Critical Feed Rate Research Articles

Analytical study and experimental investigation on delamination in drilling of CFRP laminates using twist drills

Delamination in composite laminates is one of the most common and critical defects, it would significantly reduce the stiffness and strength of components. This paper aims at reducing the drilling-induced delamination to improve hole quality. First, drilling tests are carried out to examine the distribution and amplitude of thrust force on drill under different cutting conditions. Empirical thrust force models are developed using experimental data. Then, analytical models of critical thrust force (CTF) are separately established considering two drilling states, considering the actual interactions between the drill and laminate. Anisotropy of composite, tool geometry and load distribution on drill are considered. Comparisons with experimental data and existing models show that the proposed models are capable of predicting CTFs under different loading conditions. In addition, the critical feed rates at different ply locations are determined. This study can provide guidance for the selection of cutting parameters to reduce drilling-induced delamination.

Analysis of contact area in a continuous application-and-peel test method for prepreg tack

The relationship between prepreg tack and the degree of intimate contact (DoIC) between prepreg and a rigid substrate was explored in the context of a continuous application-and-peel test method. Tack for a unidirectional prepreg tape was characterised for different surface combinations and varying test parameters (material feed rate, temperature) at a constant compaction pressure. Application of the prepreg to a transparent rigid substrate (glass), was carried out at matching test conditions to the prepreg tack measurements. Optical microscopy was utilised to acquire images of the contact area at the prepreg-glass interface. Image analysis of the micrographs enabled quantification of the contact area. The time- and temperature-dependent viscoelastic behaviour of the resin was explored directly on the prepreg using oscillatory parallel plate rheometry, and time-temperature superposition was applied to construct both tack and DoIC master curves. The shifted DoIC data showed that true contact area increases with decreasing shifted feed rates, until maximum contact area is achieved. Similarly, tack increases with decreasing shifted feed rates. However, at a critical feed rate, the bond failure mechanism switches from adhesive to cohesive failure. In cohesive failure, tack decreases with decreasing feed rate despite the high levels of DoIC.

Study on Critical Feed Rate of Delamination Based on Thrust Force of Chisel Edge during CFRP Drilling

Study on Critical Feed Rate of Delamination Based on Thrust Force of Chisel Edge during CFRP Drilling

Analytical study of delamination damage and delamination-free drilling method of CFRP composite

Abstract Drilling of composite materials is a challenge in aviation and aerospace field. Delamination often occurs at the hole exit which can reduce the strength of the structure. In this study, to reduce delamination, an analytical model with thermal effect is proposed to predict critical delamination condition of composite. Anisotropy of the composite, thermos-mechanical deformation and load cases of the tool are taken into consideration. Numerical simulation and punching experiments with accurate uncut plies are conducted to validate the rational of the analytical model. The value of critical thrust force and the effect of drilling temperature on that are obtained from the model. In addition, a thrust force model based on the tool geometry is presented with the feed rate per tooth. According to the proposed model, critical feed rate per tooth for delamination-free drilling with different uncut plies and drill diameters can be obtained

Performance evaluation of a giant magnetostrictive rotary ultrasonic machine tool

A giant magnetostrictive rotary ultrasonic machine tool (GMRUMT) with a large and stable amplitude output was developed. The purpose of this study was to comprehensively evaluate the performance and technological characteristics of the GMRUMT by conducting large amplitude experiments of rotary ultrasonic machining. Combined with the transducers’ characteristic curves of vibration amplitude versus frequency, the GMRUMT has the advantages of greater amplitude, higher power, and better stability compared with the conventional piezoelectric actuated rotary ultrasonic machine tool. The vibration stability of the GMRUMT during the machining process was evaluated by carrying out the rotary ultrasonic face milling of quartz glass and the measurement of the actual ultrasonic amplitude. The processing performance of the GMRUMT was evaluated by obtaining the cutting force, the critical feed rate, and the edge-chipping size at the exit hole via rotary ultrasonic drilling experiments. The tool life was evaluated by observing the abrasive wear of the tool. Finally, the GMRUMT was studied in a stable amplitude output condition via tuning to verify the machining advantages of the GMRUMT.

Effect of surface-active media on chip formation in micromachining

Conventionally metal working fluids are employed to reduce tool wear and energy consumption in the metal cutting process. This paper presents a new method to achieve the same purpose by reducing chip thickness and cutting forces through introducing the mechanochemical effect, also referred to as Rehbinder effect, by applying the surface-active media on the workpiece surface before machining. Orthogonal cutting tests were conducted on the copper samples to understand the underlying mechanism of material deformation in the presence of surface-active media. Theoretical explanations for the mechanochemical phenomenon have also been provided. Moreover, the studies were extended to the face turning operation to examine the effectiveness of surface-active medium in a more general case study. It was found that the effectiveness of surface-active media is diminished with decrease in feed rate and enhanced with decrease in cutting velocity. A maximum of 60% reduction in cutting force can be achieved at the critical feed rate of 50 mm/min with low cutting speeds in face turning. This paper not only advances the understanding of the mechanochemical effect in micromachining but also for the first time correlates the surface effect with machining parameters and tool geometry. As validated, the significant reduction in cutting forces open new avenues for further development of new surface-active media for applications in machining processes.

Experimental validation of chip production rate as a tool wear identification in micro-endmilling

Lack of tool wear data prevents the analytical tool wear models in micro-endmilling process proposed in the past from being properly evaluated. And the methods for tool wear data collection of a micro-endmill are scarce, due to the difficulty and tedium involved in repeatedly measuring tool wear during a cutting process. Although there are many aspects to micro-endmill wear, including flank wear, crater wear, and built-up edge, the cutting edge radius wear is a particularly important measure of wear in micro-milling owing to the influence of the minimum chip thickness effect on cutting forces and stability of the cutting process. This research proposes a more efficient and labor-saving method for tool wear data collection in which the cutting edge radius of a micro-endmill is determined by monitoring the rate of chip production during a cutting process. And micro-endmilling experiments are conducted to validate our proposed tool wear data collection method. It is found that the chip production rate decreases gradually as the uncut chip thickness becomes smaller. The number of chips tends to have a larger drop when the feedrate crosses the critical feedrate which corresponds to the minimum chip thickness.

Reliability of Cutting Edge Radius Estimator Based on Chip Production Rate for Micro End Milling

In this paper, the reliability of a new online cutting edge radius estimator for micro end milling is evaluated. This estimator predicts the cutting edge radius by detecting the drop in the chip production rate as the cutting edge of a micro end mill slips over the workpiece when the minimum chip thickness (MCT) becomes larger than the uncut chip thickness (UCT), thus transitioning from the shearing to the ploughing dominant regime. This study proposes a method of calibrating the cutting edge radius estimator by determining two parameters from training data: a ‘size filtering threshold’ that specifies the smallest-size chip that should be counted, and a ‘drop detection threshold’ that distinguishes the drop in the number of chips at the actual critical feedrate from the number drops at the other feedrates. This study then evaluates the accuracy of the calibrated estimator from testing data for determining the ‘critical feedrate’—the feedrate at which the MCT and UCT will be equal. It is found that the estimator is successful in determining the critical feedrate to within 1 mm/s in 84% of trials.

Hydrogeology of Crystal Rocks in the Planning and Construction of Underground Structures

Underground excavations usually have a significant impact on hydrogeological runoff. Water seepage affects underground construction and the long-term behaviour of the structure. The flow paths can be altered by underground activity and the spring areas can become infiltration zones. Concerning the healthy massif, permeabilities near a cavity can increase or decrease. The increase in permeability through excavations in an environment of shear stress and associated water inlets, have a lapse, which can lead to problems in mechanized tunnels when falling below a critical feed rate. When a rock mass is drained, deformations occur, which are reflected in surface settlements; the drainage of the massif causes changes in the physical parameters and the chemical composition of the groundwater. The characteristics of the leaks found in tunnels and drains do not reflect the state of the unaffected groundwater. Numerous parameters in the forecast can lead to over- or underestimations of the actual flow rates of water seepage. In addition to a forecast, it is recommended to carry out sensitivity and risk analyses, in particular to exceeding the value of the forecast and when assessing environmental compatibility, the worst possible scenario

Critical thrust force and critical feed rate in drilling flax fibre composites: A comparative study of various thrust force models

Past experimental and analytical studies for determination of critical thrust force that contributed toward minimal delamination damage in drilling synthetic fibre reinforced composites have failed to consider the recently developed natural fibre composites. From this point of view, critical thrust force and feed rate in drilling of flax fibre composites were comprehensively investigated. Three different thrust force models that were based on linear elastic fracture mechanics and properties of the composites; namely, Mode I interlaminar fracture toughness, Young modulus and Poisson ratio, were applied in order to determine the critical thrust force values. Empirical relationships between various feed rates and spindle rotational speeds with the drilling thrust force were established via regression analyses. These relationships would aid in predicting the critical feed rate while drilling the aforementioned composites. The results reveal that critical feed rate in between the range of 0.15–0.18 mm/rev was desirable for minimisation of the onset delamination damage on the composite laminates. However, the stiffer crystalline structure of the flax fibres as compared to the synthetic fibres inflicted toward the presence of delamination damage below the critical feed rate and thrust force values.

Influence of pulse repetition rate and pulse energy on the heat accumulation between subsequent laser pulses during laser processing of CFRP with ps pulses

Heat accumulation between subsequent laser pulses (HAP) is one of the major reasons for the formation of a heat-affected zone during laser processing of carbon fiber-reinforced plastics (CFRP) with picosecond (ps) laser pulses. In the case of CFRP, the formation of a so-called matrix evaporation zone (MEZ) can be observed. Based on a theoretically derived equation that describes the heat accumulation effect and presuming one-dimensional heat flow, an expression for the critical feed rate at which the HAP effect sets in was derived as a function of the pulse repetition rate and the pulse energy. This expression provides a simple dependency of the critical feed rate on the pulse energy and the repetition rate that is a useful tool for process development. To verify this relation, we used dedicated experimental conditions that ensured one-dimensional heat flow and irradiated the material with fluences below the ablation threshold to ensure that the absorbed energy completely remains in the material and is not removed by an ablation process. By doing so, we were able to confirm the validity of the theoretical model. During the actual cutting of CFRP, however, hence, when material is removed, the theoretically predicted dependency of the critical feed rate on the pulse energy and the repetition rate deviates from experimental results when one assumes the fraction of pulse energy that is left in the workpiece as heat (hence not removed with the ablated material) to be constant. The difference, therefore, is most likely attributed to the fact that the fraction of heat remaining in the material after each ablation processes, the so-called residual heat, depends on both the pulse repetition rate and the pulse energy. These dependencies are reported for the first time for pulsed laser processing of CFRP within the presented study.

Filament Breakage Monitoring in Fused Deposition Modeling Using Acoustic Emission Technique.

Polymers are being used in a wide range of Additive Manufacturing (AM) applications and have been shown to have tremendous potential for producing complex, individually customized parts. In order to improve part quality, it is essential to identify and monitor the process malfunctions of polymer-based AM. The present work endeavored to develop an alternative method for filament breakage identification in the Fused Deposition Modeling (FDM) AM process. The Acoustic Emission (AE) technique was applied due to the fact that it had the capability of detecting bursting and weak signals, especially from complex background noises. The mechanism of filament breakage was depicted thoroughly. The relationship between the process parameters and critical feed rate was obtained. In addition, the framework of filament breakage detection based on the instantaneous skewness and relative similarity of the AE raw waveform was illustrated. Afterwards, we conducted several filament breakage tests to validate their feasibility and effectiveness. Results revealed that the breakage could be successfully identified. Achievements of the present work could be further used to develop a comprehensive in situ FDM monitoring system with moderate cost.

Investigations on the critical feed rate guaranteeing the effectiveness of rotary ultrasonic machining

Rotary ultrasonic machining (RUM) is a well-known and efficient method for manufacturing holes in brittle materials. RUM is characterized by improved material removal rates, reduced cutting forces and reduced edge chipping sizes at the hole exit. The aim of this study is to investigate the critical feed rate to guarantee the effectiveness of RUM. Experimental results on quartz glass and sapphire specimens show that when the feed rate exceeds a critical value, the cutting force increases abruptly, accompanied by a significant decrease of ultrasonic amplitude. An analytical model for the prediction of critical feed rates is presented, based on indentation fracture mechanic and the theory of impact of vibrating systems. This model establishes the theoretical relationships between the critical feed rate, idling resonant ultrasonic amplitude and spindle speed. The results predicted by the analytical model were in good agreement with the experimental results.

Thermo-mechanical modelling of FRP cross-ply composite laminates drilling: Delamination damage analysis

Abstract Among other factors, thrust force, feed rate, twist drill bit chisel edge and point angle are the principal factors responsible for delamination drilling-induced damage during thermo-mechanical deformation. Hence, in this paper, an analytical thermo-mechanical model is proposed to predict critical feed rate and critical thrust force at the onset of delamination crack on CFRP composite cross-ply laminates, using the principle of linear elastic fracture mechanics (LEFM), classical plate theory, cutting mechanics and energy conservation theory. The delamination zone (crack opening Mode I) is modelled as an elliptical plate. The advantages of this proposed model over the existing models in literature are that the influence of drill geometry (chisel edge and point angle) on push-out delamination are incorporated, and mix loads condition are considered. The forces on chisel edges and cutting lips are modelled as a concentrated (point) and uniformly distributed loads, resulting into a better prediction. The model is validated with models in the literature and the results obtained show the flexibility of the proposed model to imitate the results of existing models.

Advances in the surface defect machining (SDM) of hard steels

This paper reports the realisation of precision surface finish (Ra 30nm) on AISI 4340 steel using a conventional turret lathe by adapting and incorporating a surface defect machining (SDM) method [Wear, 302 (2013) 1124–1135]. Conventional ways of machining materials are limited by the use of a critical feed rate (experimentally determined as 0.02mm/rev), beyond which no appreciable improvement in the machined quality of the surface is obtained. However, in this research, the novel application of an SDM method was used to overcome this minimum feed rate limitation ultimately reducing it to 0.005mm/rev and attaining an average machined surface roughness of 30nm. From an application point of view, such a smooth finish is well within the values recommended in the ASTM standards for total knee joint prosthesis. Further analysis was done using SEM imaging, white light interferometry and numerical simulations to verify that adapting SDM method provides improved surface integrity by reducing the extent of side flow, microchips and weldment during the hard turning process.

Experimental Study on the Formation Mechanism of Serrated Chip of TC11 Titanium Alloy

Serrated chips were a universal phenomenon and it had a significant effect on cutting process. In this paper, single tooth dry cutting of TC11 titanium alloy were conducted on a five-axis CNC machine using Stellram milling cutter with a diameter of 32 mm. Both chip macroscopic morphology and microcosmic morphology of TC11 titanium alloy were studied in-depth examining the impact of cutting speed and feed rate to the serrated chips. The results showed that: with the increase of cutting speed, the chip morphology of titanium alloy TC4 experienced a change process from the hair bar to a “C” shape and then become a long strip. The critical cutting speed of TC11 for serrated chips was found at 80 m/min and the critical feed rate to form serrated chip was defined as 0.05mm/z. With the increasing of cutting speed, the extent of chip serration was significantly enhanced. Increasing cutting speed can decrease the feed rate to form serrated chips.

Critical thrust and feed prediction models in drilling of composite laminates

Drilling induced delamination has been recognized as a major problem during drilling of composite materials. The size of the delamination zone has been shown to be related to the thrust force. However, thrust force strongly depends on drilling parameters and it is not possible to control it directly. Thrust force can be correlated with feed rate, the most important parameter affecting thrust force. This paper presents analytical models to predict critical thrust force and feed rate at the onset of delamination. The model proposed is based on elastic fracture mechanics, classical plate bending theory and the mechanics of oblique cutting. An advantage of this model over other proposed models is that to avoid delamination via thrust monitoring, the thrust force will need to be sensed and used in adaptive control, while optimal feed rate can be used directly in CNC command generation to maximize productivity.

Drilling of thick composite material with a small-diameter twist drill

Composite materials are increasingly used in aeronautics. The assembly of aircraft structures requires the machining of different parts. The drilling of thick composite plates is a particularly delicate operation because of the fast rise in the temperature of the tool and the material. This causes defects at the hole entry, on the wall of the hole, and at the exit of the plate. These defects reduce the resistance to structural failure, and therefore, it is important to find a solution to minimize and to remove them if possible. The cutting conditions are the main parameters that affect the quality of the machining. This paper focuses on the choice of the feed rate corresponding to the critical case of delamination determined in the case of a twist drill with a small diameter. The feed rate is calculated by means of two models developed in the laboratory. The first model is analytical; it allows determining the critical thrust force at delamination at the exit of the hole as a function of the characteristics of the material and the load applied by the tool. The second allows predicting the drilling thrust force based on the feed rate. The combination of these two models can determine the critical feed rate with respect to the delamination at the exit of the hole.

Theoretical modeling of the critical conditions for ductile-regime milling of single crystalline silicon

This paper introduces a new method to build a general model for estimating the critical micromilling conditions for single crystalline silicon by considering micromilling process variables based on the generally-accepted indentation model. The analytical results show that cutting edge radius and the feed rate affect the critical conditions greatly. Furthermore, the step over percentage of the cutter radius affects the critical feed rates and the machining efficiency. It shows that an appropriate cutting edge radius is the key factor to achieve high machining efficiency while ductile-regime micromilling conditions are satisfied.

Dislocation Density and Grain Size Evolution in the Machining of Al6061-T6 Alloys

This study focuses on addressing the severe plastic deformation (SPD) behavior and the effects of machining parameters on microstructure alternations in machined surface created from high-speed machining. A finite element (FE) model is proposed to predict the orthogonal machining of Al6061-T6 alloys at high speeds. By extracting strains, strain rates, stresses, and temperatures from this model, a dislocation density-based model is incorporated into it as a user-defined subroutine to predict dislocation densities and grain sizes in machined surface. The predicted results show that dislocation densities decrease with the depths below the machined surface, but grain sizes present an opposite tendency. Higher cutting speeds are associated with thinner plastic deformation layers. Dislocation densities decrease with cutting speeds, but grain sizes increase with cutting speeds in machined surface. Dislocation densities decrease initially and then increase with feed rates. There exists a critical feed rate to generate the maximum SPD layer in machined surface. Tool rake angle has a great impact on the depth of plastic deformation layer. Thus, it affects the distributions of dislocation densities and grain sizes. A large negative rake angle can induce an increased dislocation density in machined surface. The predicted chip thicknesses, cutting forces, distributions of dislocation densities, and grain sizes within the range of machining parameters have good agreement with experiments in terms of chip morphology, cutting forces, microstructure, and microhardness in chip and machined surface.