Jamal Y. Sheikh-Ahmad

Machining Damage in Edge Trimming of CFRP

Conventional machining processes such as turning, milling, drilling, abrasive cutting, and grinding are commonly used to bring composite parts to final shape and assembly requirements. However, due to the layered nature of these materials, their machining may generate undesirable defects such as delamination and high surface roughness. The service life of composite components is believed to be highly dependent on machining quality and damage due to machining may result in scraping expensive parts. In this work, an experimental investigation was conducted to determine the effect of spindle speed, feed rate, and tool condition on machining quality of carbon fiber reinforced polymer (CFRP) composites during edge trimming operation. Machining quality was quantified in terms of average delamination depth and surface roughness. Delaminations were also characterized by their type and frequency of occurrence. It was found that average delamination depth and surface roughness increase with an increase in feed rate and an increase in cutting distance and decrease with an increase in spindle speed. There is a strong relationship between delamination damage and effective chip thickness. The cutting conditions for best machining quality are high spindle speed and low feed rate, which correspond to small effective chip thickness. The most frequent delamination type was found to be Type I/II.

MULTIPLE REGRESSION AND COMMITTEE NEURAL NETWORK FORCE PREDICTION MODELS IN MILLING FRP

This work utilizes the mechanistic modeling approach for predicting cutting forces and simulating the milling process of fiber-reinforced polymers (FRP) with a straight cutting edge. Specific energy functions were developed by multiple regression analysis (MR) and committee neural network approximation (CN) of milling force data and a cutting model was developed based on these energies and the cutting geometry. It is shown that both MR and CN models are capable of predicting the cutting forces in milling of unidirectional and multidirectional composites. Model predictions were compared with experimental data and were found to be in good agreement over the entire range of fiber orientations from 0 to 180°. Furthermore, CN model predictions were found to greatly outperform MR model predictions.

Hole Quality and Damage in Drilling Carbon/Epoxy Composites by Electrical Discharge Machining

This study investigates the material removal mechanisms and machining damage in drilling of carbon fiber epoxy composite by electrical discharge machining (EDM). Detailed investigation of the morphology of the machined surfaces and elemental analysis were conducted inside a scanning electron microscope. Machining damage was characterized by the extent of delamination, hole taper, and the average width of the heat-affected zone (HAZ). The effect of pulse-on time and gap current on machining damage was also investigated. It was found that material removal occurred mainly in the form of decomposition of the polymer matrix and thermally induced fracture of the carbon fibers. Vaporization of the carbon fibers due to spark and Joule heating is also a possible mechanism. The width of HAZ was found to be influenced the most by pulse-on time where the minimum HAZ occurred for intermediate pulse-on time. Furthermore, the width of HAZ and hole taper in EDM were found to be comparable to or less than those obtained by laser cutting.

Optimization of Process Parameters in Diamond Abrasive Machining of Carbon Fiber-Reinforced Epoxy

Edge trimming of carbon fiber-reinforced composites was conducted using diamond abrasive cutters, and the effect of feed rate, spindle speed, and depth of cut on machining quality was investigated. Cutting forces, specific cutting energy, surface roughness, and workpiece temperature were measured and analyzed. It was found that depth of cut is the most important parameter to influence machinability. Trimming with low equivalent chip thickness values was found to be the most suitable in terms of the level of machining responses and machining damage. The cutting temperatures were found to exceed the glass transition temperature of the epoxy matrix when machining with large depth of cut.

Machinability of carbon/epoxy composites by electrical discharge machining

Through-thickness drilling of carbon/epoxy laminate by electrical discharge machining (EDM) was conducted using copper and graphite electrodes. Gap current and pulse-on time were varied and their effect on machinability of the composite laminate was assessed in terms of material removal rate, electrode wear, delamination factor, hole taper and deviation in hole size. It was found that the highest material removal rates were obtained with graphite electrode at the highest current and highest pulse-on time. On the other hand, these conditions also corresponded to the highest electrode wear rate, highest delamination damage and largest deviation in hole size. Hole taper was the least at these settings. An optimum set of conditions for EDM drilling could not be found within the ranges of parameters tested. However, due to the interaction between process parameters it is found that at intermediate levels of pulse-on time levels of delamination damage and deviation in hole size are the smallest. Therefore, effective EDM drilling with high MRR and low damage may be performed at high currents and intermediate pulse-on time.

Effect of edge trimming on failure stress of carbon fibre polymer composites

Finish edge trimming is often required to bring fibre reinforced polymer composites to final dimensional specifications. However, due to the inhomogeneous nature of these materials, their machining may generate undesirable defects such as delamination and surface roughness. These effects may result in compromising the mechanical strength of the machined component. In this work, an experimental investigation was conducted to determine the effect of cutting parameters on machining quality of multidirectional carbon/epoxy laminates during edge trimming operation. Machining quality was quantified in terms of delamination depth and surface roughness. Delaminations were also characterised by their type and frequency of occurrence. Tensile tests were also conducted on machined samples to determine their failure stress. Correlations were made between delamination depth and failure stress. It was found that machining conditions which promote delamination also result in reducing the failure stress of the machined sample.

Model for predicting cutting forces in machining CFRP

Cutting forces affect machining quality and part integrity. Delamination is directly related to the cutting forces, and hence predicting and controlling the cutting forces would lead to controlling and preventing delamination. This work utilises the mechanistic modelling approach for predicting cutting forces in the milling process of carbon fibre reinforced composites. Specific energy functions were determined by regression analysis of experimental data and a cutting model was developed. It is shown that the model is capable of predicting cutting forces in milling of both unidirectional and multidirectional laminates. Model predictions were found to be in good agreement with experimental results.

Tool wear in machining processes for composites

Abstract: This chapter discusses the phenomena of tool wear in machining composite materials with various types of cutting tool materials. A discussion of viable cutting tool materials is first given and the important properties required for cutting composites are highlighted. This is followed by a discussion of specific wear mechanisms that arise in machining metal matrix and polymer matrix composites. The most common forms or types of tool wear are described. The effects of composite matrix, reinforcement phases and amount, and cutting process parameters on tool wear and tool life are also discussed.

An experimental study on end milling of titanium alloy (Ti-6Al-4V) under dry and minimum quantity lubrication conditions

This paper presents the results of an optimisation study of low speed machining of Ti-6Al-4V alloy by technique of order preference by similarity to ideal solution (TOPSIS). End milling was conducted under dry and minimum quantity lubrication (MQL) conditions and at different levels of cutting speed (20 to 40 m/min), feed (0.05 to 0.15 mm/rev) and axial depth of cut (0.4 to 1.2 mm) using a full factorial design. Machinability was evaluated by measuring surface roughness and cutting forces. The micro-hardness below the machined surface for some selected combinations of cutting conditions was also measured. It was found that the feed per rev and depth of cut have the greatest influence on cutting forces and surface roughness under dry and MQL conditions. The minimum cutting forces and minimum surface roughness are obtainable at cutting speed of 30 mm/min, feed of 0.05 to 0.1 mm/rev and depth of cut of 0.4 mm under MQL condition. It was also found that these conditions result in the minimum alteration of micro-hardness beneath the machined surface.

Study of finish milling of titanium alloy (TI6AL4V)

End milling experiments on titanium alloy (Ti6Al4V) using carbide insert-based cutting tool were carried out under dry cutting conditions on a CNC machining centre. The cutting speeds selected for the experiments were between 20 and 50 m min−1 and feed rates were from 0.02 to 0.08 mm/tooth, while the axial depth of cut was kept constant at 1.0 mm. For the range of cutting speeds and feeds, measurements of cutting forces, surface roughness and cutting temperature were performed. From the experimental study, it was found that cutting speed of 30 m min−1 and feed rate of 0.02 mm/tooth could be used for machining Ti alloy because of lower cutting forces. Moreover it is found that cutting speed has significant effect on temperature when compared to feed/tooth. Micro hardness beneath the machined surface has been measured and found that it is a function cutting force and temperature. Tool life of the cutting insert is also found at the optimal cutting speed.