SAN-OM-29
Effect of cutting conditions on surface roughness during trimming using new tool
Cutting tool materials :
The chip formation mechanism is different in machining of CFRPs and metals because the compression shearing or fracture of the fiber reinforcement and matrix occur in machining of CFRPs while the plastic deformation take place in cutting of metals. The chip formation mechanism requires a sharp tool with a large positive rake angle. Hardness and toughness of the cutting tool are necessary to overcome the abrasiveness of the fibers and the loads which are produced during fracture of fibers. There are limited ranges of tool materials which satisfy these conditions. Normally, with an increase in hardness, the toughness decreases and vice versa. According to Sheikh-Ahmad (2009), the cutting tool materials are categorized in various types as listed in following paragraphs:
• Cemented tungsten carbides
Cemented tungsten carbides are composed of hard carbide particles (WC) and metallic binder material cemented together. High hardness and toughness are the main properties of submicron carbides to make them suitable for machining of composites. In machining with this tool material, the cutting forces are low and the most common form of wear is abrasion due to inhomogeneous nature of composite materials. Moreover; in this case, the fluctuated cutting forces could increase the chipping of the cutting tool.
• Coated carbides
In order to improve tool wear resistance of cemented carbide tool, the hard ceramic coatings are used. The coatings are mostly formed by two methods; first method is chemical vapor deposition (CVD). In this method, single layer and multilayer coatings of TiC, TiN, TiCN, and Al2O3 are used at high temperature. The second method is physical vapor deposition (PVD) is carried out at low temperature. PVD coatings have the advantages such as finer microstructures, higher toughness at high temperatures and they are utilized in case of high cutting forces. In machining of composites, using coated carbides doesn’t improve considerably the wear resistance, tool life and edge chipping.
• Ceramics
Ceramic tools are composed of sintered alumina (Al2O3) at high temperatures and pressures. Low toughness and chipping are most common problems in ceramic tools when the loads are heavy or interrupted; they tend to fail by chipping. In addition, it is difficult to produce them with sharp edges.
• Polycrystalline diamond
Polycrystalline diamond tool has good thermal conductivity, low friction coefficient, high tool life and it produces the good surface finish however it has some disadvantages such as cutting tool size limit, fracture susceptibility and cost of production. Polycrystalline diamond is composed of compacted PCDs and small amounts of metallic binder, such as cobalt at high temperature and pressure. Applications of this tool are in machining of abrasive materials, interrupted and rough machining. It was found that this material tool generates less delamination when the cutting tool has low tool wear.
• Polycrystalline cubic boron nitride
Polycrystalline cubic boron nitride (PCBN) tools are composed of a sintered mixture of CBN crystals and metallic or ceramic binder such as TiC and TiN. The major form of tool wear is microchipping so, these tools are not useful for cutting of inhomogeneous materials or the interrupted cutting.
• Diamond coated carbides
Low-pressure diamond synthesis produced by CVD allows processing of diamond as a tool coating. This coating leads to a great tool life improvement in machining of abrasive materials such as carbon fiber composites. The failure mode of diamond-coated carbides is associated with uniform wear of the diamond film and film delamination.
METHODOLOGY:
Introduction
In this research, the main objective was to study the effect of tool wear on the resulting quality of CFRP during trimming. Therefore, it was necessary to carry out tool life experiments in order to verify this effect. In tool life test, tool wear was obtained as a function of cutting length. In this work, to properly relate the quality of the trimmed laminates as a function of the length of cut expressed in linear meters, two setups were used, one to generate tool wear of several meters and transform the laminate in “chips and dust”, and the other to trim test coupons dedicated to full inspection and related to the length of wearing cut achieved at each step of an iterative process. Thus, in each tool life experiment, the cutting tool machined 100 mm using short cut set-up and then 900 mm using long cut setup. On the other hand, at first, each tool life experiment started by machining of short panel and then long panel was trimmed alternately. This process was repeated until the tool wear reached the flank wear criterion. This tool life limit indicates that a cutting tool, reaches the end of its useful life. In this work, the tool life criterion was considered an average maximum flank wear of 0.3 mm according to ISO 8688-2 standard. Figure 2.1 shows the cutting process in the tool life experiment. Figure 2.2 shows short cut and long cut experimental set-ups. In terms of tool life measurement, before each short cut, the tool wear was also measured using optical microscope.
In this study, the second objective was to verify the effect of cutting conditions on the cutting forces, tool life, surface integrity and surface roughness in tool life tests. To study this effect, during each short cut, the cutting forces were measured using a dynamometer table. Before each short cut, the cutting tool was removed in order to measure the tool wear. Moreover, for each coupon (32 mm wide and 100 mm long) which was obtained from the short cut, the surface finish was inspected using SEM micrograph in terms of surface integrity. In terms of surface roughness, the surface finish was evaluated using profilometer.
Preliminary tests and tool life test conditions
In this work, the preliminary cutting parameters were selected according to Bérubé (2012) who proposed the best operational conditions including feed rate and cutting speed ranges for similar tool type and composite materials. Three feed rates and three cutting speeds including minimum, intermediate and maximum values of feed rate and cutting speed were chosen from those ranges.
Surface Roughness:
In this study, surface roughness was measured using a profilometer Mitutoyo SJ-400 SURFPAK. The surface roughness measurement criteria were Ra, Rt and Rv (profile roughness parameters). All results were obtained and analysed from the -45˚ plies for the upmilling side of the coupons, using a longitudinal method over a distance of 14 mm. The roughness measurement was repeated three times on -45˚ ply. The average values of Ra, Rt and Rv were calculated separately. The up milling and down milling sides of the coupons for many cases were analysed and compared together. For example, Figure 4.27 shows the machined surfaces of two coupons (up milling and down milling sides). In this figure, the bottom surface was generated using the up milling mode (Ra is 1.73 μm) while the top surface was produced using the down milling mode (Ra is 10.76 μm). It indicates that in terms of surface roughness, up milling side was better than down milling side. So in this work, we decided to measure and analyse, the up milling side of the coupons. In addition, for the up-milling side of the coupons, all measurement results were obtained from the -45˚ plies because the trimming operation generated the worst surface roughness on the -45˚ plies according to Bérubé (2012).
Cutting Forces:
Due to inhomogeneous nature of FRPs, cutting forces are oscillating and periodic. They are periodic because during the milling of composites, the cutting tool cuts the fibers and matrix phases alternatively, so the cutting force has different magnitudes during machining. The periodic cutting forces are also affected by the periodic changes of fiber angles relative to the speed. The cutting forces generated during the machining processes are very important, especially when machining of CFRP. Controlling these cutting forces is one of the keys to decrease the quality issues or defects induced in CFRP machined components. The cutting forces are affected by many factors, such as material properties, cutting conditions and tool geometry.
J.F. Chatelain et al. (2012) studied the effect of tool geometry on the cutting forces during trimming of multilayered CFRP laminates. They carried out some experiments using crosscut tool with positive negative helix angle, grooved teeth tool and carbide standard end mill. The cross-cut and grooved teeth tools were compared to the carbide standard endmill in terms of the cutting forces. At first, some experiments were carried out using three tools at different cutting conditions in order to find stable conditions. For the obtained stable conditions, the force amplitude and profile were compared together. It was shown that the cross cut geometry was more sensitive to instability than standard and grooved geometries while the cross cut geometry produced a compressive thrust force which was the opposite of the standard geometry. Furthermore it was found that the special grooves decreased the axial force to almost a zero value. Also, they showed that the grooves considerably decreased the fluctuation of the feed and normal cutting forces.
Wang et al. (2011) verified the cutting performance of CFRPs using PCD tool during milling. They showed that the cutting force was the main factor affecting surface roughness during milling of CFRP. They observed that when the cutting forces increased up to 250 N, roughness values increased while the cutting forces increased above 250 N, roughness values decreased.
Zaghbani et al. (2012) proposed a comprehensive analysis of cutting forces during trimming of multilayer CFRP laminates. It was shown that the cutting force profile didn’t depend on the fiber orientation. They also found that fiber composite properties generated a non-linear variation of average cutting forces. They used the estimated average forces attained in experiments in order to obtain the average cutting force coefficients which were utilized in a predictive cutting force. Finally they showed that the predicted and the measured cutting forces had a good agreement at different speeds and feeds. This predicted cutting force model could be used to estimate the possible damage during trimming of CFRPs however it wasn’t a fully predictive model.
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Table des matières
INTRODUCTION
CHAPTER 1 LITERATURE REVIEW
1.1 Introduction
1.2 Mechanical properties of CFRPs
1.3 Cutting tool materials
1.4 Machining of CFRPs
1.4.1 Chip formation
1.4.2 Delamination
1.4.3 Surface quality and surface damage
1.4.4 Effect of trimming operation on the mechanical properties
1.4.5 Cutting Forces
1.4.6 Tool wear and tool life
CHAPTER 2 METHODOLOGY
2.1 Introduction
2.2 Preliminary tests and tool life test conditions
2.3 Machining set-up and equipment for experiments
2.3.1 Cutting tool
2.3.2 Workpiece material
2.3.3 Experimental set-up
2.3.4 CNC cutting machine
2.3.5 Force acquisition
2.4 Measurement set-up and equipment for quality evaluation
2.4.1 Roughness measurement
2.4.2 Tool wear measurement
2.4.3 Surface integrity evaluation
CHAPTER 3 STUDY AND MODELING OF MACHINING PARAMETERS EFFECT ON CUTTING FORCES AND SURFACE ROUGHNESS (PRELIMINARY TESTS)
3.1 Introduction
3.2 Influence of cutting parameters on Fx (Normal force)
3.3 Influence of cutting parameters on Fy (Feed force) .
3.4 Influence of cutting parameters on Fz (Axial force)
3.5 Influence of cutting parameters on Ft (Total force)
3.6 Influence of cutting parameters on Ra (Surface roughness)
3.7 Conclusion
CHAPTER 4 TOOL WEAR EFFECT ON QUALITY OF TRIMMED LAMINATES
4.1 Analysis of tool life tests
4.1.1 Tool wear analysis
4.1.2 Effect of fiber orientation on tool wear profile
4.1.3 Cutting forces analysis
4.1.4 Effect of tool wear on cutting force
4.1.5 Machined surface integrity
4.1.6 Excessive tool wear and material integrity
4.1.7 Surface Roughness
4.1.8 Effect of tool wear on roughness
4.2 Analysis of preliminary tests
4.2.1 Effect of cutting conditions on cutting forces durin g trimming using new tool
4.2.2 Effect of cutting conditions on surface roughness during trimming using new tool CONCLUSION
RECOMMENDATIONS
ANNEX I OPTICAL MICROSCOPE TYPE VHX-600+500F SPECIFICATIONS
ANNEX II PUBLISHED JOURNAL I
Abstract
Introduction
Methodology
Results and Discussions
Cutting force and tool wear
Surface Integrity
Surface Roughness
Conclusions
Acknowledgment
ANNEX III SUBMITTED JOURNAL II
LIST OF BIBLIOGRAPHICAL REFERENCES
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