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An experimental investigation on the process parameters influencing machining forces during milling of carbon and glass fiber laminates
Accepted Manuscript An Experimental Investigation on the Process Parameters Influencing Machining Forces during Milling of Carbon and Glass Fiber Laminates N. Rajesh Mathivanan, B.S. Mahesh, H. Anup Shetty PII: DOI: Reference:
S0263-2241(16)30157-9 http://dx.doi.org/10.1016/j.measurement.2016.04.077 MEASUR 4027
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
14 May 2015 11 March 2016 27 April 2016
Please cite this article as: N. Rajesh Mathivanan, B.S. Mahesh, H. Anup Shetty, An Experimental Investigation on the Process Parameters Influencing Machining Forces during Milling of Carbon and Glass Fiber Laminates, Measurement (2016), doi: http://dx.doi.org/10.1016/j.measurement.2016.04.077
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An Experimental Investigation on the Process Parameters Influencing Machining Forces during Milling of Carbon and Glass Fiber Laminates Rajesh Mathivanan N., Mahesh B. S., Anup Shetty. H* Dept. of Mechanical Engg., P. E. S. Institute of Technology, Bangalore - 85, INDIA. *Corresponding author email: [email protected] *Tel : +91 9731809601
Abstract Milling is the most feasible machining operation for producing slots and keyways with a well defined and high quality surface. Milling of composite materials is a complex task owing to its heterogeneity and the associated problems such as surface delamination, fiber pullout, burning, fuzzing and surface roughness. The machining process is dependent on the material characteristics and the cutting parameters. An attempt is made in this work to investigate the influencing cutting parameters affecting milling of composite laminates. Carbon and glass fibers were used to fabricate laminates for experimentations. The milling operation was performed with different feed rates, cutting velocity and speed. Numerically controlled vertical machining canter was used to mill slots on the laminates with different cutting speed and feed combinations. A milling tool dynamo meter was used to record the three orthogonal components of the machining force. From the experimental investigations, it was noticed that the machining force increases with increase in speed. For the same feed rate the machining force of GFRP laminates was observed to be very minimal, when compared to machining force of CFRP laminates. It is proposed to perform milling operation with lower feed rate at higher speeds for optimal milling operation. Keywords: Milling; Machining force; Composite laminates; Cutting parameters.
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1. Introduction Composite materials such as Carbon Fiber Reinforced Plastics (CFRP) and Glass Fiber Reinforced Plastics (GFRP) are characterised for having excellent properties, like light weight, high strength to weight ratio and high specific stiffness to weight ratio. These properties are very much imperative and make them attractive for automotive and aerospace applications [1-2]. These composite materials are optimal replacement materials for various other industrial products. As GFRP and CFRP composites have various industrial applications the composites will undergo various machining operations like drilling and milling. Achieving an acceptable surface quality with conventional methods of machining is found to be extremely difficult due to the composite’s anisotropic and non-homogeneous nature. It is very important to understand the mechanisms of material removal and the kinetics of machining process affecting the performance of cutting tools for achieving the desired quality of the machined surface. With regard to quality of machined component during drilling, the principal drawbacks are usually, surface delamination, fiber pullout, burning, fuzzing and inadequate surface roughness of the drill hole. Similarly there are many drawbacks when the laminates undergo milling operation. But since, very less number of research studies has been done on milling of CFRP and GFRP composite laminates, hence it creates a research gap to investigate the laminates under various cutting and load conditions during milling process. To control the geometrical tolerance in composite laminates, it is essential to perform machining operations. Milling is usually performed in CFRP and GFRP laminates to cut slots and groves. Milling is used as a corrective operation to produce a well defined and high quality surface [3]. Precision machining needs to be performed to ensure the dimensional stability and interface quality [4]. Milling is extensively used for making slots and keyways which are vulnerable to the stress concentration and source of defects. Hence the cutting
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parameters, machining forces and the strength of the material must be elaborately studied. Davim et al. [5] carried out the experimentation on milling of CFRP and found out the results that the machining force (F) in the work piece increases with the feed rate and decreases with cutting velocity. Considering the same cutting parameters (cutting speed and feed rate), they indicated that the feed rate is the cutting parameter which has greater influence on machining force in the work piece. When investigating the machining conditions of fiber reinforced polymers, Bhatnagar et. al. [6] have revealed that the cutting forces are dependent on the fiber angle as well as the cutting direction, while Jenarthanan et al. [7] indicated that the ratio of the cutting force and thrust force differ and behave in a cyclic fashion. These cutting characteristics are also found to be effective and influence the thrust force and torque as presented by Santhanakrishnan et al. [8]. Palanikumar et al. [9] have attempted to assess the influence of machining parameters on surface roughness in machining GFRP composites and concluded that the feed rate has more influence on surface roughness and it is followed by cutting speed .The same results were obtained by the tests conducted by Praveen et al. [10] The extensive works have been done on occurrence of delamination of the top layers during the machining of CFRP tape, with the focus being on the process of contour milling. The occurrence and propagation of delamination were studied by milling slots in CFRP specimens having different fibre orientations. The results showed that delamination is highly dependent on the fibre orientation and the tool sharpness [11]. Zhang et al. [12] investigated damage that occurs when drilling CFRP component and found that delamination depends on angle between cutting direction and fiber direction. The damage analysis of delamination factor while machining using different drill viz, twist drill, candle stick drill and saw drill was
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predicted and evaluated by Tsao and Hocheng [13] based on Taguchi’s method and the analysis of variance (ANOVA). Ramulu [14] evaluated the effects of tool geometry and operating conditions from an analysis of chip formation, cutting force, and machined surface topography. Three distinct mechanisms in the edge trimming of fibre-reinforced composite material including a combination of cutting, sheafing, and fracture along the fibre/matrix interface were observed during the machining process. Ramulu indicates that the machining characteristics may not only affect bulk properties but also influence the initiation and propagation of failure in composite laminates. Arokiadass et. al. [15] studied the influence of machining parameters on surface roughness (Ra) through the response surface methodology, which indicate that the feed rate was the most dominant parameter affecting the surface roughness. The effects of process parameters on delamination during high-speed drilling with cemented carbide twist drills on CFRP composite laminates were investigated with the help of full factorial design of experiments [16]. The investigation revealed that the delamination tendency decreases with increase in cutting speed and lower feed rate. Devi et. al. [17] has developed a methodology for predicting the cutting forces by transforming specific cutting energies from orthogonal cutting to oblique cutting, which is capable of predicting the cutting forces in helical end milling of unidirectional and multidirectional composites laminates. The effect of cutting parameters and tool geometry using conventional machining and also on the phenomena associated with unconventional machining of composite material in order to make damage free machining of composite materials has been reviewed by Dhiraj and Singh [18]. While the advances on machining solutions of hybrid composite laminates considering the multiple aspects of cutting responses and physical phenomena generated is analyzed by Jinyang et. al. [19].
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From the above research work, it is imperative to investigate more on the force optimisation and comparative studies between CFRP and GFRP. Comparative studies between CFRP and GFRP will provide material characteristics applicable for critical applications, by analysing their cutting conditions, process parameters and machining conditions. The emergence of modern engineering process using new composite materials blended with advanced manufacturing techniques prompted us to go in depth in this work. Composite materials are voraciously used in the field of Aerospace, automotive, medical devices etc, which are highly critical to variable load and safety conditions. Machining of fiber reinforced composites is an important activity in the integration of these advanced materials into engineering applications. The machining of these composites poses a great threat by having so many defects which can be seen regularly in drilling and milling. Optimising the machining force by reducing these defects using various cutting parameters was the prime intention behind carrying out this work. So the main goal of this work is to investigate and discuss some experimental results obtained during milling operation of CFRP and GFRP laminate and compare the effect of machining force between CFRP and GFRP laminates.
2. Specimen Fabrication Hand layup/wet layup method is used for the fabrication of CFRP and GFRP laminates with 100 x100 mm cross sectional area. Even though there are many methods by which these test specimens can be fabricated, hand lay-up technique is chosen as it is most economical and best suits for custom made low volume specimens. Composite materials manufactured by hand lay-up method are particularly very important for aerospace applications. The primary element of CFRP is a carbon filament with woven fabric and EGlass woven fabric was chosen for GFRP laminates. Araldite LY 556 epoxy resin with HY
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951 grade has been used as curing hardener. Each layer of the carbon fiber and glass fiber is of 0.2 mm and 0.3mm of thickness respectively and the stacking sequence used with an orientation of +/- 90° placed alternatively for 62 ply of carbon woven fabric and 50 ply of Eglass woven fabric to attain the specimens thickness of 15mm. The fiber volume fraction of the specimen was found to be 65%. During the first stage, the prepreg carbon fibers and glass fibers are carefully laid out on a table to make sure that fibers orientation meets the design requirement. During the second stage, pieces of prepreg materials are cut out and placed on the top of each other on a shaped tool to form a laminate. The layers could be placed in different directions to produced desired strength. After the layers of required number has been properly placed, the tooling and attached laminates are vacuum bagged for removing entrapped air from the laminates part. For final solidification the part is kept for the post curing in a hot oven at the 200° for 2 hours. After curing of the laminate it is allowed to cool at room temperature. The mechanical properties of the laminates prepared, is presented in table 1. Table. 1 Mechanical Properties of the laminates Sl. No 1 2 3 4 5 6
Properties Young’s modulus GPa Poisson’s ratio Ultimate tensile strength MPa Ultimate compressive strength MPa Shear strength MPa Shear modulus GPa
CFRP 70 0.1 600 570 90 5
GFRP 25 0.2 440 425 40 4
3. Experimental Investigation. 3.1 Methodology: The experiments have been carried on CFRP and GFRP laminates of 15mm thickness using Cemented Carbide (K10) end milling tool (Fig.1) with 10 mm diameter. The proportion of carbide phase in this tool is 80% of the total weight of the tool material. Tungsten Carbide (WC) is the most common hard phase, and Cobalt (Co) alloy as the most common binder 6
phase. These two materials form the basic structure of cemented carbide tool. The two fluted end mill presents the following geometry: a 30° helix angle, rake angle of 10°, clearance angle of 9° and flute length of 15mm. A key feature of this material is the potential to vary its composition so that the resulting physical and chemical properties ensure maximum resistance to wear. Cemented carbide tool is selected for experimentation based on the literature survey.
Fig 1. Cemented Carbide tool (K10)
A CNC Vertical machining centre “MITSUBHISHI- Litz Hitech CV1200A” with 8KW spindle power and a maximum spindle speed of 10000 rpm was used to perform the experiments.
The
controlling
interface
in
this
equipment
is
developed
by
MITSUBISHI/FANUC. The length of X, Y and Z axis are 600mm, 400mm and 400mm respectively. The fixation of the composite material and milling tool dynamometer was made as shown in Fig. 2, to ensure that the minimum vibrations occur and displacements are eliminated while machining the components. No external cooling agent was used during machining of the composite materials.
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Fig.2. Fixation of composite material and Milling tool dynamometer.
A milling tool Dynamometer having three digital indicators with a maximum range of 50 Kgf was used to acquire the three orthogonal components of machining forces (F) such as Fx, Fy and Fz. The machining were made on each CFRP and GFRP laminate and the machining force was recorded and analysed for each slot with respect to the parameters such as feed rate, speed and cutting velocity. The machined CFRP and GFRP laminates are shown in Fig.3.
Fig 3. Specimens after milling operation (CFRP and GFRP laminates) 8
3.2 Parameters and their levels: The three levels of cutting speed, feed and cutting velocities are shown in table 2. The three levels (Low, average and high) were provided to the parameters based on design of experiments and can be analyzed using ANOVA. The plan of experiments consists of nine tests by varying the spindle speed, cutting velocity and the feed rate. The parameters are being chosen based on extensive study on the stable measurement of the orthogonal machining forces while machining on CNC vertical milling machine. These parameters helped to obtain optimum machining forces by suiting with the geometrical dimensions of the work pieces.
Table.2. Assignment of levels to the factors Levels X1
X2
X3
40
80
120
Spindle Speed (rpm)
2000
4000
6000
Feed rate (mm/rev)
0.02
0.04
0.06
Parameters Cutting velocity (m/min)
4. Results and discussions The results of milling tests allowed the evaluation of two types of composite laminates CFRP and GFRP manufactured by hand lay-up and were machined using a cemented carbide (K10) end mill. The slot length of 20 mm with a depth of 5 mm is selected for machining. These parameters are selected after deliberated study on the keyways carved on the doors and other components of a car. The large flange rivets used on doors are about 14-20 mm length. The machinability was evaluated by machining force in the workpiece (F). The machining force value was determined by the following equation 1. (1) 9
Where Fx, Fy and Fz the 3 orthogonal components of machining force in the workpiece (F).
Fig.4. Measurement of three orthogonal components of machining force (F).
4.1 Influence of cutting parameters on machining force in milling of CFRP laminates. The results of the machining force in the workpiece (F) for the milling of CFRP as a function of cutting parameters are shown in the table 3 below.
Table.3. Values of machining force (F) as function of cutting parameters for CFRP. Test samples C 01 C 02 C 03 C 04 C 05 C 06 C 07 C 08 C 09
Velocity (mm/rev) 40 40 40 80 80 80 120 120 120
Speed (rpm) 2000 2000 2000 4000 4000 4000 6000 6000 6000
Feed rate (mm/rev) 0.02 0.04 0.06 0.02 0.04 0.06 0.02 0.04 0.06
Machining force F (N) 36.70 44.95 62.81 48.05 54.95 68.67 57.20 65.6 73.2
10
80
73.2
Machining Force (Fm)
70 65.6
60 50 40
57.2 48.05
68.67 62.81
54.95 44.95
Speed 1 (2000 rpm)
36.7
Speed 2 (4000 rpm)
30
Speed 3 (6000 rpm) 20 10 0 0.02
0.04 Feed Rate
0.06
Fig. 5. Machining force (F) in the CFRP workpiece as a function of feed rate
The behaviour of the machining force (F) in the workpiece with the feed for different cutting speed values are shown in fig. 5. The graph illustrates the history of feed rate and the machining force. It is realised from the curves that the machining force increases with increase in speed by keeping feed rate constant. This is evident from the graph as the machining force kept on increasing for the change of feed rate when the speed is kept constant. When the particular feed is considered, it is certain from the force histories that the machining force increases drastically when the feed rate is increased to a higher value. The supporting evidence for the above inference can be extracted from the curves in the figure that for a particular feed rate of 0.02, the machining force has increased from 36.7 to 48.05 when the speed changed from 2000 rpm to 4000 rpm. In the next leg it was increased from 48.05 to 57.2. This will allow us to categorically say that the increase in machining force was uniform. But while machining the slots of C07, C08 and C09, at higher feed rate of 0.06, the drastic increase in machining force is seen. It was 62.81 at the speed of 2000 rpm which is 11
almost double the value of machining force at feed rate of 0.02. This shows that at higher feed rate the laminate shows more stiffness and stress for the external force applied while machining.
Fig. 6. Machining force (F) in the CFRP work piece as function of Cutting velocity
The histories of machining force and cutting velocity are shown in the fig.6. The curves in the figure enable us to conclude that with increase in feed rate, the machining force increases by keeping velocity constant. It is evident from the graph that for the velocity of 40mm/rev, the value of the machining force (F) increased from 36.7 to 44.95mm/rev with the increase of feed from 0.02 to 0.04 respectively. But for the higher feed of 0.06 there was a steep increase of machining force from 44.95 to 62.81 which means to say that machining of a laminate requires more force when machined at higher feed rate. The same trend can be seen for a velocity of 80 and 120mm/ rev also. This allow us to draw an inference that feed rate influences more on machining force when the laminate undergoes milling operation.
It is very well known fact that to optimally cut a material the cutting force or the machining force must be as much less as it can. But if it is more, then it is attributed to the properties of a laminate. And being CFRP, a strong and stiff material it offers more stress
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when it is cut at higher cutting velocity. That is the reason for increase in machining force at the higher velocity. So we can draw an inference that feed rate 0.02 can be recommended to cut at various cutting velocities to get the economical machining force in milling. From the machining values, which are shown in table 3, it is seen that with the instantaneous change in cutting velocity, the F initially decreases but recovers as feed increases. The evidence for the above conclusion is that the machining force decreased from 62.81 to 48.05 when cutting velocity increased from 40 to 80 mm/rev. and then it increased from 48.05 to 54.95 with increase in feed. The same trend is seen when the cutting velocity is changed to higher value. It can also be seen that the maximum machining force is developed at higher speed. This shows that the main factor which influences the machining force in milling is the Feed rate.
80 70
6.59%
9.32% 19.5% 65.6
60
19.04% 57.02
50 40
30
30.92% 48.05
22.2
73.2
68.67
62.81
54.95
44.95
36.7
20 10 0 0.02
Speed 1 (2000 rpm)
0.04
Speed 2 (4000 rpm)
0.06
Speed 3 (6000 rpm)
Fig.7. Increase in percentage of machining force as a function of feed rate.
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From the fig. 7, the percentage increase of machining force with successive increase in speed by keeping feed rate constant is seen. We can evidence from the above graph that at higher feed rate the percentage increase in machining force decreases with increase in speed. The reason to the decrease in increase of percentage is that the top layers are strong and can endure more than the core layers of CFRP laminates.
4.2 Influence of cutting parameters on machining force in milling of GFRP. The results of the machining force in the workpiece (F) for the milling of GFRP as a function of cutting parameters are shown in the table 4 below. Table.4. Values of machining force (F) as function of cutting parameters for GFRP. Test samples G 01 G 02 G 03 G 04 G 05 G 06 G 07 G 08 G 09
Velocity (m/min) 40 40 40 80 80 80 120 120 120
Speed (rpm) 2000 2000 2000 4000 4000 4000 6000 6000 6000
Feed rate (mm/rev) 0.02 0.04 0.06 0.02 0.04 0.06 0.02 0.04 0.06
Machining force F (N) 24.02 29.43 36.53 32.70 36.02 50.02 36.70 41.62 57.20
Machining Force (Fm)
70 60 57.2 50
50.02
40 30
36.7 32.7
20
24.02
41.62 36.02
36.53
29.43
Speed 1 (2000rpm) Speed 2 (4000 rpm) Spped 3 (6000 rpm)
10 0 0.02
0.04
0.06
Feed Rate
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Fig.8. Machining force in the GFRP workpiece (F) as a function of feed rate. The behaviour of the machining force and feed rate is shown in the Fig .8. It is analysed from the curves of the graph that the machining force (F) increases with increase in speed by keeping feed rate constant. The histories show that the machining force increased uniformly when the speed changed for a particular feed rate. That is to say that it increased from 24.02 to 32.7 when the speed changed from 2000 to 4000 rpm. And in the next level the machining force increased to 36.7 for the speed of 6000 rpm. The same trend is seen when the milling is done at the feed rate of 0.04 for test samples G02, G05 and G08, respectively. But for the feed rate of 0.06 there is a steep increase of machining force from 36.53 to 57.2 for the speed of 2000 and 4000rpm and comparatively low increment for the next level of speed 6000 rpm.
Fig.9. Machining force in the GFRP workpiece (F) as a function of Cutting velocity.
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The Machining force versus cutting velocity graph for milling of GFRP laminate is shown in the fig.9. It is witnessed from the graph that by keeping the cutting velocity constant, the machining force increases more steeply at higher feed rate. For the cutting speed of 40mm/rev, the machining force increased from 24.02 to 29.43 when the feed changed from 0.02 to 0.04. Further when feed increased from 0.04 to 0.06, the machining force increased to 36.53. This shows that there is a uniform increase in machining force for the particular speed. Same condition is seen for the Cutting velocity of 80mm/rev, for test samples G04, G05 and G06. But for 120mm/rev, there is a massive increase of machining force from 41.62 to 57.2 when the feed was changed from 0.04 to 0.06 respectively. This shows that machining force develops more at higher feed rate. And as has been said, the process will be ideal if cutting force is less for a particular material unless the material is strong and has more stiffness. So the ideal condition for machining a GFRP laminate is by machining with different speed by keeping feed rate constant. From the table 4, the values enable us to state that the machining force initially decreases with increase in cutting velocity. But gradually it recovers as feed increases. When the cutting velocity increased instantaneously from 40 to 80mm/rev, the machining force value decreased from 36.53 to 32.70 for test samples G03 and G04 and it recovered as feed increased. The same trend can be seen for the change of cutting velocity from 80 to 120 mm/rev
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70
Machining Force (Fm)
60 14.3% 57.2
50
50.02 40
12.23% 36.7
30
36.13
32.7
15.54 41.62 21.94 36.02
36% 36.53
29.43
24.02
20 10 0
0.02
Speed 1 (2000 rpm)
0.04
Feed Rate
Speed 2 (4000 rpm)
0.06
Speed 3 (6000 rpm)
. Fig.10. Increase in percentage of machining force (F) as a function of feed rate. From the graph shown in Fig.10, the inference is drawn that the percentage increase in machining force (F) decreases when the speed increases. But for the second iterative the percentage increase will decrease in a marginal value.
4.3 Comparison between the influence of cutting parameters on machining forces in CFRP and GFRP. The comparative representation of machining forces as function of feed rate for different speeds (2000, 4000, 6000 rpm) are show in fig.11, 12, and13 respectively. The thorough investigation on the machining values of both CFRP and GFRP laminates declares that for a particular feed rate, the machining force of GFRP is very less compared to machining force of CFRP laminate which shows that the Carbon fiber reinforced plastic is more stiff and stronger than the Glass fiber reinforced plastic. The value of machining force attained highest (73.2) while milling of CFRP laminates and the lowest (24.02) is attained while milling of GFRP laminate. 17
CFRP laminate endures more and develops more stress for the external force applied on it than a GFRP laminate. Therefore while milling of GFRP the maximum F was 57.20 for test sample G09 and minimum was 24.02 for test sample G01. For CFRP, the machining force was maximum of 73.2 and minimum of 36.70 recorded. When the percentage increase in machining force is compared between CFRP and GFRP laminate, it is seen that the percentage increase of cutting force is more in CFRP than GFRP laminate which is the another evident to show that CFRP material offers more stress, stronger and stiffer than GFRP material. When the cutting velocity is changed to the higher velocity instantaneously, the machining force decreases more intensely in CFRP than GFRP but it recovers as the feed rate gets increased in both materials. This shows that instantaneously change in cutting velocity decreases the cutting speed.
2000 rpm Machining Force (Fm)
70 60
62.81
50 44.95
40
30 20
36.7 24.02
36.53 29.43
Speed 1 (GFRP) Speed 1 (CFRP)
10 0 0.02
0.04
0.06
Feed Rate (f)
Fig.11. Comparison of CFRP and GFRP laminates for the speed of 2000rpm
Fig.11 is a graph plotted for the different Machining force values obtained at different feed rates for both CFRP and GFRP composite laminates at a speed of 2000 rpm. From the graph it is analysed that at the speed of 2000 rpm the machining force for GFRP composite material gradually increases from 24.02 to 29.43 with increase of feed from 0.02 to 0.04 and it 18
increases from 29.43 to 36.53when the feed was increased from 0.04 to 0.06. In the case of CFRP composite laminate, first there is a gradual increase of machining force from 36.7 to 44.95, when the feed is increased from 0.02 to 0.04respectively. But there is a steep increase in machining force from 44.95 to 62.81, when feed is increased from 0.04 to 0.06 respectively.
4000 rpm Machining Force (Fm)
80 70 68.67
60
50 40 30
54.95 50.02
48.05 32.7
Speed 2 (GFRP)
36.02
Speed 2 (CFRP)
20 10 0 0.02
0.04
0.06
Feed rate (F)
Fig.12. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. The graph plotted for the different Machining force values obtained at different feed rates for both CFRP and GFRP composites at a speed of 4000 rpm is shown in fig.12. From the curves it is evident that both GFRP and CFRP laminates behave in the same way for the speed of 4000 rpm. It let us to say that there is an increase of 14 newtons of machining force when both materials are machined with increased feed rate from 0.04 to 0.06. They both behave in a same trend as far as the speed 4000 rpm is concerned.
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6000 rpm Machining Force (Fm)
80 70
73.2
65.6
60 50
57.2
40 30
36.7
57.2 41.62
Speed 3 (GFRP) Speed 3 (CFRP)
20 10 0 0.02
0.04
0.06
Feed rate (F)
Fig.13. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. The Machining force (F) values obtained at different feed rates for both CFRP and GFRP composites at a speed of 6000 rpm is shown in fig. 13. From this graph it is evident that at the speed of 6000 rpm for GFRP composite laminate there is a gradual increase of machining force from 36.7 to 41.62 when the feed is increased from 0.02 to 0.04 but there is a steep increase of machining force i.e. 41.62 to 57.2 when the feed rate was increased from 0.04 to 0.06. In the case of CFRP, the same trend followed as the curves are similar in nature. For a particular feed rate, the machining force was more while machining CFRP than GFRP laminate.
5. Conclusion In the light of the experimental results obtained, the following conclusion can be drawn from the milling of both CFRP and GFRP composite laminates.
The feed rate and speed have statistical and physical significance on the machining force in the work piece. Especially the feed rate has more influence on the machining force.
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The machining force increases with increase in speed when the feed rate is kept constant in milling of CFRP & GFRP laminates. The maximum stress was observed at higher feed rate.
It is suggested to use lower feed rates with higher speeds for optimal and economical milling of CFRP & GFRP laminates.
The instantaneous change in cutting velocity decreases the machining force initially but recovers as feed increases.
For the same feed rate the machining force of GFRP laminates is observed to be very less when compared to machining force of CFRP laminates.
The percentage increase of cutting force is more in CFRP than GFRP laminate which is evident that CFRP material offers more stress, strength and stiffness than GFRP material.
References [1] Smith W.F., Principles of Materials science and engineering. Mc graw Hill, 1990, 699724. [2] User manual for machining carbon fiber reinforced plastic composites (CFRP), Sandvik publication, 2009. [3] Jahannmir S., Ramulu M., Koshy P., Machining Of Ceramic and Composites. Marceldekker Inc, Newyork, 2000, 267-297. [4] Ferreira, J.R., Coppini, N.L., Miranda, G.W., Machining Optimisation in Carbon fiber reinforced Composites materials. J. Mater. Process. Technol. 1999, 135-140. [5] Paulo Davim, J., Pedro Reis., Damage and Dimension Precision on milling carbon fiber reinforced plastic using design experiments. J. Mater. Process. Technol. 2005, 160–167.
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[6] Bhatnagar N., Ramakrishnan N., Naik N.K., Komanduri, R., On the Machining of Fiber Reinforeced Plastic (FRP) Composites laminates. Int. J. Mach. tools manuf. 1995, 35, 701716. [7] Jenarthanan M.P., Jeyapaul R., Evaluation of machinability index on milling of GFRP composites with different fibre orientations using solid carbide endmill with modified helix angles. Int. J. Eng. Sci. 2014, 6, 1-10. [8] Santhanakrishnan G., Krishnamurthy R., Malhota S.K., Machinability characteristics of fibre reinforced plastics composites. J. Mater. Process. Technol. 1988, 17, 195–204. [9] Palanikumar, K., Karunamoorthy, R., Karthikeyan., Latha, B., Optimization of machining parameters in turning GFRP composites using a carbide (K10) tool based on the taguchi method with fuzzy logics. Met. Mater. Int. 2006, 12, 483-491. [10] Praveen Raj, P., ElayaPerumal, A., Ramu, P., Prediction of Surface roughness and Delamination in End Milling of GFRP using ANSYS. Indian J. Eng. Mater. Sci. 2013, 6, 107-120. [11] Hintze W.F., Dirk Hartmann, Christoph Schutte, Occurrence And propagation of delamination during the machining of carbon fiber reinforced plastic (CFRP)- An experimental study. Compos. Sci. Technol. 2011, 71, 1719-1726. [12] Zhang H., Chen W., Zhang L., Assessment of exit defects in Carbon Fiber reinforced plastic plates caused by drilling. Key Eng. Mater. 2001, 196, 43-52. [13] TSao C.C. and Hocheng H, Taguchi analysis of delamination associated with various drill bits in drilling of composite material. Int. J. Mac Tools & Mnf. 2004, 44, 1085–90 [14] Ramulu M., Machining and surface integrity of fiber reinforced plastic composites, University of Washington, Seatle, USA., Sadhana, June 1997, 22, Part 3, 449-472.
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[15] Arokiadass R., Palaniradja K., Alagumoorthi N., Predictive modeling of surface roughness in end milling of composite materials, Scholars Research Lib, Appl. Sci. Res., 2011, 228-236. [16] Kyu yvol Park, Jin ho choi, Dai gil jee, Delamination-Free and High Efficiency Drilling of Carbon Fiber Reinforced Plastics, J. Compos. Mater., 2013, 13. [17] Gaitonde V. N., Karnik S.R., Campos Rubio J., Esteves Correia, Paulo Davim,J., Analysis of parametric influence on delamination in high-speed drilling of carbon fiber reinforced plastic composites, J. Mater. Process. Technol.,2008, 203, 431–438. [18] Devi Kalla, Jamal Sheikh, Janet Twomey, Prediction of cutting forces in helical end milling fiber reinforced polymers, Int. J. Mach. Tools Manuf, 2010, 50, 882–891. [20] Dhiraj kumar and K. K. Singh, An approach towards damage free machining of CFRP and GFRP composite materials: A Review, Adv. Compos. Mater., 2015, 24. [21] Jinyang, Makaddem Ali, Mohammed El Mansari, Recent advances in drilling hybrid FRP/Ti composite: A State of the art review, Compos. Struct., Jan 2016,316-338.
List of figures Fig.1. Cemented carbide tool (K10) Fig.2. Mounting of composite laminate and milling tool dynamometer. Fig.3. Specimens after milling operation (CFRP and GFRP laminates) Fig.4. Direction of measurement of three orthogonal components (F). Fig.5. Machining force (F) in the CFRP laminate as a function of feed rate. Fig.6. Machining force (F) in the CFRP work piece as function of Cutting velocity Fig.7. Increase in percentage of machining force during milling of CFRP laminates Fig.8. Machining force (F) in the GFRP laminate as a function of feed rate
23
Fig.9. Machining force (F) in the GFRP laminate as a function of Cutting velocity Fig.10. Increase in percentage of machining force during milling of GFRP laminates Fig.11. Comparison of CFRP and GFRP laminates for the speed of 2000 rpm Fig.12. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. Fig.13. Comparison of CFRP and GFRP laminates for the speed of 6000 rpm.
List of Tables Table.1. Mechanical Properties of the laminates Table.2. Assignment of levels to the factors Table.3. Values of machining force (F) as function of cutting parameters for CFRP. Table.4. Values of machining force (F) as function of cutting parameters for GFRP.
24
Figures
Fig. 1. Cemented carbide tool (K10)
Fig. 2. Mounting of composite laminate and milling tool dynamometer.
Fig. 3.Specimens after milling operation (CFRP and GFRP laminates) 25
Fig. 4. Direction of measurement of three orthogonal components (Fm).
80 73.2
Machining Force (Fm)
70 65.6
60 57.2
54.95
48.05
44.95
50
68.67 62.81
40 36.7 30 Speed 1 (2000 rpm)
20
Speed 2 (4000 rpm) 10 Speed 3 (6000 rpm) 0 0.02
0.04
0.06
Feed Rate (mm/rev)
Fig. 5. Machining force (Fm) in the CFRP laminate as a function of feed rate.
26
Fig. 6. Machining force (F) in the CFRP work piece as function of Cutting velocity
80 6.59%
70
Machining Force (Fm)
9.32% 60
19.5% 19.04% 57.02
50 40 30
30.92%
48.05
65.6
73.2
68.67
62.81
22.2% 54.95 44.95
36.7
20 10 0 0.02
0.04
0.06
Feed rate (mm/rev) Speed 1 (2000 rpm)
Speed 2 (4000 rpm)
Speed 3 (6000 rpm)
Fig. 7. Increase in percentage of machining force during milling of CFRP laminates
27
Machining Force (Fm)
70 60 57.2 50
50.02
40 30
36.7 32.7
20
24.02
41.62 36.02
36.53
29.43
Speed 1 (2000rpm) Speed 2 (4000 rpm) Spped 3 (6000 rpm)
10
0 0.02
0.04
0.06
Feed Rate Fig.8. Machining force in the GFRP workpiece (F) as a function of feed rate.
Fig.9. Machining force in the GFRP workpiece (F) as a function of Cutting velocity.
28
70
Machining Force (Fm)
60 14.3% 57.2
50 36% 40
15.54% 12.23%
30 20
36.7
36.13% 32.7
50.02
41.62 36.53
21.94% 36.02 29.43
24.02
10 0 0.02
0.04
0.06
Feed Rate (mm/rev) Speed 1 (2000 rpm)
Speed 2 (4000 rpm)
Speed 3 (6000 rpm)
Fig. 10. Increase in percentage of machining force during milling of GFRP laminates Milling at 2000 rpm 70
Machining Force (Fm)
60
62.81
50 44.95
40
36.7
36.53
30 29.43
20
24.02 Speed 1 (GFRP)
10
Speed 1 (CFRP) 0 0.02
0.04 Feed Rate (mm/rev)
0.06
Fig. 11. Comparison of CFRP and GFRP laminates for the speed of 2000 rpm
29
Milling at 4000 rpm 80
Machining Force (Fm)
70 68.67
60 54.95
50
50.02
48.05
40 30
32.7
36.02
20
Speed 2 (GFRP)
10 Speed 2 (CFRP)
0 0.02
0.04
0.06
Feed rate (mm/rev)
Fig.12. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. Milling at 6000 rpm 80
Machining Force (Fm)
70
73.2 65.6
60 50
57.2
40 30
57.2 41.62
36.7
20
Speed 3 (GFRP) 10 Speed 3 (CFRP)
0 0.02
0.04
0.06
Feed rate (mm/rev)
Fig.13. Comparison of CFRP and GFRP laminates for the speed of 6000 rpm.
30
Tables
Table. 1 Mechanical Properties of the laminates Sl. No
Properties
CFRP
GFRP
1
Young’s modulus GPa
70
25
2
Poisson’s ratio
0.1
0.2
3
Ultimate tensile strength MPa
600
440
4
Ultimate compressive strength MPa
570
425
5
Shear strength MPa
90
40
6
Shear modulus GPa
5
4
Table.2. Assignment of levels to the factors Levels Parameters
X1
X2
X3
40
80
120
Spindle Speed (rpm)
2000
4000
6000
Feed rate (mm/rev)
0.02
0.04
0.06
Cutting velocity (m/min)
Table.3. Values of machining force (Fm) as function of cutting parameters for CFRP. Test samples
Velocity (mm/rev)
Speed (rpm)
Feed rate (mm/rev)
Machining force Fm (N)
C 01
40
2000
0.02
36.70
C 02
40
2000
0.04
44.95
C 03
40
2000
0.06
62.81
C 04
80
4000
0.02
48.05
C 05
80
4000
0.04
54.95
C 06
80
4000
0.06
68.67
C 07
120
6000
0.02
57.20
C 08
120
6000
0.04
65.6
C 09
120
6000
0.06
73.2
31
Table.4. Values of machining force (Fm) as function of cutting parameters for GFRP. Test samples
Velocity (m/min)
Speed (rpm)
Feed rate (mm/rev)
Machining force Fm (N)
G 01
40
2000
0.02
24.02
G 02
40
2000
0.04
29.43
G 03
40
2000
0.06
36.53
G 04
80
4000
0.02
32.70
G 05
80
4000
0.04
36.02
G 06
80
4000
0.06
50.02
G 07
120
6000
0.02
36.70
G 08
120
6000
0.04
41.62
G 09
120
6000
0.06
57.20
32
S0263-2241(16)30157-9 http://dx.doi.org/10.1016/j.measurement.2016.04.077 MEASUR 4027
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
14 May 2015 11 March 2016 27 April 2016
Please cite this article as: N. Rajesh Mathivanan, B.S. Mahesh, H. Anup Shetty, An Experimental Investigation on the Process Parameters Influencing Machining Forces during Milling of Carbon and Glass Fiber Laminates, Measurement (2016), doi: http://dx.doi.org/10.1016/j.measurement.2016.04.077
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An Experimental Investigation on the Process Parameters Influencing Machining Forces during Milling of Carbon and Glass Fiber Laminates Rajesh Mathivanan N., Mahesh B. S., Anup Shetty. H* Dept. of Mechanical Engg., P. E. S. Institute of Technology, Bangalore - 85, INDIA. *Corresponding author email: [email protected] *Tel : +91 9731809601
Abstract Milling is the most feasible machining operation for producing slots and keyways with a well defined and high quality surface. Milling of composite materials is a complex task owing to its heterogeneity and the associated problems such as surface delamination, fiber pullout, burning, fuzzing and surface roughness. The machining process is dependent on the material characteristics and the cutting parameters. An attempt is made in this work to investigate the influencing cutting parameters affecting milling of composite laminates. Carbon and glass fibers were used to fabricate laminates for experimentations. The milling operation was performed with different feed rates, cutting velocity and speed. Numerically controlled vertical machining canter was used to mill slots on the laminates with different cutting speed and feed combinations. A milling tool dynamo meter was used to record the three orthogonal components of the machining force. From the experimental investigations, it was noticed that the machining force increases with increase in speed. For the same feed rate the machining force of GFRP laminates was observed to be very minimal, when compared to machining force of CFRP laminates. It is proposed to perform milling operation with lower feed rate at higher speeds for optimal milling operation. Keywords: Milling; Machining force; Composite laminates; Cutting parameters.
1
1. Introduction Composite materials such as Carbon Fiber Reinforced Plastics (CFRP) and Glass Fiber Reinforced Plastics (GFRP) are characterised for having excellent properties, like light weight, high strength to weight ratio and high specific stiffness to weight ratio. These properties are very much imperative and make them attractive for automotive and aerospace applications [1-2]. These composite materials are optimal replacement materials for various other industrial products. As GFRP and CFRP composites have various industrial applications the composites will undergo various machining operations like drilling and milling. Achieving an acceptable surface quality with conventional methods of machining is found to be extremely difficult due to the composite’s anisotropic and non-homogeneous nature. It is very important to understand the mechanisms of material removal and the kinetics of machining process affecting the performance of cutting tools for achieving the desired quality of the machined surface. With regard to quality of machined component during drilling, the principal drawbacks are usually, surface delamination, fiber pullout, burning, fuzzing and inadequate surface roughness of the drill hole. Similarly there are many drawbacks when the laminates undergo milling operation. But since, very less number of research studies has been done on milling of CFRP and GFRP composite laminates, hence it creates a research gap to investigate the laminates under various cutting and load conditions during milling process. To control the geometrical tolerance in composite laminates, it is essential to perform machining operations. Milling is usually performed in CFRP and GFRP laminates to cut slots and groves. Milling is used as a corrective operation to produce a well defined and high quality surface [3]. Precision machining needs to be performed to ensure the dimensional stability and interface quality [4]. Milling is extensively used for making slots and keyways which are vulnerable to the stress concentration and source of defects. Hence the cutting
2
parameters, machining forces and the strength of the material must be elaborately studied. Davim et al. [5] carried out the experimentation on milling of CFRP and found out the results that the machining force (F) in the work piece increases with the feed rate and decreases with cutting velocity. Considering the same cutting parameters (cutting speed and feed rate), they indicated that the feed rate is the cutting parameter which has greater influence on machining force in the work piece. When investigating the machining conditions of fiber reinforced polymers, Bhatnagar et. al. [6] have revealed that the cutting forces are dependent on the fiber angle as well as the cutting direction, while Jenarthanan et al. [7] indicated that the ratio of the cutting force and thrust force differ and behave in a cyclic fashion. These cutting characteristics are also found to be effective and influence the thrust force and torque as presented by Santhanakrishnan et al. [8]. Palanikumar et al. [9] have attempted to assess the influence of machining parameters on surface roughness in machining GFRP composites and concluded that the feed rate has more influence on surface roughness and it is followed by cutting speed .The same results were obtained by the tests conducted by Praveen et al. [10] The extensive works have been done on occurrence of delamination of the top layers during the machining of CFRP tape, with the focus being on the process of contour milling. The occurrence and propagation of delamination were studied by milling slots in CFRP specimens having different fibre orientations. The results showed that delamination is highly dependent on the fibre orientation and the tool sharpness [11]. Zhang et al. [12] investigated damage that occurs when drilling CFRP component and found that delamination depends on angle between cutting direction and fiber direction. The damage analysis of delamination factor while machining using different drill viz, twist drill, candle stick drill and saw drill was
3
predicted and evaluated by Tsao and Hocheng [13] based on Taguchi’s method and the analysis of variance (ANOVA). Ramulu [14] evaluated the effects of tool geometry and operating conditions from an analysis of chip formation, cutting force, and machined surface topography. Three distinct mechanisms in the edge trimming of fibre-reinforced composite material including a combination of cutting, sheafing, and fracture along the fibre/matrix interface were observed during the machining process. Ramulu indicates that the machining characteristics may not only affect bulk properties but also influence the initiation and propagation of failure in composite laminates. Arokiadass et. al. [15] studied the influence of machining parameters on surface roughness (Ra) through the response surface methodology, which indicate that the feed rate was the most dominant parameter affecting the surface roughness. The effects of process parameters on delamination during high-speed drilling with cemented carbide twist drills on CFRP composite laminates were investigated with the help of full factorial design of experiments [16]. The investigation revealed that the delamination tendency decreases with increase in cutting speed and lower feed rate. Devi et. al. [17] has developed a methodology for predicting the cutting forces by transforming specific cutting energies from orthogonal cutting to oblique cutting, which is capable of predicting the cutting forces in helical end milling of unidirectional and multidirectional composites laminates. The effect of cutting parameters and tool geometry using conventional machining and also on the phenomena associated with unconventional machining of composite material in order to make damage free machining of composite materials has been reviewed by Dhiraj and Singh [18]. While the advances on machining solutions of hybrid composite laminates considering the multiple aspects of cutting responses and physical phenomena generated is analyzed by Jinyang et. al. [19].
4
From the above research work, it is imperative to investigate more on the force optimisation and comparative studies between CFRP and GFRP. Comparative studies between CFRP and GFRP will provide material characteristics applicable for critical applications, by analysing their cutting conditions, process parameters and machining conditions. The emergence of modern engineering process using new composite materials blended with advanced manufacturing techniques prompted us to go in depth in this work. Composite materials are voraciously used in the field of Aerospace, automotive, medical devices etc, which are highly critical to variable load and safety conditions. Machining of fiber reinforced composites is an important activity in the integration of these advanced materials into engineering applications. The machining of these composites poses a great threat by having so many defects which can be seen regularly in drilling and milling. Optimising the machining force by reducing these defects using various cutting parameters was the prime intention behind carrying out this work. So the main goal of this work is to investigate and discuss some experimental results obtained during milling operation of CFRP and GFRP laminate and compare the effect of machining force between CFRP and GFRP laminates.
2. Specimen Fabrication Hand layup/wet layup method is used for the fabrication of CFRP and GFRP laminates with 100 x100 mm cross sectional area. Even though there are many methods by which these test specimens can be fabricated, hand lay-up technique is chosen as it is most economical and best suits for custom made low volume specimens. Composite materials manufactured by hand lay-up method are particularly very important for aerospace applications. The primary element of CFRP is a carbon filament with woven fabric and EGlass woven fabric was chosen for GFRP laminates. Araldite LY 556 epoxy resin with HY
5
951 grade has been used as curing hardener. Each layer of the carbon fiber and glass fiber is of 0.2 mm and 0.3mm of thickness respectively and the stacking sequence used with an orientation of +/- 90° placed alternatively for 62 ply of carbon woven fabric and 50 ply of Eglass woven fabric to attain the specimens thickness of 15mm. The fiber volume fraction of the specimen was found to be 65%. During the first stage, the prepreg carbon fibers and glass fibers are carefully laid out on a table to make sure that fibers orientation meets the design requirement. During the second stage, pieces of prepreg materials are cut out and placed on the top of each other on a shaped tool to form a laminate. The layers could be placed in different directions to produced desired strength. After the layers of required number has been properly placed, the tooling and attached laminates are vacuum bagged for removing entrapped air from the laminates part. For final solidification the part is kept for the post curing in a hot oven at the 200° for 2 hours. After curing of the laminate it is allowed to cool at room temperature. The mechanical properties of the laminates prepared, is presented in table 1. Table. 1 Mechanical Properties of the laminates Sl. No 1 2 3 4 5 6
Properties Young’s modulus GPa Poisson’s ratio Ultimate tensile strength MPa Ultimate compressive strength MPa Shear strength MPa Shear modulus GPa
CFRP 70 0.1 600 570 90 5
GFRP 25 0.2 440 425 40 4
3. Experimental Investigation. 3.1 Methodology: The experiments have been carried on CFRP and GFRP laminates of 15mm thickness using Cemented Carbide (K10) end milling tool (Fig.1) with 10 mm diameter. The proportion of carbide phase in this tool is 80% of the total weight of the tool material. Tungsten Carbide (WC) is the most common hard phase, and Cobalt (Co) alloy as the most common binder 6
phase. These two materials form the basic structure of cemented carbide tool. The two fluted end mill presents the following geometry: a 30° helix angle, rake angle of 10°, clearance angle of 9° and flute length of 15mm. A key feature of this material is the potential to vary its composition so that the resulting physical and chemical properties ensure maximum resistance to wear. Cemented carbide tool is selected for experimentation based on the literature survey.
Fig 1. Cemented Carbide tool (K10)
A CNC Vertical machining centre “MITSUBHISHI- Litz Hitech CV1200A” with 8KW spindle power and a maximum spindle speed of 10000 rpm was used to perform the experiments.
The
controlling
interface
in
this
equipment
is
developed
by
MITSUBISHI/FANUC. The length of X, Y and Z axis are 600mm, 400mm and 400mm respectively. The fixation of the composite material and milling tool dynamometer was made as shown in Fig. 2, to ensure that the minimum vibrations occur and displacements are eliminated while machining the components. No external cooling agent was used during machining of the composite materials.
7
Fig.2. Fixation of composite material and Milling tool dynamometer.
A milling tool Dynamometer having three digital indicators with a maximum range of 50 Kgf was used to acquire the three orthogonal components of machining forces (F) such as Fx, Fy and Fz. The machining were made on each CFRP and GFRP laminate and the machining force was recorded and analysed for each slot with respect to the parameters such as feed rate, speed and cutting velocity. The machined CFRP and GFRP laminates are shown in Fig.3.
Fig 3. Specimens after milling operation (CFRP and GFRP laminates) 8
3.2 Parameters and their levels: The three levels of cutting speed, feed and cutting velocities are shown in table 2. The three levels (Low, average and high) were provided to the parameters based on design of experiments and can be analyzed using ANOVA. The plan of experiments consists of nine tests by varying the spindle speed, cutting velocity and the feed rate. The parameters are being chosen based on extensive study on the stable measurement of the orthogonal machining forces while machining on CNC vertical milling machine. These parameters helped to obtain optimum machining forces by suiting with the geometrical dimensions of the work pieces.
Table.2. Assignment of levels to the factors Levels X1
X2
X3
40
80
120
Spindle Speed (rpm)
2000
4000
6000
Feed rate (mm/rev)
0.02
0.04
0.06
Parameters Cutting velocity (m/min)
4. Results and discussions The results of milling tests allowed the evaluation of two types of composite laminates CFRP and GFRP manufactured by hand lay-up and were machined using a cemented carbide (K10) end mill. The slot length of 20 mm with a depth of 5 mm is selected for machining. These parameters are selected after deliberated study on the keyways carved on the doors and other components of a car. The large flange rivets used on doors are about 14-20 mm length. The machinability was evaluated by machining force in the workpiece (F). The machining force value was determined by the following equation 1. (1) 9
Where Fx, Fy and Fz the 3 orthogonal components of machining force in the workpiece (F).
Fig.4. Measurement of three orthogonal components of machining force (F).
4.1 Influence of cutting parameters on machining force in milling of CFRP laminates. The results of the machining force in the workpiece (F) for the milling of CFRP as a function of cutting parameters are shown in the table 3 below.
Table.3. Values of machining force (F) as function of cutting parameters for CFRP. Test samples C 01 C 02 C 03 C 04 C 05 C 06 C 07 C 08 C 09
Velocity (mm/rev) 40 40 40 80 80 80 120 120 120
Speed (rpm) 2000 2000 2000 4000 4000 4000 6000 6000 6000
Feed rate (mm/rev) 0.02 0.04 0.06 0.02 0.04 0.06 0.02 0.04 0.06
Machining force F (N) 36.70 44.95 62.81 48.05 54.95 68.67 57.20 65.6 73.2
10
80
73.2
Machining Force (Fm)
70 65.6
60 50 40
57.2 48.05
68.67 62.81
54.95 44.95
Speed 1 (2000 rpm)
36.7
Speed 2 (4000 rpm)
30
Speed 3 (6000 rpm) 20 10 0 0.02
0.04 Feed Rate
0.06
Fig. 5. Machining force (F) in the CFRP workpiece as a function of feed rate
The behaviour of the machining force (F) in the workpiece with the feed for different cutting speed values are shown in fig. 5. The graph illustrates the history of feed rate and the machining force. It is realised from the curves that the machining force increases with increase in speed by keeping feed rate constant. This is evident from the graph as the machining force kept on increasing for the change of feed rate when the speed is kept constant. When the particular feed is considered, it is certain from the force histories that the machining force increases drastically when the feed rate is increased to a higher value. The supporting evidence for the above inference can be extracted from the curves in the figure that for a particular feed rate of 0.02, the machining force has increased from 36.7 to 48.05 when the speed changed from 2000 rpm to 4000 rpm. In the next leg it was increased from 48.05 to 57.2. This will allow us to categorically say that the increase in machining force was uniform. But while machining the slots of C07, C08 and C09, at higher feed rate of 0.06, the drastic increase in machining force is seen. It was 62.81 at the speed of 2000 rpm which is 11
almost double the value of machining force at feed rate of 0.02. This shows that at higher feed rate the laminate shows more stiffness and stress for the external force applied while machining.
Fig. 6. Machining force (F) in the CFRP work piece as function of Cutting velocity
The histories of machining force and cutting velocity are shown in the fig.6. The curves in the figure enable us to conclude that with increase in feed rate, the machining force increases by keeping velocity constant. It is evident from the graph that for the velocity of 40mm/rev, the value of the machining force (F) increased from 36.7 to 44.95mm/rev with the increase of feed from 0.02 to 0.04 respectively. But for the higher feed of 0.06 there was a steep increase of machining force from 44.95 to 62.81 which means to say that machining of a laminate requires more force when machined at higher feed rate. The same trend can be seen for a velocity of 80 and 120mm/ rev also. This allow us to draw an inference that feed rate influences more on machining force when the laminate undergoes milling operation.
It is very well known fact that to optimally cut a material the cutting force or the machining force must be as much less as it can. But if it is more, then it is attributed to the properties of a laminate. And being CFRP, a strong and stiff material it offers more stress
12
when it is cut at higher cutting velocity. That is the reason for increase in machining force at the higher velocity. So we can draw an inference that feed rate 0.02 can be recommended to cut at various cutting velocities to get the economical machining force in milling. From the machining values, which are shown in table 3, it is seen that with the instantaneous change in cutting velocity, the F initially decreases but recovers as feed increases. The evidence for the above conclusion is that the machining force decreased from 62.81 to 48.05 when cutting velocity increased from 40 to 80 mm/rev. and then it increased from 48.05 to 54.95 with increase in feed. The same trend is seen when the cutting velocity is changed to higher value. It can also be seen that the maximum machining force is developed at higher speed. This shows that the main factor which influences the machining force in milling is the Feed rate.
80 70
6.59%
9.32% 19.5% 65.6
60
19.04% 57.02
50 40
30
30.92% 48.05
22.2
73.2
68.67
62.81
54.95
44.95
36.7
20 10 0 0.02
Speed 1 (2000 rpm)
0.04
Speed 2 (4000 rpm)
0.06
Speed 3 (6000 rpm)
Fig.7. Increase in percentage of machining force as a function of feed rate.
13
From the fig. 7, the percentage increase of machining force with successive increase in speed by keeping feed rate constant is seen. We can evidence from the above graph that at higher feed rate the percentage increase in machining force decreases with increase in speed. The reason to the decrease in increase of percentage is that the top layers are strong and can endure more than the core layers of CFRP laminates.
4.2 Influence of cutting parameters on machining force in milling of GFRP. The results of the machining force in the workpiece (F) for the milling of GFRP as a function of cutting parameters are shown in the table 4 below. Table.4. Values of machining force (F) as function of cutting parameters for GFRP. Test samples G 01 G 02 G 03 G 04 G 05 G 06 G 07 G 08 G 09
Velocity (m/min) 40 40 40 80 80 80 120 120 120
Speed (rpm) 2000 2000 2000 4000 4000 4000 6000 6000 6000
Feed rate (mm/rev) 0.02 0.04 0.06 0.02 0.04 0.06 0.02 0.04 0.06
Machining force F (N) 24.02 29.43 36.53 32.70 36.02 50.02 36.70 41.62 57.20
Machining Force (Fm)
70 60 57.2 50
50.02
40 30
36.7 32.7
20
24.02
41.62 36.02
36.53
29.43
Speed 1 (2000rpm) Speed 2 (4000 rpm) Spped 3 (6000 rpm)
10 0 0.02
0.04
0.06
Feed Rate
14
Fig.8. Machining force in the GFRP workpiece (F) as a function of feed rate. The behaviour of the machining force and feed rate is shown in the Fig .8. It is analysed from the curves of the graph that the machining force (F) increases with increase in speed by keeping feed rate constant. The histories show that the machining force increased uniformly when the speed changed for a particular feed rate. That is to say that it increased from 24.02 to 32.7 when the speed changed from 2000 to 4000 rpm. And in the next level the machining force increased to 36.7 for the speed of 6000 rpm. The same trend is seen when the milling is done at the feed rate of 0.04 for test samples G02, G05 and G08, respectively. But for the feed rate of 0.06 there is a steep increase of machining force from 36.53 to 57.2 for the speed of 2000 and 4000rpm and comparatively low increment for the next level of speed 6000 rpm.
Fig.9. Machining force in the GFRP workpiece (F) as a function of Cutting velocity.
15
The Machining force versus cutting velocity graph for milling of GFRP laminate is shown in the fig.9. It is witnessed from the graph that by keeping the cutting velocity constant, the machining force increases more steeply at higher feed rate. For the cutting speed of 40mm/rev, the machining force increased from 24.02 to 29.43 when the feed changed from 0.02 to 0.04. Further when feed increased from 0.04 to 0.06, the machining force increased to 36.53. This shows that there is a uniform increase in machining force for the particular speed. Same condition is seen for the Cutting velocity of 80mm/rev, for test samples G04, G05 and G06. But for 120mm/rev, there is a massive increase of machining force from 41.62 to 57.2 when the feed was changed from 0.04 to 0.06 respectively. This shows that machining force develops more at higher feed rate. And as has been said, the process will be ideal if cutting force is less for a particular material unless the material is strong and has more stiffness. So the ideal condition for machining a GFRP laminate is by machining with different speed by keeping feed rate constant. From the table 4, the values enable us to state that the machining force initially decreases with increase in cutting velocity. But gradually it recovers as feed increases. When the cutting velocity increased instantaneously from 40 to 80mm/rev, the machining force value decreased from 36.53 to 32.70 for test samples G03 and G04 and it recovered as feed increased. The same trend can be seen for the change of cutting velocity from 80 to 120 mm/rev
16
70
Machining Force (Fm)
60 14.3% 57.2
50
50.02 40
12.23% 36.7
30
36.13
32.7
15.54 41.62 21.94 36.02
36% 36.53
29.43
24.02
20 10 0
0.02
Speed 1 (2000 rpm)
0.04
Feed Rate
Speed 2 (4000 rpm)
0.06
Speed 3 (6000 rpm)
. Fig.10. Increase in percentage of machining force (F) as a function of feed rate. From the graph shown in Fig.10, the inference is drawn that the percentage increase in machining force (F) decreases when the speed increases. But for the second iterative the percentage increase will decrease in a marginal value.
4.3 Comparison between the influence of cutting parameters on machining forces in CFRP and GFRP. The comparative representation of machining forces as function of feed rate for different speeds (2000, 4000, 6000 rpm) are show in fig.11, 12, and13 respectively. The thorough investigation on the machining values of both CFRP and GFRP laminates declares that for a particular feed rate, the machining force of GFRP is very less compared to machining force of CFRP laminate which shows that the Carbon fiber reinforced plastic is more stiff and stronger than the Glass fiber reinforced plastic. The value of machining force attained highest (73.2) while milling of CFRP laminates and the lowest (24.02) is attained while milling of GFRP laminate. 17
CFRP laminate endures more and develops more stress for the external force applied on it than a GFRP laminate. Therefore while milling of GFRP the maximum F was 57.20 for test sample G09 and minimum was 24.02 for test sample G01. For CFRP, the machining force was maximum of 73.2 and minimum of 36.70 recorded. When the percentage increase in machining force is compared between CFRP and GFRP laminate, it is seen that the percentage increase of cutting force is more in CFRP than GFRP laminate which is the another evident to show that CFRP material offers more stress, stronger and stiffer than GFRP material. When the cutting velocity is changed to the higher velocity instantaneously, the machining force decreases more intensely in CFRP than GFRP but it recovers as the feed rate gets increased in both materials. This shows that instantaneously change in cutting velocity decreases the cutting speed.
2000 rpm Machining Force (Fm)
70 60
62.81
50 44.95
40
30 20
36.7 24.02
36.53 29.43
Speed 1 (GFRP) Speed 1 (CFRP)
10 0 0.02
0.04
0.06
Feed Rate (f)
Fig.11. Comparison of CFRP and GFRP laminates for the speed of 2000rpm
Fig.11 is a graph plotted for the different Machining force values obtained at different feed rates for both CFRP and GFRP composite laminates at a speed of 2000 rpm. From the graph it is analysed that at the speed of 2000 rpm the machining force for GFRP composite material gradually increases from 24.02 to 29.43 with increase of feed from 0.02 to 0.04 and it 18
increases from 29.43 to 36.53when the feed was increased from 0.04 to 0.06. In the case of CFRP composite laminate, first there is a gradual increase of machining force from 36.7 to 44.95, when the feed is increased from 0.02 to 0.04respectively. But there is a steep increase in machining force from 44.95 to 62.81, when feed is increased from 0.04 to 0.06 respectively.
4000 rpm Machining Force (Fm)
80 70 68.67
60
50 40 30
54.95 50.02
48.05 32.7
Speed 2 (GFRP)
36.02
Speed 2 (CFRP)
20 10 0 0.02
0.04
0.06
Feed rate (F)
Fig.12. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. The graph plotted for the different Machining force values obtained at different feed rates for both CFRP and GFRP composites at a speed of 4000 rpm is shown in fig.12. From the curves it is evident that both GFRP and CFRP laminates behave in the same way for the speed of 4000 rpm. It let us to say that there is an increase of 14 newtons of machining force when both materials are machined with increased feed rate from 0.04 to 0.06. They both behave in a same trend as far as the speed 4000 rpm is concerned.
19
6000 rpm Machining Force (Fm)
80 70
73.2
65.6
60 50
57.2
40 30
36.7
57.2 41.62
Speed 3 (GFRP) Speed 3 (CFRP)
20 10 0 0.02
0.04
0.06
Feed rate (F)
Fig.13. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. The Machining force (F) values obtained at different feed rates for both CFRP and GFRP composites at a speed of 6000 rpm is shown in fig. 13. From this graph it is evident that at the speed of 6000 rpm for GFRP composite laminate there is a gradual increase of machining force from 36.7 to 41.62 when the feed is increased from 0.02 to 0.04 but there is a steep increase of machining force i.e. 41.62 to 57.2 when the feed rate was increased from 0.04 to 0.06. In the case of CFRP, the same trend followed as the curves are similar in nature. For a particular feed rate, the machining force was more while machining CFRP than GFRP laminate.
5. Conclusion In the light of the experimental results obtained, the following conclusion can be drawn from the milling of both CFRP and GFRP composite laminates.
The feed rate and speed have statistical and physical significance on the machining force in the work piece. Especially the feed rate has more influence on the machining force.
20
The machining force increases with increase in speed when the feed rate is kept constant in milling of CFRP & GFRP laminates. The maximum stress was observed at higher feed rate.
It is suggested to use lower feed rates with higher speeds for optimal and economical milling of CFRP & GFRP laminates.
The instantaneous change in cutting velocity decreases the machining force initially but recovers as feed increases.
For the same feed rate the machining force of GFRP laminates is observed to be very less when compared to machining force of CFRP laminates.
The percentage increase of cutting force is more in CFRP than GFRP laminate which is evident that CFRP material offers more stress, strength and stiffness than GFRP material.
References [1] Smith W.F., Principles of Materials science and engineering. Mc graw Hill, 1990, 699724. [2] User manual for machining carbon fiber reinforced plastic composites (CFRP), Sandvik publication, 2009. [3] Jahannmir S., Ramulu M., Koshy P., Machining Of Ceramic and Composites. Marceldekker Inc, Newyork, 2000, 267-297. [4] Ferreira, J.R., Coppini, N.L., Miranda, G.W., Machining Optimisation in Carbon fiber reinforced Composites materials. J. Mater. Process. Technol. 1999, 135-140. [5] Paulo Davim, J., Pedro Reis., Damage and Dimension Precision on milling carbon fiber reinforced plastic using design experiments. J. Mater. Process. Technol. 2005, 160–167.
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[6] Bhatnagar N., Ramakrishnan N., Naik N.K., Komanduri, R., On the Machining of Fiber Reinforeced Plastic (FRP) Composites laminates. Int. J. Mach. tools manuf. 1995, 35, 701716. [7] Jenarthanan M.P., Jeyapaul R., Evaluation of machinability index on milling of GFRP composites with different fibre orientations using solid carbide endmill with modified helix angles. Int. J. Eng. Sci. 2014, 6, 1-10. [8] Santhanakrishnan G., Krishnamurthy R., Malhota S.K., Machinability characteristics of fibre reinforced plastics composites. J. Mater. Process. Technol. 1988, 17, 195–204. [9] Palanikumar, K., Karunamoorthy, R., Karthikeyan., Latha, B., Optimization of machining parameters in turning GFRP composites using a carbide (K10) tool based on the taguchi method with fuzzy logics. Met. Mater. Int. 2006, 12, 483-491. [10] Praveen Raj, P., ElayaPerumal, A., Ramu, P., Prediction of Surface roughness and Delamination in End Milling of GFRP using ANSYS. Indian J. Eng. Mater. Sci. 2013, 6, 107-120. [11] Hintze W.F., Dirk Hartmann, Christoph Schutte, Occurrence And propagation of delamination during the machining of carbon fiber reinforced plastic (CFRP)- An experimental study. Compos. Sci. Technol. 2011, 71, 1719-1726. [12] Zhang H., Chen W., Zhang L., Assessment of exit defects in Carbon Fiber reinforced plastic plates caused by drilling. Key Eng. Mater. 2001, 196, 43-52. [13] TSao C.C. and Hocheng H, Taguchi analysis of delamination associated with various drill bits in drilling of composite material. Int. J. Mac Tools & Mnf. 2004, 44, 1085–90 [14] Ramulu M., Machining and surface integrity of fiber reinforced plastic composites, University of Washington, Seatle, USA., Sadhana, June 1997, 22, Part 3, 449-472.
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[15] Arokiadass R., Palaniradja K., Alagumoorthi N., Predictive modeling of surface roughness in end milling of composite materials, Scholars Research Lib, Appl. Sci. Res., 2011, 228-236. [16] Kyu yvol Park, Jin ho choi, Dai gil jee, Delamination-Free and High Efficiency Drilling of Carbon Fiber Reinforced Plastics, J. Compos. Mater., 2013, 13. [17] Gaitonde V. N., Karnik S.R., Campos Rubio J., Esteves Correia, Paulo Davim,J., Analysis of parametric influence on delamination in high-speed drilling of carbon fiber reinforced plastic composites, J. Mater. Process. Technol.,2008, 203, 431–438. [18] Devi Kalla, Jamal Sheikh, Janet Twomey, Prediction of cutting forces in helical end milling fiber reinforced polymers, Int. J. Mach. Tools Manuf, 2010, 50, 882–891. [20] Dhiraj kumar and K. K. Singh, An approach towards damage free machining of CFRP and GFRP composite materials: A Review, Adv. Compos. Mater., 2015, 24. [21] Jinyang, Makaddem Ali, Mohammed El Mansari, Recent advances in drilling hybrid FRP/Ti composite: A State of the art review, Compos. Struct., Jan 2016,316-338.
List of figures Fig.1. Cemented carbide tool (K10) Fig.2. Mounting of composite laminate and milling tool dynamometer. Fig.3. Specimens after milling operation (CFRP and GFRP laminates) Fig.4. Direction of measurement of three orthogonal components (F). Fig.5. Machining force (F) in the CFRP laminate as a function of feed rate. Fig.6. Machining force (F) in the CFRP work piece as function of Cutting velocity Fig.7. Increase in percentage of machining force during milling of CFRP laminates Fig.8. Machining force (F) in the GFRP laminate as a function of feed rate
23
Fig.9. Machining force (F) in the GFRP laminate as a function of Cutting velocity Fig.10. Increase in percentage of machining force during milling of GFRP laminates Fig.11. Comparison of CFRP and GFRP laminates for the speed of 2000 rpm Fig.12. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. Fig.13. Comparison of CFRP and GFRP laminates for the speed of 6000 rpm.
List of Tables Table.1. Mechanical Properties of the laminates Table.2. Assignment of levels to the factors Table.3. Values of machining force (F) as function of cutting parameters for CFRP. Table.4. Values of machining force (F) as function of cutting parameters for GFRP.
24
Figures
Fig. 1. Cemented carbide tool (K10)
Fig. 2. Mounting of composite laminate and milling tool dynamometer.
Fig. 3.Specimens after milling operation (CFRP and GFRP laminates) 25
Fig. 4. Direction of measurement of three orthogonal components (Fm).
80 73.2
Machining Force (Fm)
70 65.6
60 57.2
54.95
48.05
44.95
50
68.67 62.81
40 36.7 30 Speed 1 (2000 rpm)
20
Speed 2 (4000 rpm) 10 Speed 3 (6000 rpm) 0 0.02
0.04
0.06
Feed Rate (mm/rev)
Fig. 5. Machining force (Fm) in the CFRP laminate as a function of feed rate.
26
Fig. 6. Machining force (F) in the CFRP work piece as function of Cutting velocity
80 6.59%
70
Machining Force (Fm)
9.32% 60
19.5% 19.04% 57.02
50 40 30
30.92%
48.05
65.6
73.2
68.67
62.81
22.2% 54.95 44.95
36.7
20 10 0 0.02
0.04
0.06
Feed rate (mm/rev) Speed 1 (2000 rpm)
Speed 2 (4000 rpm)
Speed 3 (6000 rpm)
Fig. 7. Increase in percentage of machining force during milling of CFRP laminates
27
Machining Force (Fm)
70 60 57.2 50
50.02
40 30
36.7 32.7
20
24.02
41.62 36.02
36.53
29.43
Speed 1 (2000rpm) Speed 2 (4000 rpm) Spped 3 (6000 rpm)
10
0 0.02
0.04
0.06
Feed Rate Fig.8. Machining force in the GFRP workpiece (F) as a function of feed rate.
Fig.9. Machining force in the GFRP workpiece (F) as a function of Cutting velocity.
28
70
Machining Force (Fm)
60 14.3% 57.2
50 36% 40
15.54% 12.23%
30 20
36.7
36.13% 32.7
50.02
41.62 36.53
21.94% 36.02 29.43
24.02
10 0 0.02
0.04
0.06
Feed Rate (mm/rev) Speed 1 (2000 rpm)
Speed 2 (4000 rpm)
Speed 3 (6000 rpm)
Fig. 10. Increase in percentage of machining force during milling of GFRP laminates Milling at 2000 rpm 70
Machining Force (Fm)
60
62.81
50 44.95
40
36.7
36.53
30 29.43
20
24.02 Speed 1 (GFRP)
10
Speed 1 (CFRP) 0 0.02
0.04 Feed Rate (mm/rev)
0.06
Fig. 11. Comparison of CFRP and GFRP laminates for the speed of 2000 rpm
29
Milling at 4000 rpm 80
Machining Force (Fm)
70 68.67
60 54.95
50
50.02
48.05
40 30
32.7
36.02
20
Speed 2 (GFRP)
10 Speed 2 (CFRP)
0 0.02
0.04
0.06
Feed rate (mm/rev)
Fig.12. Comparison of CFRP and GFRP laminates for the speed of 4000 rpm. Milling at 6000 rpm 80
Machining Force (Fm)
70
73.2 65.6
60 50
57.2
40 30
57.2 41.62
36.7
20
Speed 3 (GFRP) 10 Speed 3 (CFRP)
0 0.02
0.04
0.06
Feed rate (mm/rev)
Fig.13. Comparison of CFRP and GFRP laminates for the speed of 6000 rpm.
30
Tables
Table. 1 Mechanical Properties of the laminates Sl. No
Properties
CFRP
GFRP
1
Young’s modulus GPa
70
25
2
Poisson’s ratio
0.1
0.2
3
Ultimate tensile strength MPa
600
440
4
Ultimate compressive strength MPa
570
425
5
Shear strength MPa
90
40
6
Shear modulus GPa
5
4
Table.2. Assignment of levels to the factors Levels Parameters
X1
X2
X3
40
80
120
Spindle Speed (rpm)
2000
4000
6000
Feed rate (mm/rev)
0.02
0.04
0.06
Cutting velocity (m/min)
Table.3. Values of machining force (Fm) as function of cutting parameters for CFRP. Test samples
Velocity (mm/rev)
Speed (rpm)
Feed rate (mm/rev)
Machining force Fm (N)
C 01
40
2000
0.02
36.70
C 02
40
2000
0.04
44.95
C 03
40
2000
0.06
62.81
C 04
80
4000
0.02
48.05
C 05
80
4000
0.04
54.95
C 06
80
4000
0.06
68.67
C 07
120
6000
0.02
57.20
C 08
120
6000
0.04
65.6
C 09
120
6000
0.06
73.2
31
Table.4. Values of machining force (Fm) as function of cutting parameters for GFRP. Test samples
Velocity (m/min)
Speed (rpm)
Feed rate (mm/rev)
Machining force Fm (N)
G 01
40
2000
0.02
24.02
G 02
40
2000
0.04
29.43
G 03
40
2000
0.06
36.53
G 04
80
4000
0.02
32.70
G 05
80
4000
0.04
36.02
G 06
80
4000
0.06
50.02
G 07
120
6000
0.02
36.70
G 08
120
6000
0.04
41.62
G 09
120
6000
0.06
57.20
32