Igor Đukić

The article examines the main and interactive effects of thermal modification (TM), relative humidity (RH), and direction of vibration on the density, modulus of elasticity (MOE) and shear modulus of spruce wood. Samples were thermally modified at 180, 200, and 230 °C and then equilibrated at 20%, 44%, 76%, and 88% RH. The MOE in the longitudinal direction and the shear moduli GLR and GLT were calculated from the natural frequencies of the first three lateral vibration modes of free-free specimen. EL-R and GLR were determined from the vibration frequencies when the specimens vibrated laterally in the longitudinal-radial plane, while EL-T and GLT were determined from vibration frequencies when the specimens vibrated in longitudinal-tangential plane. Density, MOE, and shear moduli decreased at the highest TM level by averages of 16%, 9.8% and 9.7%, respectively. Acoustic coefficients such as the sound velocity (c), the sound radiation coefficient (R) and the characteristic acoustic impedance (ACE) were determined. On average, c and R increased as a function of RH from 1% to 3% and from 10% to 15%, respectively, while ACE decreased from 7% to 13%. The test material was classified for making soundboards for musical instruments.

The article presents the method for an objective determination of threshold value, needed for filtering out the anatomical roughness signal, with filter based on fast Fourier Transform (FFT), from surface roughness profile after machining. The method...

The increasing use of modified wood justifies the intention to use it in joints made by rotary welding with dowels. Thermal modification, for example, increases the dimensional stability of wood by reducing its hygroscopicity and water absorption, but it is difficult to glue or to rotary weld due to the appearance of cracks. This can be avoided by changing the optimal welding parameters, which on the other hand directly affects the reduction of the pull-out force by more than 25 % depending on the type of the modified base. In the case of welding wooden dowels into wood modified with citric acid, the reduction in pull-out force is even greater. Due to the significant reduction, the use of this type of modification for wood welding is questionable. When citric acid modification is extended to the dowels, the dowel becomes a problem due to its lower tensile strength, which is even lower than the pull-out force of the welded dowel.

The article presents an analysis of the influence of simulated circular saw teeth marks profile in relation to the anatomical roughness of solid oak wood (Quercus robur L.) on machining roughness after sawing along the grain with a circular saw in order to better understand this relationship and to improve the knowledge how this two factors can be separated for further analysis in real-world sawing and to try to quantify the minimum theoretical tooth mark height that could be distinguished from anatomical roughness of wood. The signal-to-noise ratio (SNR) of simulated circular saw teeth mark signal in relation to measured anatomical roughness signal of solid oak wood was used to determine the level at which saw teeth marks could have a meaningful impact on machined surface roughness. From the analysis it can be concluded that SNR values even as low as −13 dB can have enough information to be used for frequency analysis and filtering out of tooth marks signal. Averaging and longer sampling lengths provide much better SNR values and provide the frequency spectrum of the structural roughness signal of solid oak wood with the highest amplitude of less than 0.5 μm. If more favorable SNR values are to be obtained, sampling lengths should be much longer than what is standardly used and recommended in ISO 3274: 1996 (in this research cumulative sampling length of 600 mm was used).

The article presents theoretical analysis of machined surface roughness after sawing on circular saw and implementation of fast Fourier transform (FFT) as a possible simple filtering method for filtering out just the saw blade and saw tooth influence on the surface roughness. Surface roughness profile is represented as a signal that can be obtained as a sum of complex periodic signals that represent theoretical profile of tooth marks and lateral movement of tooth due to saw lateral movement and signals that represent structural roughness of wood combined with machining roughness, represented as a Gaussian noise. The application of FFT based filtering on such a signal can be effectively used to extract the main frequency components due to tool influence on total surface signal and the time domain of filtered signals display can then be obtained by use of the inverse Fourier transform. In order to test the theoretical assumptions, the machining tests in sawing of solid oak wood (Quercus robur L.) and medium density fiberboard (MDF) was conducted. Machined surface roughness was measured and analyzed in accordance with theoretical assumptions. It was concluded that a combination of discrete Fourier transform of surface roughness profile and standard roughness parameters can give a more complete representation of machined surface roughness after sawing with circular saws and that filtering of surface roughness profile signal with FFT filter can be used as a simple and effective method in quantifying tool influence on machined surface roughness after sawing on circular saw in varying machining conditions and on different workpiece material.