Drilling of carbon composites using a one shot drill bit. Part I: Five stage representation of drilling and factors affecting maximum force and torque

The thrust force and torque produced during drilling contain important information related to the quality of the hole and the wear of the drill bit [1]. In this paper, the force and torque produced during drilling of carbon fibre using a ‘one shot’ drill bit is investigated. The signals in the time domain were divided into stages and common problems and defects associated with each stage discussed. It is also shown how tool wear and thickness of the workpiece affect the thrust force and torque throughout the drilling process. The findings of this paper are used to develop a mathematical model of the maximum thrust force and torque as described on Part II of this paper and are a valuable reference for future optimisation of drilling carbon composites with a ‘oneshot’ drill bit.     

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.     

Modeling and tool wear in drilling of CFRP

This paper presents the prediction and evaluation of thrust force in drilling of carbon composite material. In order to extend tool life and improve quality of hole drilling, a better understanding of uncoated and coated tool behaviors is required. This paper describes the development of a phenomenological model between the thrust force, the drilling parameters and the tool wear. The experimental results indicate that the feed rate, the cutting speed and the tool wear are the most significant factors affecting the thrust force. The model can then be used for tool-wear monitoring. The model presented here is verified by experimental tests.     

High-throughput drilling of titanium alloys

Experiments of high-throughput drilling of Ti–6Al–4V at 183 m/min cutting speed and 156 mm³/s material removal rate (MRR) using a 4 mm diameter WC–Co spiral point drill were conducted. The tool material and geometry and drilling process parameters, including cutting speed, feed, and fluid supply, were studied to evaluate the effect on drill life, thrust force, torque, energy, and burr formation. The tool wear mechanism, hole surface roughness, and chip light emission and morphology for high-throughput drilling were investigated. Supplying the cutting fluid via through-the-drill holes has proven to be a critical factor for drill life, which can be increased by 10 times compared to that of dry drilling at 183 m/min cutting speed and 0.051 mm/rev feed. Under the same MRR of 156 mm³/s with a doubled feed of 0.102 mm/rev (91 m/min cutting speed), over 200 holes can be drilled. The balance of cutting speed and feed is essential to achieve long drill life and good hole surface roughness. This study demonstrates that, using proper drilling process parameters, spiral point drill geometry, and fine-grained WC–Co tool material, the high-throughput drilling of Ti alloy is technically feasible.     

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.     

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.     

Dry and minimum quantity lubrication drilling of cast magnesium alloy (AM60)

Dry and minimum quantity lubrication (MQL) drilling of cast magnesium alloy AM60 used in the manufacturing of lightweight automotive components have been studied. The maximum and average torque and thrust forces measured during drilling using distilled water (H₂O-MQL) and a fatty acid-based MQL fluid (FA-MQL), both supplied at the rate of 10 ml/h, were compared with those generated during flooded (mineral oil) drilling. Tool life during dry drilling was inadequately short, due to excessive magnesium transfer and adhesion to the (HSS steel) drill causing drill failure in less than 80 holes. The use of MQL reduced magnesium adhesion and built-up edge formation, resulting in an increase in tool life as well as reductions in both average torque and thrust forces—prompting a performance similar to that of flooded drilling. The maximum temperature generated in the workpiece during MQL drilling was lower than that observed in dry drilling, and comparable to flooded condition. The mechanical properties of the material adjacent to drilled holes, as evaluated through plastic strain and hardness measurements near the holes, revealed a notable softening in the case of dry drilling, but not for MQL drilling. MQL drilling provided a stable drilling performance, which was evident from the uniform torque and force patterns throughout the drilling cycles and also resulted in desirable machining characteristics, including a smooth hole surface and short chip segments.     

碳纤维复合材料旋转超声椭圆振动套磨制孔技术研究

针对航空航天领域中碳纤维复合材料在制孔过程中易出现分层、毛刺和刀具磨损快等问题,提出了一种新型旋转超声椭圆振动套磨制孔技术。分析了旋转超声椭圆振动套磨制孔机理,研制出了小型化旋转超声椭圆振动套磨制孔系统,并进行了碳纤维复合材料(CFRP)套磨制孔实验。结果表明:相比于普通套磨,超声椭圆振动套磨极大改善了切屑粉尘和料芯的排出,延长了刀具使用寿命;明显降低了钻削力和扭矩分别约25%和30%;有效降低了复材孔分层和毛刺的制孔缺陷,改善了孔表面粗糙度和平整性,提高了加工质量。     

Generalized modeling of drilling vibrations. Part I: Time domain model of drilling kinematics, dynamics and hole formation

This two part paper presents a comprehensive exercise in modeling dynamics, kinematics and stability in drilling operations. While Part II focuses on the chatter stability of drilling in frequency domain, Part I presents a three-dimensional (3D) dynamic model of drilling which considers rigid body motion, and torsional–axial and lateral vibrations in drilling, and resulting hole formation. The model is used to investigate: (a) the mechanism of whirling vibrations, which occur due to lateral drill deflections; (b) lateral chatter vibrations; and (c) combined lateral and torsional–axial vibrations. Mechanistic cutting force models are used to accurately predict lateral forces, torque and thrust as functions of feedrate, radial depth of cut, drill geometry and vibrations. Grinding errors reflected on the drill geometry are considered in the model. A 3D workpiece, consisting of a cylindrical hole wall and a hole bottom surface, is fed to the rotating drill while the structural vibrations are excited by the cutting forces. The mechanism of whirling vibrations is explained, and the hole wall formation during whirling vibrations is investigated by imposing commonly observed whirling motion on the drill. The time domain model is used to predict the cutting forces and frequency content as well as the shape of the hole wall, and how it depends on the amplitude and frequency of the whirling vibration. The model is also used to predict regenerative, lateral chatter vibrations. The influence of pilot hole size, spindle speed and torsional–axial chatter on lateral vibrations is observed from experimental cutting forces, frequency spectra and shows good similarity with simulation results. The effect of the drill–hole surface contact during drilling is discussed by observing the discrepancies between the numerical model of the drilling process and experimental measurements.     

Time domain simulation of torsional–axial vibrations in drilling

A time domain model of the torsional–axial chatter vibrations in drilling is presented. The model considers the exact kinematics of rigid body, and coupled torsional and axial vibrations of the drill. The tool is modeled as a pretwisted beam that exhibits axial and torsional deflections due to torque and thrust loading. A mechanistic cutting force model is used to accurately predict the cutting torque and thrust as a function of feedrate, radial depth of cut, and drill geometry. The drill rotates and feeds axially into the workpiece while the structural vibrations are excited by the cutting torque and thrust. The location of the drill edge is predicted using the kinematics model, and the generated surface is digitized at discrete time intervals. The distribution of chip thickness, which is affected by both rigid body motion and structural vibrations, is evaluated by subtracting the presently generated surface from the previous one. The model considers nonlinearities in cutting coefficients, tool jumping out of cut and overlapping of multiple regeneration waves. Force, torque, power and dimensional form errors left on the surface are predicted using the dynamic chip thickness obtained from the exact kinematics model. The stability of the drilling process is also evaluated using the time domain simulation model, and compared with extensive experiments. This paper provides details of the mathematical model, experimental verification and simulation capabilities. Although the surface finish from unstable cutting can be predicted realistically, the actual drilling stability cannot be determined without including process damping.     

Cutting edge rounding: An innovative tool wear criterion in drilling CFRP composite laminates

An evenly and smoothly distributed abrasion wear, observed along the entire cutting edge of an uncoated carbide drill bit in drilling CFRPs, is due to the highly abrasive nature of the carbon fibres. A very few researchers have only quoted this wear mode as being responsible for giving rise to the rounding of the cutting edge, or its bluntness. However, this wear feature has seldom been investigated, unlike the conventional flank wear in practice. This paper offers a new approach in unveiling and introducing the cutting edge rounding (CER) – a latent wear characteristic as a measure of sharpness/bluntness – of uncoated cemented carbide tools during drilling CFRP composite laminates. Four different types of drills (conventional and specialised) were tested to assess the applicability and relevance of this new wear feature. Mechanical loads (drilling thrust and torque) were recorded, and the hole entry and exit delamination were quantified. For the utilised tools, the accruing magnitude of CER was also recorded, in parallel with studying their conventional flank wear. Very appreciable correlations between the CER and the drilling loads, and also the quantitative delamination results are observed. Results reveal that this new wear type develops almost similarly for the selected tools and is practically independent of their respective conventional flank wear patterns. Moreover, a distinct, non-zero magnitude of the CER for a very fresh tool state may provide researchers with some lucid information in further studying the results during wear tests, more emphatically. The CER correlations with quantitative delamination results are noticed quite comparable to those of the conventional flank wear via statistical linear regression analyses.     

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.     

Machining parameters for an intelligent machining system for composite laminates

Composite laminates exhibit very high in-plane strengths but are plagued by delamination damage when subjected to machining. This is due to their poor transverse strengths and low delamination fracture toughness. In drilling, delamination is initiated when the thrust force exceeds a threshold value, particularly at the critical entry and exit locations of the drill bit. To minimize damage, therefore, it is important to monitor process variables such as the machining forces and the position of the tool relative to the workpiece. The availability of a suitable model coupled with an intelligent control scheme would be a large advancement in the machining of composite laminates. This paper explores the development of such models for machining of composites and for coupling the models to intelligent control strategies. Using a machining center, a series of drilling experiments were conducted on carbon fiber-reinforced composite laminates to determine key process parameters for various cutting conditions. An intelligent machining scheme is proposed as the basis for the design of a new machine tool.     

Thrust force, torque, and tool wear in drilling the bulk metallic glass

The thrust force, torque, and tool wear in drilling of Zr-based bulk metallic glass (BMG) material are investigated. Drilling the BMG at high speed generates the chip light emission, high tool temperature, and severe tool wear. At low spindle speed, the BMG work-material builds up at the major and margin cutting edges and may break the drill. A range of feasible spindle speed and feed rate for the efficient drilling of BMG without the detrimental chip light emission and cutting edge work-material build-up has been identified in this study. Under the same drilling condition, the WC-Co tool generally requires less thrust force and about the same torque than the high-speed steel tool. The progressive wear of the major and margin cutting edges for BMG drilling is examined. Severe drill wear is associated with the bright BMG chip light emission. Without chip light emission, the drill wear is visible but not severe. This study concluded that precision holes in BMG could be generated with proper selection of tooling and process parameters.     

Mechanistic models of thrust force and torque in step-drilling of Al7075-T651

In the aerospace industry, burr removal is an important and expensive part of the manufacturing process. One approach to minimizing burrs is to lower the thrust force in drilling through suitable modification of the drill geometry such as the use of step drills. This paper focuses on the modeling of thrust force and torque for step drills. A mechanistic model capturing the various material removal mechanisms, i.e. oblique cutting, orthogonal cutting, and indentation, active on different sections of the step drill is developed. Subsequently, a series of experiments is conducted to calibrate and validate the model. The validation results show that the predicted thrust and torque values are in good agreement with measured values, although the torque is slightly underestimated. The validated model was further used to investigate the effects of step drill geometry parameters on the thrust force and torque. The model predictions suggest that the thrust force increases and the torque decreases for larger secondary point angles and inner diameters.     

Identification of thrust force models for vibratory drilling

Among the scientific problems in drilling, determining thrust forces for any drill geometry and any workmaterial remains an issue. This question is especially critical in the context of the self-vibratory drilling (SVD) technology, since thrust force governs the trajectory of cutting lips. In order to predict the dynamical behaviour of the self-vibratory drilling head (SVDH), a numerical simulator has been developed in the laboratory. In order to predict the relevant behaviour of the SVD operations, a good accuracy of the thrust force model is highly necessary. The aim of the paper is to propose an experimental methodology to identify a thrust force model for one pair “drill–workmaterial” depending on the uncut chip thickness. This methodology proposes to divide cutting edges into several parts in order to define the local contribution of this part to the macroscopic thrust force. Local models are identified during the penetration of the drill. From a limited number of experiments with various feeds, it becomes possible to identify all the coefficients of an accurate thrust force model.     

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.     

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.     

Drill geometry and operating effects when cutting small diameter holes in CFRP

The paper details experimental results when drilling small holes (1.5 mm diameter cemented carbide drills with varying end point and helix geometry) in thin quasi-isotropic, unbacked carbon fibre reinforced plastic (CFRP) laminate (typical cutting time 0.4 s/hole). The study utilised an L12 Taguchi fractional factorial orthogonal array with analysis of variance (ANOVA) employed to evaluate the effect of drill geometry and drilling conditions on tool life and hole quality. Main effects plots and percentage contribution ratios (PCR) are detailed in respect of response variables and process control factors. More conventionally, tool wear and cutting force data are plotted/tabulated, together with micrographs of hole entry/exit condition and internal hole damage. Drill geometry and feed rate in general had the most effect on measured outputs. Thrust force was typically below 100 N at test cessation; however, drill wear progression effectively doubled the magnitude of force from test outset. Entry and exit delamination factors (F_d) of 1.3 were achieved while the maximum number of drilled holes for a tool life criterion VB_Bmax of ≤100 μm was 2900 holes using a stepped, uncoated drill with a feed rate of 0.2 mm/rev.     

A decision fusion algorithm for tool wear condition monitoring in drilling

Tool wear monitoring of cutting tools is important for the automation of modern manufacturing systems. In this paper, several innovative monitoring methods for on-line tool wear condition monitoring in drilling operations are presented. Drilling is one of the most widely used manufacturing operations and monitoring techniques using measurements of force signals (thrust and torque) and power signals (spindle and servo) are developed in this paper. Two methods using Hidden Markov models, as well as several other methods that directly use force and power data are used to establish the health of a drilling tool in order to avoid catastrophic failure of the drill. In order to increase the reliability of these methods, a decision fusion center algorithm (DFCA) is proposed which combines the outputs of the individual methods to make a global decision about the wear status of the drill. Experimental results demonstrate the effectiveness of the proposed monitoring methods and the DFCA.