Performance of multi-margin coated tools in one-shot drilling of metallic-composite stack materials under varying feed rate and pecking conditions

The paper details an investigation into the influence of operating conditions and drill margin design on tool wear, hole accuracy and chip morphology when single-shot drilling Ti-6Al-4V/CFRP/Al7050 stack materials (6.35mm diameter holes). A full factorial experiment involving 3 factors (feed rate, margin design and pecking strategy), each at 2 levels (total of 8 tests) was undertaken. Statistical analysis techniques incorporating analysis of variance (ANOVA) and main effects plots were employed to identify the significance of variable parameters on response measures. All holes were found to be undersized when utilising triple margin drills irrespective of feed rate or peck cycle, with average diameters typically ~20μm smaller compared to those produced using the single margin design. The use of a pecking strategy together with triple margin drills however resulted in superior hole cylindricity, although none of the variables were statistically significant at the 5% level. In contrast, margin design had a substantial impact on minimising hole out of roundness, especially in the Ti and CFRP sections with corresponding percentage contribution ratios (PCR’s) of ~96% and 70% respectively. Lower wear rates were generally observed when drilling at the higher feed rate level regardless of pecking strategy or margin design. An analysis of the swarf revealed that packing and folding of the Ti and Al chips were prominent, particularly when operating with the triple margin drills due to a smaller curvature in the flutes, which impeded chip evacuation.