Evaluation of the relationship between pilot hole dimensions and screw holding strength in softwood furniture elements
DOI:
https://doi.org/10.22320/s0718221x/2026.30Keywords:
Screw holding strength, pilot hole diameter, pilot hole depth, softwood, grain directionAbstract
The influence of pilot-hole geometry on screw holding strength in solid softwood furniture elements remains insufficiently understood, particularly when wood species, pilot-hole dimensions, and grain direction are considered simultaneously. This study aimed to investigate the effects of pilot-hole diameter and depth on the screw holding strength of selected softwood species commonly used in furniture manufacturing and to identify the best-performing pilot-hole configurations among the tested levels for different grain directions. The materials examined were Larix decidua Mill. (larch), Pinus sylvestris L. (Scotch pine), Abies sp. (fir), Picea orientalis L. (spruce) and Cedrus libani A.Rich (cedar). A total of 900 specimens were prepared, comprising 5 wood species, 9 pilot hole configurations (3 diameter by 3 depth), and 10 replications for each condition. Screw withdrawal tests were conducted under static tensile loading, with 450 specimens tested parallel to the grain and 450 perpendicular to the grain. The results indicated that, for the withdrawal tests parallel to the grain, the highest mean screw holding strength was obtained with a pilot-hole diameter corresponding to 50 % of the screw diameter, while pilot-hole depth had no significant main effect. In case of the withdrawal tests perpendicular to the grain, highest mean screw holding strength was obtained with a pilot-hole diameter corresponding to 75 % of the screw diameter, whereas pilot-hole depth again showed no significant main effect. Wood species and pilot-hole diameter significantly affected screw holding strength, and all two-way and three-way interactions were significant. Cedar exhibited the highest mean screw holding strength in both loading orientations. These findings highlight the importance of considering pilot-hole diameter in relation to wood species and grain direction for screw connections in solid softwoods.
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