Resistance drilling: the effect of needle wear

Semi-non-destructive drilling resistance instruments have been widely adopted for timber testing and forestry for wood quality assessments. The accuracy of a drilling resistance tool for predicting wood density may be affected by needle (drill) wear, and there is limited information regarding the wear behaviour of drill bits in wood resistance drilling. It is unknown when changes in needle sharpness and diameter become critical for quantifying wood density. To measure the effect of Resi needle wear, an IML Resi PD 500 was used to obtain cross-sectional traces of Southern Pine (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Sénéclauze)) average outerwood density: 519 kg/m³ and Spotted Gum (Corymbia citriodora subsp. variegata (F. Muell.)) average outerwood density: 779 kg/m³ logs. Southern Pine was tested with feed speeds of 200 cm/min at 3500 rpm and 100 cm/min at 5000 rpm. Spotted Gum was tested with a feed speed of 70 cm/min at 5000 rpm. A new needle was used for each setting. The effect of needle wear was tested after 0, 100, 200, 300, 400, 500, and 600 m of drilling depth. There was no effect on drilling resistance values up to 600 m of drilling length with feed speed 200 cm/min and 3500 rpm settings for Southern Pine. This would result in 2000 drilling measurements of 30 cm diameter trees. With alternative setting for Southern Pine with a setting of 100 cm/min feed speed and 5000 rpm, the increase in amplitudes was not significant until 300 m of drilling, after which, at 400 m, the amplitude increased by 2% (9.8 kg/m³), and after 600 m, the amplitude increased by 3.1% (15.2 kg/m³). For the hardwood Spotted Gum, the amplitude decreased by 0.9% (2.3 kg/m³) after 100 m, and after 300 m, the amplitude decreased by 3.3% (8.2 kg/m³). After 300 m, it was not possible to drill Spotted Gum due to extensive needle wear. The study highlighted the importance of needle wear in resistance drilling, providing guidance for reliable needle use to improve measurement accuracy and wood quality assessment.