MECHANISMS OF SURFACE IRREGULARITY FORMATION IN CNC MILLING OF SOLID WOOD

Abstract

Achieving high surface quality in CNC milling of solid wood remains a critical challenge due to the complex interaction between machining parameters and the heterogeneous structure of wood. This study investigates the causes of surface irregularities formed during CNC milling of three representative wood species – spruce (Picea abies L.), beech (Fagus sylvatica L.), and oak (Quercus robur L.). The primary objective was to analyse the influence of technological parameters, particularly feed speed and rotational speed, on surface roughness and waviness, and to relate these findings to the microscopic analysis. Samples were milled under controlled conditions using a 5-axis CNC machining centre, with feed speeds ranging from 10 m·min⁻¹ to 18 m·min⁻¹ and rotational speeds from 16,000 rpm to 20,000 rpm. Surface quality and micro-scopic analysis were evaluated using a Keyence VHX-7000 digital microscope. Roughness pa-rameters (Ra, Rz, Rp, Rv, and Rk) and waviness parameters (Wa and Wz) were analysed along-side microscopic observations of defects such as torn fibres, fuzziness, and tool marks. The results demonstrated that feed speed had a dominant effect on surface quality, with increasing feed speed leading to higher roughness values. Conversely, higher rotational speeds contributed to a reduction in surface irregularities. The optimal combination of parameters was identified at 20,000 rpm and 10 m·min⁻¹, where the lowest roughness and minimal defects were observed. The results showed that the effects of the technological parameters are mediated by feed per tooth. Additionally, the study confirmed that wood species and anatomical structure significantly influence surface formation, with denser wood species generally producing smoother surfaces. The findings provide a comprehensive understanding of the mechanisms behind sur-face irregularities and contribute to the optimization of CNC milling parameters for improved surface quality in wood machining.