Surface Roughness and Wettability Variation : The effect of Cutting Distance during Milling of Pinus Radiata Wood

The variation of the machined surface properties is usually described in terms of changes in cutting parameters. However, the effect of the cutting distance should be considered, as it further shows the infl uence of the cutting tool wear. In order to assess the effect of the cutting distance on roughness and wettability of Pinus radiata wood surfaces, three conditions of feed speed and cutting speed were applied on cutting distance of 0 to 10000 meters. This study describes the effect of machining parameters on the behavior of roughness and wettability through the cutting distance, and the interaction of both surface properties is also discussed. The variation of surface roughness in function of the cutting distance is not directly related to variations in the surface wettability. It was, therefore, concluded that is was not reliable to consider only the surface roughness to explain the wetting behavior of a heterogeneous surface such as wood.


INTRODUCTION 1. UVOD
The characterization of wood cutting surfaces is very important as it normally interacts with adhesives, paints and varnishes.Roughness and wettability are surface properties that are normally used to assess surface quality.The characterization of surface quality depends on many infl uencing factors and can be related both to wood properties and machining conditions (Magoss, 2008).Dundar et al., (2008) and Magoos (2008) found that an increase in cutting speed decreased the surface roughness.On the other hand, by the increasing of the feed speed, the surface roughness is increased (Keturakis and Juodeikienė, 2007; Hernandez and Cool, 2008).Walinder (2000) explained that the intrinsic wood surface structure combined with the irregular surface structure resulting from different machining processes might cause variation in wood wettability.Hernandez and Cool (2008) found that an increment in feed speed affected the wetting of a surface.The authors considered that better wetting was associated with higher surface roughness.In a comparison of different types of wood surface machining, planed, sanded and disc sawn surfaces, Santoni and Pizzo (2011) found that the high roughness, produced in the sanding process made more wettable the sanding surfaces.Cool and Hernandez (2011) also measured better wettability on rough surfaces compared to smoother ones.
Nevertheless, the effect of those cutting parameters on surface properties is not constant due to the wear on the cutting tool by the effect of the cutting distance.This cutting distance was considered for limited cutting distances or the wear was reproduced artifi cially.The wood milling tools undergo wear during the cutting process, the mass of the tools decreases and the geometrical parameters change (Keturakis and Lisauskas, 2010).Surface quality is particularly sensitive to the changes in wear profi le geometry (Sheikh and Mc-Kenzie, 1997).Itaya and Tsuchiya (2003) cited by Aknouche et al. (2009) concluded that the direct consequence of the cutting edge wear is the gradual loss of its ability to cut the machined material.These conditions also result in a poor wood surface quality.Keturakis and Juodeikienė (2007) modelled artifi cially the cutting edges of the knives and found that a decrease on tool edge blunt radius and feed rate during milling decreases the surface roughness of birch wood.Gilewicz et al. (2010), evaluated the wear by the effect of the cutting distance until 6000 meter on the surface properties and found almost a linear increase of R z together with the tool wear.This study is intended to provide information on the variation of surface properties depending on the cutting distance in order to support decisions as to when to renew the cutting tool.
Therefore, the main objective of this study was to describe the behavior of roughness and wettability on Pinus radiate wood surfaces in function of the cutting distance during its machining.

MATERIJAL I METODE
In this study, Pinus radiata D. Don samples, see table 1, with a mean EMC of 10.9 % (NCh176/1) and a mean density of 470 kg•m -3 (NCh176/2) according to Chilean standard, were machined using a single-spindle shaper (milling) machine with variable cutting speed and feed, with three hydro centered mounting cutter-heads of 6 knives HS6-5-2 (High Speed Steel type M2) with 26° clearance angle and 15° of rake angle each.
The machining takes into account a tool wear at a cutting distance of 0 to 10000 meters (linear cutting meters of material to be cut), with specifi c wear conditions, see Table 1.
Also in order to control and measure the tool wear, the cutting edge recession was measured with the help of a magnifying glass and software.Each 2000 meters of cutting distance, the machining conditions were adjusted, see Table 1, to assess the surface quality through the roughness and wettability surface properties.The mean peak-to-valley height (R z ) parameter, in accordance with ISO 4287 (1997) standard, was used to measure the roughness surface using a Mitutoyo SJ-201 apparatus.On the other hand, wettability was measured by the principle of contact angle using a DSA25 Krüss device.

REZULTATI I RASPRAVA
Table 2, below, shows the variation of average surface roughness in function of the cutting distance associated with the machining parameters.Independent of the machining conditions, the average roughness decreases from its initial level at 0 meters until 2000 meters and then increases again until the cutting distance of 8000 meters; then it shows a gentle tendency to diminish.When the roughness is compared in function of the cutting speed, roughness tends to diminish as the cutting speed v c increases.On the other hand, when the comparison is related to feed speed, there is a slight increase of the roughness as feed speed v f increases.These trends become more consistent when the comparison is made among conditions of v f (22 to 38 m•min -1 ) and as the cutting distance increases.Table 3 shows the average contact angle in function of the cutting distance and machining conditions.In general, it does not show a clear trend through the cutting dis-tance, or with the cutting speed.However, when comparison is made based on the cutting distance of 0 meter and 10000 meter, the contact angle changes depending on the feed speed.In case of feed speed of 22 and 38 m•min -1 , the surface wetting tends to decrease with the cutting distance, i.e. higher contact angle at 10000 meters than 0 meters.
The statistical analysis, ANOVA, see Table 4, shows that the cutting distance (L) is the most significant factor for roughness (R z ).It is followed in importance by the cutting speed (v c ), and only then by the feed speed (v f ).In the case of wettability (contact angle), the most signifi cant factor of variance is the feed speed (v f ) followed by the cutting distance (L) and the cutting speed (v c ) as the least important.Moreover, Pearson correlation shows similar tendencies.R z behavior is most related to the effect of the cutting distances (L) followed by the cutting velocity (v c ) and only then by the feed speed (v f ).In the case of contact angle θ, the most related factor is the feed speed (v f ) followed by the cutting distance and only then by the cutting velocity, see Table 5.
Based on the factor signifi cance, the behavior of surface properties, roughness and wettability (contact angle), are presented below in function of the cutting distance.The fi gures 1 to 4 represent the values as (+ main, -median, ┬ ┴ min -max and ° outliers).
Figure 1 shows the general behavior of surface roughness parameter R z in function of the cutting distance (L).As can be seen, the R z values decrease from 0 meter to 2000 meters, and then increase until 8000 meters, to decrease again when the cutting distance of 10000 meters has been completed.The above is consistent with a Duncan test that identifi es three different groups of roughness through the cutting distance (L), 0 to 2000, 4000 to 8000, and 10000 meters, at a significance level of 0.05 %.Second in importance is the infl uence of the cutting speed (v c ), Figure 2 shows the well known effect of the cutting speed on roughness; it decreases as the cutting speed increases.
This effect on roughness appears consistent throughout the cutting distance.Nevertheless, the values of roughness for the same machining conditions change as the cutting distance accumulates.For example, up to the cutting speed of 44 m/s, at the cutting distance of 0 meter, the R z is slightly greater than 25 μm, but after the cutting distance of 8000 meters, the same machining conditions no longer generate the same level of roughness; it changes up to 25 μm.
The effect of cutting distance (L) and feed speed (v f ) on surface wettability (contact angle) of Pinus radiate wood throughout the cutting distance are shown in Figure 3 and 4. If the whole experiment design is considered, see Figure 3, it can be seen that the median and the average contact angle decrease until the minimum level when the cutting distance has reached 4000 meters, and then begin to rise steadily from 6000 meters on.
However, when the feed speed is plotted through the cutting distance, see Figure 4, its signifi cance on wetting is evident.
Figure 4 shows how the contact angle evolves three different paths as the cutting distance increases.It could be said that the feed speed of 30 m•min -1 produced the most wettable surface, while the feed speed of 22 m•min -1 produced less wettable surface.
Regarding the wear of the cutting tool, Figure 5 shows a comparison of cutting edges at the cutting distance of 0 meters and a dull condition of the knives after 10000 meters.The total average accumulated wear was of 3.5 μm.Both the Rake Face and the Clearance Face showed modifi cations to its original geometry.As shown in Figure 5, the most variable region is the so called land wear located on the Clearance Face.Future studies should focus on measuring this region through the cutting distance.
In this study, the behavior of roughness and wetting is described through the cutting distance.As it is well known, wettability is a thermodynamic response  as e.g.surface roughness, and conclude that differences in polarity between freshly cut cell walls and native inner lumen surfaces correlate with chemical heterogeneity.Apparently, the wear of the cutting tool, related to the cutting distance, produced a variation in the exposure of the chemical components of the wood ultrastructure and determined the thermodynamic behavior of the machined surface.

CONCLUSION 4. ZAKLJUČAK
The monitoring of the surface properties through the cutting distance has allowed a correlation to be to physical and chemical properties of wood surfaces (Tshabalala, 2005).Apparently, chemical properties would be playing a more important role than physical properties, and their infl uence on wettability of a wood surface would be related in part to the wear of the cutting tool as a result of the cutting distance.This assumption is based on the null correlation found among roughness and wettability in this study.

Figure 1 Figure 2 2 .
Figure 1 Surface roughness parameter R z in function of cutting distance considering the whole experiment Slika 1. Parametar hrapavosti površine R z u ovisnosti o duljini rezanja tijekom cijelog eksperimenta

Figure 3 Figure 4 Figure 5
Figure 3 Contact angle θ in function of cutting distance considering the whole experiment Slika 3. Kontaktni kut kvašenja površine θ u ovisnosti o duljini rezanja tijekom cijelog eksperimenta Hansson et al. (2011) indicate that surface roughness failed to correctly predict the contact angle on a heterogeneous surface with chemical or topological heterogeneities covered by the droplet.Frybort et al. (2014) explain that variability in wettability of a surface is largely determined by surface chemistry, besides other factors such Contact angle / kontaktni kut, °C utting distance / duljina rezanja, m Contact angle / kontaktni kut, °C utting distance / duljina rezanja,