Abstract
Several previous studies have investigated the effects of heat treatment on the chemical composition, along with the physical and mechanical properties, of wood from various species. However, the effects of these property changes upon the machining properties and surface quality of machined wood have been studied much less. The main goal of this work was to investigate the comparative cutting power consumption during milling and the resulting surface roughness of heat-treated and untreated beech wood (Fagus sylvatica L.). Several cutting regimes were tested by combining different values of rotation speed, feed speed, and cutting depth. The cutting power and the processing roughness were assessed and compared. The results clearly showed that the cutting power involved in the milling of heat-treated beech wood was up to 50% lower than that of untreated wood, but the processing roughness was slightly higher.
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Milling of Heat-Treated Beech Wood (Fagus sylvatica L.) and Analysis of Surface Quality
Mihai Ispas, Lidia Gurau,* Mihaela Campean, Murat Hacibektasoglu, and Sergiu Racasan
Several previous studies have investigated the effects of heat treatment on the chemical composition, along with the physical and mechanical properties, of wood from various species. However, the effects of these property changes upon the machining properties and surface quality of machined wood have been studied much less. The main goal of this work was to investigate the comparative cutting power consumption during milling and the resulting surface roughness of heat-treated and untreated beech wood (Fagus sylvatica L.). Several cutting regimes were tested by combining different values of rotation speed, feed speed, and cutting depth. The cutting power and the processing roughness were assessed and compared. The results clearly showed that the cutting power involved in the milling of heat-treated beech wood was up to 50% lower than that of untreated wood, but the processing roughness was slightly higher.
Keywords: Heat-treated beech wood; Machinability; Cutting power; Surface quality; Processing roughness; Waviness
Contact information: Transilvania University of Brasov, Faculty of Wood Engineering, Universitatii Str. 1, 500036 Brasov, Romania; *Corresponding author: lidiagurau@unitbv.ro
INTRODUCTION
Research regarding the heat treatment of wood has increased significantly in recent years as a result of the ongoing search to improve wood properties, along with the desire to take advantage of this technique in order to employ wood in outdoor applications. Many aspects of heat-treated wood, such as dimensional stability, durability, mechanical properties, equilibrium moisture content, mass loss, wettability, color change, chemical modifications, and others (Esteves and Pereira 2009) have been studied for various wood species. However, only limited studies have looked at wood machinability and the resulting wood quality (de Moura and Brito 2008; Budakci et al. 2013; Tu et al. 2014; Kubs et al. 2016). Milling is one important operation that usually precedes sanding. A good quality of milling is required in order to get the best results from the operations that follow.
Because of the chemical changes that wood undergoes during heat treatment, its density decreases, most mechanical strengths are weakened, and its brittleness increases with the deterioration of fracture properties due to the loss of amorphous polysaccharides (Esteves and Pereira 2009). Thus, heat-treated wood is more susceptible to mechanical damage during further processing, and it sometimes requires adapted technological conditions compared to untreated wood of the same species.
According to the ThermoWood Handbook (Finnish ThermoWood Association 2003), milling heat-treated, resinous wood can be regarded as similar to working with hard, brittle hardwoods. The sharpness of the cutters is important, in order to avoid tearing, especially when milling across the grain. The greatest problems with tearing, as well as enhanced and uneven, accidental, vibrational waviness, occur at the beginning and the end of the milling path, when the cutter gets into and comes out of the wood.
Previous studies on the machinability of heat-treated beech wood have shown that the most important factors that affect the cutting power during the milling of heat-treated wood as compared to untreated wood are the cutting speed, the rake angle of the cutter, and the feed speed (Kubs et al. 2016). The cutting power represents the mechanical work at processing released per second. It is determined by measuring the electric power absorbed by the machine motor during processing as well as at idle run (Eq. 1)
Mandic et al. (2010) investigated the influence of the temperature (170, 190, and 210 °C) used during the heat-treatment of beech samples on the cutting power during milling. Only the samples treated at 190 °C and 210 °C were cut with a significantly lower cutting power than the untreated ones, and the cutting power increased significantly at feed speeds above 8 m/min.
However, none of the previously published works found a clear correlation between the cutting power and the cutting parameters used during the processing of heat-treated wood. This lack of a correlation might be the result of variability of the wood samples, failure to observe the importance of preparing the samples with strictly oriented fibers, or the sensors’ performance and limitations.
Some authors have compared the surface roughness of heat-treated wood with that of untreated wood, where the samples were planed prior to the heat treatment, but not afterwards. The measurements of surface roughness after heat treatment indicated slightly lower roughness for Turkish river red gum wood, Eucalyptus camaldulensis (Unsal and Ayrilmis 2005), red-but maple, Acer trautvetteri Medw. (Korkut and Guller 2008), Turkish hazel, Corylus colurna L. (Korkut et al. 2008), European Hophornbeam, Ostrya carpinifolia Scop. (Korkut et al.2009), and Rowan wood, Sorbus aucuparia L. (Korkut and Budakci 2010).
However, evaluating the surface roughness of heat-treated wood after machining is more interesting because in real practice the heat treatment precedes processing (milling, drilling, turning, sanding, etc.). The modifications that wood undergoes during a heat treatment, such as mass loss, might have an important impact on the surface roughness, seen as the result of wood-tool interactions, in a different way than in the above studies.
Budakci et al. (2013) determined the roughness (Ra) perpendicular to the grain of Eastern beech wood (Fagus orientalis L) heat-treated at 140 C and 160 C (for 3, 5, and 7 h), after milling at a rotation speed of 6000 rpm, with a 4-m/min feed speed and a 1-mm cutting depth, using two types of cutters (star blade and razor blade). The roughness values (Ra), as measured by a stylus with a 5-m tip radius, for heat-treated wood were slightly higher (up to 8%) than those for untreated wood, and they increased with increasing duration of the heat-treatment.
Kvietkova et al. (2015) investigated the roughness (Ra), measured along the feed direction by the stylus method, after milling beech wood (Fagus sylvatica L.). After varying the rotation speed, feed speed, clearance angle, rake angle, and cutting angle of the milling cutter, the authors concluded that the thermal treatment had no significant influence upon the average roughness of the milled surfaces. There was, however, a significant effect of the cutting speed and feed speed. The lowest value of Ra was found with a clearance angle of 20° and the highest value of cutting speed (40 m/s). The heat treatment (190 °C for only 1 h) may have been insufficient for detecting an effect of the treatment on surface roughness. The need for a robust filter for analysis of the roughness of wood surfaces was confirmed and tested by more recent research of Tan et al. (2012) and Piratelli-Filho et al. (2012). However, no previous research on the surface roughness of heat-treated wood has used a robust filter. Rather, all of those authors applied a simple Gaussian filter inherent in most measuring instruments on the market, which indicates the need for further and more precise analysis.
Objective
The main objective of this study was to investigate the cutting power during milling with various cutting regimes, along with the subsequent surface roughness, of beech wood strips (Fagus sylvatica L.) heat-treated by the ThermoWood method at 200 °C for 2.5 h, in comparison with untreated wood manufactured under the same conditions. The influence of different rotation speeds, feed speeds, and cutting depths upon the cutting power and the surface roughness were investigated. An interesting element of novelty was brought by the more robust filtering procedure used to evaluate the surface roughness, by adding more roughness parameters than used in the literature (Rt, Rk, Rpk, and Rvk) and by including an analysis of the influence of the longer wavelength components in the primary profile (Pt). Furthermore, the study included roughness and primary profiles computed in MathCAD, which allowed a visual comparison.
EXPERIMENTAL
Materials, Methods, and Equipment
The wooden material used within the experiments consisted of 400-mm × 50-mm × 28-mm beech wood (Fagus sylvatica L.) samples. Half of the samples were heat-treated in superheated steam in an industrial-scale TekmaWood kiln, manufactured by TekmaHeat Corporation (Lahti, Finland), according to the schedule presented in Table 1. The other half of the samples were kept untreated as controls.
Table 1. Heat-Treatment Schedule
The average mass loss of the samples due to this heat treatment was 13.18% ± 1.36%.
All of the samples were conditioned for 4 weeks at 20 °C and 55% relative humidity (RH) before being processed. The average moisture content of the samples after conditioning was 3% ± 0.2% for the heat-treated strips and 8% ± 0.5% for the untreated controls.
The samples were then processed by means of a conventional milling cutter head 125, B118 with 6 cutters with 30-mm × 12-mm × 1.5-mm carbide-tipped removable plates (Fig. 1), on a vertical milling machine type MNF10 produced by UMARO SA (Roman, Romania).
Two different rotation speeds (n), five different feed speeds (u), and three cutting depths (h) were used:
n1 = 3300 rpm; n2 = 4818 rpm
u1 = 4.5 m/min; u2 = 9 m/min; u3 = 13.5 m/min; u4 = 18 m/min; u5 = 22.5 m/min
h1 = 1 mm; h2 = 2 mm; h3 = 3 mm.
Sets of ten heat-treated samples and respectively untreated samples were used for each milling condition. Milling was performed on the specimen edge.
The cutting power (P) was calculated according to Eq. 1:
P = Pt – P0 (1)
| where Pt is the total power consumed by the electric motor during processing (kW), and P0 is the power consumed by the electric motor during the idle run (kW). Both Pt and P0 were measured by means of a three-phase transducer for active power connected directly into the electrical circuit of the machine motor (Fig. 2).
The roughness measurements were carried out immediately after milling using a MarSurf XT20 instrument (Fig. 3) manufactured by MAHR GMBH (Gottingen, Germany), endowed with a scanning head MFW 250 with a tracing arm in the range of 500 m and a stylus with a 2-m tip radius and 90 tip angle, which measured the specimens at a lateral resolution of 5 m, at a speed of 0.5 mm/s, and using a low scanning force of 0.7 mN. For each milling condition, 6 profiles, 10 mm long, were randomly scanned on each specimen’s milled edge across the grain (across the feed direction) to measure the roughness parameters. This length was restricted by the edge size of the specimens, but it should have been enough to cover wood growth variability (earlywood and latewood areas). According to de Moura (2006), values of roughness measured across the grain are usually higher than those measured along the grain. |
Fig. 2. Experimental stand for measuring the cutting power during milling |
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| Fig. 3. MarSurf XT20 instrument used to measure the roughness parameters of the processed surface |
Two other profiles, 50 mm long, were randomly measured along the grain (in the feed direction) in order to analyze the waviness of the surface caused by process kinematics. This longer length was chosen so that longer wave irregularities, such as waviness, could be detected.
The instrument had MARWIN XR20 software (Gottingen, Germany) installed for processing the measured data.
The sequence of operations for an individual profile began with removing the form error by best fitting a polynomial regression to the dataset. At this stage, a primary profile was obtained containing waviness and roughness.
The roughness profiles were obtained by filtering each profile using a new and robust filter, RGRF (Robust Gaussian Regression Filter), contained in ISO 16610-31 (2010). The cutoff used was 2.5 mm, as recommended for wood in previous research by Gurau et al. (2006) and several other researchers (Unsal and Ayrilmis 2005; Korkut et al. 2008; Korkut et al. 2009; de Moura et al. 2011).
A range of roughness parameters was calculated for the roughness profiles taken across the grain (across the cutting direction), such as Ra, Rq, and Rt, from ISO 4287 (1997). Parameters Rk, Rpk, and Rvk from ISO 13565-2 (1996) were included in this study. Although useful for variable surfaces as wood, these parameters have not been tested by previous researchers in the case of heat-treated wood. For the primary profiles taken along the grain, a primary profile parameter, Pt, was calculated from ISO 4287 (1997), in order to obtain a magnitude for the combined kinematic waviness and roughness after processing.
A standard description of the above mentioned roughness parameters is given below (ISO 4287). A profile is represented by a vector of length n of ordinate values Zi.
The ISO 13562-2 standard defines a procedure to obtain the Rk, Rpk, and Rvk profile parameters below, but it is beyond the scope of this paper to describe it in detail. Abbot-curve parameters from the ISO 13565‑2 are presented as follows:
| Rk | The core roughness depth is the depth of the core profile within an evaluation length, excluding the height of the protruding peaks and deep valleys. |
| Rpk | The reduced peak height is the average height of the protruding peaks above the roughness core profile. |
| Rvk | The reduced valley depth is the average depth of the valleys projecting through the roughness core profile |
Wood anatomy is known to bias not only the filtering process, but also the evaluation of the processing roughness parameters of wood, especially when the magnitude of inherent wood irregularities is greater than that caused by processing alone. Processing roughness, even for a diffuse, porous species such as beech, should be separated from anatomical irregularities if the effect of processing is to be properly evaluated (Gurau et al. 2015). However, if anatomy is not removed from the measured profile, the Rk parameter is a good approximation of the processing roughness (Westkämper and Riegel 1993; Gurau 2004; Sharif and Tan 2011). This parameter (Rk) measures the core roughness of a profile, and it should be sensitive to wood processing and surface heat treatment.
Mean parameters Ra and Rq are common roughness indicators, but alone, they do not provide sufficient information about wood surface topography. Furthermore, it is expected that they are influenced by wood anatomy if wood irregularities are kept in the evaluation. Similarly, Rt (measuring the total height of profile), as well as Rpk (measuring the reduced peak height associated with fuzziness) or Rvk (measuring the reduced valley depths associated with wood anatomical valleys), is expected to be sensitive to variations in local wood anatomy, but also to defects occurring during processing, such as pull-out fibers or gaps caused by an improper processing schedule.
For each processing condition and roughness parameter observed in this study, a mean value and the standard deviation were calculated and included in a table.
Individual roughness profiles taken across and along the grain (feed direction) were computed in MathCAD. In order to visualize the results, the core roughness (processing roughness) was separated from the other surface irregularities by upper and respectively lower thresholds with a method described by Gurau et al. (2005) to allow visual comparisons between milling conditions and wood treatment.
ANOVA and Duncan’s multiple range tests were performed to test significant differences between datasets.
RESULTS AND DISCUSSION
Cutting Power
Table 2 presents the active cutting power as a function of the variable cutting conditions. The power values recorded during the processing of heat-treated wood were lower than those of the untreated controls. This result was expected because of the strength loss due to the heat treatment.
The most significant reductions (up to 50%) of the cutting power at the milling of heat-treated wood compared to untreated wood were recorded with low rotation speed (n1 = 3300 rpm), high feed speeds (u4 = 18 m/min and u5 = 22.5 m/min), and low cutting depths (h1 = 1 mm and h2 = 2 mm).
The values in Table 2 and Fig. 4 clearly show that the cutting power increased with increasing rotation speed, feed speed, and cutting depth.
When the rotation speed was increased from 3300 rpm to 4818 rpm, the increase in cutting power was not noticeable for untreated wood. However, for the heat-treated wood, the increase was noticeable, ranging up to 30% at the same feed speed and cutting depth.
Figure 4 also shows that the cutting power increased with increasing feed speed at constant rotation speed and cutting depth. The dependency function was linear, both for untreated beech wood (R2 = 0.96 to 0.99) and heat-treated beech wood (R2 = 0.92 to 0.99).
Table 2. Cutting Power (in kW) (with Mean Value x̅ and Standard Deviation ) As a Function of Cutting Conditions for Heat-Treated and Untreated Beech Wood
As expected, the cutting power also increased with increasing cutting depth. The correlation found at constant rotation speed and feed speed corresponded to a linear function (R2 = 0.95 to 0.99) in all cases. An example is presented in Fig. 5.
Surface Roughness
The analysis of surface roughness was limited to the depth of cut h1 = 1 mm in combination with feed speeds from 4.5 m/min to 18 m/min and both rotation speeds (n1 and n2) for heat-treated and untreated beech wood. It was observed that for increased depths of cut (h2, h3) combined with high feed speeds (u3, u5), the surfaces of both heat-treated and untreated beech wood presented milling defects such as pull-off fibers, and therefore, roughness measurements would have been irrelevant. The same was true for a depth of cut h1 = 1 mm combined with the highest feed speed (u5 = 22.5 m/min). The measured values are contained in Table 3.
Table 3. Roughness Parameters—Mean Value and (Standard Deviation) for Heat-Treated (HT) and Untreated (NT) Beech Wood Processed by Milling with Four Different Feed Speeds and Two Rotation Speeds at a Cutting Depth of 1 mm
It is important to mention that the processed beech samples were randomly cut without considering the radial or tangential surfaces. For this reason, wood variability likely had an impact on the roughness results similar to that reported by Kantay and Űnsal (2002). A good quality cut is usually obtained when cutting to the grain, mainly in the tangential section of boards (de Moura et al. 2014). However, a generally increasing trend of surface roughness with increasing feed speed was observed for the cutting depth h1 = 1 mm at both rotation speeds (n1and n2) for all roughness parameters.
When comparing the mean values, the heat-treated wood had a slightly higher roughness in comparison with the untreated wood for both rotation speeds n1 and n2, and the surface quality had, generally, a larger variability as measured by standard deviation.
Examples of roughness profiles recorded for heat-treated and untreated wood processed by milling with a rotation speed n1 = 3300 rpm, feed speed u3 = 18 m/min, and cutting depth h1= 1 mm, are given for comparison in Fig. 6 (Ra = 8.85 m; Rk = 12.5 m; Rt = 93.17 m) and Fig. 7 (Ra = 6.79 m; Rk = 11.02 m; Rt = 69.25 m).
Fig. 6. Roughness profile of heat-treated beech wood processed by milling with a rotation speed n = 3300 rpm, feed speed u = 18 m/min, and cutting depth h = 1 mm. Thresholds with continuous horizontal lines delimit the processing roughness.
Fig. 7. Roughness profile of untreated beech wood processed by milling with a rotation speed n = 3300 rpm, feed speed u = 18 m/min, and cutting depth h = 1 mm. Thresholds with continuous horizontal lines delimit the processing roughness.
The core roughness, expressing the contribution of processing, is delimited by thresholds. Features extending below the lower threshold are anatomical cavities, but they can also be surface defects, such as gaps caused by a pulled group of fibers that detached from the surface during cutting. Such a defect is visible for the heat-treated wood, towards the left side of the profile. These kinds of defects were observed spreading along the processed surface of heat-treated wood, especially for high feed speeds of 13.5 m/min and 18 m/min. Reiterer and Sinn (2002) studied the fracture properties of heat-treated wood, and they noticed a lower resistance against fractures and higher brittleness in comparison with untreated wood. This behavior of heat-treated wood has been associated with the loss of amorphous polysaccharides due to degradation (Phuong et al. 2007).
The Ra roughness parameter was taken as a reference because it was the most used parameter in the literature. For this parameter, the differences between wood treatments in Table 3 were significant (at 5% significance level), as measured by ANOVA and Duncan multiple range tests (Table 4), for feed speeds 4.5 m/min and 18 m/min, but were not significant for feed speeds 9 m/min and 13.5 m/min, for both rotation speeds (n1 and n2). The surface roughness, measured by Ra, increased with the feed speed and the difference as relate to the control (4.5 m/min) was significant for both rotation speeds and for both: treated and untreated wood. Other authors also found an increase in surface roughness, measured by the Raparameter, with increasing feed speed in the plane milling of beech (Kvietkova et al. 2015).
For the Rk parameter, the differences were not significant between heat-treated and untreated wood or between processing with various feed speeds for the lower rotation speed (n1). For the higher rotation speed (n2), however, the differences between heat-treated and untreated wood were significant at high feed speeds (13.5 m/min and 18 m/min).
Table 4. Influence of the Heat Treatment and Feed Speed for Two Rotation Speeds on Roughness Parameters Ra and Rk for Milled Beech, with Statistical Analysis from Duncan’s Multiple Range Test
Note: Groups with the same letters in columns indicate that there was no statistical difference (p < 0.05) between the samples according to Duncan’s multiple range tests.
Increasing the rotation speed from 3300 rpm to 4818 rpm decreased the surface roughness for both treated and untreated wood. In the case of untreated wood, these differences were significant for feed speeds of 4.5 m/min and 18 m/min, but they were not significant for feed speeds of 9 m/min and 13.5 m/min, as judged from the Ra and Rk parameters (Table 5). However, the differences were significant for Ra for all feed speeds in the case of heat-treated wood, with the exception of the differences for feed speed 18 m/min. Other authors have also found the decrease of surface roughness with an increase in cutting speed to be statistically significant for Ra (Kvietkova et al. 2015). Although Rk was generally smaller for the rotation speed 4818 rpm, than 3300 rpm, these differences were not statistically significant for the heat treated wood.
Table 5. Influence of Rotation Speed and Feed Speed for Heat-Treated and Untreated Beech Wood Processed by Milling on Roughness Parameters Ra and Rk
Note: Groups with the same letters in columns indicate that there was no statistical difference (p<0.05) between the samples according to Duncan’s multiple range tests
The influence of the kinematic waviness was measured by the Pt parameter (total height of the primary profile) in Table 3. The heat-treated wood had a smaller Pt than the untreated wood, which means that milling caused a smaller depth amplitude in the heat-treated wood. Generally, Pt increased with the feed speed, and it was smaller when a lower cutting speed was used. This result was comparable to the findings of Gaff et al. (2015), who measured the waviness as expressed by the Wa (arithmetical mean deviation of the waviness profile) parameter for plane milling of birch wood. Wa is similar to Ra, but applies to the waviness profile. The waviness profile derives from the primary profile from which the shortest wavelength irregularities (roughness) are subtracted.
Figure 9 shows an example of the smaller depth waviness observed in the primary profiles of heat-treated beech, which can be compared with the higher waviness of untreated beech in Fig. 8 for a feed speed of 18 m/min, a rotation speed of 3300 rpm, and a cutting depth of 1 mm.
Fig. 8. Primary profile of untreated beech processed by milling with a cutting speed n1 = 3300 rpm, feed speed u = 18 m/min, and a cutting depth h = 1 mm. Measurement was taken along the cutting direction.
Fig. 9. Primary profile of heat-treated beech processed by milling with a cutting speed n1 = 3300 rpm, feed speed u = 18 m/min, and a cutting depth h = 1 mm. Measurement was taken along the cutting direction.
It has to be mentioned that the results obtained in this paper refer to heat-treated beech wood (Fagus sylvatica), with the ThermoWood method at 200 °C for 2.5 h in comparison with untreated wood. It is very likely that variation in the treating method, temperature, and duration of treatment may change the values of the cutting power and roughness parameters. Currently, the authors are studying the effects of the heat treatment duration on the cutting power and surface roughness of beech wood, and this will be the subject of a further paper.
CONCLUSIONS
The main conclusions regarding the influence of different cutting parameters upon the consumed power and resulting surface roughness at milling of heat-treated beech wood (Fagus sylvatica), with the ThermoWood method at 200 °C for 2.5 h compared to untreated beech can be summarized as follows:
- The cutting power during the milling of heat-treated beech wood was up to 50% lower than that of untreated wood.
- The cutting power during milling increased with increasing rotation speed, feed speed, and cutting depth for both untreated and heat-treated wood. All correlations were linear.
- The surface roughness of heat-treated beech processed by milling was slightly higher than that of untreated wood (as measured by Ra, Rq, Rt, Rk, and Rk+Rpk+Rvk). This result was significant (for p < 0.05 significance level) for Ra for both rotation speeds n1 = 3300 rpm and n2 = 4818 rpm and for feed speeds of 4.5 m/min and 18 m/min.
- The surface roughness of heat-treated and untreated beech wood increased with the milling feed speed, but the roughness values for feed speeds of 9 m/min and 13.5 m/min were not significantly different.
- An increase in the rotation speed from n1 = 3300 rpm to n2 = 4818 rpm decreased the surface roughness for both heat-treated and untreated beech. This result was significant for Ra and Rk, for untreated wood for feed speeds of 4.5 m/min and 18 m/min. For heat-treated wood, the differences in Ra were significant for all feed speeds, with the exception of 18 m/min.
- The influence of kinematic waviness, as measured by Pt, increased with the feed speed for both heat-treated and untreated beech. The total peak-to-valley height in the primary profiles, measured along the cutting direction, for each feed speed was smaller in the case of heat-treated wood for both rotation speeds. These differences could also be visualized on primary profiles plotted in MathCad.
ACKNOWLEDGMENTS
We hereby acknowledge the structural funds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, ctr. No 11/2009) for providing the infrastructure used in this work.
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Article submitted: July 2, 2016; Peer review completed: August 21, 2016; Revised version received and accepted: August 29, 2016; Published: September 8, 2016.
DOI: 10.15376/biores.11.4.9095-9111