Лесотехнически Университет

Резюме

Equivalent continuous sound pressure level L_Aeq is the main variable characterizing the physiological effects of noise on humans. The technological equipment for woodworking and furniture industry includes the woodworking milling machines, which at the same time are characterized with high levels of noise emissions.

The current study investigates the changes in equivalent sound pressure level measured at workplace during milling of details from common beech wood (Fagus Sylvatika L). The influence of the construction and rotational speed of the cutting tool have been studied.

It was found that depending on the cutting speed and the construction characteristics of the cutting tool, the equivalent sound pressure level L_Aeq for eight hour working day varies from 74 dB(A) tо 89 dB(A).

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Experimental studies during milling of solid wood were carried out. The current study was performed at the Laboratory of Woodworking Machinery, University of Forestry, Sofia. The measurements were carried out using universal milling machine FD – 3 (ZDM – Plovdiv). The correlations between fundamental factors influencing the milling process and target functions, such as cutting force and power, specific cutting work, specific power consumption have been investigated. On the basis of the analysis of the obtained results, practical recommendations have been proposed.

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In this paper, the absorption properties of wood materials from Scots pine (Pinus Silvestrys L.), which are widely used in flooring and wall linings, have been experimentally studied. The frequency-dependent sound absorption coefficient (αp) was determined in octagonal frequency bands with an average geometric frequency of 100 Hz to 2000 Hz. The tests were carried out using an impedance tube, according to the requirements of BDS EN ISO 10534-1:2006. The obtained results are of a particular interest from both scientific and practical point of view, given that the sound absorption coefficient has a decisive impact on the reverberation time. The latter is the main acoustic parameter of a room depending on its intended purpose.

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The current study investigates the changes in the surface quality of experimental samples of Scots pine (Pinus sylvestris L.) wood during a milling process, performed with different rotation speed of the cutting tool (n), feed rates (U) and different thicknesses of the removed layer (h). On the basis of the performed experiments, graphical dependencies, representing the relationship between the different factors have been derived. In order to achieve a higher quality of the milled (processed) surfaces, practical recommendations for the optimal values of the evaluated factors have been suggested. The surface roughness of the material (surface) was measured with a roughness tester, type „SurftestSJ-210“ (Mitutoyo, Japan).

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This study presents the results of conducted investigations of the forced spatial vibrations of a wood shaper, caused by the wear of the cutting tool. The paper is based on a specific mechanical – mathematical model, developed by the authors, which allows studying of vibrations of this type of machinery. In this model the wood shaper is regarded as a system of three rigid bodies, which are connected by elastic and damping elements with each other and with the motionless floor. This study renders an account the mass, inertia, elastic and damping properties and geometric parameters of the machine. A necessary system of matrix differential equations is compiled and analytical solutions are presented. The results of the numerical investigations are presented. They are obtained through modern software and by using parameters of a particular machine. The results show that the amplitudes of the body’s vibrations are relatively small but the amplitudes of the spindle’s vibrations are significant especially at resonance zones.

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An approach for computing and research of the duration of regimes for autoclave steaming of non-frozen wooden prisms at limited heat power of the steam generator in cases of dispatching interventions into the regimes in order to obtain a duration that is suitable for the subsequent cutting of the veneer from the heated plasticized wood has been suggested. Using own 2D unsteady model implemented in the approach, calculations of the change of the duration of regimes for plasticizing of beech prisms in an autoclave have been carried out. The variables used during the computer simulation were, as follows: initial wood temperature of 0 °C, wood moisture content of 0.6 kg·kg⁻¹, and cross-section dimensions of the prisms 0.4 × 0.4 m; steaming autoclave with a diameter of 2. 4 m, length of 9.0 m, and loading level of 50% for the cases of lowering by the dispatcher the maximal temperature of the basic steaming regime from 130 ^oC to 120, 110, and 100 ^oC in the 3^rd, 5^th, and 7^th hour of this regime.

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Studies on the durability of tungsten carbide (TC) grooving and rebate cutter head and assessment of the quality of its sharpening have been conducted. The linear wear of the blade and the blunting radius when processing of massive coniferous wood and chipboard were studied. Recommendations were made regarding the preliminary preparation of the cutter heads and their effective operation.

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An approach for computing the average moisture content of the whole quantity of wood materials in given batch subjected to steaming in an autoclave has been suggested. This value of the wood moisture content is needed for calculation and automatic realization of optimal energy saving regime for autoclave steaming of the materials. The approach is based on two own 2D mathematical models: one of the temperature distribution in non-frozen prismatic wood materials during their steaming and other – of the heat balance of the autoclave. The calculated by the models increase in the steaming medium temperature in the beginning of basic regime is compared with the real increase of that temperature, which is measured with a sensor in the automatic control system. After proper processing of the obtained differences between measured and calculated temperatures with the software package Table Curve, an equation for calculating the average moisture content of the wood materials in the batch loaded in an autoclave is derived. The application of the suggested approach for the cases of autoclave steaming of beech prisms in the production of veneer is presented.

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With the help of our own non-stationary model, the defrosting times of frozed beech logs with a diameter of 0.4 m, initial temperature of −10^oC, −20^oC, −30^oC, and moisture content of 0.6 kg∙kg^-1 were determined at water temperatures in the boiling pit equal to 80^oC. Using the determined logs’ defrosting durations, the change in energy required for the entire plasticization process and that for each of the 5 components of the pit’s thermal balance was calculated. Computer simulations were performed for a well-insulated concrete pit with working volume of 20 m³ and degree of filling with logs f equal to 25%, 50%, and 75%. It was found that at f = 75% the total energy consumption of the pit increases from 163.6 kWh.m^-3 to 177.6 kWh∙m^-3 when the initial temperature of the logs decreases from −10^oC to −30^oC. The thermal efficiency of the studied pit decreases almost proportional to the decreasing of f, mainly due to the increase in the specific energy required to heat the water in the pit.

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The current study aims to assess the level of noise emission generated by the CNC milling machine Rover A.3.30 (Biesse, Italy). The change of the А-weighted sound pressure level (L_pA) measured at the operator's position at idle mode and during milling operations was investigated in relation to cutting tool rotation speed (n) and radial depth of cut (h) when processing particle boards, medium density fiberboards (MDF) and plywood. Sound pressure levels were determined using the precision digital noise meter CEL-620B1 (CASELLA, UK), taking into account the correction coefficients, accounting for the influence of background noises and sound field characteristics. The results show that at idle mode, without movement of the units along the x and y axes, the noise emission level was L_pA = 72,6 dB(A), which is significantly below the upper safe limit of 85 dB(A) for eight hours of exposure. It was found that the A-weighted sound pressure level during cutting mode changed from 79,5 dB(A) to 85,6 dB(A), depending on the characteristics of the cutting process. The lowest level was registered when processing particle boards, followed by that of processing MDF and the highest level was measured for plywood. On the basis of the performed experiments, graphical dependencies representing the relationship between the different factors were compiled.

Резюме

With the help of our own non-stationary model, the temperature field and defrosting durations of frozen beech logs with a diameter of 0.4 m, initial temperature of −10 ^oC, and moisture content of 0.4 kg∙kg^-1, 0.6 kg∙kg^-1, and 0.8 kg∙kg^-1 were determined at water temperatures in the boiling pit equal to 80 ^oC. Using the determined durations, the change in energy required for carrying out of the entire defrosting process and that for each of the 5 components of the pit's thermal balance was calculated. Computer simulations were performed for a well-insulated concrete pit with working volume of 20 m³ and degree of filling with logs f equal to 25%, 50%, and 75%. It was found that at maximum possible value f = 75% the total energy consumption of the pit increases from 145.1 kWh.m^-3 to 180.2 kWh∙m^-3, i.e. by 24.2% when the moisture content of the frozen logs increases from 0.4 kg∙kg^-1 to 0.8 kg∙kg^-1. With a decrease in f, the thermal efficiency of the pit decreases almost proportional to f, mainly due to the increase in the specific energy required to heat the water in the pit.

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A mathematical description of the energy Q_ice-bw required for melting the temperature dependent part of frozen bound water in wood, using a software package Table Curve 2D v.5.01 has been presented. This package allows for the selection of equations, which provide the best similarity between the calculated with them values of Q_ice-bw during thermal treatment of wood containing ice and the respective numerical data obtained with an adequate temperature-energy model of Q_ice-bw. The type and coefficient values of the Table Curve equations for Q_ice-bw in this work have been determined for the case of thermal treatment of frozen beech wood with an initial temperature of −1 ^oC, −10 ^oC, −20 ^oC, −30 ^oC, −40 ^oC and moisture content of 0.4 kg·kg^-1, 0.6 kg·kg^-1, and 0.8 kg·kg^-1. The equations obtained for Q_ice-bw can be used for development and automatic implementation of scientifically based energy-efficient regimes for steaming or boiling of frozen wood materials with different parameters.

Резюме

In this paper, the surface quality of scots pine (Pinus sylvestris L.) wood specimens, processed with a CNC woodworking drilling and milling machine with a negative face mill helix, has been investigated. The changes in the surface roughness, depending on the changes in main factors determining the cutting process, such as cutting tool rotation speed (n); feed speed (Vf) and radial depth of cut (h), have been evaluated. The quality of the processed surfaces was evaluated by the roughness parameter Rz, measured with a portable roughness tester, model Surftest SJ-210 (Mitutoyo, Japan).  
Based on the experiments performed, graphical dependencies, presenting the influence of the individual factors on the quality of the processed surface, were derived. A regression model was derived that could be used for quantitative prediction of the surface roughness of Scots pine wood specimens when processed with a CNC machine, depending on the changes of the variable factors (n, Vf, h).