EFFECT OF DRILL WEAR ON CUTTING FORCES IN THE DRILLING OF METAL-MATRIX COMPOSITES

Holes were drilled in 6061 T-6 aluminum alloy as well as in a particle reinforced metal matrix composite (MMCp) consisting of 20 vol.% SiC particles, 12 mu m in diameter, in a 6061 aluminum matrix (6061/SiC/20p, or Duralcan(R) F3S.20S). HSS (high speed steel) drills of 10 mm diameter were used, and measurements were made of thrust (normal force), torque and flank wear for several feed rates and drill speeds. It was found that when drilling Duralcan(R) with unworn drills, bath torque and thrust varied with feed rate raised to the power 0.81, as for classical materials. When flank wear V-b became significant, torque varied linearly with V-b and with f(0.8) but no empirical relation with physically meaningful parameters was found to fit the thrust data. Speed had no significant effect on wear or on drilling forces. Flank wear proceeded linearly with depth of material drilled, or with the total distance passed by the lip or cutting edge of the drill. A linear relation between both thrust and torque against flank wear was observed, so that either thrust or torque may be measured to give an indication of wear of the drill. The linear relation between torque and wear of the drill implies a linear variation of specific cutting energy with flank wear. Drilling forces are controlled by the matrix material and not by the particles.