feed rate. After experimental validation tests, both models were able to predict Ra with an accuracy of about 95% and 94%, respectively. In addition, a higher surface roughness with increasing feed rate was reported.
Although most researchers investigate the effect of process parameters values on surface roughness, the effect of milling strategy, such as symmetric or asymmetric milling on the surface quality during face milling has rarely been investigated. Bağci and Aykut [14] compared symmetric and asymmetric face milling strategies, showing that during symmetric milling a lower value of roughness was obtained. Pimenov et al. [15] investigated the effect of the relative position of the milling cutter, which characterizes the face milling process kinematics and affects the contribution of up and down milling. It was found that this factor directly affected milling vibrations, cutting forces and surface roughness and the lowest values of vibrations and cutting forces were observed for the case of up-milling. Finally, they were able to optimize the relative position of the milling cutter for obtaining better machining conditions such as reducing the roughness parameter.
In the present work, an experimental study is conducted with a view to determine the effect of feed rate values on workpiece surface quality during face milling. Contrary to most of the works in the relevant literature, both 2D and 3D surface roughness parameters are determined not only in a single position or plane but in three different parallel planes on the feed direction in order to be able to determine the variation of surface roughness with the distance from the symmetric plane.

EXPERIMENTAL CONDITIONS
The experimental tests in the present paper were carried out regarding face milling of normalized C45, 1.0503 grade carbon steel workpieces, using a single cutting insert at various feed rate values. More specifically, milling experiments were performed in a Perfect Jet MCV-M8 CNC vertical machining center with a face milling head with only one round coated carbide insert of RCKT1204M0-PM type mounted on it. The cutting length was 50 mm and the width of cut 58 mm on the milled surfaces of the samples. During the experimental procedure the feed per tooth fz values ranged from 0.1 to 0.8 mm/tooth and the other process parameters such as the cutting speed vc and depth of cut ap were held constant at 200 m/min and 0.8 mm, respectively. The process parameters' values for the face milling experiments are also presented in Table 1.
The face milling head had a nominal diameter (Dt) of 68 mm and the rake (γο) and relief angle (αο) of the insert were 0° and 7°, respectively. The hardness of the workpiece material was HB207 and the tensile strength Rm was 580 MPa [16]. The surfaces were produced with symmetrical setting of face milling under dry machining conditions. The 2D and 3D roughness measurements were carried out on an AltiSurf 520 threedimensional surface roughness measuring device and a confocal chromatic sensor was employed to perform the measurements. The evaluation of the results, as well as the calculation of 2D and 3D roughness parameters after the measurements, was conducted using AltiMap Premium software. Roughness was measured in three planes parallel to the direction of feed with the one plane being the middle plane and the two planes being at equal distance (20 mm) from that and towards different sides. It is to be noted that the last two planes will be denoted afterwards as entry and exit side plane. The measurements were performed twice in each plane with a 20 mm separation between them, as presented in Figure 1. In accordance with ISO 4288:1998, the lengths of the 2D measurements were 4 mm, whereas for the 3D evaluations, the surface roughness on areas of 2.5 x 2.5 was recorded. The aforementioned lengths and areas are illustrated with lines and squares, respectively, in Figure 1, in respect to the milling head rotation and the workpiece movement directions.

RESULTS
After the face milling experiments were carried out, surface roughness measurements were performed on the machined samples, each repeated three times and then the results were processed by the aforementioned software in order to determine the values of surface roughness parameters. In Tables 2 and  3, as well as Figure 2, the average values of 2D and 3D roughness parameters, namely Ra, Rz, Sa and Sz for the three parallel planes and five different feed rate values are displayed. In addition to the average values of 2D and 3D surface roughness parameters, the recorded 2D and 3D surface roughness profiles will be afterwards presented in Figures 3 and 4 for fz = 0.1, 0.3 and 0.8 mm/tooth, in order to show more directly the effect of different feed rate values on the surface topography during face milling.

DISCUSSION
The topography of the machined surfaces during face milling is considerably affected by the kinematics of this process. More specifically, during face milling, a rotating milling head is used, on which the cutting insert is mounted, and the workpiece moves with a fixed velocity relative to the cutting tool. Furthermore, during symmetrical face milling, where the axis of the cutting tool is moving along the symmetrical plane of the machined part, the action of the cutting insert in the first half of the chip removal process corresponds to the up-milling process, whereas in the second half the chip removal process corresponds to a down-milling process. For that reason, after the face milling experiments, it is important to conduct surface roughness measurements not only on the symmetrical or middle plane, but also in parallel planes on two opposite sides. Based on the experimental results, presented in Tables 2 and 3 and depicted in Figure 2, it can be observed that surface roughness values, especially Ra and Sa generally increase with an increase of feed rate, as it was anticipated. Regarding Ra, this increase is gradual up to 0.4 mm/tooth and then Ra is increased considerably for fz = 0.8 mm/tooth. Sa values remain practically unchanged up to 0.4 mm/tooth and then increase sharply, and the same trend is observed for Rz and Sz as well. The increasing trend of surface roughness with higher feed rate values and the sharp rise of its values at fz = 0.8 mm/tooth can be also seen in the 2D surface roughness profile, depicted in Figure 3. From Figure 3, it becomes evident that the height of the profiles becomes much larger at the highest feed rate value.
When the surface roughness values in the case with the lowest and highest feed rate values were compared on the symmetrical plane, it was found that Ra increased by 3.98 times, Rz by 3.83 times, Sa by 2.82 times and Sz by 2.44 times. On the entry side, Ra increased by 2.82 times, Rz by 2.51 times, Sa by 3.01 times and Sz by 2.89 times. Moreover, on the exit side, Ra increased by 2.49 times, Rz by 2.49 times, Sa by 1.19 times and Sz by 1.07 times. From these findings, it can be observed that the effect of increasing feed rate values was more important on the symmetrical plane than the other two planes, on which the increase of roughness values regarding Ra and Rz was almost similar and lower than that on the symmetrical plane. When the surface roughness values between different planes are compared, it can be clearly seen that the highest overall values, for the highest feed rate value are recorded on the symmetrical plane. However, in the other cases the surface roughness is not always higher on the symmetrical plane and the highest values can be observed on the entry or exit side as well. Regarding the difference between Ra and Sa values between the symmetrical plane and other two planes, it becomes smaller for fz = 0.3 mm/tooth, implying that the roughness profile becomes more homogenous in this case, as average roughness values are almost similar in all planes. The variations in surface topography of the milled workpieces under different feed rates can be also seen in the 3D surface roughness graphs of Figure 4. At first, it can be understood that the shape of the grooves is indicative of the location of each measurement, as arc shaped grooves occur in the entry and exit side, whereas on the symmetrical plane almost straight grooves are formed. However, differences regarding the grooves dimensions are more important for surface quality. The difference between the profiles are not only in the height of the profile, which is much bigger in the case with fz = 0.8 mm/tooth, but also in the spacing between the micro-grooves formed on the surfaces. As it was aforementioned, the kinematics of the face milling process directly affect the shape of the profile, with a higher feed rate leading to larger spacing between adjacent grooves and also higher height of the profile. In the case with the lowest feed rate, the 3D profile seems less smooth, as irregularities of the surface are comparable to the dimensions of the grooves, whereas for higher feed rates, as the grooves become deeper and spacing between them is increased, these irregularities become less significant. However, the adverse result of surface quality deterioration at the highest feed rate value becomes obvious by comparing the 3D profiles at every parallel plane.

CONCLUSIONS
In the present work, symmetrical face milling experiments on steel workpieces at different feed rate values were carried out with the use of a single round insert, with a view to determine the effect of feed rate on surface topography of the milled parts. The surface quality was examined in every case in three planes parallel to the feed direction and both 2D and 3D surface roughness parameters were determined. From the analysis of the experimental results, various important conclusions were drawn.
The values of all surface roughness parameters showed a variation with feed rate, with a considerable rise for feed values over 0.4 mm/tooth, whereas the variation was minimal for lower values of feed. From these findings, it became clear that feed value should be selected lower than 0.4 mm/tooth in order to achieve an acceptable surface quality, with Ra lower than 1 μm. The highest increase of roughness values was observed on the symmetrical plane, as well as the largest overall values of every surface roughness parameter. Finally, it was found that the minimum difference of Ra and Sa values between the symmetrical and the other two planes was achieved for fz = 0.3 mm/tooth.