On the characterization of machined surfaces of Perspex samples under different conditions in surface finishing F. Fazlullah, M. S. Kaiser, and S. Reaz Ahmed
Citation: AIP Conference Proceedings 1980, 030020 (2018); doi: 10.1063/1.5044299 View online: https://doi.org/10.1063/1.5044299 View Table of Contents: http://aip.scitation.org/toc/apc/1980/1 Published by the American Institute of Physics
On the Characterization of Machined Surfaces of Perspex Samples under Different Conditions in Surface Finishing F. Fazlullah1, a), M. S. Kaiser2 and S. Reaz Ahmed1 Department of Mechanical Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh 2 Directorate of Advisory, Extension and Research Services, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh 1
a)
Corresponding author:
[email protected]
Abstract. The present study deals with the characterization of machined surfaces and chips generated during surface finishing of Perspex samples under different machining conditions. The machining is carried out using a horizontal pushcut type shaper machine with a single point V-shaped HSS cutting tool, where the cutting speed and depth of cut are varied while the stroke length is kept constant. The machined surfaces are characterized in terms of a number of mechanical properties measured directly on the surfaces, which include surface roughness, hardness and temperature. Attempt is also made to characterize the surfaces by the shape and size of the chips generated from the surface. The chip formation characteristics are investigated using digital photographs as well as optical microstructures. The results show that the best surface finish can be obtained for 2mm depth of cut at a cutting speed of 13.3 m/min. At lower cutting speeds, continuous chips are formed from the machined surfaces, while discontinuous chips are observed under higher cutting speeds.
INTRODUCTION Perspex, the trade name of Lucite, is usually produced from methyl methacrylate monomer. It is one of the most useful materials in the workshop because it can be used to make precision engineering components for both domestic and industrial products. Typical applications include signs, glazing, safety screening, roof-lighting, furniture, lighting fittings and a great many industrial parts [1, 2]. Many plastic products require some degree of machining and finishing after they have been processed and are put in the market place. Machining of polymeric materials has increasingly become necessary when the quantity of precious items does not justify the cost of tooling for moulds or extrusion dies, or when a product needs a costly dimensional accuracy. A material property, commonly known as machinability, is basically an indicator of an engineering material which describes how easy or difficult to machine the material using a cutting tool to achieve an acceptable surface finish. In general, during machining operation, cutting speed, feed rate and depth of cut show maximum influence on quality measure, such as, surface roughness [3, 4]. Machinability is found to differ for different types of plastics. The machining of polymers and polymer-based composites has become one of the most important manufacturing processes, especially when the polymers cannot be processed by melting due to their high melt viscosity, or in the case of manufacturing of small quantities, complex components [5, 6]. Despite the importance of the machining of polymers, fundamental information on their behavior during the process is still inadequate. The major limitation is related to the fact that polymer processing is highly time and temperature dependent, and exhibit a wide range of properties during machining [7]. The aim of this paper is to characterize the machined surfaces of commercially available Perspex samples under different machining conditions in surface finishing in terms of mechanical properties as well as chip formation. Mechanical properties like, surface roughness, hardness, surface temperature, etc. are measured during the machining in which the cutting speed and depth of cut are varied while the stroke length is kept constant. The
International Conference on Mechanical Engineering AIP Conf. Proc. 1980, 030020-1–030020-8; https://doi.org/10.1063/1.5044299 Published by AIP Publishing. 978-0-7354-1696-3/$30.00
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characteristic of chip formation during the process is also analyzed in terms of optical microstructures of the chips as well as direct optical images.
EXPERIMENTAL PROCEDURE Machining operation was carried out with commercially available Perspex samples of dimension 15 mm x 75 mm x 150 mm. For the purpose of machining, a commercial shaper machine with a 60°, V-shaped High-Speed Steel (HSS) tool bit was used at various strokes per minute (11, 48, 70 and 99) to maintain the corresponding cutting speeds of 3.0, 13.3, 19.4 and 27.5 m/min, respectively. The cutting speed, depth of cut and feed rate were considered as the process parameters for the present investigation, as they are the most influencial parameters in finishing a surface under shaper machine. The selected cutting speeds covered the entire range from the lowest to highest ones available for the given machine. The V-shaped HSS tool was selected because it is one of the most common tools used in practice. Also, the symmetry of the tool makes it possible to perform shaping operation from either direction, i.e., left to right or right to left. The machining times required to finish the sample surface under the selected cutting speeds were 5.4, 1.2, 0.8 and 0.6 min, respectively. Different values of depth of cut were used (0.5, 1.0, 2.0 and 4.0 mm) for a constant feed rate of 0.254 mm/stroke. Surface roughness (Center Line Average Roughness Ra) of the machined surfaces was measured under different machining conditions with a surface-roughness measuring instrument (Talysurf). For each value of surface roughness, the average of minimum three readings has been considered. The surface temperature of the samples was directly measured during machining at the tool-tip and material interface using a digital pyrometer. A total of fifteen hardness measurements were performed at different locations of each sample using Durometer Hardness tester to present the corresponding average hardness of the machined surface. The dimensions of the generated chips were taken to calculate the corresponding chip deformation and were also photographed in as-received condition. The micrographs of different chip and machined surfaces were taken with an optical microscope. The photograph of the experimental set-up is shown in Figure 1. Chip deformation was estimated using the following equation and the associated right-angle triangle assumed at the tool-tip with half of the tool angle, θ = 30o:
Tool post
HSS Tool
Perspex specimen
Digital pyrometer
Figure 1. Photograph of the machining setup for shaping the surface
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TABLE 1. Tool geometry and cutting condition Back rake angle (degree) Clearance angle (degree) Notch radius (mm) Feed rate (mm/stroke) Depth of cut (mm) Cutting speed (m/min)
5 8 0.8 0.254 0.5, 1.0, 2.0 and 4.0 3.0, 13.3, 19.4 and 27.5
RESULTS AND DISCUSSIONS Figure 2 shows the variation of average surface roughness with increasing cutting speeds at different depths of cut. From the figure, it can be seen that the best surface finish is obtained at 2mm depth of cut, as the average surface roughness is the lowest and remains the same throughout the entire span of cutting speed. At other depths of cut, the surface roughness has an erratic relation with cutting speed. It can also be seen that, as the depth of cut increases from 0.5mm to 2mm, the average surface roughness decreases to its minimum average value, and then starts increasing with the increase of depth of cut. So, it can be said that surface roughness mainly depends on the depth of cut as opposed to cutting speed for a given tool [8]. With the increase of depth of cut, the heat generated at the tool-work interface increases, which eventually increases the shear forces. The increased heat (the temperature in the cutting zone exceeds Tg) causes the Perspex sample to change from a completely brittle material to a transition state where the material assumes a soften condition. The combined effect of these two parameters is what determines the average surface roughness at a particular depth of cut. At lower depths of cut, the material remains in an amorphous, glass state, thereby giving increased average surface roughness due to brittle fracture. At 2mm depth of cut, the material reaches its transition state, where the shear forces and viscoelasticity of the material is such that the specimen can shear off easily and uniformly, which eventually results in the lowest surface roughness. At higher depth of cut (4mm) with the highest cutting speed (27.5m/min) and lowest machining time (0.6 min), the roughness suddenly jumps compared to those of other speeds. At this condition, the generated heat and shear force are maximum, which causes the material to assume a rubbery state of the specimen (highest viscous behavior) and hence greatest softening of the material. This increased softness ultimately helps in gumming and lumping of the material at various locations non-uniformly, thereby resulting in increased roughness. 5.5 0.5 mm 1.0 mm 2.0 mm 4.0 mm
5.0
Roughness Ra, m
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0
0
5
10
15
20
25
30
Cutting speed, m/min
FIGURE 2. Variation of surface roughness with cutting speed at different depth of cut.
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Figure 3 shows the variation of surface temperature with cutting speeds at different depths of cut. For all cases, the surface temperature is found to increase with the increase of cutting speed. The similar trend is also observed for the case of depth of cut, as for any cutting speed, the temperature increases with the increase of depth of cut. However, the response of temperature with increasing cutting speeds is quite different for different depths of cut. With increasing cutting speeds, the dissipation of frictional power is increased. As a result, the surface temperature increases with increasing cutting speeds. The increased cutting forces are also the reason why the temperature rise increases for increasing cutting speeds [9]. It is observed from Figure 4 that, as the cutting speed increases, the surface hardness decreases. Most likely, it is due to the increase in temperature during machining, as the increase in cutting speed generates heat, which eventually makes the material surface soften. Also, an increase in depth of cut produces increased cutting temperature, which, in turn, decreases the hardness of machined surface. For the case of higher depth of cut together with higher cutting speed, the surface hardness is found to increase, which indicates that the surface materials experience severe strain hardening induced by plastic deformation during machining [10]. 70 0.5 mm 1.0 mm 2.0 mm 4.0 mm
o
Temperature, C
60 50 40 30 20
0
5
10
15
20
25
30
Cutting speed, m/min
FIGURE 3. Variation of surface temperature with cutting speed at different depth of cut. 90
Hardness, DH
85 80 75 0.5 mm 1.0 mm 2.0 mm 4.0 mm
70 65
0
5
10
15
20
25
30
Cutting speed, m/min
FIGURE 4. Variation of surface hardness with cutting speed at different depth of cut.
Figure 5 shows the measured relationship between chip deformation and cutting speed at different depth of cut. For all cases, it is seen that the chip deformation increases with increasing cutting speed. Again, for a given cutting
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speed, the deformation is also found to change with the depth of cut. The depth of cut also has an impact on how the chip deformation responds to increasing cutting speeds. At higher depths of cut, the deformation increases gradually with cutting speed, while the rise is much more prominent at lower depths of cut. Chip deformation basically depends on the heat generated at the work-tool interface, amount of heat conducted to the chip, the volume (mass) of the chip, the cutting forces and the structure of the material. Since, at lower depths of cut, the thickness and thus the volume of the chip is small, the temperature rise of the chip is higher for the same amount of heat generated, which, in turn, causes more softening of the chip and thus more deformation. As the chip volume is directly proportional to depth of cut (uncut chip thickness), the rise in temperature of the chip is less for a given cutting speed, thereby leading to less deformation. Again, at lower depth of cut, increasing cutting speeds causes a much faster rate of chip deformation due to less volume of the chip under higher rise in temperature. But, at higher depth of cut, the volume of the chip is higher; that is why, increasing cutting speeds could not cause much rise in temperature of the chip. As a result, less deformation is observed, which is verified in our study at 4mm depth of cut [11]. 250 0.5 mm 1.0 mm 2.0 mm 4.0 mm
Chips deformation, %
200 150 100 50 0
0
5
10
15
20
25
30
Cutting speed, m/min
FIGURE 5. Variation of chips deformation with cutting speed at different depth of cut. Figure 6 shows the change of surface hardness with the actual temperature of the Perspex sample. With increasing
temperature, the surface hardness is found to decrease following nearly a linear law. This reduction is due to thermal softening of the sample. At room temperature, the structure of Perspex is amorphous with chains intertwining among themselves. As temperature is increased the tangled chains start to untangle and place themselves further apart resulting in a decrease in surface hardness [12]. 90
Hardness, DH
85 80 75 70 65
10
20
30
40
50
60
70
80
90
100 110
o
Temperature, C
FIGURE 6. Variation of surface hardness as a function of actual temperature of the surface.
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Figure 7 shows the photographs of chips formed at different depths of cut and cutting speeds. It is evident that increasing the cutting speed causes the formation of cracks on the chips. As the depth of cut is increased, this behavior is exaggerated together with the increase of cutting speed, and eventually the continuous chip becomes fully discontinuous. The formation of cracks that ultimately leads to discontinuous chips is due to the fact that Perspex is a highly amorphous polymer in which the chains are physically entangled to each other at random points. This tangling phenomenon causes the material to be extremely brittle. Increasing cutting speed has two inverse effects. One is the slight increase in shear stress at the primary shear zone that causes the material resistance to change, and the other is the reduction of shear stress due to increased temperature. The combined effect of these two ultimately gives rise to cracks in the chips. The photographs also verify the pervious results of chip deformation in that the increase in chips width is greater at the lower depth of cut, and gradually decreases as depth of cut is increased until we use the highest one, i.e., 4mm, where almost no increase in chips width is observed [13].
(a)
(b)
(c)
(e)
(f)
(g)
(d)
(h)
FIGURE 7. Photo micrograph of the chips generated due to machining at different cutting speed and depth of cut: (a) 3.0m/min at 0.5mm, (b) 3.0m/min at 1.0mm, (c) 3.0m/min at 2.0mm, (d) 3.0m/min at 4.0mm, (e) 27.5m/min at 0.5mm, (f) 27.5m/min at 1.0mm, (g) 27.5m/min at 2.0mm and (h) 27.5m/min at 4.0
Figure 8 shows the micrographs of the machined surfaces obtained under machining at various depths of cut and cutting speeds. Considering all the photographs, it is evident that, at lower cutting speed, there are small loose white particles present which subsequently vanish at the higher speed. These loose particles are the result of abrasive and chemical (oxide) wear on the machined surfaces [14]. At 2mm depth of cut, almost the same types of scratches are observed at the highest and lowest cutting speeds. In addition, at this depth of cut, the surface finish is better than those obtained with all the other machining conditions. The next favorable condition is found for surfaces shown in figures (b) and (f), which is obtained with 1 mm depth of cut; some mild scratch lines can be seen in both the figures. The machined surface obtained with 4mm depth of cut and 27.5 m/min cutting speed is identified to be the worst in terms of surface finish, on which considerably deep and wide scratches are observed.
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(a)
(e)
(b)
(f)
(c)
50m
(g)
(d)
(h)
FIGURE 8. Optical microstructure of the machined surfaces generated at different cutting speed and depth of cut: (a) 3.0m/min at 0.5mm, (b) 3.0m/min at 1.0mm, (c) 3.0m/min at 2.0mm, (d) 3.0m/min at 4.0mm, (e) 27.5m/min at 0.5mm, (f) 27.5m/min at 1.0mm, (g) 27.5m/min at 2.0mm and (h) 27.5m/min at 4.0
CONCLUSIONS Machined surfaces of Perspex samples are investigated to determine the influence of various conditions of surface finishing on the relevant mechanical properties as well as the formation of chips. Measured values of surface roughness, hardness, temperature as well as microstructure of the surfaces reveal that the machining parameters, especially, cutting speed and depth of cut have significant role in defining the state of machined surfaces. The temperature of machined surfaces of Perspex samples is found to increase with the increase of both the depth of cut and the cutting speed, which, in turn, causes the surface hardness to assume reduced values. The best surface finish has been obtained for a depth of cut of 2mm with 13.3 m/min cutting speed. At lower cutting speeds, continuous chips are formed from the machined surfaces, while discontinuous chips are observed when machined with higher depth of cut and higher cutting speeds. Microstructures of the machined surfaces also verify the formation of cracks at higher cutting speeds and depths of cut.
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