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Koleda, P., Barcík, Š., and Nociarová, A. (2018). "Effect of technological parameters of machining on energy efficiency in face milling of heat-treated oak wood," BioRes. 13(3), 6133-6146.

Abstract

This paper examines the influence of technological parameters on electrical power in the plane milling of native and modified oak wood. Milling was performed under various cutting conditions, including cutting speeds of 20, 40, and 60 m s-1, feed rates of 6, 10, and 15 m min-1, and cutting edge angles of 15, 20, and 30° on five different samples of oak wood. The wood was native and heat-treated at temperatures of 160, 180, 210, and 240 °C. An analysis of variance and post-hoc Duncan test revealed the influence of the examined parameters on the energy consumption of milling, whereby the cutting speed was the most statistically significant parameter and was directly dependent on the speed of the asynchronous motor and the moment transmission to the miller spindle.


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Effect of Technological Parameters of Machining on Energy Efficiency in Face Milling of Heat-Treated Oak Wood

Peter Koleda,* Štefan Barcík, and Adriana Nociarová

This paper examines the influence of technological parameters on electrical power in the plane milling of native and modified oak wood. Milling was performed under various cutting conditions, including cutting speeds of 20, 40, and 60 m s-1, feed rates of 6, 10, and 15 m min-1, and cutting edge angles of 15, 20, and 30° on five different samples of oak wood. The wood was native and heat-treated at temperatures of 160, 180, 210, and 240 °C. An analysis of variance and post-hoc Duncan test revealed the influence of the examined parameters on the energy consumption of milling, whereby the cutting speed was the most statistically significant parameter and was directly dependent on the speed of the asynchronous motor and the moment transmission to the miller spindle.

Keywords: ThermoWood; Milling; Technological parameters; Input power

Contact information: Department of Manufacturing and Automation Technology, Faculty of Environmental and Manufacturing Technology, Technical University in Zvolen, Studentska 26, 96053 Zvolen, Slovakia;

* Corresponding author: peter.koleda@tuzvo.sk

INTRODUCTION

Wood has a very wide range of uses, especially in construction, furniture, paper, and transportation. A very important feature of wood is its natural durability in various exterior and more demanding indoor exposures. The properties of wood materials can be changed and improved by thermal and hydrothermal wood treatment at high temperatures of 150 to 260 °C (Boonstra et al. 2007). The high temperatures degrade wood’s structural polymers to form new water-insoluble substances and substances with a toxic or repellent effect against biological pests of wood. Strength and some mechanical properties decrease in heat-treated wood due to the decrease in density (Vančo et al. 2017a), as well as the disruption of hemicellulose and increased hydrophobicity of the surface (Barcík and Homola 2004; Niemz et al. 2010). The mechanical properties are significantly reduced if the heat treatment of the wood is carried out in an inert environment without access of oxygen, such as in a vacuum, nitrogen, or oil (Reinprecht 2008; Safin et al. 2015).

At temperatures above 150 to 170 °C, in addition to plasticizing processes, the chemical structure of the treated wood begins to change significantly. Hydrophilic functional groups begin to disappear in structures of the polysaccharides, lignin, and accompanying materials. Depolymerization and condensation reactions are carried out in conjunction with partial carbonization of the wood and the release of flammable gases. Due to the aforementioned changes in the heat-treated wood, the wood becomes more resistant to biological pests and its hygroscopicity decreases (Zobel and Sprangue 1998; Bengtsoon et al. 2003; Reinprecht 2008).

Working heat-treated wood, either by machine or manually, has its disadvantages compared to ordinary wood. When machining heat-treated wood, the blade must be well sharpened; because cutting surfaces are smoother, the cutting force is reduced. The problem with machining heat-treated wood is the formation of fine dust that pollutes the work environment. The dust can also cause health problems for service personnel. Therefore, it is necessary to capture the resulting fraction during machining using special suction hoods to avoid inhalation of this dust. Another unpleasant fact about machining heat-treated wood is the specific odor generated by the release of the aromatic compounds (Reinprecht and Vilholdová 2008). The course of the blade in milling is a cycloid because the cutting speed is much higher than the feed rate. The cutting path is a circle (Prokeš 1982; Lisičan 1996). In practice, it is very important that the entire woodworking process proceeds with the smallest energy demand, while attaining the desired properties and quality of the machined surface (Thiede et al. 2012). Several factors influence the power demand of machinery, such as the selection of the appropriate cutting tool material, the geometry of the cutting tool, optimal cutting conditions (cutting speed, feed rate, tooth movement), and the cutting power.

Cutting input and output power are the basic criteria for the evaluation of woodworking machines. The energy demand of the cutting process is most frequently observed by means of cutting power (Barcík and Rehák 2009). Cutting power is the power that is required to allow the tool blades to cut off chips. It is the result of the scalar component of the force vector and the cutting speed vector, which is shown in Eq. 1,

 (1)

where Pc is cutting power (W), Fc is vector of force (N), and vc is cutting speed vector (m s-1).

With the known milling technology parameters, the cutting power can be determined as shown in Eq. 2 (Olteanu et al. 2013),

 (2)

where ap is the cut depth (mm), ae is the cut width (mm), vf is feed rate (mm min-1), and kc is specific cutting force (N mm-2).

The input power of the motor Pp is the product of the voltage, current, and power factor. Power is an important parameter necessary to determine energy costs and to determine the load of electric power cables. Input power is calculated in Eq. 3,

 (3)

where U is electric voltage (V), I is electric current (A), and cos φ is power factor (-).

If the machine is connected in a three-phase system, the power input of the electric motor is calculated from Eq. 4, which shows it as the sum of inputs per phase (Barcík 2009).

 (4)

In Eq. 4, U1,2,3 are phase electric voltages (V), I1,2,3 are electric currents (A), and cos φ1,2,3 are power factors (-).

The goal of this experiment was to determine the dependence of total input power for the milling of selected wood species on the temperature of the heat treatment of wood.

EXPERIMENTAL

Samples of 96-year-old Quercus robur from Vlčí jarok (Budča, Slovak Republic) were used in the experimental tests (Koleda et al. 2017).

Lumber measuring 25 mm thick was cut from logs with a diameter of 350 to 400 mm using a band saw. Subsequently, the wood was dried to a moisture of 10%. The dried wood was cut to obtain a lateral tangential lumber with a 110 mm width. By aligning on the grinding and pull milling cutters, the thickness was adjusted to 20 mm. Cuts were made using a 500-mm circular saw, followed by heat treatment in a high temperature furnace at Forestry Faculty in Russia. Two cuts were left natural, and the other eight were heat-treated to the appropriate temperature (160 °C, 180 °C, 210 °C, and 240 °C). The heat treatment process is illustrated in Fig. 1. The time intervals of the individual phases and wood density are shown in Table 1 (Vančo et al. 2017b). The heat-treated samples had a humidity of 3 to 6%, which was measured with the Wagner L6006 humidifier (Kazan, Russia), prior to experimental measurement (Barcík et al. 2014).

Fig. 1. Phases of thermal modification

Table 1. Times of Thermal Modification and Wood Density

Fig. 2. Equipment for thermal modification – high temperature furnace

Fig. 3. Wood samples before measurement

The high-temperature furnace for the thermal treatment is shown in Fig. 2, and the samples prepared for milling are shown in Fig. 3.

All practical tests were carried out by an experimental device (Figs. 4 and 5), which is in the development workshops of Technical University in Zvolen. The experiment was carried out using the following operations and machinery:

  • planar milling – lower spindle miller FVS
  • feed of material – feed device Frommia
  • current and voltage measurement – Power Analyzer DW6090