The influence of the profiled milling parameters to the quality of the surface.
Brenci, Luminita Maria ; Cismaru, Ivan ; Cosereanu, Camelia 等
1. INTRODUCTION
The surface quality was quantitative assessed by the roughness
measured on the surface. The actual research is generally oriented to
analyze the quality of the flat sanded surfaces (Ratnasingam et al.
2002, Gurau et. al. 2005) or coated ones (Boieriu et al. 2008). A
particular characteristic of the wooden surfaces is the non-homogenous
structure compared to other materials, thing that makes the process of
assessment the quality of the surface more difficult to be done,
especially in the conditions where a unitary technical specification for
measuring the wood surface roughness doesn't exist yet. The most
difficult problem in measuring the roughness of the wooden surfaces
occurs in case of complex profiles (concave or convex ones) (Brenci
2006, Salca et. al. 2008). The roughness of the flat surfaces can be
measured by stylus or by laser or light beam equipment without problems
(Brenci et. al. 2008), whilst the roughness on the profiles can be
measured only by laser or light beam equipment. An optical sensor has
been used for present research, in order to determine the roughness of
the beech and oak wood profiles after milling phase. The roughness was
measured first perpendicular to the grains on a concave wooden profile
and then parallel to the grains.
The research done in this paper is only the first step in studying
the quality of the profiles after milling phase, depending on the
processing parameters, next step to be done being the research on the
quality of the profiles for the most common species of wood used in
furniture manufacturing and than for those less studied so far.
2. APPLIED METHODS
Two species of wood with different anatomical structure have been
analyzed and they are beech and oak wood. The samples have been milled
using two rotation speeds combined with three feed speeds, as can be
seen in Tab. 1. The wood profile presented in Fig. 1 has been analyzed
on a concave surface of 6 mm radius and a length of 9 mm perpendicular
to the grains and on a length of 50 mm parallel to the grains. Problems
rise when detaching the chips on feed speed direction.
[FIGURE 1 OMITTED]
The wood structure and splitting tendency in the direction of the
chip removal at the knife contact is a result of the cutting force
components. This is the reason the milling in the reverse direction of
the feed speed was considered only the quality of the surface depending
on the correlation between the feed speed and the direction of tool
rotation.
The samples have been measured using a light beam roughness
equipment FRT type (Fries Research and Technology), made in Germany,
provided with light beam of 2 urn diameter and the measurement method
was applied according to EN ISO 4287:1997 technical specification.
Two species of hardwood with different anatomical structure have
been analyzed: oak wood, with a porous structure, numerous and big
vessels in the early wood and beech wood, more homogeneous than the
previous one, with small vessels uniformly distributed in its structure.
The roughness values of the concave surface measured perpendicular
and parallel to the grains are expressed by Rz.
3. RESULTS AND DISCUSSIONS
The diagrams in Fig. 2. a show the values of the roughness measured
for the two species of wood in the conditions of different rotation
speeds and feed speeds. It can be observed that the value of the
roughness obtained for the oak wood is higher than that obtained for the
beech wood. This fact can be explained by the anatomical structure of
the oak wood, less homogenous than that of the beech wood, with big and
numerous pores.
A solution to "diminish" the effects of the wood
structure when assessing the surface roughness could be that of
"filtering" the measuring results by removing the defects of
structure, the results being those presented in Fig. 2. b. The method of
measuring the roughness perpendicular to the grains may introduce errors
because of the small length measured and due to the possibility of
hitting a non-favorable zone with vessels (in case of oak wood). The
defects of the oak wood structure have not been filtered in the above
mentioned analysis and they appear on the roughness profile as seen in
Fig. 3.
Increased rotation speeds do not have as result the effect of
improving the quality of the surface (as a normal situation).
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The anatomical structure proves to be a dominant one, so that the
problem of optimizing the quality of the surface when milling the oak
wood becomes an open issue to be discussed, just because of the big
sizes of the wood vessels. The anatomical structure in case of oak wood
alters the smoothness of the surface and an unfiltered 3D measurement of
the surface roughness on an area of 9 x 9 [[mm.sup.2]] shows the sizes
of the vessels (Fig. 4. a), whilst the filtered 3D roughness profiles in
case of oak wood (Fig. 4. b) and beech wood (Fig. 4. c) at a rotation
speed of 4852 rot/min and a feed speed of 23.74 m/min look smoother.
That's why, a measuring method along the grains of the wood is
recommended, in order to measure on a length big enough (50 mm) to cover
areas with and without pores. The resulted values of the roughness
measured parallel to the grains on the concave surface were higher in
case of oak compared to beech wood, especially for the rotation speed of
4852 rot/min and feed speed of 12.7 m/min (Fig. 5. a.). In case the
defects of anatomical structure of oak wood are removed (Fig. 5. b.),
the value of Rz parameter decreases.
In case of assessing the roughness on the longitudinal direction of
the profile, the recorded values were higher because of the sum of the
technological factors of influence competing together. These factors are
as follows: the heights of the irregularities, radial and frontal stroke
of the spindle and of the profiled milling tool, vibrations in the
kinematics chain, etc.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
4. CONCLUSIONS
The study on the profiled milled surfaces has shown that the
parameters of the milling operation and also the direction of
measurement have a significant influence on the resulted values of the
roughness. Thus, a lower rotation speed (4852 rot/min) and a higher feed
speed (23,74 m/min) are recommended in order to obtain better results of
the roughness (Rz), the results being more accurate when measuring
parallel to the grains (in the other case the results being
contradictory). From the analysis of the relationship between the
anatomical structure (vessel sizes), measuring methods and surface
quality, has resulted that the recommended measuring method of the
roughness is along the grains, having thus the possibility to choose
areas with different anatomical structure of wood.
In case of assessing the wood surface, a "filtering"
operation is mandatory for the obtained data, so that the influence of
the wood structure to be eliminated as much as possible.
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Slovakia, October 2008, DAAAM International, Vienna
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Tab. 1. Milling parameters of the samples
Wood Rotation speed
species n = 4852 [rot/min] n = 6594 [rot/min]
Feed speed [m/min]
Beech/ Oak 6.5 12.7 23.7 6.5 12.7 23.7
