Effects of machining parameters on raised grain occurring after the application of water-based finishes

The application of water-based finishes to wood surfaces is known to increase surface roughness by raising the grain. This phenomenon is detrimental in the appearance of the finished wood surface. To eliminate it, light denibbing is needed prior to applying the second coat of finish. The objective of this study is to optimize wood machining to minimize raised grain caused by water-based coatings and to avoid, or at least, to minimize the sanding operation between each coat of finish. Experiments were done on beech samples. Complete experimental design was defined to generate several surface qualities from “smooth” to “rough” which are linked to the average thickness of the removed chips. The planing parameters were the cutting mode, cutting direction, feed speed, and rotation speed. To compare with existing practices in joinery industries, specimens were also sanded. Two layers of water based coating were applied. Half of the samples were sanded after the first coat and the other half were not. Following each coat, surface roughness was measured and raised grain was evaluated through tactile tests. Results show that sanding operation is responsible for more raised grain than planing. Whereas for planing, high chip removal thickness gives more raised grain after coating application. The effects of the other parameters on raised grain occurence are not found to be statistically significant. The average roughness (Sa), quadratic mean roughness (Sq), reduced peak height (Spk) and the material volume of the peak section (Vmp) are the roughness parameters best correlated with tactile evaluation of grain raising, but the correlations are weak.