MILLING TOOLS
3 RESULTS AND DISCUSSION
2
1
Z Z Z Z
R
zZ
(2)
Rt - maximum height of the profile:
v p
t
R R
R
(3)
Figure 5: Profile roughness parameters Ra, Rz and Rt [3].
3 RESULTS AND DISCUSSION
Fig. 6 shows average values of the profile roughness parameter Ra for chip thickness of 0.15 mm, where the values are decreasing with increasing rake angle. It can be seen from this figure that the minimum values and thus the best surface is produced at tissue orientation angle of 90°, that is, at the radial surface while the angle of 30° produces the highest value. However, it should be noted that in turning as well as in the circumferential milling where the slowly rotating piece is milled with a milling cutter, a specific orientation angle cannot be avoided, i.e., the work piece has to be milled from all sides.
Figure 6: The profile roughness parameter Ra for the chip thickness of 0.15 mm, different rake angles and wood tissue orientations (
The Role of Technological and Geometrical Parameters in the Construction of Wood Milling Tools
The same trend as for Ra also applies to the profile roughness parameter Rz
(fig. 7): that is, the values are decreasing with increasing rake angle, and are the lowest at the rake angle of 50°.
Figure 7: The profile roughness parameter Rz for chip thickness of 0.15 mm, different rake angles and wood tissue orientations (
In wood machining process it often happens that the otherwise high-quality machined surface has torn grain, which means that such a surface is useless. Fig. 8 shows parameter Rt, which tells us the maximum distance between the highest peak and the lowest valley, which in our case corresponds to chip torn or grain surface. It can be seen in the figure that the values are the lowest at the greatest rake angle, i.e., the surface has the best quality.
Figure 8: Profile roughness parameter Rt for chip thickness of 0.15 mm, different rake angles and wood tissue orientations (
But a distinction has to be made between cutting in the directions of 90°-0°
and 0°-90°. In the first case, cutting with greater rake angles can result in the cleavage of the tissue in front of the tip of the blade bellow the cutting plane as described in the introduction. On the contrary in our case (0°-90°), cutting at low rake angle results in tearing wood because the tip of the blade does not cut the tissue, but it pushes it in front of the blade which can lead to torn grain below the cutting plane. Such a profile can be seen in fig. 9a, where the cut was made at the rake angle of 10°, tissue orientation angle of 30°
and chip thickness of 0.15 mm, whereas fig. 9b shows a profile for the rake angle of 40°.
Figure 9: Profile roughness measurement, chip thickness 0.15 mm, wood tissue orientation 30°; a) rake angle of 10° and b) rake angle of 40°.
Figure 10: Profile roughness parameter Ra at rake angles of 10° (dashed line) and 40° (solid line) for different wood tissue orientations and chip
thicknesses (h).
Fig. 10 shows parameter Ra for different tissue orientations and chip thicknesses cutting at the rake angles of 10° and 40°. The rake angle of 40°
was chosen because in the case of the cutting knife made from the tungsten
The Role of Technological and Geometrical Parameters in the Construction of Wood Milling Tools
carbide the minimum tool angle of 45° is required, which means that only 5°
remains for the clearance angle. The picture shows the fact that was already highlighted in fig. 6, i.e., the worst quality surface results at the tissue orientation angle of 30°. In addition, the figure clearly shows that the surface quality increases with decreasing chip thickness. This was expected and is in line with common practice, and shows that the surface quality is better at smaller chip thicknesses. This is particularly true for cutting at the rake angle of 10°, while cutting at the rake angle of 40° Ra is less sensitive to tissue orientation and chip thickness.
Figure 11: Profile roughness parameter Rz at rake angles of 10° (dashed line) and 40° (solid line) for different wood tissue orientations and chip
thicknesses (h).
Figure 12: Profile roughness parameter Rt at rake angles of 10° (dashed line) and 40° (solid line) for different wood tissue orientations and chip
thicknesses (h).
Fig. 11 and fig. 12 show Rz and Rt respectively for the same cutting conditions as shown in fig. 10. Both figures show practical no dependence of
means that cutting with rake angle of 40° makes the surface smoother at smaller chip thicknesses since Ra is smaller (fig. 10), but the amount of torn grain is the same, i.e., there is no torn grain at any chip thickness (fig. 11 and 12).
4 CONCLUSION
Within this research, the influence of rake angle, chip thickness and orientation of tissue in the orthogonal cutting in the direction of 0°-90° were investigated. The research has shown that the best surface quality can be achieved by cutting at large rake angles where the surface quality slightly fluctuates with wood tissue orientation angle, being the worst and the best at the angles of 30° and 90° respectively, where 90° correspond to the radial surface. However, it should be noted that normally different tissue orientation angles cannot be avoided, since the work piece has to be milled from all sides.
The research has also shown that at the rake angle of 40° parameter Ra
increases with chip thickness while Rz stays the same, which means that at a smaller chip thickness the surface is otherwise smoother as compared to a greater chip thickness, but there is the same amount of grained surface. A quite different situation is in cutting with rake angle of 10° where parameters Rz and Ra increase with chip thicknesses, which means rougher surface with a greater amount of grained surface. It should be noted that in case of a rougher surface the surface can be grinded. If, however, the surface is highly grained with a great amount of torn chips the surface cannot be grinded because too much material has to be removed. It follows from the research conducted that the use of tools which are normally used for cutting in the longitudinal direction, i.e., in the direction of 90°-0°, is not optimal for cutting in the direction of 0°-90°. Due to the chip formation process, described in the introductory part, the rake angle in a longitudinal milling (in the direction of 90°-0°) has to be around 20°. At greater rake angles the chip is formed by the cleavage of tissue in front of the tip of the knife. If the cut is made against the grain, the tissue can split below the cutting plane in front of the tip of the blade which produces a chipped surface.
However, when cutting in 0°-90° direction, the stiffness of the tissue in the lateral direction is lower than in longitudinal direction, and thus the chipped surface cannot occur when greater rake angles are used, while there may be tissue tear-offs below the cutting plane when small rake angles are used where greater compressive stresses are present.
The Role of Technological and Geometrical Parameters in the Construction of Wood Milling Tools
ACKNOWLEDGMENTS
The work is part of the research project V4-1419 ‘Rational use of hardwoods with a focus on beech wood’ supported by the Ministry of Agriculture, Forestry and Food of the Republic of Slovenia (MKGP) and the Slovenian Research Agency (ARRS).
REFERENCES
[1] Koch, P. (1985) Utilization of hardwoods growing on southern pine sites.
Agriculture handbook no. 605. Washington, U.S. Department of Agriculture, Forest Service.
[2] Merhar, M., Bučar, B. (2012) Cutting force variability as a consequence of exchangeable cleavage fracture and compressive breakdown of wood tissue. Wood Sci. Technol., 46(5): 965-977.
[3] https://in.misumi-ec.com/contents/tech/press/20.html.