Effect of Cutting Parameters on Vertical Force in AWJ Cutting
A. I. Hassan1* and R. Kovacevic**
*Benha University, Department of Mechanical Engineering, Benha, Egypt1
**Southern Methodist University, Department of Mechanical Engineering, Research Center for Advanced Manufacturing (RCAM), 3101 Dyer St., Dallas, Texas 75275-03372, USA
In abrasive waterjet (AWJ) cutting, the vertical cutting force is much lower compared to other conventional machining processes. The present paper describes a model that predicts the vertical cutting force in AWJ using waterjet pressure. The experimental results were found in good agreement with the theoretical vertical cutting force to a reasonable degree of accuracy. The results show that as pressure increases, the vertical cutting force increases while both the traverse rate and the abrasive flow rate have a slight effect on the vertical force over a wide range of experimental values.
KEYWORDS Abrasive waterjet cutting, cutting parameters, vertical cutting force
Aeff effective nozzle area
dm mixing tube diameter, mm
dn waterjet nozzle diameter, mm
F theoretical theoretical vertical force, N
ma abrasive flow rate, g/s
P waterjet pressure, MPa
S stand off distance, mm
u traverse rate, mm/min
α contraction number
Difficult-to-machine materials pose problems while being machined using traditional machining processes because of the excessive cutting forces and high cutting temperatures which lead to rapid tool wear that reduces productivity and increases machining cost. Due to its low cutting force, low cutting temperatures, and deep kerfing capabilities, abrasive waterjet (AWJ) cutting is used in many versatile engineering applications especially difficult-to-machine materials which are cut easily using AWJ. There is no contact between AWJ nozzle and the workpiece material and this solves the tool wear problem in traditional machining of difficult-to-machine materials. One of the most advantageous characteristics of the AWJ is that the vertical cutting force, which is defined as the force exerted by the AWJ perpendicular to the workpiece surface, has smaller values that could reach one twentieth of its counterpart in traditional machining processes. However, due to its complex physics and in order to be integrated in modern computer integrated manufacturing (CIM) systems, the process has to be controlled to insure reliable productivity.
The present paper describes the effect of the AWJ cutting parameters on vertical cutting force measured by a specially designed dynamometer.
2 LITERATURE REVIEW
In an early study on the vertical cutting force in pure waterjet cutting, Decker et al. (1) suggested a model for jet forces based on jet energy. It was found that the jet force increases with an increase in waterjet pressure and waterjet nozzle diameter and it is affected by the nozzle geometry. Based on Decker’s model, a study by König et al. (2) showed the importance of the ratio of abrasive flow rate to water flow rate in affecting jet forces. A simplified hydrodynamic model for the theoretical vertical cutting force in AWJ cutting was developed using a contraction number. The model predicted the vertical cutting force to a reasonable accuracy. The theoretical vertical cutting force (F theoretical) is given by:
where, Aeff is the effective nozzle area, α is a contraction number = 0.7 in case of AWJ, P is the waterjet pressure, and dn is the waterjet nozzle diameter. Kovacevic (3) used the vertical cutting force signal, measured by a piezoelectric crystal force sensor, to control the depth of cut in AWJ cutting by an experimental model using curve fitting. This is important in AWJ milling where the depth of cut must be homogeneous. Five AWJ parameters; pressure, traverse rate, abrasive flow rate, mixing tube diameter and standoff distance were changed in a wide range of values and their effects on the depth of cut and the vertical force were obtained. The workpiece normal force was found to increase with increasing waterjet pressure, abrasive flow rate and mixing tube diameter. Whereas, it decreases with increasing stand off distance and it is only slightly affected by traverse rate. A large increase in the magnitude of the normal force indicates the presence of nozzle wear and shows that the depth of cut is exceeding the acceptable limit. This phenomenon could be utilized in monitoring of nozzle wear (3). Due to small vertical force values, some parameters such as abrasive flow rate and traverse rate have only a slight effect on the vertical force as shown from the experimental results. Moreover the other two parameters; standoff distance and mixing tube diameter are practically restricted to optimized values and could not be changed over a wide range of values. Kovacevic et al. (4) used the vertical cutting force signal, measured by the same experimental set-up as in (3), to control the surface waviness of the workpiece surface in AWJ cutting in a manner similar to that used in (3) for the depth of cut monitoring using auto regression moving average model. Four AWJ parameters; pressure, traverse rate, abrasive flow rate, and standoff distance were changed in a wide range and their effects on the waviness and the vertical force were obtained. Due to small vertical force values obtained, some parameters such as abrasive flow rate and traverse rate have only a slight effect on the vertical force. Fekaier et al. (5) used the vertical cutting force signal to control surface finish by considering AWJ cutting parameters. This study was only limited to cutting wear mode, slow traverse rates and thin workpieces. The developed model could not be applied to the deformation wear mode as the deviation between experimental and theoretical results was pronounced. The effect of the traverse rate on the vertical force was exaggerated by using a large scale. Precise examination of the traverse rate curves show that they are approximately sticking to each other suggesting that there is little role for the traverse rate in affecting the vertical force. No work was actually done in surface roughness control using the recorded vertical force signals (5). Tazibt et al. (6) used a new approach for modeling the abrasive waterjet machining process of ductile materials, based on experimental results, in order to obtain metal removal rates as a function of particle velocity. It has been shown that the material is cut because a threshold water pressure is exceeded. A correlation relating material removal rate to the particle velocity was derived. The model is only applied to the cutting wear mode in ductile materials. However the formula derived for the vertical force underestimated its value by approximately 60% if compared with the results of the present study and the results of König et al. (2). Momber (7) conducted an analysis of energy transformation efficiency using vertical force measurements. The developed model is a correlation between vertical force and energy transfer efficiency coefficient. Validation was done by vertical force measurments for both WJ & AWJ. Hassan et al. (8) showed that AWJ starts to interact with the workpiece material causing a sudden rise in the value of the vertical cutting force. Afterwards, the vertical cutting force becomes stable in the stable cutting zone. The magnitude of the vertical cutting force in the stable cutting zone increases as a result of the increase in pressure due to the increase in the abrasive particle velocity. The depth of cut in the workpiece material increases by an increase in the vertical cutting force. The experimental results were used to describe a linear relationship between the vertical cutting force and the depth of cut. Moreover, the depth of cut could be on-line monitored if the force signal is continuously recorded. Anantharamaiah et al. (9) used fluid dynamics simulation to obtain an impulsive impact force for the discrete portion of the waterjet. The peak of this impulsive impact force is found to be 4 times greater than that of the continuous portion.
3 EXPERIMENTAL WORK
A three-axis CNC AWJ machine was used in the present work. The machine has the following specifications: work table movements: X-axis: 1219 mm, Y-axis: 1219 mm and Z-axis movement: 200 mm. The high pressure intensifier has a maximum pressure ratingof 60,000 psi (414 MPa) and power of 30 hp. A Paser 3TM abrasivejet cutting system was also used. The waterjet nozzle used has an inside diameter of 0.33 mm, while the mixing tube has an inside diameter of 1.02 mm and length of 76 mm. As an abrasive material; Garnet of Mesh No. 80 was used. The experimental set-up for vertical force measurement is shown in Fig. 1. A strain gauge dynamometer was specially designed and installed on the table of the AWJ machine that measures the vertical cutting force. The signal from the dynamometer was transmitted to an amplifier and then to a data acquisition system at a sampling rate of 0.01 second. The force signal was analyzed by LabVIEW software.
Carbon steel AISI 1018 in the cold-rolled condition containing 0.15 % C was chosen as the workpiece material. The dimensions of each specimen are: 100 mm in length, 13 mm in width, and 130 mm in height. Vertical cutting force was measured for each sample from the beginning of the experiment up to a length of cut of 38 mm to ensure steady-state AWJ cutting conditions. Afterwards, each sample was machined by face milling to reveal the kerf surface, and three measurements of the vertical cutting force are taken in the steady state cutting zone and then averaged. There is a number of cutting parameters that significantly affect the AWJ performance. Among these parameters; the waterjet pressure (P), traverse rate (u) and abrasive flow rate (ma) were varied, and the resultant vertical force was measured for each experiment. Other parameters such as the waterjet nozzle diameter, mixing tube diameter, stand off distance, abrasive grain size, and abrasive material were kept constant. The AWJ cutting conditions used are listed in Table 1. The range of values was selected from reference (10).
4. RESULTS AND DISCUSSION
The experimental results at abrasive flow rate of 3.1-3.7 g/s are shown in Fig. 2 below.
It is clear from this figure that as waterjet pressure increases, the vertical cutting force sharply increases. This trend is the same for all values of traverse rate, from 20 mm/min to 300 mm/min. This result is due to increased abrasive particle velocity as waterjet pressure increases. It is apparent from Fig. 2 that there is a slight variation in vertical cutting force as the traverse rate increases from 20 mm/min to 300 mm/min. It could be concluded that the most important parameter in affecting the vertical cutting force is waterjet pressure whereas the traverse rate and abrasive flow rate have a slight effect on the vertical cutting force. This means that the vertical cutting force is slightly affected by both the abrasive exposure time of the kerf as the traverse rate varies and the number of abrasives entering the kerf as the abrasive flow rate varies. Figure 2 indicates one of the most beneficial advantages of AWJ cutting which is the low value of the vertical cutting force. If the value of force is compared with traditional cutting processes, it would be at least 20 times less. A comparison of the experimental results of the vertical force with the theoretical vertical force based on König’s model (2) indicates that the vertical force is predicted to a reasonable degree of accuracy as shown in Fig. 2. The model slightly overestimates the vertical force in the small pressure range; from 150 to 200, and slightly underestimates the vertical force, in the high pressure range; from 300 to 350.
Figure 3 shows the effect of waterjet pressure on vertical force at abrasive flow rate of 5.2-5.7 g/s. It is obvious from Fig. 3 that as waterjet pressure increases; the vertical cutting force sharply increases for all values of traverse rate. This is similar to the behavior of the vertical force at abrasive flow rate of 3.1-3.7 g/s as in Fig. 2. It is apparent from Fig. 3 that there is a slight variation in vertical cutting force as the traverse rate increases from 20 mm/min to 300 mm/min. The model slightly overestimates the vertical force at both small and high pressure.
Figure 4 shows the effect of waterjet pressure on vertical force at abrasive flow rate of 7.5 g/s and figure 5 shows the effect of waterjet pressure on vertical force at abrasive flow rate of 8.7 g/s. It is obvious from both figures that as waterjet pressure increases; the vertical cutting force sharply increases for all values of traverse rate. There is a slight variation in vertical cutting force as the abrasive flow rate increases from 3.1 g/s, Fig. 2, to 8.7 g/s, Fig. 5.
From Figs 4 and 5 it is clear that the model accurately predicts the vertical force at abrasive flow rate of 7.5 g/s and 8.7 g/s. The same trend of the effect of waterjet pressure on the vertical cutting force obtained in Fig. 2 to Fig. 5 was obtained in the case of pure waterjet as seen in Fig. 6. A comparison between Figs. 2-5 with Fig. 6 shows that the vertical cutting force is slightly higher in the case of pure waterjet, Fig. 6, compared with abrasive waterjet, Figs. 2-5.
The effect of AWJ cutting parameters; e.g. pressure, traverse rate and abrasive flow rate on the vertical cutting force was studied in the present paper. The following conclusions could be drawn:
1. The most important parameter in affecting the vertical cutting force is waterjet pressure. As waterjet pressure increases, the vertical cutting force sharply increases. This trend is the same for traverse rate, from 20 mm/min to 300 mm/min and for abrasive flow rate from 3.1 g/s to 8.7 g/s. It was found out that the traverse rate and abrasive flow rate have a slight effect on the vertical cutting force.
2. The vertical force is predicted to a reasonable degree of accuracy at low abrasive flow rate and to a higher degree of accuracy at high abrasive flow rate.
3. The vertical cutting force is slightly higher in the case of pure waterjet compared with abrasive waterjet.
The authors would like to express their gratitude to the Binational Fulbright Commission in Egypt for its senior research grant for the first author and the Research Center for Advanced Manufacturing, Mechanical Engineering Department, Southern Methodist University for its financial support of the experimental work. The authors are also grateful to Mr. Michael Vallant for his help in the design of the test rig.
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