Influence of thermal modification of ash wood (fraxinus excelsior l.) And machining parameters in CNC face milling on surface roughness using response surface methodology (RSM)

Abstract

The objective of this research was to analyse the effect of thermal modification of ash wood (Fraxinus excelsior L.) at moderate temperature of 160oC and three processing parameters: spindle speed, feed rate and depth of cut in CNC face milling operation on surface quality, expressed by arithmetic surface roughness parameter (Ra). In order to determine material properties, moisture content (MC), density, swelling, anti-swelling efficiency (ASE) and contact angle for both untreated and thermo-treated ash wood have been measured. Highly effective, incomplete 33 Box-Behnken factorial design was made, with three levels of cutting speed: 8.000, 12.000, and 16.000 rpm; three levels of feed rate: 1.000, 1.500 and 2.000 mm/min; and three levels of depth of cut: 2, 4, and 6 mm. According to the above design matrix, all groups of 50x50x30mm samples have been machined with two machining strategies: raster and offset. Surface roughness parameter Ra was measured per each run. Response - surface analysis (RSM) was applied to the parameter Ra for all sets of samples. The 3-D response surface plots, polynomial equations and ANOVA tables have been obtained per each observed input variable, for both machining strategies (raster and offset). The results indicated that the thermal modification of ash wood at 160 oC improved it`s physical properties: decreased MC, improved wood density, improved ASE and increased wood hydrophobicity. Polynomial equations and ANOVA tables showed different behaviour of untreated and treated ash wood regarding changing of machining parameters in experimental space. Offset processing strategy, gave better results in the quality of wood surface, than raster processing strategy for all types of samples. Thermal modification of ash wood at 160oC improved surface quality after machining for both processing strategies.