Abstract
The impact of heat-treatment and plane milling parameters were evaluated relative to the surface quality of native and thermally modified oak wood (Quercus robur L.). Experimental wood samples were treated via ThermoWood technology at Volga State University of Technology in Yoshkar-Ola, Russia. The investigation focused on the impact of the tool (rake angle = 15°, 20°, and 30°), material (native wood, wood heat-treated at 160 °C, 180 °C, 200 °C, and 220 °C), cutting speed (20 m/s, 40 m/s, and 60 m/s), and feed rate (20 m/min, 40 m/min, and 60 m/min) on the machined surface quality (mean arithmetic deviation of the surface). The roughness measurement was performed via a contactless method with a laser profilometer. An analysis of variance and post-hoc Duncan test revealed the influence of the examined parameters on the surface roughness in the following order from highest to lowest: rake angle, cutting speed, heat treatment temperature, and feed rate. This research is part of a study of the properties of woodworking thermally modified wood that is focused on measuring the quality and energy of the machining process.
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Effects of Technical and Technological Parameters on the Surface Quality when Milling Thermally Modified European Oak Wood
Michal Korčok,* Peter Koleda, Štefan Barcík, and Marek Vančo
The impact of heat-treatment and plane milling parameters were evaluated relative to the surface quality of native and thermally modified oak wood (Quercus robur L.). Experimental wood samples were treated via ThermoWood technology at Volga State University of Technology in Yoshkar-Ola, Russia. The investigation focused on the impact of the tool (rake angle = 15°, 20°, and 30°), material (native wood, wood heat-treated at 160 °C, 180 °C, 200 °C, and 220 °C), cutting speed (20 m/s, 40 m/s, and 60 m/s), and feed rate (20 m/min, 40 m/min, and 60 m/min) on the machined surface quality (mean arithmetic deviation of the surface). The roughness measurement was performed via a contactless method with a laser profilometer. An analysis of variance and post-hoc Duncan test revealed the influence of the examined parameters on the surface roughness in the following order from highest to lowest: rake angle, cutting speed, heat treatment temperature, and feed rate. This research is part of a study of the properties of woodworking thermally modified wood that is focused on measuring the quality and energy of the machining process.
Keywords: Plane milling; Physical properties; Surface roughness; ThermoWood; Quercus robur
Contact information: Department of Manufacturing and Automation Technology, Faculty of Environmental and Manufacturing Technology, Technical University in Zvolen, Študentská 26, Zvolen, 960 53, Slovakia;
* Corresponding author: korcokmichal@gmail.com
INTRODUCTION
Heat-treated wood has been extensively manufactured for more than 10 years, and its production has been introduced to many Western European countries in response to changing chemical wood treatment legislation (International ThermoWood Association 2003). Finland pioneered the production of thermally modified wood called ThermoWood® in 1990. Later, ThermoWood® began to be produced in the Netherlands, Germany, Austria, and France (International ThermoWood Association 2003; Gaff et al. 2015).
Heat treatment is a method of modifying the wood structure by means of high temperature, humidity or a bath of oil. The technology itself depends on the country where it was developed. The main differences between these methods are based on the materials used (e.g. wood species, fresh or dried wood, moisture content, dimensions), process conditions applied (e.g. one or two process stages, wet or dry process, heating medium, oxygen or nitrogen as sheltering gas, heating and cooling down velocity) and the equipment necessary for treatment (e.g. process vessel, kiln). (Boonstra et al. 2007; Boonstra 2008). This has a positive effect on wood’s properties. The primary aim of thermally modifying wood is to prepare a material that balances the following benefits: a lower hygroscopicity; higher dimensional stability; higher resistance to wood-decaying and discolouring fungi, moulds, and ligniperdous insects; maintaining or improving the aesthetics (colour, minimal cracks, gloss, texture, etc.); and preservation or improvement of the mechanical properties (strength hardness, stiffness, etc.) (Požgaj et al. 1997; Bengtsoon et al. 2003; Niemz et al. 2010; Barcík and Gašparík 2014).
Milling is the process of machining wood with a rotating multi-wedge tool (milling cutter, milling head, shank cutter, etc.). In such processes the nominal thickness of the chips changes from the minimum to maximum value at conventional milling, or vice versa, from maximum to minimum at climb milling (Fig. 1). The milling process changes the width or shape of a milled wood material. The vector of feed rate is in the opposite direction of the circumferential speed vector of the rotating cutting tool during conventional milling. Speed vectors have the same directions at climb milling (Horáček 1998; Lisičan 2007; Barcík et al. 2014; Koleda et al. 2014, 2016).
Fig. 1. Milling methods: (a) conventional milling and (b) climb milling
The precision in terms of the shape and dimensions of a workpiece is mainly affected by the rigidity of the tool, vibrations of the entire feed and cutting mechanism, and accuracy of the blade adjustment (Siklienka and Kminiak 2013). The precision of surface roughness is a result of repetitive cutting edge activity on the workpiece surface, it depends mainly on cutting speed, depth of cut, feed rate and geometry of cutting tool. The precision of surface roughness of the milled parts requires increased attention for the following surface technological treatment. Suitable cutting conditions can achieve an improved surface quality during woodworking (Prokeš 1982; Kačíková and Kačík 2011, Budakçı et al. 2013) and optimization of energy consumption (Kubš et al. 2017; Koleda et al. 2018).
The aim of the study was to determine the influence of the heat treatment temperatures of European oak and the subsequent plane milling parameters on the quality of milled surface. This research is part of a study of the properties of woodworking thermally modified wood that is focused on measuring the quality and energy of the machining process.
EXPERIMENTAL
Materials
Samples of Quercus robur wood with an average age of 96 years from Vlčí jarok (Budča, Slovakia) were used in the experimental tests. The samples were made via ThermoWood® technology at the Volga State University of Technology in Yoshkar-Ola, Russia. The mechanical woodworking of samples with the dimensions 500 mm × 110 mm × 20 mm and their subsequent heat treatment at temperatures of 160 °C, 180 °C, 210 °C, and 240 °C were performed using the technologies described by Koleda et al. (2018). Similarly, the density measurements and cutting conditions were as in Koleda et al. (2018). Milling was performed at the Technical University in Zvolen, Slovakia on a lower spindle milling machine FVS (Czechoslovakia Music Instruments, Hradec Králové, Czech Republic) and feeding mechanism ZMD 252/137 (Frommia, Fellbach, Germany). Table 1 shows the technical parameters of the milling machine employed for the study.
Table 1. Technical Parameters of the Lower Spindle Milling Machine FVS and of the Feeder
A double-blade wood cutter block with a rake angle (γ) of 15°, 20°, and 30° and interchangeable blades was used for milling (Fig. 2) to a cutting depth of 1 mm. The cutting tool geometry and as well as the cutting speed (20 m/s, 40 m/s, and 60 m/s) and feed rate (6 m/min, 10 m/min, and 15 m/min) were the same as those used by Koleda et al. (2018).
Fig. 2. Milling cutters with rake angles of 15° (1), 20° (2), and 30° (3)
Surface Roughness Measurement
The surface roughness was measured using an LPM-4 laser profilometer (KVANT Ltd., Slovak Republic) (Fig. 3) with the basic parameters in Table 2.
Fig. 3. Laser profilometer (LPM-4) used for the experimental measurements of Ra
The measured values of arithmetic average of the roughness (Ra) were processed and displayed on a computer using the LPMView 1.2 measuring software (KVANT Ltd., Slovak Republic). Subsequently, the data was processed with STATISTICA 10 software (StatSoft CR s.r.o., Prague, Czech Republic). Using this program, the resulting graphs (arithmetic means and variance) and dependencies were generated via a single-factor and multifactor analysis of variance (ANOVA).
Table 2. Basic Parameters of the Profilometer
RESULTS AND DISCUSSION
Influence of the Thermal Treatment
Figure 4 shows that the roughness of the surface after the milling of heat treated wood varied considerably. The highest surface roughness value was measured on the sample treated at 240 °C, whereas the highest surface quality was found with the sample treated at 160 °C. It was therefore evident that the surface roughness increased with an increasing thermal treatment temperature. It was also evident that the best quality was achieved with thermal treatment at 160 °C and 180 °C, compared with the values of the native wood.
Fig. 4. Effect of the thermal treatment on the material surface roughness
Influence of the Feed Rate
Multifactor ANOVA results for the effect of feed rate on the surface roughness after milling are shown in Fig. 5. The surface roughness tended to increase with an increasing feed rate. The greatest surface roughness occurred on native wood at a feed rate of 15 m/min. In contrast, the highest wood surface quality was achieved on wood that was heat treated at 160 °C and milled at a feed rate of 10 m/min.
Fig. 5. ANOVA for the dependence of surface roughness on the feed rate
Influence of the Cutting Speed
Multifactor ANOVA results for the dependence of surface roughness on the cutting speed are shown in Fig. 6. It was evident that the highest surface quality was achieved on a sample heat-treated at 160 °C. The surface roughness values were almost identical at 40 m/s and 60 m/s cutting speeds. The highest surface roughness value was obtained on a sample heat treated at 210 °C at a cutting speed of 60 m/s.