The effect of chisel length and associated pilot hole on delamination when drilling composite materials

Abstract

Drilling-induced delamination often occurs both at the entrance and the exit of the workpiece during drilling of composite material. Investigators have studied analytically and experimentally that delamination in drilling can be correlated to the thrust force of the drill. With a pre-drilled pilot hole, the delamination can be reduced significantly. Early reference reported models of drilling-induced delamination, however, the effect of chisel edge length and pilot hole diameter on delamination is rarely discussed. The optimal range of chisel edge length with respect to drill diameter is derived in this paper.

Introduction

Carbon fiber-reinforced composites are well recognized for their superior mechanical properties and are widely used in aerospace, defense and transportation applications. Composite materials possess peculiar characteristics during machining. The reference of drilling of fiber-reinforced plastics reports that the quality of the machined parts is strongly dependent on drilling parameter [1], [2]. Numerous studies have examined the delamination in drilling [3], [4], [5], [6], [7]. Most of the previous research correlates the drill geometry and feed rate to delamination, which leads to severe reduction in the load-carrying capacity of the composite part. Drilling-induced delamination occurs both at the entrance and the exit planes of the workpiece, it has been correlated with the thrust force during exit of the drill [8], [9], [10]. A rapid increase in feed rate at the end of drilling will cause the cracking around the exit edge of the hole [11]. It was also stated that the larger the feeding load, the more serious the cracking. The drill geometry is also considered the most important factor that affects drill performance [12]. Several non-traditional machining processes, i.e. laser-beam drilling [13], [14], [15], water-jet drilling (with or without abrasives) [16], [17], [18], ultrasonic drilling [19], [20], electrical discharge machining (EDM) [21], have been reported as alternatives. Nevertheless, conventional drilling continues to be widely used for practical purpose. Various drilling tools are available, but the twist drill is by far the most common. The rotation and feeding of the drill bit result in relative motion between the cutting edges and the workpiece to produce chips. The efficiency of the cutting action varies, being the most efficient at the outer diameter of the drill and the least efficient at the center. In fact, the relative velocity at the drill point is zero, without cutting action. Instead, the chisel edge of the drill point pushes aside the material at the center as it penetrates into the hole. Chandrasekharan et al. developed a model to predict the thrust and torque at the different regions of cutting on a drill [22]. Their mechanistic approach exploits the geometry of the process, which is independent of the workpiece material.

Several specialized drills were developed to reduce the delamination. For example, Boeing Aircraft Co. (Seattle) developed a four-fluted spiral rotary carbide milling cutter with a unidirectional helix and a reversed-directional helix [23]. With a pilot hole, delamination can be reduced significantly. Recently, Won and Dharan investigated the effect of the chisel edge on the thrust force, and an innovative process model was developed to predict the advantage of a specimen with pre-drilled pilot hole [24]. A hole is pre-drilled to eliminate the thrust caused by the chisel edge, thus the threat for delamination is significantly reduced. The diameter of the pre-drilled hole is set equal to the length of chisel edge. The smaller diameter of the pilot hole cannot fully cover the chisel edge, while a larger one tends to cause undesired delamination during pre-drilling. Although valuable efforts have been made for the analysis of drilling-induced delamination, little has been reported on the effect of chisel edge length (or pilot hole diameter) on delamination. An optimal range of diameter of pilot hole associated with chisel edge length is derived in this paper.