MultiScience - XXXIII. microCAD International Multidisciplinary Scientific Conference University of Miskolc, 23-24 May, 2019, ISBN 978-963-358-177-3
FEM ANALYSIS ON THE IMPACT CONDITIONS OF THE INSERT IN FACE MILLING
István Sztankovics assistant lecturer
University of Miskolc, Institute of Manufacturing Science ABSTRACT
One development direction in face milling of planer surfaces is the study of the feed increase. Many researchers are working on the analysis of the roughness and topographical parameters of machined surface and on the examination of the forces during cutting and on the determination of the dynamic load of the cutting edge. In this paper the cutting force components are analysed in different insert orientations in face milling during the start of the cutting.
Keywords: cutting force, face milling, FEM 1. INTRUDUCTION
The expectations by the increasing market demands for the procedures applied in manufacturing processes requires the intensification of productivity and the increase of the machined surface per unit of time. One of the possible solutions to achieve these is the increase of the secondary feed motion besides the change in the primary cutting motion. This direction can be observed as well in the development of face milling, the widely used procedure to machine flat surfaces. As the studies of Kurkut et al. shows [1], it is subservient to adjust the cutting speed in the range of its optimal value to avoid the unwanted built up layer on the cutting insert. However, as Varga et al. have shown in their studies on aluminium workpiece, the parameters of the surface topography and flatness can get significantly worse values with the increase of the feed [2]. The regression analysis model applied by Sheth et al. proved that the machined surface error can be lowered with the correct choice of the cutting speeds and feed rates [3].
The high-feed face milling is widely studied recently for the increase of productivity, where the ap/fz ratio is typically lower than 1 [4]. The chip removal characteristic alters in high-feed milling compared to the traditionally used relations in face millings, since in this case the role of the edge on the face of the tool increases.
An increasingly prominent issue is the roughness estimation of the to be machined surface, specially in face milling, where the generated topography is inhomoheneous.
Numerous milling experiments were done to analyse the alteration of the surface roughness. On the one hand the relation between the theoretical and real roughness was analysed as a function of the cutting parameters [5,6], on the other hand the effect of the ap/fz ratio alteration was studied in constant chip cross-section [7,8]. The research of Nagy et al. showed that the characteristic parameters of surface topography will increase according to the applied insert geometry by decreasing the ap/fz ratio [9]. However the studies of Kundrák et al. on the cutting forces proved, that the specific values of the different directional cutting forces will decrease with the
DOI: 10.26649/musci.2019.053
decrease of the ap/fz ratio in high-feed milling [10]. From this it can be stated, that the specific load on the insert can be lowered with the increase of the feed, which can have a positive effect on the productivity of material removal.
It can be concluded from the former studies that the research of the high-feed procedure can be the solution for the expectations of face milling. However, the dynamic load of cutting insert will be high during the process, because of the discontinuous removal of the inconstant cross-sectional chip. This dynamic effect depends on the position of the cutting insert, as the study of Karpuschewski et al.
shows [11]. According to their analysis, the axial and radial rake angle affects the resultant cutting forces in a significant extent. They recommend the increase of the former and decrease of the latter. The reason for this is the change in the character of the dynamic load on the cutting insert as a function of these rake angles [12].
Figure 1
Interpretation of the impact forms [12]
For the description of the impact forms the four corners of the to be removed chip cross-section were marked in Figure 1 as S, T, U and V in order [12]. The art of impact can be surface (SVUT), linear (UV, ST, UT, SV) and point-like (U, T, V, S).
The conditions for the radial (γf) and axial (γp) rake angles to fulfil these cases can be found in Table 1, where the tool orientation is described by the angle of contact (φ).
Table 1
The arts of impact as a function of the radial (γf) and axial (γp) rake angles γp > 0° γp = 0° γp < 0°
γf > φ S ST T
γf = φ SV SVUT UT
γf < φ V UV U
Three cases are presented in Figure 2 from these. The impact on the tip of the cutting edge is shown in the left side of Figure 2, when the axial rake angle (γp) is positive and the radial rake angle (γf) is greater than the angle of contact. With the decrease of the rake angles the impact art alters to the surface-like form when γp reaches 0° and γf reaches the angle of contact (φ) in the middle of Figure 2.
S SVUT U
Figure 2
The insert position with different radial (γf) and axial (γp) rake angles
With the further decrease of the inspected angles the art of impact turns into the point- like form again in the right side, however the innermost point of the chip cross-section will touch the workpiece firstly in this case. The change in the spatial position of the cutting insert marked with black can be observed in the three section of Figure 2.
The aim of my study in this paper is to analyse the effect of the radial and axial rake angle alteration on the cutting forces in face milling with Finite Element Method.
Based on the results the practical experiments van be planned.
2. EXPERIMENTAL CONDITIONS
Finite Element Method simulations were run for the analysis of chip removal with the ThirdWave AdvantEdge 7.5 software.
The material and geometry of the cutting insert was adjusted for the OCKX 0606 AD- TR marked Garant cutting tool. The characteristic of this insert is the 45° major cutting edge angle and the 0° minor cutting edge angle. The diameter of the milling head was 120 mm.
The workpiece material was C45 steel with 200 HV hardness, its geometry was adjusted that way, so the side of the workpiece was coincident with the symmetry plane of the cutting tool (therefore φ = 0°)
Table 2 shows the 12 setups which were used in the study. The axial rake angle (γp) was -12°, 0° and 12°, while the radial rake angle (γf) was adjusted to 0° and 10°
values. From the process parameters, the cutting speed was chosen to 200 m/min according to the manufacturer recommendations. The feed and cutting speed were adjusted to 0.15 mm and 1.5 mm values to achieve ap/fz ratios of 10 and 0.1 for my analysis.
Table 2
Setup conditions for the varied parameters fz
[mm]
ap
[mm]
ap/fz
[-]
γf
[°]
γp
[°]
Abbrevation
of impact Art 1
0.15 1.5 10
0
-12 UT Line
2 0 STUV Surface
3 12 SV Line
4
10
-12 T Point
5 0 ST Line
6 12 S Point
7
1.5 0.15 0.1
0
-12 UT Line
8 0 STUV Surface
9 12 SV Line
10
10
-12 T Point
11 0 ST Line
12 12 S Point
3. RESULTS
From the output results of the FEM software, the cutting force components in the tool coordinate system (main cutting force: Fc, radial force: Ff, axial: Fz) and the torque were analysed in this paper. I marked the analysed section on the result graph of the software in Figure 3, which is related to the tool-workpiece impact. The maximal values of each force component were determined for the 12 setups (Table 3).
Figure 3
The results of the FEM software in Case 2 and the analysed range
Table 3
The maximal values calculated from the FEM software outputs
1 2 3 4 5 6 7 8 9 10 11 12
Ff
(N) 159.5 138.6 115.5 18.2 27.0 36.0 45.2 38.9 40.4 -106.2 -89.5 -73.5 Fc
(N) 927.3 804.2 700.5 736.8 722.7 612.3 862.0 712.9 593.8 860.5 687.1 568.9 Fz
(N) 489.8 149.6 127.7 237.5 241.9 157.4 441.7 276.4 132.3 437.1 283.7 154.8 M
(Nm) 56.1 48.6 42.4 44.5 43.5 36.8 50.4 42.2 35.3 50.6 40.3 33.5
4. DISCUSSION
The results for ap/fz > 1 (1-6 setups) can be seen in Figure 4-5. I conclude from Figure 4.a, that the feed directional force will be significantly higher with the increase of the radial rake angle (4…5-fold increase depending on the radial rake angle). With the increase of γp the Ff value will decrease in γf = 0° and it will increase with γf = 10°.
The change in the main cutting speed can be seen in Figure 4.b. This force value will decrease with the increase of the axial rake angle in both values of γf. The radial rake angle has no significant effect on Fc. In case of the axial force (Figure 5.a) the
maximum value was resulted in -12° axial rake angle and 0°radial rake angle (2..3- fold higher than in the other 5 values). The Fz will decrease with the increase of γp, while the increase of the radial rake angle will increase this component in most cases.
a) b)
Figure 4
The acting forces on the plane of the machined surface in 0°and 10° radial rake angles (γf) (ap/fz = 10)
a) b) Figure 5
The axial force (Fz) and the Toque as a function of axial rake angle (γp) in 0°and 10° radial rake angles (γf) (ap/fz = 0.1)
Figure 6 and 7 shows the results for the cases (7-12), where the feed value was higher than the value of the depth of cut (ap/fz < 1).
I conclude based on Figure 6.a, that the feed directional force will be nearly two times higher with the increase of the radial rake angle (γp), and furthermore its direction will also change. The axial rake angle has a reducing effect on Ff.
It can be seen in Figure 6.b, that the main cutting force decreases with the increase of the axial rake angle in both values of γf. The radial rake angle has no important effect on Fc in this setup as well.
With the increase of the axial rake angle to 0°and 12°, the axial cutting force will decrease by 33% firstly and by 50% secondly (Figure 7.a). The increase of the radial rake angle does not affect the Fz value. The torque load on the tool changes according to the alteration of the main cutting force as seen in Figure 7.b.
a) b)
Figure 6
The acting forces on the plane of the machined surface (Ff, Fc) in 0°and 10° radial rake angles (γf) (ap/fz = 0.1)
a) b) Figure 7
The axial force (Fz) and the Toque as a function of axial rake angle (γp) in 0°and 10° radial rake angles (γf) (ap/fz = 0.1)
The effect of the application of high-feed on the analysed parameters is shown in Figure 8 and 9. In these diagrams the percentage change are shown based on the ap/fz>1 cases.
I conclude from Figure 8.a, that the feed directional force will be higher in ap/fz<1 ratio with 10° radial rake angle, the maximum of this difference is in -12°axial rake angle, where the increase is 5-fold. However, in 0° radial rake angle the Ff will be half of its value in high-feed milling, than in ap/fz>1.
The main cutting force (Figure 8.b) and torque (Figure 9.b) will increase by 17% in high-feed milling in -12°radial rake angle and 10°axial rake angle. In the remaining setups a 10% decrease can be observed in these values.
There is no change in the Fz force in γp=12° according to Figure 9.a. The better value is resulted in γf=0° in -12°axial rake angle, while it is resulted in γf=10° in 0° axial rake angle. An 80% increase can be observed in both cases in the value of Fz.
a) b)
Figure 8
Changes in the forces acting on the machined surface (Ff, Fc) as a function of axial rake angle (γp) in 0°and 10° radial rake angles (γf)
a) b) Figure 9
Change in the axial force (Fz)and in the Toque
as a function of axial rake angle (γp) in 0°and 10° radial rake angles (γf)
Finally, the experimental setups were compared based on the values in Table 3. First, I ranked the 12 case according to the Ff, Fc, Fz and M values, then I summed up their standing for each setup. The final order between the studied cases were determined based on this sum, and therefore the most favourable setup was identified. Table 4 shows this ranking of the experimental setups.
Table 4
Order of the experimental setups based on the cutting forces and torque
Case 1 2 3 4 5 6 7 8 9 10 11 12
Ranking
Ff 12 11 10 1 2 3 6 4 5 9 8 7
Fc 12 9 5 8 7 3 11 6 2 10 4 1
Fz 12 3 1 6 7 5 11 8 2 10 9 4
M 12 9 6 8 7 3 10 5 2 11 4 1
Sum 48 32 22 23 23 14 38 23 11 40 25 13
Order 10 7 4 5 5 3 8 5 1 9 6 2
In Table 5 the experimental setups in the determined order are presented. The most favourable setups are when the axial rake angle equals 12° (γp = 12°). From those 4 cases the first two resulted by the high-feed procedure variant. The worst performing setups were typically those, which had -12°axial rake angle. By means of the art of the impact the dynamic load on the cutting insert were the lowest, when the cutting edge on the face of the tool (SV) or the tool tip (S) reaches the workpiece firstly. The worst performing case was that, when the upper portion of the cutting insert (UT) started the cutting.
Table 5
The adjusted parameters for the cases in the final order
Order 1 2 3 4 5 5 5 6 7 8 9 10
Case 9 12 6 3 8 5 4 11 2 7 10 1
ap/fz [-] 0.1 0.1 10 10 0.1 10 10 0.1 10 0.1 0.1 10
γf [°] 0 10 10 0 0 10 10 10 0 0 10 0
γp [°] 12 12 12 12 0 0 -12 0 0 -12 -12 -12 Abbreviation
of impact SV S S SV STUV ST T ST STUV UT T UT
Type of
impact Line Point Point Line Surface Line Point Line Surface Line Point Line
5. SUMMARY
The effect of the cutting insert position was studied on the dynamic load of the tool by the alteration of the radial and axial rake angle in traditional and high-feed face milling. I concluded with Finite Element Method analysis, that the most favourable variant for the minimal dynamic load is when the tool tip of the insert contacts firstly the workpiece. Secondly the forces can be lowered with the decrease of the ap/fz ratio by the correct choice of the axial and radial rake angles.
ACKNOWLEDGEMENTS
The described study was carried out as part of the EFOP-3.6.1-16-00011 “Younger and Renewing University – Innovative Knowledge City – institutional development of the University of Miskolc aiming at intelligent specialisation” project implemented in the framework of the Szechenyi 2020 program. The realization of this project is supported by the European Union, co-financed by the European Social Fund.
The authors greatly appreciate the support of the National Research, Development and Innovation Office – NKFIH (No. of Agreement: OTKA K 116876).
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