Effects of special drill bits on drilling-induced delamination of composite materials

Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for joining structures. Delamination is among the serious concerns during drilling. Practical experience proves the advantage of using such special drills as saw drill, candle stick drill, core drill and step drill. The experimental investigation described in this paper examines the theoretical predictions of critical thrust force at the onset of delamination, and compares the effects of these different drill bits. The results confirm the analytical findings and are consistent with the industrial experience. Ultrasonic scanning is used to evaluate the extent of drilling-induced delamination. The advantage of these special drills is illustrated mathematically as well as experimentally, that their thrust force is distributed toward the drill periphery instead of being concentrated at the center. The allowable feed rate without causing delamination is also increased. The analysis can be extended to examine the effects of other future innovative drill bits.     

Effect of pilot hole on thrust force by saw drill

The applications of composite materials are numerous, especially in the structural parts of aerospace, automotive and marine industries. Owing to the marked anisotropy and macroscopic heterogeneity of composite materials, the mechanics of machining used is different when compared to metals. Delamination is one of the most concerns of applying the fiber-reinforced composite materials in various industries. A hole is pre-drilled to eliminate the thrust caused by the chisel edge of twist drill; thus, the threat for delamination is significantly reduced. Saw drills eliminate the chisel and utilize the peripheral distribution of the thrust in drilling. An analytical approach to identifying the role of the pilot hole was proposed to reduce the thrust force-induced delamination during saw drilling. The predicted critical thrust force is in good agreement with the experimental values.     

Effect of deviation on delamination by saw drill

Various cutting techniques are available to drill holes, but drilling is the most common way in secondary machining of composite materials owing to the need for structure joining. Twist drills are widely used in the industry to produce holes rapidly and economically. Since the twist drill has a chisel edge, increasing the length of a chisel edge will result in an increase in the thrust force generated. Whereas, a saw drill has no chisel edge; it utilizes the peripheral distribution of the thrust force for drilling. As a result, the saw drill can achieve better a machining quality in drilling composite laminates than twist drill. The deviation of cutting edge that occurs in saw drill would result in an increase of thrust force during drilling, causing delamination damage when drilling composite materials in particular. A comprehensive model concerning delamination induced by the thrust force of a deviation saw drill during drilling composite materials has been established in the present study. For a deviation saw drill, the critical thrust force that triggers delamination increases with increasing β. A lower feed rate has to be used with an increasing deviation saw drill in order to prevent delamination damage. The results agree with real industrial experience. A guide for avoiding the drill deviation during drill regrinding or drill wear has been proved analytically by the proposed model, especially when the deviation ratio (β) affects the critical thrust force. This approach can be extended to examine similar deviation effects of various drills.     

Effect of eccentricity of twist drill and candle stick drill on delamination in drilling composite materials

Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for structure joining. Delamination is one of the serious concerns during drilling. Practical experience shows that an eccentric twist drill or an eccentric candle stick drill can degrade the quality of the fiber reinforced material. Comprehensive delamination models for the delamination induced by an eccentric twist drill and an eccentric candle stick drill in the drilling of composite materials have been constructed in the present study. For an eccentric twist drill and an eccentric candle stick drill, the critical thrust force that will produce delamination decreases with increasing point eccentricity ξ. The results agree with industrial experience. The need for control of drill eccentricity during drill regrinding has been proved analytically by the proposed models.     

Evaluation of drilling parameters on thrust force in drilling carbon fiber reinforced plastic (CFRP) composite laminates using compound core-special drills

Drilling is the mostly used secondary machining of the fiber reinforced composite laminates, while the delamination occurs frequently at the drill exit in the workpiece. In the industrial experiences, core drill shows better drilling quality than twist drill. However, chip removal is a troublesome problem when using the core drill. Conventional compound core-special drills (core-special drills and step-core-special drills) are designed to avoid the chip removal clog in drilling. But the cutting velocity ratio (relative motion) between outer drill and inner drill is null for conventional compound core-special drills. The current study develops a new device and to solve the problems of relative motion and chip removal between the outer and inner drills in drilling CFRP composite laminates. In addition, this study investigates the influence of drilling parameters (cutting velocity ratio, feed rate, stretch, inner drill type and inner drill diameter) on thrust force of compound core-special drills. An innovative device can be consulted in application of compound core-special drill in different industries in the future.     

Reducing drilling-induced delamination in composite tube by magnetic colloid back-up

Drilling is an indispensible machining process for building a load-carrying structure of composite materials. Delamination defect is often produced at the exit of drilling, which threatens the service safety of the structure. There are back-up methods to reduce delamination when drilling the open flat-plate composite structure, but none for drilling into the curved-surface or hollow-shape structures. This study describes an innovative method using electromagnet and the deformable inexpensive colloid mixed with iron powder to produce magnetic back-up force at drilling exit to suppress delamination in industrial tube parts. The delamination extent can be reduced by 60–80%. The optimal volume ratio of powder-to-colloid is found 1:3.     

Comprehensive analysis of delamination in drilling of composite materials with various drill bits

Beside the twist drill, the effects of various drill geometries were rarely discussed in analytical fashion. This study presents a comprehensive analysis of delamination in use of various drill types, such as saw drill, candle stick drill, core drill and step drill. In this analysis, the critical thrust force at the onset of delamination is predicted and compared with the twist drill.     

The effect of pilot hole on delamination when core drill drilling composite materials

Removal of chips is a serious problem when core drill drilling polymer composites. As the chip is formed it moves to the inner hole of core drill. A hole is pre-drilled to eliminate the thrust caused by the removal chip, thus the threat for delamination is significantly reduced. The diameter of the pre-drilled hole is set equal to the inner hole of core drill. A smaller diameter of pilot hole cannot solve the problem of removal chips, 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 pilot hole diameter on delamination for core drills. The design of drill tools can be improved using obtained results.     

Delamination reduction in drilling composite materials by active backup force

Composites have been widely employed in various industries due to their outstanding mechanical properties and corrosion resistance. Drilling is an indispensible operation for building a load-carrying structure. Delamination, however, is among the serious concerns in drilling composite-based components in practice. This paper describes a novel method for the reduction of delamination during drilling of composites by active backup force. The applied backup force contributes to suppression of the growth of the delamination at drilling exit by 60–80%. The proposed novel drilling technique reveals the potential for fabrication of composite components at low cost and minor delamination with high feed rate.     

A neural network thrust force controller to minimize delamination during drilling of graphite-epoxy laminates

Delamination is a well-recognized problem associated with drilling fiber-reinforced composite materials (FRCMs). The most noted problems occur as the drill enters and exits the FRCM. Since drilling is often a final operation during assembly, any defects introduced in parts through the drilling process that result in the part being rejected represent an expensive loss. Studies based on linear-elastic fracture mechanics theory have proposed critical cutting and thrust forces in the various drilling regions that can be used as a guide in preventing crack growth or delamination. Using these critical force curves as a guide, a thrust force controller was developed to minimize the delamination while drilling a graphite-epoxy laminate. A neural network control scheme was implemented which required a neural network identifier to model the drilling dynamics and a neural network controller to learn the relationship between feed rate and the desired thrust force. Experimental results verifying the validity of this control approach as well as the robustness of the design are presented. Visual measurements of the delamination zones were used to quantify the benefits of the thrust force controlled drilling process versus the conventional constant feed rate drilling process.     

Experimental studies of step drills and establishment of empirical equations for the drilling process

Various sizes of step drills were manufactured by a CNC grinder machine and used in the drilling process with different speeds and feed rates to produce single step holes in S1214 free machining steel. The performance of step drills was compared with that of conventional twist drills in the drilling of the free machining steel for the same task. The influences of drill size, feed rate and cutting speed on the performance of step drills were studied. Experimental results show that for better cutting performance, the small diameter should not be less than 60% of the large diameter. Also, most of the changes in the characteristics of the thrust force were influenced by the smaller drill of the step drill. On the other hand, the small diameter part of the step drill only contributed about 30% of the torque. From the experimental results, empirical equations for drilling thrust force and torque have been established for step drills.     

Drilling of carbon composites using a one shot drill bit. Part II: empirical modeling of maximum thrust force

In order to extend tool life and improve quality of hole drilling in carbon composite materials, a better understanding of ‘one shot’ hole drilling is required. This paper describes the development of an empirical model of the maximum thrust force and torque produced during drilling of carbon fiber with a ‘one shot’ drill bit. Shaw's simplified equations are adapted in order to accommodate for tool wear and used to predict maximum thrust force and torque in the drilling of carbon composite with a ‘one shot’ drill bit. The mathematical model is dependent on the number of holes drilled previously, the geometry of the drill bit, the feed used and the thickness of the workpiece. The model presented here is verified by extensive experimental data.     

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

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.     

The effect of vibratory drilling on hole quality in polymeric composites

The anisotropy of fiber-reinforced plastics (FRP) affects the chip formation and thrust force during drilling. Delamination is recognized as one of the major causes of damage during drilling of fiber reinforced plastics, which not only reduces the structural integrity, but also has the potential for long-term performance deterioration. It is difficult to produce good quality holes with high efficiency by conventional drilling method. This research concerning drilling of polymeric composites aims to establish a technology that would ensure minimum defects and longer tool life. Specifically, the authors conceived a new drilling method that imparts a low-frequency, high amplitude vibration to the workpiece in the feed direction during drilling. Using high-speed steel (HSS) drill, a series of vibratory drilling and conventional drilling experiments were conducted on glass fiber-reinforced plastics composites to assess thrust force, flank wear and delamination factor. In addition, the process-status during vibratory drilling was also assessed by monitoring acoustic emission from the workpiece. From the drilling experiments, it was found that vibratory drilling method is a promising machining technique that uses the regeneration effect to produce axial chatter, facilitating chip breaking and reduction in thrust force.     

Mechanical model for predicting thrust and torque in vibration drilling fibre-reinforced composite materials

Delamination is a dramatic problem associated with drilling fibre-reinforced composite materials (FRCMs), which, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration. The key to solving the problem lies in reducing the thrust force of drilling. In this paper, a theoretical analysis for predicting mean values of thrust and torque in vibration drilling FRCMs is presented. The model is based on mechanics of vibration cutting analysis and the continuous distributions of thrust and torque along the lip and the chisel edge of a twist drill. The result of a simulation study has shown a very good agreement between the theoretical predictions and the experimental evidence. On the same cutting conditions, the thrust and the torque by the vibration drilling method are reduced by 20–30 percent, compared with conventional drilling.     

Significantly enhanced drilling ability of the orthopedic drill made of Zr-based bulk metallic glass composite

Zr-based bulk metallic glass composite (BMGC) presents superior unique properties, including high hardness, high fracture strength, high toughness, large elastic limit, excellent corrosion resistance, and was believed as a promising material for medical-tool applications. In this study, the 4 mm diameter rods of ZrCuAlAgSi-based (Zr-based) BMGC containing ex-situ Ta particles were successfully fabricated by two-step arc melting and suction casting method. These Zr-based BMGC rods was ascertained their amorphous nature by X-ray diffraction and differential scanning calorimetry analyses, and then machined into the orthopedic drill bits with 2 mm in diameter. The drilling tests of the commercial and Zr-based BMGC orthopedic drill bits were conducted by a specially designed indentation-drilling rig. The data of thrust force as a function of drilling distance between drill and porcine bone was recorded and analyzed to evaluate the drilling ability of the Zr-based BMGC made and commercial orthopedic drills, respectively. As a result, the Zr-based BMGC made drill presents 73% reduced thrust force than the commercial one, this indicates that the Zr-based BMGC made drill has less friction force and performs much better drilling ability.     

The performance of hydrogenated and non-hydrogenated diamond-like carbon tool coatings during the dry drilling of 319 Al

Aluminium alloys, though widely used in the automotive industry, are difficult to machine, particularly by drilling and tapping without the use of metal removal fluids, because of aluminium's strong tendency to adhere to the cutting tool. Tribological tests have revealed that carbon-based tool coatings, such as diamond-like carbon (DLC), promise an improved performance due to their low friction and adhesion. However, the tribological performance of DLC coatings depends on both their hydrogen content and the testing environments. Hence the experimental approach taken in this study was designed to understand the cutting performance of hydrogenated DLC (H-DLC) and non-hydrogenated DLC (NH-DLC) tool coatings during the dry drilling of a 319 Al (Al–6%Si) alloy. An experimental drilling station was built to measure torque and thrust force changes using a cutting speed of 2500 rpm and a feed rate of 0.25 mm/rev. The cutting performance was assessed by measuring the torques and thrust forces generated during the drilling of the first 150 holes or by drill failure—depending on which occurred first. The results indicated that superior cutting performance was achieved, in both torque and thrust force responses, using DLC-coated drills rather than uncoated high-speed steel (HSS) drills. The uncoated HSS drills failed after drilling only 49 holes as a result of excessive aluminium adhesion. At least 150 holes could be drilled using the DLC-coated drills, and both the torque and thrust forces generated during drilling were lower than those with uncoated HSS drills. In addition, a smaller proportion of holes exhibited abrupt increases in torque (at the end of the drilling cycle) during drilling with the DLC-coated drills. Scanning electron microscopy (SEM) investigations showed that the H-DLC drill flutes displayed minimal aluminium clogging—resulting in lower torque. H-DLC coating also diminished metal transfer and buildup edge formation on the drill's flank face and cutting edge. Thus, torque and thrust force measurements, supported by metallographic data, indicated that H-DLC-coated drills provided better dry drilling performance than NH-DLC.     

A study of an improved cutting mechanism of composite materials using novel design of diamond micro-core drills

Core drilling at small diameters in carbon composite materials is largely carried out using diamond electroplated tools consisting of hollow shafts and simplistic geometries that are likely to work in an abrasional/rubbing mode for material removal. The paper reports a step change in the performance of small diameter core drilling by facilitating a shearing mechanism of the composite workpiece through the utilisation of a novel tool design. This has been achieved by laser producing core drills from solid polycrystalline diamond, incorporating controlled cutting edges where the geometries are defined. To evaluate the efficiency of the shearing vs. abrasion/rubbing cutting mechanisms, a critical comparison between the novel (defined cutting edges) and the conventional electroplated tools (randomly distributed micro-grains) has been made with reference to thrust forces, tool wear mechanisms and their influences on the hole quality (e.g. delamination, fibre pullout). This work has been augmented by studies using high-speed thermal imaging of the two tool types in operation. The examinations have shown that, based on the concept of defined cutting edges in solid diamond, there is the possibility to make significant improvements in core drilling performance, (ca. 26% lower thrust force, minimal tool surface clogging, lower drilling temperatures) resulting in improved cleanliness of fibre fracture and a reduced tendency of material delamination.     

Dedicated drill design for reduction in burr and delamination during the drilling of composite materials

In the drilling of composite materials such as carbon fiber reinforced plastic, burrs and delamination occur during machining. This study proposed a design of a drill tool with a shape that suppresses burrs and delamination during the drilling of composite materials. The tool shape (tip, groove, land, etc.) is determined, and a nick shape that cuts off chips and reduces heat generation is adopted. A finite element analysis and an experimental evaluation involving 4600 holes were conducted. The maximum errors in the hole diameter and roundness were 15 μm and 0.016, respectively. Further, the maximum burr height was observed to be 80 μm or less, and delamination was not observed in the experiment.     

Effect of laminate configuration and feed rate on cutting performance when drilling holes in carbon fibre reinforced plastic composites

Composites use in the aerospace industry is expanding, in particular carbon fibre reinforced plastics (CFRP) for structural components. Machinability can however be problematic especially when drilling, due to CFRP's inherent anisotropy/in-homogeneity, limited plastic deformation and abrasive characteristics. Following a brief review on composites development and associated machining, the paper outlines experimental results when twist drilling 1.5 mm diameter holes in 3 mm thick CFRP laminate using tungsten carbide (WC) stepped drills. The control variables considered were prepreg type (3 types) and form (unidirectional (UD) and woven), together with drill feed rate (0.2 and 0.4 mm/rev). A full factorial experimental design was used involving 12 tests. Response variables included the number of drilled holes (wear criterion VB_Bmax ≤ 100 μm), thrust force and torque, together with entry and exit delamination (conventional and adjusted delamination factor values calculated) and hole diameter. Best results were obtained with woven MTM44-1/HTS oven cured material (3750 holes) while the effect of prepreg form on tool life was evident only when operating at the higher level of feed rate. Thrust forces were typically under 125 N with torque values generally below 65 Nmm over the range of operating parameters employed. Finally, the delamination factor (F_d) measured at hole entry and exit ranged between ～1.2–1.8 and 1.0–2.1 respectively.