This article focuses on the plane milling of thermally modified birch wood while taking into account technological parameters that have substantial effects on the processed wood surface’s average waviness profile deviation (Wa). The milling process was affected by the cutting speed, which varied from 20 to 60 m/s, with a feed rate of 4, 8, and 11 m/min. The results obtained on the set of thermally modified test specimens, were compared with the results obtained on test specimens without heat treatment. The surface finish was measured using various milling parameters. The material removal was 1 mm per pass. The results indicate that the thermal processing of wood did not significantly influence the arithmetic average deviation of the roughness profile (Ra). Cutting speed and feed rate had the most significant effects among the monitored factors. The lowest arithmetic average deviation of the roughness profile (Ra) was determined at a feed rate of 4 m/min and cutting speed of 40 m/s. An increase in cutting speed led to a decrease in the average roughness, while increased feed rate had the opposite effect.
Effect of Selected Parameters on the Surface Waviness in Plane Milling of Thermally Modified Birch Wood
Milan Gaff,a, * Monika Kvietková,a Miroslav Gašparík,a Lukáš Kaplan,a and Štefan Barcík b
This article focuses on the plane milling of thermally modified birch wood while taking into account technological parameters that have substantial effects on the processed wood surface’s arithmetic mean deviation of the waviness profile (Wa). The milling process was affected by the cutting speed, which varied from 20 to 60 m/s, with a feed rate of 4, 8, and 11 m/min. The results obtained on the set of thermally modified test specimens, were compared with the results obtained on test specimens without heat treatment. The surface finish was measured using various milling parameters. The material removal was 1 mm per pass. The results indicate that the thermal modification of wood did not significantly influence the arithmetic mean deviation of the waviness profile (Wa). Cutting speed and feed rate had the most significant effects among the monitored factors. The lowest arithmetic mean deviation of the waviness profile (Wa) was determined at a feed rate of 4 m/min and cutting speed of 40 m/s. An increase in cutting speed led to a decrease in the average roughness, while increased feed rate had the opposite effect.
Keywords: Surface waviness; ThermoWood; Technological parameters; Plane milling
Contact information: a: Department of Wood Processing, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 – Suchdol, 16521, Czech Republic; b: Department of Machinery Control and Automation, Faculty of Environmental and Manufacturing Technology, Technical University in Zvolen, Študentská ulica 26, Zvolen, 960 53, Slovakia;
* Corresponding author: firstname.lastname@example.org
Wood has a very wide range of uses, primarily in construction, the furniture industry, the paper industry, and transport. One of wood’s most important properties is its natural durability, especially regarding its resistance to biological pests. For all machining processes, it is necessary to select an appropriate tooling material, one with its cutting edge suitably treated to protect it from wear (Békés et al. 1999).
Milling is machining with a rotary tool (cutter, milling head, etc.), where the nominal shaving thickness increases from zero to a maximum figure, and the material is fed at an angle to the cutter’s axis of rotation (Lisičan 1996). It is the most widespread method for producing flat and contoured surfaces, grooves, finger joints, and the like. The main movement is rotation, which is made by the cutter, while the work piece (or with some machines the headstock) makes the remaining movements.
The purpose of milling is to process or machine a component through cutting (a process that forms shavings) into the required dimensions, shape, and surface finish. Milling is a very widely used wood processing technology. It is possible to mill along the edge of the cutter, which is known as peripheral milling. Depending on the rotation direction, peripheral milling can be divided into two categories: conventional milling, where the cutter rotates against the direction of feed, and climb milling, in which it turns in the direction of the feed (Řasa and Gabriel 2000).
Selecting the type and subsequent maintenance of the tool (cutter) is very important and also requires attention. An advantage of milling is its relatively high output capacity and high-quality surface finish. It is used to machine flat, contoured, and rotating surfaces. Milling is also used to machine grooves of various profiles, as well as to create finger joints.
Fig. 1. Theoretical calculation of thickness and length of milled chip from peripheral milling; Vf is feed rate (m/min), Vc is cutting speed (m/s), hmax is maximum thickness (mm), hs is cut shaving thickness (mm), ap is cut depth (m), fz is feed per tooth (mm), n is tool rotation frequency (cutter rotating speed) (min−1), D is tool (cutter) diameter (m), R:ap is cut depth–tool radius ratio, φ is tooth cutting angle (°), and φstr is central angle (°)
ThermoWood is thermally modified wood that obtains its innovative internal structure through thermal and moisture treatment. This method was invented and registered many years ago in Finland. ThermoWood is modified using only heat and steam, and the entire production process is completely environmentally friendly. Thermal treatment positively affects and improves not only the durability, but also other physical and mechanical properties (Maulis 2009).
During thermal treatment, the wood is thermally and hydrothermally treated at high temperatures ranging from 150 to 260 °C (Kačíková and Kačík 2011). Greater durability and resistance to fungi and other biological pests characterize thermally modified wood. Wood modified in this manner exhibits very good properties, which are similar to those of rare tropical wood, a fact that attracts great attention for this modern material, primarily among more demanding customers and those who prefer luxury products. This material has increased in popularity because its beneficial properties cannot be found in natural wood growing in Europe. Additionally, ThermoWood has both interior and exterior uses.
Machining is a technological process in which the workpiece is cut into the required shape at certain defined dimensions with the required surface finishes. For thermally modified wood, it is important that the machine tools are well sharpened and the finished surfaces are as smooth as possible. Thermal treatment even facilitates the tool’s penetration capabilities into the workpiece.
All machining, including of course milling, is designed with the intention of achieving the smoothest surface possible. A surface is defined as the interface between two distinct environments – the basic material and its surroundings. This interface defines the overall appearance of the given material. The product’s surface finish can generally be defined as a set of properties specified by the producer, consumer, and price-setting bodies according to various criteria (Prokeš 1982). The overall finish of the machined surface is influenced by many factors. The main factor affecting surface finish is the tool, in terms of both the material from which it’s made and its service life. Among the criteria used, the machining process concerns shape precision, the workpiece’s surface finish, and dimensional precision (Ondra 1998). Each technological operation that disturbs the workpiece’s original properties and integrity leaves characteristic irregularities on the machined surface.
According to ČSN EN ISO 4287 (1999), these irregularities can cause microscopic changes in the machined surface’s roughness, as well as creating macroscopic transformations such as waviness, scores, troughs, and torn out fibers. Surface finish, which consists of roughness, waviness, and lay, decisively affects the properties and behavior of the component in operation (e.g., the course of wear, fatigue properties, bonding strength, and kinematic and dynamic surface connections).
Vibrations, deformations in the workpiece, and material hardening can cause waviness. It is considered to be an effect from the machine, such as from imbalanced grinding wheels, imprecision in guide parts, or low stiffness or imprecision in guide screws. The waviness profile is a profile acquired by gradually applying the λf and λc filters to the primary profile. It is acquired by suppressing long-wave components from the primary profile using λf, and short wave components using λc by means of a band-pass filter (Studený and Kusmič 2007).
Fig. 2. Course of waviness in a profile
Waviness appears on the surface with regularly repeating peaks and troughs of almost identical shapes and dimensions. The surface finish parameter of waviness is understood as very small deviations from the required (ideal) shape or dimension, which nevertheless substantially affects further processing of the piece, specifically its surface finish.
Silver birch (Betula pendula Roth) from Poľana, which is east of Zvolen, Slovakia, was used. The parts of the wood chosen for sample preparation were in the middle of the wood, between the pith and bark. These parts were cut into 100-cm-long pieces. Defectless samples with dimensions 40 × 100 × 500 mm were used for the experiments. All samples were conditioned for 4 months in a conditioning room (ϕ = (65 ± 3) % and t = (20 ± 2) °C) to achieve 12% equilibrium moisture content (EMC). Birch wood had a density of 550 kg/m3 in oven-dry state.
After conditioning, the samples were divided into two groups. The first group contained samples intended for thermal treatment and the second group consisted of reference samples of native wood. The whole investigation involved 450 samples.
Wood samples were placed on a metal grate and subsequently placed into the thermal chamber model S400/03 (LAC Ltd., Czech Republic) (technical parameters are indicated in Table 1). The heat treatment was carried out in three phases according to ThermoWood® process developed by VTT, Finland. The first phase consisted of drying the wood and heating the chamber to the desired temperature, from 160 to 240 °C using steam as a protective vapor. In the second stage, the desired temperature was maintained for the specified time (5 h) (Table 2). In the third and last phase, the chamber and wood were gradually cooled. During this phase, the wood is re-moisturized in order to achieve the end-use moisture (5 – 7 %). The thermally modified samples were then conditioned (ϕ = (65 ± 3) % and t = (20 ± 2) °C) for three weeks. Before experiments, all samples were machined to final thickness (25 mm) using a DHM 630P thickness planer (Holzmann, Germany). Native and thermally-modified samples (final dimensions 25 × 100 × 500 mm) were prepared for the plane milling process.
Table 1. Parameters of Thermal Chamber
Table 2. Duration of Thermal Treatment
The milling process was carried out using an FVS mill with a STEFF 2034 automatic feeder (Maggi, Italy). Cutter parameters and individual cutting angles were as follows.
FVS with feed system STEFF 2034 Cutter Head
- Input power 4 kW, – Rake angle γ 25, 20, 15°
- RPM 3000, 4500, 6000, – Cutting angle of wedge β 45°,
- Cutting speed 20, 40, 60 m/s, – Clearance angle α 20, 25, 30,
- Feed speed 4, 8, 11 m/min. – Cutting angle δ 65, 70, 75.
The waviness profile was measured according to ISO 4287 (1997) and ISO 4288 (1996) using a Form Talysurf Intra roughness meter (Taylor-Hobson, UK). The measurement was carried out in three tracing lengths oriented in a parallel direction with respect to the length of the sample and the feed direction. The track length was 50 mm. The waviness profile was evaluated based on the arithmetic mean deviation of the waviness profile Wa.
Evaluation and Calculation
The influence of factors on waviness was statistically evaluated using ANOVA, mainly by Fisher’s F-test, in STATISTICA 12 software (Statsoft Inc.; USA).
The density was determined before and after treatment. Density was calculated according to Eq. 1 from ISO 13061-2 (2014),
where ρw is the density of the test sample at certain moisture content w (kg/m3); mw is the mass (weight) of the test sample at certain moisture w (kg); aw, bw, and lw are the dimensions of the test sample at certain moisture w (m); and Vw is the volume of the test sample at a certain moisture w (m3).
The moisture content of samples was determined according to ISO 13061-1 (2014) and Eq. 2,
where w is the moisture content of the sample (%), mw is the mass (weight) of the test sample at certain moisture w (kg), and m0 is the mass (weight) of the oven-dry test sample (kg). Drying to an oven-dry state was also carried out according to ISO 13061-1 (2014).
RESULTS AND DISCUSSION
Based on the significance levels given in Table 3, the feed rate can be considered to have had a significant effect, whereas the cutting speed and thermal treatment did not exhibit significant effects. Furthermore, the interaction among the three monitored factors did not result in a significant effect.
The data presented in Table 3 clearly indicate that cutting speed did not have a significant effect on the arithmetic mean deviation of the waviness profile (Wa). These results confirm the results shown in Fig. 3.
Costes and Larricq (2002) also found that the increase of cutting speed does not have any effect on the value waviness profile (Wa).
The values shown in Fig. 4 clearly illustrate the significant effect that feed rate had on the monitored characteristic, as the arithmetic mean deviation of the waviness profile (Wa) increased as the feed rate increased. Keturakis and Juodeikienė (2007) also found the same effect of the feed speed on the monitored characteristic.
Table 3. Individual Factors’ Effects on Arithmetic Mean deviation of the Waviness Profile
Fig. 3. Cutting speed’s effect on arithmetic mean deviation of the waviness profile
Fig. 4. Feed rate’s effect on arithmetic mean deviation of the waviness profile
The values in Fig. 5 clearly indicate higher arithmetic mean deviations of the waviness profile for thermally modified wood in the range of 160 to 210 °C, although these differences were small and slightly outside the limit for statistical significance. These conclusions were also found in the works of Budakçı et al. (2011, 2013). (2013). Figure 6 illustrates that the machined material’s resulting waviness was not affected after the birch wood was thermally treated.
Fig. 5. Thermal treatment’s effect on arithmetic mean deviation of the waviness profile
Fig. 6. Combined effect of cutting speed, feed rate, and thermal treatment arithmetic mean deviation of the waviness profile
Based on these graphical outputs, the effect of cutting speed on the monitored variables is apparent. The arithmetic mean deviation of the profile waviness decreased with increased cutting speed. In contrast, increases in the monitored parameter were observed after increases in the feed rate. For high-quality machining, it is necessary for the cutting speed to be sufficiently high relative to feed rate, as higher feed rates resulted in diminished quality of the machined surface finish.
- Based on the results, it can be stated that thermal treatment did not have a significant effect on surface finish, expressed as the arithmetic mean deviation of the waviness profile of birch wood in plane milling.
- Increases in cutting speed during plane milling decreased the arithmetic mean deviation of the waviness profile.
- Changes in feed rate had the opposite effect, as increasing this parameter increased the arithmetic mean deviation of the waviness profile.
- Cutting speed affected the finish of the machined surface. To achieve a higher-quality surface finish, it is necessary to select a high cutting speed, as well as the lowest possible feed rate.
The authors are grateful for the support of the Internal Grant Agency of the Faculty of Forestry and Wood Science, project No. B05/15, “Properties of laminated materials based on wood and non-wood components.”
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Article submitted: June 2, 2015; Peer review completed: September 19, 2015; Revised version received and accepted: September 20, 2015; Published: September 24, 2015.