Investigation of experimental study of end milling of CFRP composite

1 Introduction

The usage of carbon fiber-reinforced plastic (CFRP) composites is growing in a variety of engineering and industrial applications, including aircraft, space, automotive, sporting goods, and marine engineering due to their significant advantages over other materials. They provide high rupture resistance, very good fatigue strength, very good heat and electricity conductivity, and smaller specific mass. CFRP composite materials do also not clearly show cracks and fatigue. Therefore, it is widely used in military and commercial aircraft industry. For instance, A380 aircraft’s central fuselage section is made of 50% CFRP composites. Boeing 787 has 80% composite by volume. The materials, listed by weight, are 50% advanced composites, 20% aluminium, 25% titanium and steel, and 5% other materials. The reason for using these materials is to reduce the costs of scheduled and nonroutine maintenances [1–4].

In automotive, aircraft, and space industries, machining of composite components is a major manufacturing operation. Milling is one of the machining processes most frequently used in the manufacturing part of composites. However, the milling of composite components remains a challenge. There have been several studies in the literature on damage, surface roughness, and cutting forces in milling of CFRP composites [5–7]. Hocheng et al. [8] presented a preliminary scientific study on the milling of CFRP composites. They observed chip characteristics and evaluated machinability of CFRP composites as a function of fiber direction and cutting condition. The milled surface parallel to laminate is found to be quite smooth for different cutting conditions. They found it to be quite smooth of milled surface parallel to laminate for difficult cutting conditions. The influence of the machining processes of carbon/epoxy and glass/epoxy composites on their mechanical performances is investigated by Ghidossi et al. [9]. As a consequence of their study, it may be said that there is a significant effect of machining processes on composite parts strength for certain sets of parameters. Hintze et al. [10] have report which mainly investigates the milling of CFRP as a function of increasing tool wear. In this experimental study, they investigated the correlation between wear and fiber orientation on the delamination and described occurrence of delamination of the top layers during milling of CFRP composites using new and worn tools. The experiment results show that delamination in milling CFRP composite materials are dependent on the fiber orientation and tool sharpness. Raj et al. [11] carried out a study of surface roughness and delamination factor in use of coated and uncoated K10 end mills under various cutting conditions. Ahmad et al. [12] conducted an experimental investigation to determine the effect of spindle speed, feed rate, and tool condition on delamination depth and surface roughness of carbon fiber-reinforced polymer composites during edge trimming operation. Delaminations were also characterized by their type and frequency of occurrence. Surface roughness and delamination are quantified as functions of cutting speed and feed rate. The optimum cutting parameters for working with this cutting tool are also identified. They found that delamination and surface roughness increase with an increase in feed rate. Delamination and surface roughness decrease with an increase in spindle speed. Davim and Reis [13] investigated the influence of cutting parameters such as cutting speed and feed rate on the surface roughness, delamination factor, and international dimensional precision in milling laminate plates of CFRP using cemented carbide end mills. They also established an empirical relationship between cutting parameters and surface roughness and delamination factor. Erkan et al. [14] conducted a study that evaluated the cutting parameters such as cutting speed, feed rate, and depth of cut on damage in end milling glass fiber-reinforced polymer (GFRP) composites using two-, three-, and four-flute end mills. The objectives of their study are to determine optimal milling parameters to minimize the surface damage during milling of GFRP composite material and to predict the surface damage depending on milling process parameters using artificial neural network (ANN) model. ANN modeling of a cutting process was used in predicting the damage factor. To reduce number of expensive and time-consuming experiments, they recommended the usage of ANN in predicting the damage factor in end milling of GFRP composite materials.

Davim et al. [15] presented a study of evaluating the cutting parameters related to machining force in the work piece, delamination factor, surface roughness, and international dimensional precision in milling GFRP composite materials (Viapal VUP 9731 and ATLAC 382-05). Evaluation of machining parameters of hand layup GFRP related to machining force was also carried out on milling using cemented carbide (K10) end mill. Azmi et al. [16] conducted an experimental study to evaluate the constitutive relationship between different machining parameters and tool wear, tool life, machining quality, and machining forces using uncoated cemented carbide end mill tool. In their study, cutting speed was recognized as the key machining parameter in influencing the tool life followed by feed rate and fiber orientation. Kalla et al. [5] studied prediction of cutting forces in milling CFRP composites. They used mechanistic modeling techniques for simulating the cutting of CFRP composites. They also developed a force prediction model in helical end milling of unidirectional and multidirectional composites with different fiber orientations from 0° to 180°. Azmi et al. [17] investigated the effects of feed rate, spindle speed, and depth of cut on different machinability characteristics of GFRP composites using Taguchi design of experimental (DOE) method. The aim of their study is to explain the end milling machinability of GFRP composites with respect to different machinability characteristics, namely machining forces, surface roughness, and tool life.

The aim of this study is to discuss and find out the influence of milling parameters such as cutting speed, feed rate, and number of end mill flute on delamination factor, surface roughness, and cutting force in milling of CFRP composites using cemented carbide end mill with different number of the flutes.

2 Delamination types and delamination factor

Delamination is a damage that can result from the machining of fiber-reinforced composite materials. In milling/trimming of fiber-reinforced plastic composites, there are different types of deformation on the machined edge of the CFRP, and they can be classified. Colligan and Ramulu [7] defined four types of delamination at edge trimming process. These four types of delamination are described as types I, II, I/II, and III. Types of delamination are shown schematically in Figure 1. Type I delamination occurs in areas in which the surface fibers are broken and removed some distance inward from the machined edge. Type II delamination consists of the uncut fibers that protrude outward from the machined edge. Type I/II delamination occurs in areas in which the fibers are broken inward and extend outward from machining edge. Type I/II is a kind of combination of both types I and II. Type III delamination is described as fibers that are partially attached or cracks that are parallel to the machined surface [7].

Figure 1

Types of delamination in fiber-reinforced plastic.

Delamination is the most significant drawback during the end milling of fiber-reinforced plastic composites. To determine the delamination factor around the machined edge, the maximum damage (W_max) in the delamination zone was measured (Figure 2). The value of delamination factor (F_dw) can be determined by [13, 15]

Figure 2

A photograph of measurement of damage width.

where W_max is the width of maximum damage (mm) and W is the nominal width of the cut (mm).

3 Experimental setup and process parameters

The CFRP composite plates used in this experimental study were produced by sheet compression moulding method. These CFRP composite plates have 16 layers of carbon fiber mats with a fiber orientation of 0/90°. The CFRP composite plate has some mechanical properties such as elasticity module of 20 N/mm², tensile strength of 280 N/mm², bend strength of 350 N/mm², and pressure strength of 500 N/mm². The CFRP composite was provided in the size of 350×300×5 mm. The work piece material specimens having size of 170×100×5 mm were cut from the plate for the milling operations.

The end milling tests were carried out using HUMMER VMC-1000 CNC milling vertical machining center with a spindle power of 15 kW and a maximum speed of 8000 rpm. The three orthogonal components of machining forces, such as Fx, Fy, and Fz, have been measured by Kistler piezoelectric dynamometer (Type 9257B). The data read from the dynamometer were transferred to a national instrument data acquisition card after being amplified by Kistler 8 channel amplifiers. The obtained data were processed in software Kistler DynoWare version 2.5.3. Experimental setup is shown in Figures 3 and 4.

Figure 3

Schematic setup used in CNC vertical machining center.

Figure 4

Experimental setup for end milling.

In milling tests, cemented carbide end mills with diameter of 10 mm, which had three and four flutes, were used. The machining parameters used in the end milling experiments and properties of end mill are given in Table 1. All tests were run without coolant. The damage in end milling of CFRP composite material was measured using a microscope, Nikon Epiphot 200 optical microscope, with 10× magnification and 1 μm resolution. The milled surfaces were measured using surface texture measuring instrument of Taylor-Hobson Surtronic 3+.

4 Results and discussion

4.1 Influence of cutting speed, feed rate, and end mill flutes on machining quality

Machining quality depended significantly on cutting parameters and tool geometry. In end milling operation, delamination is an important criterion. A series of milling tests are conducted to assess the effect of cutting parameters on delamination in end milling CFRP composites using different tools. The damages in CFRP composite plates milled using three- and four-flute end mill at different cutting parameters are shown in Figure 5. Experimental results of the delamination for end milling of CFRP composites with different cutting parameters are demonstrated in Figure 6.

Figure 5

Different types of deformation in milled CFRP.

Figure 6

Influence of the cutting parameters on delamination factor at different number of flutes: (A) three flutes and (B) four flutes.

In Figure 5, severe damages and uncut fibers on the milled surface appear. While type III delamination occurs at end milling using three-flute end mill, types I and I/II delamination occur at end milling by four-flute end mill.

From the results of Figure 6, it was observed that the delamination factor is significantly affected by feed rate. The delamination factor increased with increasing feed rate. As the lowest delamination factor was obtained by feed rate of 100 mm/min, the highest delamination factor was obtained by feed rate of 200 mm/min. The similar behavior was also found for different cutting speed. The averaged delamination factor increased with increasing cutting speed for different feed rate values. This result was also mentioned by various researchers [13, 14]. They explained that best results of the delamination factor were obtained at lower feed rates. Erkan et al. [14] explained that delamination was to affect increasing plastic deformation rate at higher cutting speeds. It was also observed that the delamination factor decreased at the same cutting parameters with increasing number of flutes. This is attributed to decreasing delamination factor due to participation of more cutting edges of end mill in a revolution [14].

Surface roughness has attracted a lot of attention as an important machinability parameter [18]. Surface roughness is closely tied to the accuracy or tolerance of a machine component [1]. Geometric irregularities and metallurgical alterations of machined surfaces are important aspects to be defined and controlled. Therefore, surface finish and surface integrity must be defined, measured, and maintained. The final finish on a machined surface is a function of tool geometry, tool material, the run out errors in tool, mechanical properties of work piece material, machining process, speed, feed, depth of cut, and cutting fluid. Surface finish is also related to the process variability [1]. The effect of cutting parameters and number of end mill flute on surface roughness (Ra) is shown in Figure 7.

Figure 7

Influence of the cutting parameters on surface roughness at different number of flutes: (A) three flutes and (B) four flutes.

From Figure 7, it can be inferred that surface roughness increases with increasing the feed rate. Surface roughness decreases with increasing the cutting speed. To obtain a fine surface roughness, it is necessary a higher cutting speed and a low feed be applied. It can be observed that four-flute end mill provide a better surface roughness than the three-flute end mill.

4.2 Influence of cutting speed, feed rate, and end mill flutes on cutting forces

In machining, force is required to deform and cut the material plastically. This force is the cutting force component. They depend on certain factors and are very sensitive to material composition, hardness, microstructure, geometry and cutting tool materials used, machining parameters, heat generation, and machine stability. The cutting force component is important to understand how it varies with changes in the machining parameters, tool materials and geometry, tool wear, etc. For this purpose, several experiments were performed. According to the results obtained from these experiments, the value of the machining force determined the average of the resulting three components of the cutting force in the work piece. Equation (2) was used to obtain the resulting value of the machining force. The effect of different cutting parameters and number of end mill flute on machining force is shown in Figure 8.

Figure 8

Influence of the cutting parameters on cutting force at different number of flutes: (A) three flutes and (B) four flute.

(2)F=Fx2+Fy2+Fz2. (2)

From Figure 8, it can be inferred that the cutting force increased with increasing feed rate. The results agree with the industrial practice [1]. In general, the use of a low feed rate reduces cutting force due to low cutting temperature. The cutting forces remain nearly constant when speed is increased. The tool geometry plays a role in determining the cutting forces. The cutting forces decrease with increasing number of end mill flutes. It can be concluded that to achieve a lower cutting force value for low feed rate and four flute end mill, increasing number of flute end mill is desirable in a cutting tool because it leads to lower cutting forces and reduces tool tip temperatures [1].

5 Conclusions

In this paper, an experimental investigation has been conducted to study the effect of different parameters such as cutting speed, feed rate, and flute number of end mill on the end milling CFRP composite materials using cemented carbide end mills. The following conclusions can be drawn from the experimental investigation.

• Damage in end milling of CFRP materials was affected by cutting parameters and tool geometry. Delamination factor increased with increasing feed rate and cutting speed and decreased with increasing flute number of end mill.

• End milling with three- and four-flute end mill produced type III and type I-I/II uncut fibers on the milled surface, respectively.

• Surface roughness in direction end milling increased with increasing feed rate and decreased with increasing cutting speed for both three-flute end mill and four-flute end mill.

• Cutting force increased with an increase in the feed rate and remained nearly constant with increase in cutting speed for both end mills.

• The number of flute of end mill was another important factor having effect on damage, surface roughness, and cutting force.

• With four-flute end mill, best results were obtained from this experimental study.