Wear behavior of special tools in the drilling of CFRP composite laminates

Abstract

Mechanical drilling of carbon fiber reinforced polymers (CFRPs) has been notoriously difficult due to the high abrasiveness of reinforcing fibers and the anisotropic behavior of the constituents leading to poor hole quality and rapid tool wear. Special drills featured by novel geometrical designs have been proved as a promising alternative to conventional twist drills for the machining improvement of these fibrous composites due to their capability to suppress delamination formation and ensure superior surface quality. The present work aims to address the wear characteristics of two types of special tools including a candlestick drill and a step drill when machining CFRPs. The novelty of the current work lies in identifying the underlying mechanisms controlling the wear development of the special tools and in quantifying their wear effects on the composite machining responses such as delamination extents, hole wall morphologies, and dimensional accuracy. Progressive abrasion wear is confirmed as the dominant wear pattern for both drills, while premature microchipping occurs for the step drill even after the tool makes only few composite holes. For both tools, the drill edge segments residing close to the tool periphery basically undergo the most severe abrasion wear due to the highest instantaneous cutting speed involved. The results discussed in this paper highlight the effectiveness of the functionally-designed tool geometries in improving the wear resistance of drill bits and emphasize the vital role of the tool wear in affecting the composite machinability.