Numerical Simulation of the Gap Flow Field for CEDMMG Compound Machining Engineering Ceramics [Sensors & Transducers (Canada)]
(Sensors & Transducers (Canada) Via Acquire Media NewsEdge) Abstract:
Engineering ceramics are introduced to satisfy the requirements of highprecision manufacturing industries. However, they are difficult to machine due to their high strength, high hardness, high toughness and high thermal resistance. In this paper, the circumferential electrical discharge milling and mechanical grinding (CEDMMG) compound machining process is presented. The high material removal rate and good surface quality of machining engineering ceramics with the compound process can be obtained. Moreover, the gap flow field of the compound process is numerically simulated based on the liquidsolid twophase flow theory. The results show that the working fluid flow can be accelerated, and the machining debris can be ejected timely due to the rapid rotation of the tool, so the process performance can be enhanced.
Copyright © 2012 IFSA.
Keywords: Numerical simulation, Gap flow field, Engineering ceramics, Compound machining.
(ProQuest: ... denotes formulae omitted.)
1. Introduction
Recent years many new engineering ceramics are introduced to satisfy the requirements of highprecision manufacturing industries [1]. However, the high strength, high hardness, high toughness and high thermal resistance of these materials prevent their machining using traditional techniques [2]. Accordingly, various nontraditional machining techniques, such as laser beam cutting [3], ultrasonic machining [4] and electrical discharge machining (EDM) [5], have been proposed. Of these techniques, EDM has a lot of advantages, including a noncontact machining operation, the ability to machine thin plates without causing distortion, the ability to produce fine arrays of microholes and so on. Furthermore, EDM provides the means to machine materials with high mechanical strength and high hardness, such as silicon carbide ceramic, alumina ceramic, zirconia ceramic etc.
However, the traditional EDM shows low efficiency when machining a large surface area on engineering ceramics. The topic of how to reduce machining time and to maintain reasonable accuracy has always been of research interest. Recently, some researches have been made to improve the efficiency using EDM milling [6]. The results show that the material removal rate can be improved proportionally, but it still can't meet the demand of modern industrial applications, and the machined surface is poor. In this paper, the circumferential electrical discharge milling and mechanical grinding (CEDMMG) compound machining process is presented to machine engineering ceramics. The process employs the pulse generator used in EDM, and use s a waterbased emulsion as the working fluid. During machining, the electrical discharge milling and mechanical grinding happen alternately and they are mutually beneficial, so the high material removal rate and good surface quality of machining engineering ceramics with the compound process can be obtained.
The flow of the working fluid in the machining gap exercises a great influence on the movement of the machining debris, which affects the discharge frequency and the discharge distribution. Consequently, it is important to understand the gap flow field. During machining, the electrode and the workpiece are nontransparent, and the machining gap is very small, so it is impossible to measure the gap flow field with the measuring equipment directly. Furthermore, the flow of the working fluid in the machining gap is very complex, and it is not able to obtain the analytic solution with the relevant theory directly, so the numerical simulation of the gap flow field with the finite element method is a good way. The software of Fluent is the commercial popular computational fluid dynamics (CFD) software package, which can calculate the fluid flow, thermal transmission, and chemical reaction etc [7, 8]. In this paper, the flow of the working fluid and the movement of the machining debris in the machining gap are modeled based on the liquidsolid twophase flow theory. The gap flow field is calculated with the software of Fluent, and the velocity field and the pressure field are analyzed.
2. Establishment of the Model for the Gap Flow Field
2.1. Mathematical Modelling of the Movement for the Working Fluid and the Machining Debris in the Machining Gap
During the circumferential electrical discharge milling and mechanical grinding compound machining, the removed material from the workpiece in the form of machining debris is in the gap flow field. The machining debris is driven by the working fluid, and it will leave the machining gap by the working fluid, so the matter motion during the machining gap is the movement of the liquidsolid twophase flow composed of the working fluid and the machining debris. To solve the movement of the working fluid and the machining debris effectively, the fluid motion during the gap flow field is simplified and assumed as follows:
(1) The fluid in the gap flow field is incompressible and continuous.
(2) The gap flow field is stable, which means the physical quantity in the gap flow field is related with the space coordinate, but not related with the time.
(3) The transmission of the discharge thermal is not considered
The movement of the working fluid in the gap flow field follows the law of conservation of mass and the law of conservation of momentum. The fluid during the machining gap is incompressible, so its density is a constant. According to the law of conservation of mass, the continuity equation of the working fluid in the gap flow field can be obtained in the system of rectangular coordinates [9]:
... (1)
According to the law of conservation of momentum, the NavierStokes equation of the working fluid in the gap flow field can be obtained as follows [9]:
... (2)
... (3)
... (4)
where u, v, w are the velocity component of the working fluid in the coordinate, y coordinate and coordinate, respectively, t is the time, is the density of the working fluid, , , are the volume force of the gap flow field in the coordinate, y coordinate, and coordinate, respectively, is the pressure of the working fluid in the gap flow field, µ is the viscosity of the working fluid.
Equations (l)(4) are the fundamental dynamics equations of the working fluid in the gap flow field. The machining debris in the machining gap is driven by the working fluid, and its movement follows the Newton's second law, so the motion equation of the machining debris can be obtained as follows [9]:
... (5)
... (6)
... (7)
where up, vp, wp are the velocity component of the machining debris in the coordinate, y coordinate, and z coordinate, respectively, pp is the density of the machining debris, Fd(uup), Fd(vvp), Fd(wwp) are the drag force of the unit mass grain in the coordinate, y coordinate, and coordinate, respectively, gx, gy, gz are the acceleration of gravity of the machining debris in the coordinate, y coordinate, and coordinate, respectively, Fx, Fy, Fz are the additional force of the unit mass grain in the coordinate, y coordinate, and coordinate, respectively.
2.2. Establishment of the Simulation Model for the Gap Flow Field
The gap flow field of the circumferential electrical discharge milling and mechanical grinding compound machining is shown in Fig. 1, in which the diameter of the tool wheel is 210 mm, the rotary speed of the tool wheel is 3000 rpm, and the workpiece is SiC ceramic with the dimensions of 50x50 mm. It can be seen from Fig. 1 that the water based working fluid is flushed into the gap between the tool and the workpiece with the nozzle. The rapid rotation of the tool enables the working fluid to pass the machining gap quickly, so the discharge region can be cooled, and the machining debris can be flushed away easily, which improves the machining stability and the process performance.
The software of Fluent can calculate the working fluid flow, and its software package is composed of the Gambit and the Fluent solver. When the software of Fluent is used to calculate the fluid, the Gambit is used to structure the geometry of the flow region, generate the boundary and mesh firstly, and output the pattern that can be calculated by the Fluent solver. Then the Fluent solver calculates the flow region, and does the postprocessing. In this paper, the gap flow field is calculated with the software of Fluent, and the meshing should be done for the geometrical model shown in Fig. 1 . The structuring meshing is used to enhance the computational accuracy and save the computational time. The meshing close to the discharge region is compacting, and the meshing away from the discharge region is sparse. The meshing model of the gap flow field is shown in Fig. 2.
The SIMPLE arithmetic in the software of Fluent is used to calculate the gap flow field. The import is the velocity inlet, the working fluid is composed of 5 wt.% emulsified oil and 95 wt.% distilled water. The main component is distilled water, so the working fluid is selected as the distilled water, and the flow velocity of the inlet is 14.1 m/s. The machining debris is SiC ceramic, and its density is 3020 kg/m3, the grain size is 10 µ [10]. The boundary condition of the working fluid export is set as the fully developed flow. The machining debris completely escapes at the export.
3. Results and Analysis
3.1. Analysis of the Working Fluid Movement in the Gap Flow Field
The velocity distribution and the pressure distribution of the working fluid in the gap flow field of circumferential electrical discharge milling and mechanical grinding compound machining are shown in Figs. 3 and 4, respectively. It can be seen from Fig. 3 that the velocity of the working fluid increases greatly by the rapid rotation of the tool after it is flushed into the machining gap. The velocity of the working fluid decreases after it is flushed away the machining gap.
Fig. 4 shows the pressure distribution of the working fluid in the gap flow field of circumferential electrical discharge milling and mechanical grinding compound machining. It can be seen from Fig. 4 that the pressure at the working fluid entrance is high, the pressure decreases when the working fluid flows through the discharge region, and the pressure decreases continuously when the working fluid outflows the discharge region, which is beneficial to the machining debris ejection and the working fluid circulation.
3.2. Analysis of the Machining Debris Movement in the Gap Flow Field
Many machining debris will be produced during the circumferential electrical discharge milling and mechanical grinding compound machining SiC ceramic. The machining debris will gather on the machined surface, and arc discharge will happen if the machining debris is not ejected timely. The machining debris movement in the gap flow field is analyzed with the software of Fluent, and the result is shown in Fig. 5.
It can be seen from Fig. 5 that the machining debris is flushed away the machining gap by the rapid rotation of the tool. The velocity of the machining debris decreases after it is flushed away the machining gap. It can be concluded that the machining debris can be flushed away the machined surface timely by the rapid rotation of the tool, and the discharge stability can be enhanced.
4. Conclusions
(1) During the compound machining, electrical discharge milling and mechanical grinding happen alternately and they are mutually beneficial, so the high material removal rate and good surface quality of machining engineering ceramics with the compound process can be obtained.
(2) The gap flow field is numerically simulated, and the flow of the working fluid and the movement of the machining debris in the machining gap are modeled based on the liquidsolid twophase flow theory. The results show that the working fluid flow can be accelerated, and the machining debris can be ejected timely by the rapid rotation of the tool, so the process performance can be enhanced.
Acknowledgements
The work is partially supported by a grant from the National Natural Science Foundation of China (Grant No. 5120541 1), a grant from Shandong Provincial Natural Science Foundation of China (Grant No. ZR2012EEL15), a grant from the Fundamental Research Funds for the Central Universities (Grant No. 1 1CX0403 1 A), a grant from Taishan Scholar project of Shandong Province (TS201 10823), and a grant from Science and Technology Development Project of Shandong Province (201 1GHY1 1520).
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Renjie Ji, Yonghong Liu, Chao Zheng, Fei Wang, Yanzhen Zhang,
Yang Shen, Baoping Cai
College of Electromechanical Engineering, China University of Petroleum,
Qingdao, Shandong 266580 P. R. China.
Tel: +8653286983303, fax: +8653286983300
Email: jirenjie202@yahoo.cn
Received: 11 September 2012 /Accepted: 11 October 2012 /Published: 20 November 2012
(c) 2012 International Frequency Sensor Association
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