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.     

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.     

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.     

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 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.     

Computerized tomography and C-Scan for measuring delamination in the drilling of composite materials using various drills

Whilst drilling is the most frequently employed operation of secondary machining for structure joining, delamination is a very serious defect during drilling of fiber-reinforced composite materials,. The evaluation of the delamination damage in the material is important but rather difficult, particularly for carbon fiber-based composites, because their colour makes visual inspection difficult. A special technique using medical equipment for computerized tomography is presented in this paper. It is compared to techniques using ultrasonic and is demonstrated as a feasible and an effective tool for the evaluation of drilling-induced delamination.     

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.     

Effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill

Machining of composites has caught greater attention in manufacturing of structural parts in aerospace, automobile and sporting goods. Composite materials have advantageous features in strength and stiffness coupled with lightweight compared to the conventional metallic materials. Amongst all machining operations, drilling is the most commonly applied method for generating holes for riveting and fastening the structural assembly. Delamination is one of the serious concerns in drilling holes in composite materials at the bottom surface of the workpiece (drill exit). Quite a few references of the drilling of fiber-reinforced plastics report that the quality of cut is strongly dependent on drilling parameter as well as the drill geometry. Saw drills and core drills produce less delamination than twist drills by distributing the drilling thrust toward the hole periphery. Delamination can be effectively reduced or eliminated by slowing down the feed rate when approaching the exit and by using back-up plates to support and counteract the deflection of the composite laminate leading to exit side delaminations. The use of the back-up does reduce the delamination in practice, which its effects have not been well explained in analytical fashion. This paper predicts the effects of backup plate on delamination in drilling composite materials using saw drill and core drill. The critical drilling thrust force at the onset of delamination is calculated and compared with that without backup. The well known advantage of industrial use of backup can be understood fundamentally by the fact that the threshold thrust force at the onset of delamination is increased making the delamination less induced.     

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.     

Prediction of the location of delamination in the drilling of composite laminates

An analytical approach for the determination of the position of the onset of delamination during the drilling of composite laminates based on linear elastic fracture mechanics is presented in this paper. The critical thrust force calculated from the predicted position of the onset of delamination has a reasonable agreement with experimental results obtained in the drilling of T300/5208 graphite epoxy composite laminates reported in the literature. Thus, the analytical method developed in the present study may be applied to the drilling of composite laminates drilling with the avoidance of delamination.     

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.     

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.     

Taguchi analysis of delamination associated with various drill bits in drilling of composite material

This paper presents a prediction and evaluation of delamination factor in use of twist drill, candle stick drill and saw drill. The approach is based on Taguchi’s method and the analysis of variance (ANOVA). An ultrasonic C-Scan to examine the delamination of carbon fiber-reinforced plastic (CFRP) laminate is used in this paper. The experiments were conducted to study the delamination factor under various cutting conditions. The experimental results indicate that the feed rate and the drill diameter are recognized to make the most significant contribution to the overall performance. The objective was to establish a correlation between feed rate, spindle speed and drill diameter with the induced delamination in a CFRP laminate. The correlation was obtained by multi-variable linear regression and compared with the experimental results.     

CFRP drilling: Fundamental study of local feed force and consequences on hole exit damage.

Carbon Fiber-Reinforced by Plastic (CFRP) is now commonly used in the aircraft industry. The main challenge is to manufacture this difficult-to-cut work material, considering its quality criteria and economical aspects. Drilling is the main machining operation required for the assembly of the aircraft structure. In this paper, results are presented and discussed regarding exit delamination studied at a local scale. Because of the anisotropic properties of CFRP, the fiber cutting modes change with the composite sequence combined with the drill revolution parameters. The local feed forces generated by the cutting edge on the hole bottom may be correlated with delaminating aspects. A posttreatment method is proposed to analyze precisely these feed force and cutting torque distributions. Appropriate ply sequences are identified in order to limit the mechanical load concentration and the risk of delamination or uncut fibers     

The effect of hole drilling on fatigue and residual stress properties of shot-peened aluminum panels

The effect of hole drilling after shot peening on the fatigue life and residual stress state of selected aluminum alloys was investigated. Compared to the unpeened condition, the hole drilling after shot peening reduced the fatigue life of the 2024-T351 and 2324-T39 alloys to a small extent. On the other hand, the fatigue life of 7150-T7751 decreased considerably due to hole drilling after shot peening. A compressive residual stress was at the surface of the shot peened specimens. Hole drilling reduced the compressive residual stress by 60%. The excess dislocation density was mostly concentrated at the surface of the specimens.     

Accuracy estimation of drilled holes with small diameter and influence of drill parameter on the machining accuracy when drilling in mild steel sheet

The miniaturization of component parts has advanced in the modern industrial product sector. Corresponding with this new wave, there is a strong demand for the establishment of minute-scale techniques to manufacture products with high quality involving the drilling of small holes. Non-traditional machining techniques such as electro discharge machining can be applied to make small holes. However, there is much practical value if small holes can be made by the traditional drilling process. In this paper, a method for estimating the drilled hole accuracy using a Fourier series analysis is proposed and this method is applied to small holes 1 mm in diameter drilled in mild steel for machine structure use. As a result, it is clarified that the bending rigidity of drill and the thinning of the drill point exert a large influence on the drilled hole accuracy.     

Experimental investigation of drilling damage and stitching effects on the mechanical behavior of carbon/epoxy composites

The main purpose of composite materials drilling is the need to put together different parts of a structure, in aeronautics for example. Machining generates damages which affect mechanical properties and have to be taken into account during manufacturing process. The objective of this study is to experimentally analyze the influence of drilling on a carbon/epoxy composite, to investigate the relationships among damages, cutting forces, mechanical properties of the drilled specimens and crack propagation. Stitching and a range of spindle speed and feed have been tested when drilling with a classic twist drill. The effect of each parameter has been assessed in terms of thrust force, moment (during machining) and defects, and then linked to mechanical test results. Experimental results have shown significant influences of feed and composite configuration on delamination. Furthermore, cyclic tensile tests have shown that reducing damage and using stitching help increasing tensile strength.     

双头深孔钻床控制系统的设计与改造

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