WEAR BEHAVIOR OF DRILL BITS IN WOOD DRILLING RESISTANCE MEASUREMENTS

摘要

The objectives of this study were to investigate the wear behavior of drill bits in wood drilling resistance measurements and to understand how the blunting of the cutting edges may affect the cutting forces and ultimately the measurement results. Laboratory resistance drilling experiments were conducted using an IML-RESI PD 400 tool (IML Instrumenta Mechanik Labor GmbH, Wiesloch, Germany) and a standard spade-type drill bit. Results were based on 375 drillings made on a 2.58 m long, freshly cut, defect-free yellow birch (Betula alleghaniensis) log with an average MC of 55.5%, an average density of 710 kg/m(3), and a total cutting path length (CPL) of 5011 m. With the use of the photographic facilities of the microscope, wear and blunting parameters such as clearance and rake face wear, cutting edge rounding, wear along the bisecting line of the wedge (sharpness) angle, residual microclearance angle, wear area, and drill bit diameter were measured and calculated for initial condition of the drill bit and the conditions at incremental cutting path lengths. The initial geometry parameters of the cutting head of the drill bit had a big impact on tool wear and blunting, which affected the precision of wood density evaluation. Intensive blunting and wear of the cutting edges occurred on the clearance faces and increased proportionally with the total cutting path length. Rounding of the cutting edges and drilling resistance (torque) were relatively constant within the experimental conditions, indicating that resistance drilling measurement in wood was still accurate as the total CPL reached 5011 m (or 375 drillings). Feeding force was found to be affected by the blunting of the cutting tool and may be used to predict the service life of a drill bit.