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On ultrasonic assisted abrasive flow finishing of bevel gears
International Journal of Machine Tools & Manufacture 89 (2015) 29–38
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
On ultrasonic assisted abrasive flow finishing of bevel gears G. Venkatesh, Apurbba Kumar Sharma n, Pradeep Kumar Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, Uttarakhand 247667, India
art ic l e i nf o
a b s t r a c t
Article history: Received 18 August 2014 Received in revised form 13 October 2014 Accepted 15 October 2014 Available online 7 November 2014
Finishing of bevel gears is an important requirement in many machining shop floors. Variants of abrasive flow machining (AFM) could be plausible solutions for finishing such parts with intricate geometries. In the present work, a relatively new variant of AFM called ultrasonically assisted abrasive flow machining (UAAFM) technique was employed to finish bevel gears made of EN8 steel. An analysis of the process has been presented with suitable illustrations. A finite element simulation of the behavior of the medium during finishing of bevel gears using the UAAFM process has been presented. A 3D model was constructed to simulate the flow of medium through the outer wall of the gear tooth surface using computational fluid dynamics (CFD) approach. The velocity, pressure and temperature values along the length of the workpiece were computed for both UAAFM and the conventional AFM processes. Further, the effectiveness of the process was investigated through experimental trials by conducting a comparison study between classical AFM and UAAFM. Ultrasonic frequency, extrusion pressure, processing time and the media flow rate were considered as the input variables while improvements in surface finish and material removal were considered as the monitored outputs. Results confirm that improvements in surface roughness and material removal are significantly higher than those obtained with conventional abrasive flow machining. The study further reveals that, the applied high frequency (ultrasonic) vibration to the workpiece has the maximum influence on the process responses among the variables considered. & Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic assisted abrasive flow machining Simulation Ultrasonic assistance Bevel gear Surface finish Material removal
1. Introduction The surface quality of a gear plays a major role in its performance and reliability. Bevel gears are widely used machine elements that connect nonparallel intersecting shafts. Bevel gears are used in differential drives, valve control and precision mechanical instruments. Consequently, continued efforts are on to achieve better surface finish of such gears irrespective of size and profiles. In general, hobbing, lapping, honing, shaving and some advanced processes such as electrochemical honing [1] are some of the popular processes used in finishing of gear. The major disadvantages of these processes are that they suffer from low finishing rate and high equipment cost. Therefore, there is a need for an alternate process which has the capability of nano-level finishing at higher finishing rate. The process is expected to be effective for finishing intricate external profiles and advanced materials. In the recent years, the AFM is emerging as a prominent finishing technique for both internal and external surfaces. The basic principle of the process consists of hydraulic pressure setup that pushes an abrasive laden medium onto the part surface through n
Corresponding author. Fax: þ 91 1332 285665. E-mail address: [email protected] (A.K. Sharma).
http://dx.doi.org/10.1016/j.ijmachtools.2014.10.014 0890-6955/& Elsevier Ltd. All rights reserved.
proper tooling. The tooling holds the workpiece and guides the medium to flow in contact with the target surface as shown in Fig. 1. Many researchers had reported on AFM for machining various internal primitives such as collets, diesel injectors, microbores, micro-holes, conformal channel tooling etc. Investigations on finishing of external surfaces using this process is however limited. In case of external surface finishing, tooling design decides the extent of restriction to be imposed [2]. The author also suggested that in case of polishing gear teeth, diameter of a cylinder placed around the gear teeth determines the extent of restriction present for the flow of medium. Yongchao et al. [3] investigated the effect of AFM processing on finishing of helical gears. The AFM process has the feasibility and capability of finishing gears, however, the process appears to be a relatively slow process to meet the present day demand for higher productivity. In order to overcome its limitations, such as low finishing rate, different variants of AFM have been developed over the years. Each variant has its own contribution to the field of science in abrasive flow machining and abrasive based nano-finishing techniques [4,5]. In some variants, a strong magnetic field had been applied to the workpiece to achieve better finishing rate; while in some work, a magnetic rheological fluid was used to improve the surface finish and termed as magnetorheological abrasive flow finishing (MRAFF) [6–9]. In another variant, a centrifugal force was generated by inserting a centrifugal rod inside the workpiece. In this
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Nomenclature Va Fa Vr Fr VR FR ‘θ’ B
ω ρ fi
Pa T ∇ p t
axial velocity of an abrasive particles (m/s) axial force (N/kg) radial velocity of the workpiece (m/s) radial force vector (N/kg) resultant velocity (m/s) resultant force (N/kg) approximate angle of an abrasive scratch (deg.) amplitude (mm) angular frequency (kHz) fluid density (kg/m3) body forces (per unit volume) acting on the fluid (N/m3) Pascal component of total stress tensor (N/m2) vector differential operator pressure (Pa) time (S)
u D/Dt r Ra
velocity vector (m/s) Eulerian derivative operator Cauchy stress tensor (N/m2) arithmetic average surface roughness
Acronyms AFM UAAFM MRAFF UFP CFD FEM FBD SF MR FE-SEM TGA
abrasive flow machining ultrasonic assisted abrasive flow machining magnetorheological abrasive flow finishing ultrasonic flow polishing computational fluid dynamics finite element modeling free body diagram surface finish improvement rate material removal field emission scanning electron microscopy thermo gravimetric analysis
Fig. 1. Schematic view of the UAAFM setup used for finishing bevel gears.
process, the centrifugal force generating rod was made to rotate about its axis, which in turn, rotates the medium and thereby improves the active abrasive grain density [10,11]. In an attempt to increase the medium-workpiece interaction and hence to enhance the process capability, a few researchers used helical passageways to provide helical path to the abrasives; better finishing rates were recorded for these studies [12,13]. In some other studies workpiece was rotated and managed to give simultaneous axial and rotary motions to the magnetorheological medium inside the workpiece; the study was carried out for finishing stainless steel tubes [14]. Pusavec [15] developed a novel method of movable mandrel based abrasive flow machining process in order to overcome the low finishing rate of the AFM. The author reported finishing of gearing injection mold, improved gear performance, energy efficiency in operation and fatigue life of plastic gears. Ultrasonic assisted AFM is a new technique in which the workpiece is subjected to a reciprocating micro-motion at high frequency while in contact with the abrasive medium [13]. This is achieved by coupling the workpiece to a piezo-electric actuator (Model: A-125020) that generates micro-motion at a high frequency in the range of 5–20 kHz. Owing to the application of high frequency, the process is termed as ultrasonic assisted abrasive flow machining (UAAFM). The first ultrasonic assistance with AFM was, however, reported by Jones and Hull [16]. In this method, also called ultrasonic flow polishing (UFP), the abrasive polymer medium was pumped with a center of the ultrasonically energized tool and better finishing was recorded when compared with simple AFM. A substantial enhancement in the capability of the basic AFM process by integrating the ultrasonic effect has also been reported [13,17].
The earlier studies were, however, limited to cylindrical geometry workpieces only; further analyses of the process, including simulation of the behavior of the working medium, have not been reported. Application of the UAAFM process for finishing of any external surface was also not attempted. Considering the potential of the UAAFM process, it was decided to investigate finishing of EN8 steel bevel gears using the process. In the present work, a natural polymer material mixed with SiC abrasive particles was used as the medium [18]. Simulation of the medium flow through the bevel gear teeth surface was carried out in order to understand the medium flow behavior in the process. Finite element analysis of medium flow in both AFM and UAAFM modes were carried out in terms of medium velocity, extrusion pressure and temperature using commercially available ANSYS FLUENT tool. Further, experiments were planned considering ultrasonic frequency, extrusion pressure; processing time and medium flow rate were selected as input variables, whereas surface finish improvement rate (SF) and material removal (MR) were considered as the process responses.
2. The ultrasonic assistance mechanism The ultrasonic assisted abrasive flow machining is a new process in which the workpiece is subjected to ultrasonic vibration perpendicular to the medium flow direction [17]. In this process, a high frequency (about 5–20 kHz) with low amplitude (5–10 mm) was applied to the workpiece with the help of a piezo actuator using a specially designed fixture. Fig. 1 illustrates the fundamental concept of the UAAFM process used in this study for
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Fig. 2. (a) Schematic of a bevel gear during machining through UAAFM process, (b) free body diagram of the abrasive-workpiece system and (c) machined surface.
finishing of bevel gear. The workpiece gear was located inside the two halves of the teflon tooling. It was fixed to the tooling through mechanical coupling in such a way that it could be assisted with the ultrasonic vibrations through the actuator. The workpiece gear is, thus, in dynamic condition throughout the process. The medium was strictly restricted to flow in contact with the gear tooth face width and flank surface with the help of a specially designed tooling. A schematic of the interaction of the abrasive particles with the workpiece surface is shown in Fig. 2. As the media is constrained to flow in contact with gear teeth profiles through the special tooling, the abrasives scratch the surface asperities on the gear teeth producing micro-chips (Fig. 2(a)). A free body diagram (FBD) of the abrasive asperity contact point is presented in Fig. 2(b). Ultrasonic vibration increases the relative velocity (VR) of abrasives with which they hit the workpiece asperities [13,17]. Since the velocity of the workpiece is made more than the velocity of the abrasive particles flowing with the medium (Va), there is a considerable increase in active abrasive grain density at any given instant of time. Moreover, an additional radial force (Fr) gets added as illustrated in Fig. 2(b). The asperity peak on the rough micro surface of the gear was abraded (hit) due to the resultant force (FR). The resultant velocity and the resultant force act at an angle ‘θ’ on the asperity and the asperity gets sheared off generating a micro-chip as illustrated in Fig. 2(c). The process continues until the major asperity peaks get sheared as the medium is kept flown reciprocating through the work surface. Eventually a smooth and glazed surface is resulted. Consequently, this arrangement helps in providing better cutting conditions for the tiny cutting tools (abrasives) in terms of
more depth of penetration and better shearing; hence improvements in surface finish and material removal could be possible. Detailed analysis of the process has been given elsewhere [13]. Fig. 3(a) shows the UAAFM arrangement used for finishing of bevel gears in the present investigation. The two medium cylinders act as the reservoir for the medium that contains SiC abrasive particles. The medium cylinders get driven by the hydraulic cylinders placed next to them (Fig. 3(a)). The medium is made to move from one cylinder to the other through the workpiece contact placed inside the workpiece fixture (teflon tooling). The pinion, in relation to the actuator and setup, was made to vibrate in Z-direction perpendicular to the abrasive medium flow direction (X-axis). The magnitude of displacement is controlled in only one direction (Z-axis) using the KC-N15 amplifier and the computer (Fig. 1). The actual mounting of the bevel gear inside the Teflon tooling is shown in Fig. 3(b).
3. Simulation of the medium behavior The working medium is a homogeneous mixture of a highly viscous natural polymer (viscosity ¼730 Pa s) and SiC abrasive particles (average size: 74 mm) in a predefined ratio. The medium acts as a continuously deforming tool and enables the tiny cutting points (the abrasives) scratch the surface through which it is made to flow. Consequently, characteristics of the medium flow influence the machining in the AFM process. In the UAAFM process, an additional effect in terms of the externally applied vibration is superimposed along with the classical action of the AFM. Thus, it is important to know the behavior of medium in the UAAFM process.
Fig. 3. (a) UAAFM setup and (b) details of the Teflon tooling.
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Accordingly, a numerical model of the abrasive medium was developed using the finite element modeling (FEM) technique while flowing through (i.e., in physical contact with) the workpiece. The model was used to estimate pressure, velocity and the temperature developed while machining through the AFM and UAAFM. Computations were performed using the ANSYS FLUENT CFD tool (version-14). The Navier–Stokes equation (Eq. (1)) was used for fluid flow simulation in order to define the fluid flow characteristics.
⎛ ∂u ⎞ ρ⎜ + u . ∇u⎟ = − ∇p + ∇. T + fi ⎝ ∂t ⎠
3,552,428 Hybrid (tetrahedral, hexahedral and cubic) 10 μm 40,000 10 4
the factor Dρ /Dt becomes zero. Hence Eq. (3) reduces to
(1)
3.1. Continuity equation The law of conservation of mass, can be stated as,
⇒
(2)
∂uy ∂ux ∂uz + + =0 ∂x ∂y ∂z
(4)
Similarly, for the y-axis and z-axis, the corresponding relationships get modified. 3.2. Momentum equation According to the law of conservation of linear momentum, the rate of change of linear momentum Is identical to the sum of the external forces acting on the region. the concept of law of conservation of momentum has been used for analysis of medium in other variant of the AFM process [10]. Applying this for the UAAFM process gives
ρ
Du = ∇. σ + ρFr Dt
(5)
where, Fr ¼Radial force vector due to applied vibration measured in per unit mass, N/kg and r ¼Cauchy stress tensor, N/m2 Then, Eq. (5) can be written as
where,
D ∂ = + u. ∇ Dt ∂t
Number of cells Mesh (element type) Element size Minimum number of iterations Stopping criteria
∇. u = 0
where u is the velocity vector, m/s, ρ is the fluid density, kg/m3, p is the pressure, Pa, T is the component of total stress tensor, N/m2, fi is the body forces (per unit volume) acting on the fluid, N/m3, and ∇ is the vector differential operator. The Navier–stokes equations are strictly a statement of the conservation of momentum. in order to understand and describe the fluid flow on the outer wall of the specimen, conservation of mass, conservation of energy, and/or an equation of state were invoked in the simulation as briefly described in the following sections.
Dρ + ρ∇ . u = 0 Dt
Table 1 Details of the mesh model used in the simulation studies.
(3)
The density of carrier medium was measured and consequently assumed to be constant throughout the machining process as well as in simulation. The polymer medium used is incompressible, and therefore, u (x, t) satisfies the incompressibility constraint. Hence,
⎛ ∂u ∂σij ∂u ⎞ ρ⎜⎜ i + ui i ⎟⎟ = + ρFr ∂xj ⎠ ∂xj ⎝ ∂t
(6)
The tooling, medium inlet and outlet flow conditions and workpiece dimensions were modeled exactly to replicate the existing setup
Fig. 4. Model of the tooling used for finishing the bevel gears: (a) configuration in classical AFM, (b) specially designed tooling for UAAFM and direction of medium flow and (c) a mesh model of the medium and the tooling.
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Fig. 5. Simulation results for the distribution of medium flow properties for classic AFM: (a) pressure distribution (b) temperature distribution (c) velocity distribution and (d) velocity flow vectors at AFM conditions. Inset: zoomed view of the velocity vectors.
conditions. The same conditions were also included for performing the experimental trials. In simulation, the following processing conditions were used: working pressure of the abrasive flow¼30 105 Pa; initial density of the abrasive medium¼1260 kg/m3. The properties of the medium were: temperature¼300 K, thermal conductivity¼ 0.8 W m 1 K 1, specific heat coefficient¼ 1800 J/kg K, medium viscosity¼ 730 Pa s. The cross section of the half thickness of the bevel gear of EN8 steel material was used to demonstrate the simulated results. The half section was used to reduce the computational time. The tooling used for positioning the bevel gear in the setup and the arrangements to facilitate the medium flow through the gear teeth in AFM and UAAFM are illustrated in Fig. 4(a) and (b). The positioning of the piezo-actuator and its coupling with the bevel gear is also shown in Fig. 4(b). However, the actuator arrangement was excluded in the simulation model, instead, only micro-motion at 19 kHz and 10 mm was considered as one of the input conditions for the UAAFM mode. The sinusoidal micro-motion, applied orthogonal to the gear axis, was simulated through a user defined function. The figure also illustrates the movement of the medium through the bevel gear. The fixture was specially designed considering the constraint of medium entry into the work space owing to unequal diameters of the gear at the two ends. Accordingly, six small medium entry holes as shown in the left half of the tooling (Fig. 4) were provided and the direction of flow has been indicated. The medium, however, flows from both the directions during the operation. The simulation results were obtained for the medium flow from the large end of the gear towards the smaller end. A mesh model of the working medium while flowing through the outer wall of the workpiece bevel gear is shown in Fig. 4(c). The element sizes in the mesh at different zones of the model were varied depending on the importance of the data points and to minimize the computation time.
In the present work, the medium was discretized using hybrid elements (Fig. 4(c)). The element size was kept fine at the workpiece zone targeting higher accuracy while course mesh was used at the outer parts. The optimized grid size was selected based on the convergent time and accuracy in solution. The details of the mesh model are given in Table 1. In order to investigate the appropriate element/cell number, a grid independency study was performed by proceeding with a small element number. The optimum number of element was determined based on the consistency in the outputs (pressure, velocity and temperature distribution). The simulation was stopped when the error converged to 10 4. The output values of interest had reached steady state at this stage of iterations. The simulation results for both the machining cases (AFM and UAAFM) are shown in Figs. 5 and 6. The pressure, velocity, temperature and the direction of velocity vector profiles while flowing through the workpiece in AFM are presented in Fig. 5(a–d). The corresponding simulation results for the UAAFM process have been illustrated in Fig. 6(a–d). The results reveal that, in both AFM and UAAFM conditions, the pressure decreases with increase in length of medium flow travel (Figs. 5(a) and 6(a)). The velocity profiles, on the other hand, show that the medium velocity increases along the length of travel of the medium (Figs. 5(b) and 6 (b)). The medium pressure gets marginally dropped due to the increased effective volume once it passes along the teeth of the bevel gear and there is a corresponding increase in the flow velocity. Similar observations in AFM processing were also claimed by other author [10]. However, the changes in the values of pressure and velocity are much higher in case of the UAAFM. The maximum increase in medium velocity was recorded as 18.9%. The increased velocity, attributed to the application of the external ultrasonic vibration, helps in easy removal of the asperity peaks on
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Fig. 6. Simulation results for the distribution of medium flow properties for UAAFM: (a) pressure distribution (b) temperature distribution (c) velocity distribution and (d) velocity flow vectors at UAAFM conditions. Inset: zoomed view of the velocity vectors.
the workpiece surface as already explained in the Section 2 (Fig. 2). This also makes the finishing process reasonably faster. The simulation results also showed that (Figs. 5(c) and 6(c)) temperature of the working medium in both AFM and UAAFM was increasing along the length of travel of the medium. The maximum temperature value was computed at the end of the maximum travel of the medium (UAAFM: 308 K; AFM: 302 K), i.e. in this case, at the smaller end of the workpiece. This is due to the friction between the medium and the workpiece wall as well as the rapid cutting action by the SiC abrasives on the asperity peaks of the work surface. However, the rise in temperature of the medium is well below the thermal degradation temperature of the medium as confirmed by the Thermo Gravimetric Analysis (TGA) [18]. Therefore, the applied ultrasonic assistance will have insignificant effect on the rheological properties of the medium. Further, it is interesting to note that the velocity vectors in case of UAAFM were found traveling at an angle ‘θ’ with respect to the axis of the workpiece (Fig. 6(d), Inset); whereas no such angular deviation of the velocity vectors was observed in AFM (Fig. 5(d)). This observed deviation is attributed to the applied ultrasonic assistance along with the medium flow velocity as discussed in the Section 2 (Fig. 2). The change in the direction of the velocity vectors in the UAAFM is expected to help the tiny tool points (abrasives) in the cutting (shearing) process as explained already. The experimental results are presented in the following sections. 4. Experimental procedure Fig. 7. A view of EN8 bevel gear workpieces prior to finishing.
A horizontal type double acting multi variant abrasive flow machining setup was used for the experimental trials. A natural
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Table 2 Chemical composition of the workpiece gear. Element Weight%
C 0.1690
Si 0.1740
Mn 0.6640
P 0.0280
S 0.0220
Cr 0.0110
Al 0.0350
V 0.011
Fe Balance
Table 3 Experimental conditions used in UAAFM and AFM. Type
Applied vibration (sinewave-continuous mode)
Frequency (kHz) UAAFM 19 kHz AFM 0
Amplitude (mm) 10 um 0
Extrusion pressure ( 105Pa)
Processing time (min)
Media flow rate (cm3/min)
Abrasive size (mesh)
Abrasive concentration (wt%)
Media viscosity (Pa s)
30 30
5-10-15 5-10-15
560 560
220 220
60 60
730 730
material removal were also calculated using the following equations (Eqs. (7) and (8)). The surface roughness values of the workpiece surfaces before and after finishing were measured using an optical profiler (Model: Veeco NT1100). Measurements were taken at five different positions on the sample and average of those values was considered. The amount of material removed was quantified by measuring the mass of workpiece before and after the process using a digital weighing machine (Shimadzu, model: AUW220D, accuracy: 0.01 mg). A scanning electron microscope (FE-SEM, Model: FEL Quanta 200) was used for examining the surface topography.
SF =
Intial Ra − final Ra × 100% intial Ra
MR = [Intial weight − final weight]
Fig. 8. Effect of processing time on improvement on surface finish.
(7)
(8)
5. Results and discussions The experimental results have been presented in the following sections. Some correlations with the simulation results have presented. 5.1. Surface roughness and material removal
Fig. 9. Improvement on material removal as a function of processing time.
polymer medium (prepared as in [18]) with SiC abrasives in the ratio of 40:60 was used as the working medium. Fig. 7 shows a few EN8 bevel gear workpieces. The chemical composition of the bevel gears as presented in Table 2 were observed through a BAIRD made atomic absorption spectroscopy instrument. The trial conditions used for the finishing operations are presented in Table 3. The percentage improvement in surface roughness values and
Trials were conducted to study the effects of applied frequency on finishing of the workpieces along with other important parameters such as extrusion pressure, media flow rate, abrasive size and processing time. Surface finish improvement rate (SF) and material removal (MR) on the bevel gears were monitored as the process responses. Results were also compared with those obtained through simple AFM process. Corresponding results are presented in Figs. 8 and 9. It may be noted that SF and MR are significantly higher in UAAFM process as compared to classical AFM process. The observed low finishing rate as shown in the UAAFM while compared to AFM for the observed periods may be attributed to higher material removal and hence higher surface finish attained during the initial few cycles (o 5 min of machining) itself (Fig. 8). Machining beyond 5 min in case of UAAFM become a fine finishing operation in which rate of improvement gets reduced (Ψ2 o Ψ1, Ψ4 o Ψ3, Fig. 8) while better finish is resulted. On the other hand, in the conventional AFM, the radial force acting on the abrasive particles is negligible (Fr ¼0) leading to relatively less interaction of the abrasives with the workpiece surface. In the UAAFM process, the workpiece is in dynamic condition, which vibrates radially at a high frequency and is subjected to additional radial forces (Fr) along with the conventional AFM process (Fig. 2 (b)). The radial velocity of the workpiece gets added to the velocity of the abrasives in the axial direction and contributes towards
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Fig. 10. Topography of the machined surfaces obtained through optical profilometer: (a) prefinished, (b) AFM and (c) UAAFM.
Fig. 11. Typical SEM micrographs of gear tooth surface: before UAAFM at (a) magnification: 1000, (b) magnification: 5000; after UAAFM at (c) magnification: 1000 and (d) magnification: 5000.
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observed enhancement in surface finish improvement rate and material removal. Figs. 8 and 9 clearly show that rate of improvement in surface finish and MR in UAAFM pertaining to 5 min of machining is even higher than the values after 15 min of machining through conventional AFM although the initial surface finish in both the cases were similar (Ra ¼1.8 mm). This further clarifies that the rate of removal and surface finish improvement during the initial cycles in UAAFM are significantly higher than that of AFM. After 5 and 15 min of UAAFM, the improvements in SF and MR are 73.12% and 24 mg respectively. Initially, the as received surfaces contain high asperity peaks. Thus, while removing those peaks during the initial cycles, the average Ra value reduces relatively faster. Further processing is generally used for achieving fineness; removal of peaks becomes gradually insignificant. Continued machining thus, results in reduced improvement rate compared to initial cycle (Ψ4 o Ψ2, Fig. 8). The trend of surface finish improvement with increase in processing time was analyzed and can be correlated with the FEM velocity results (Figs. 5(a) and 6(b)) where the velocity increases with additional vibration effect thus makes the UAAFM process rapid when compared to AFM. Typical material removal characteristics observed during the UAAFM and AFM with silicon abrasive carrier while finishing the bevel gears is shown in Fig. 9. It is observed that during the initial abrasion cycles (say up to 10 min of machining), material removal (MR) is very high which however, decreases with the increase in the processing time (Ø3 { Ø1). This is due to the reduced effectiveness of the dull abrasives to further scratch the already glazed surface. The corresponding improvements in surface roughness also gets reduced significantly (Ψ3 { Ψ1, Fig. 8). Owing to the presence of radial component (Fr, Fig. 2(b)) in UAAFM, however even the dull abrasives during the later stages of machining (say beyond 10 minutes) are capable of removing micro-chips from the workpiece surfaces as indicated by a significantly high rate of MR (Ø4 4Ø3, Fig. 9). 5.2. Morphology of machined surface The finished surfaces of the EN8 steel bevel gears using UAAFM were also investigated by examining the surface texture and microstructure. The effect of the processing parameters on the surface quality was evaluated using a scanning electron microscopy and an optical profilometer. Fig. 10(a–c) illustrate the surface topographies of the workpiece surface before finishing, after finishing through conventional AFM and UAAFM processed surfaces. It is clear from the figures that the surface texture of the gears finished by the UAAFM appears glazed indicating significant improvement in surface finish. It was observed that there was a 55% rate of improvement in the surface of the gears finished through classical AFM, while an improvement rate of 73.12% was recorded after finishing the part through UAAFM for similar machining interval. The typical profiles presented show significantly reduced peaks in case the UAAFMed surface. The observed deep feed marks got swallowed reasonably during AFM, while they appear almost removed in the profile corresponding to the UAAFM. Thus, the effectiveness of the application of the additional effect in terms of the external ultrasonic vibration could be realised. The SEM analysis was also conducted on UAAFM finished bevel gear workpieces to investigate the morphology as well as to study the mechanism of UAAFM process. Fig. 11(a–b) clearly show that before UAAFM process, the work surface of the as received gear tooth contains scratches, micro-cracks, pits etc. Micro-graphs of UAAFMed surface morphology are shown in the Fig. 11(c–d). The UAAFM finished surface presents a glazed appearance, while very light feed marks (due to gear hobbing) and scratches by the medium abrasives can be identified. The asperity heights were removed during the UAAFM operation as discussed in Section 2
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(Fig. 2) and accordingly, the heights of the hobbing feed marks also got significantly reduced. It is observed from the figure that the abrasives hit the target surface with some inclination, say angle ‘θ’ (as shown schematically in Fig. 2(b) and also evident from simulation results presented at Fig. 6(d)). The angular scratching by the abrasives in the UAAFM process could be confirmed by the observed scratches in the magnified view of the UAAFMED surface (Fig. 11(d)). This is due to the applied vibrational (ultrasonic assistance) at high frequency and low amplitude. The inactive sliding and rolling abrasives now hit the target surface causing increased removal of the asperity peaks. On continued machining, the ultrasonic assistance contributes significantly on surface finish as evidenced by the glazed surface (Fig. 11(c) and (d)).
6. Conclusion In the present work, simulations as well as experimental studies of the UAAFM process on bevel gears were carried out for better understanding of the process. Results have been presented with suitable illustrations. The study reveals that the UAAFM is one of the best alternative methods for finishing gears. The following major conclusions could be drawn from the study. i. The abrasives in the UAAFM process scratch the asperity peaks of the workpiece surface at a higher velocity than in the AFM in the corresponding machining conditions. This facilitates easy and faster removal of materials which enhances the performance of the process. Finishing of bevel gears using UAAFM is more effective than conventional AFM process in terms of finishing time and improvement in surface roughness. ii. The simulation studies confirm significant changes in the medium behavior in the UAAFM process with reference to the classical AFM in terms of velocity and pressure profiles. iii. The additional ultrasonic vibration causes the abrasives to interact with the workpiece asperities to an angular shift ‘θ ’. This causes increase in the active abrasive grain density and reduction in inactive sliding and rolling of the abrasives. iv. Improvement in surface quality is rapid during the initial phases of finishing, while the material removal rate remains almost unchanged. v. Gear tooth surface morphology gets significantly improved resulting in glazed texture owing to enhanced density of active abrasive grains in a given processing time.
Acknowledgment The authors are thankful to the Department of Science and Technology, Government of India for providing financial assistance to this work through DST Project Grant no. DST-384-MID.
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machining variants and media, Proc. IMechE, Part B: J. Eng. Manuf 226 (12) (2012) 1951–1962. S. Singh, H.S. Shan, Development of magneto abrasive flow machining process, Int. J. Mach. Tools Manuf 42 (2002) 953–959. A. Sidpara, V.K. Jain, Experimental investigations into forces during magnetorheological fluid based finishing process, Int. J. Mach. Tools Manuf 51 (4) (2011) 358–362. V.K. Jain, S.C. Jayswal, P.M. Dixit, Modelling and simulation of surface roughness in magnetic abrasive finishing using non-uniform surface profiles, Mater. Manuf. Process. 22 (2) (2007) 256–270. S. Jha, V.K. Jain, Design and development of the magnetorheological abrasive flow finishing process, Int. J. Mach. Tools Manuf 44 (10) (2004) 1019–1029. R.S. Walia, H.S. Shan, P. Kumar, Finite element analysis of media used in the centrifugal force assisted abrasive flow machining process, Proc. IMechE, Part B: J. Eng. Manuf. 220 (11) (2006) 1775–1785. R.S. Walia, H.S. Shan, P. Kumar, Multi-response optimization of CFAAFM process through taguchi method and utility concept, Mater. Manuf. Process. 21 (8) (2006) 907–914. A.C. Wang, K.C. Cheng, K.Y. Chen, Y.C. Lin, Enhancing the surface precision for the helical passageways in abrasive flow machining, Mater. Manuf. Process. 29 (2) (2014) 153–159.
[13] S. Rajesha, Some Studies to Enhance the Capabilities of Abrasive Flow Machining Process (Dissertation), IIT Roorkee, India, 2011. [14] M. Das, V.K. Jain, P.S. Ghoshdastidar, The out-of-roundness of the internal surfaces of stainless steel tubes finished by the rotational–magnetorheological abrasive flow finishing process, Mater. Manuf. Process. 26 (8) (2011) 1073–1084. [15] F. Pusavec, J. Kenda, J. Kopac, The transition to a clean, dry and energy efficient polishing process: an innovative upgrade of abrasive flow machining for simultaneous generation of micro-geometry and polishing in the tooling industry, J. Clean. Prod. 76 (2014) 180–189. [16] A.R. Jones, J.B. Hull, Ultrasonic flow polishing, Ultrasonics 36 (1998) 97–101. [17] A.K. Sharma, S. Rajesha, P. Kumar, An improved ultrasonic abrasive flow machining and a device therefore, Indian Patent Number 3578/DEL/201, December 9, 2011. [18] S. Rajesha, G.Venkatesh, A.K. Sharma, P. Kumar, A Natural polymer media for abrasive flow machining Process and a process for preparation, Indian Patent Number 1349/DEL/2011, 2011.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
On ultrasonic assisted abrasive flow finishing of bevel gears G. Venkatesh, Apurbba Kumar Sharma n, Pradeep Kumar Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee, Uttarakhand 247667, India
art ic l e i nf o
a b s t r a c t
Article history: Received 18 August 2014 Received in revised form 13 October 2014 Accepted 15 October 2014 Available online 7 November 2014
Finishing of bevel gears is an important requirement in many machining shop floors. Variants of abrasive flow machining (AFM) could be plausible solutions for finishing such parts with intricate geometries. In the present work, a relatively new variant of AFM called ultrasonically assisted abrasive flow machining (UAAFM) technique was employed to finish bevel gears made of EN8 steel. An analysis of the process has been presented with suitable illustrations. A finite element simulation of the behavior of the medium during finishing of bevel gears using the UAAFM process has been presented. A 3D model was constructed to simulate the flow of medium through the outer wall of the gear tooth surface using computational fluid dynamics (CFD) approach. The velocity, pressure and temperature values along the length of the workpiece were computed for both UAAFM and the conventional AFM processes. Further, the effectiveness of the process was investigated through experimental trials by conducting a comparison study between classical AFM and UAAFM. Ultrasonic frequency, extrusion pressure, processing time and the media flow rate were considered as the input variables while improvements in surface finish and material removal were considered as the monitored outputs. Results confirm that improvements in surface roughness and material removal are significantly higher than those obtained with conventional abrasive flow machining. The study further reveals that, the applied high frequency (ultrasonic) vibration to the workpiece has the maximum influence on the process responses among the variables considered. & Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic assisted abrasive flow machining Simulation Ultrasonic assistance Bevel gear Surface finish Material removal
1. Introduction The surface quality of a gear plays a major role in its performance and reliability. Bevel gears are widely used machine elements that connect nonparallel intersecting shafts. Bevel gears are used in differential drives, valve control and precision mechanical instruments. Consequently, continued efforts are on to achieve better surface finish of such gears irrespective of size and profiles. In general, hobbing, lapping, honing, shaving and some advanced processes such as electrochemical honing [1] are some of the popular processes used in finishing of gear. The major disadvantages of these processes are that they suffer from low finishing rate and high equipment cost. Therefore, there is a need for an alternate process which has the capability of nano-level finishing at higher finishing rate. The process is expected to be effective for finishing intricate external profiles and advanced materials. In the recent years, the AFM is emerging as a prominent finishing technique for both internal and external surfaces. The basic principle of the process consists of hydraulic pressure setup that pushes an abrasive laden medium onto the part surface through n
Corresponding author. Fax: þ 91 1332 285665. E-mail address: [email protected] (A.K. Sharma).
http://dx.doi.org/10.1016/j.ijmachtools.2014.10.014 0890-6955/& Elsevier Ltd. All rights reserved.
proper tooling. The tooling holds the workpiece and guides the medium to flow in contact with the target surface as shown in Fig. 1. Many researchers had reported on AFM for machining various internal primitives such as collets, diesel injectors, microbores, micro-holes, conformal channel tooling etc. Investigations on finishing of external surfaces using this process is however limited. In case of external surface finishing, tooling design decides the extent of restriction to be imposed [2]. The author also suggested that in case of polishing gear teeth, diameter of a cylinder placed around the gear teeth determines the extent of restriction present for the flow of medium. Yongchao et al. [3] investigated the effect of AFM processing on finishing of helical gears. The AFM process has the feasibility and capability of finishing gears, however, the process appears to be a relatively slow process to meet the present day demand for higher productivity. In order to overcome its limitations, such as low finishing rate, different variants of AFM have been developed over the years. Each variant has its own contribution to the field of science in abrasive flow machining and abrasive based nano-finishing techniques [4,5]. In some variants, a strong magnetic field had been applied to the workpiece to achieve better finishing rate; while in some work, a magnetic rheological fluid was used to improve the surface finish and termed as magnetorheological abrasive flow finishing (MRAFF) [6–9]. In another variant, a centrifugal force was generated by inserting a centrifugal rod inside the workpiece. In this
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Nomenclature Va Fa Vr Fr VR FR ‘θ’ B
ω ρ fi
Pa T ∇ p t
axial velocity of an abrasive particles (m/s) axial force (N/kg) radial velocity of the workpiece (m/s) radial force vector (N/kg) resultant velocity (m/s) resultant force (N/kg) approximate angle of an abrasive scratch (deg.) amplitude (mm) angular frequency (kHz) fluid density (kg/m3) body forces (per unit volume) acting on the fluid (N/m3) Pascal component of total stress tensor (N/m2) vector differential operator pressure (Pa) time (S)
u D/Dt r Ra
velocity vector (m/s) Eulerian derivative operator Cauchy stress tensor (N/m2) arithmetic average surface roughness
Acronyms AFM UAAFM MRAFF UFP CFD FEM FBD SF MR FE-SEM TGA
abrasive flow machining ultrasonic assisted abrasive flow machining magnetorheological abrasive flow finishing ultrasonic flow polishing computational fluid dynamics finite element modeling free body diagram surface finish improvement rate material removal field emission scanning electron microscopy thermo gravimetric analysis
Fig. 1. Schematic view of the UAAFM setup used for finishing bevel gears.
process, the centrifugal force generating rod was made to rotate about its axis, which in turn, rotates the medium and thereby improves the active abrasive grain density [10,11]. In an attempt to increase the medium-workpiece interaction and hence to enhance the process capability, a few researchers used helical passageways to provide helical path to the abrasives; better finishing rates were recorded for these studies [12,13]. In some other studies workpiece was rotated and managed to give simultaneous axial and rotary motions to the magnetorheological medium inside the workpiece; the study was carried out for finishing stainless steel tubes [14]. Pusavec [15] developed a novel method of movable mandrel based abrasive flow machining process in order to overcome the low finishing rate of the AFM. The author reported finishing of gearing injection mold, improved gear performance, energy efficiency in operation and fatigue life of plastic gears. Ultrasonic assisted AFM is a new technique in which the workpiece is subjected to a reciprocating micro-motion at high frequency while in contact with the abrasive medium [13]. This is achieved by coupling the workpiece to a piezo-electric actuator (Model: A-125020) that generates micro-motion at a high frequency in the range of 5–20 kHz. Owing to the application of high frequency, the process is termed as ultrasonic assisted abrasive flow machining (UAAFM). The first ultrasonic assistance with AFM was, however, reported by Jones and Hull [16]. In this method, also called ultrasonic flow polishing (UFP), the abrasive polymer medium was pumped with a center of the ultrasonically energized tool and better finishing was recorded when compared with simple AFM. A substantial enhancement in the capability of the basic AFM process by integrating the ultrasonic effect has also been reported [13,17].
The earlier studies were, however, limited to cylindrical geometry workpieces only; further analyses of the process, including simulation of the behavior of the working medium, have not been reported. Application of the UAAFM process for finishing of any external surface was also not attempted. Considering the potential of the UAAFM process, it was decided to investigate finishing of EN8 steel bevel gears using the process. In the present work, a natural polymer material mixed with SiC abrasive particles was used as the medium [18]. Simulation of the medium flow through the bevel gear teeth surface was carried out in order to understand the medium flow behavior in the process. Finite element analysis of medium flow in both AFM and UAAFM modes were carried out in terms of medium velocity, extrusion pressure and temperature using commercially available ANSYS FLUENT tool. Further, experiments were planned considering ultrasonic frequency, extrusion pressure; processing time and medium flow rate were selected as input variables, whereas surface finish improvement rate (SF) and material removal (MR) were considered as the process responses.
2. The ultrasonic assistance mechanism The ultrasonic assisted abrasive flow machining is a new process in which the workpiece is subjected to ultrasonic vibration perpendicular to the medium flow direction [17]. In this process, a high frequency (about 5–20 kHz) with low amplitude (5–10 mm) was applied to the workpiece with the help of a piezo actuator using a specially designed fixture. Fig. 1 illustrates the fundamental concept of the UAAFM process used in this study for
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Fig. 2. (a) Schematic of a bevel gear during machining through UAAFM process, (b) free body diagram of the abrasive-workpiece system and (c) machined surface.
finishing of bevel gear. The workpiece gear was located inside the two halves of the teflon tooling. It was fixed to the tooling through mechanical coupling in such a way that it could be assisted with the ultrasonic vibrations through the actuator. The workpiece gear is, thus, in dynamic condition throughout the process. The medium was strictly restricted to flow in contact with the gear tooth face width and flank surface with the help of a specially designed tooling. A schematic of the interaction of the abrasive particles with the workpiece surface is shown in Fig. 2. As the media is constrained to flow in contact with gear teeth profiles through the special tooling, the abrasives scratch the surface asperities on the gear teeth producing micro-chips (Fig. 2(a)). A free body diagram (FBD) of the abrasive asperity contact point is presented in Fig. 2(b). Ultrasonic vibration increases the relative velocity (VR) of abrasives with which they hit the workpiece asperities [13,17]. Since the velocity of the workpiece is made more than the velocity of the abrasive particles flowing with the medium (Va), there is a considerable increase in active abrasive grain density at any given instant of time. Moreover, an additional radial force (Fr) gets added as illustrated in Fig. 2(b). The asperity peak on the rough micro surface of the gear was abraded (hit) due to the resultant force (FR). The resultant velocity and the resultant force act at an angle ‘θ’ on the asperity and the asperity gets sheared off generating a micro-chip as illustrated in Fig. 2(c). The process continues until the major asperity peaks get sheared as the medium is kept flown reciprocating through the work surface. Eventually a smooth and glazed surface is resulted. Consequently, this arrangement helps in providing better cutting conditions for the tiny cutting tools (abrasives) in terms of
more depth of penetration and better shearing; hence improvements in surface finish and material removal could be possible. Detailed analysis of the process has been given elsewhere [13]. Fig. 3(a) shows the UAAFM arrangement used for finishing of bevel gears in the present investigation. The two medium cylinders act as the reservoir for the medium that contains SiC abrasive particles. The medium cylinders get driven by the hydraulic cylinders placed next to them (Fig. 3(a)). The medium is made to move from one cylinder to the other through the workpiece contact placed inside the workpiece fixture (teflon tooling). The pinion, in relation to the actuator and setup, was made to vibrate in Z-direction perpendicular to the abrasive medium flow direction (X-axis). The magnitude of displacement is controlled in only one direction (Z-axis) using the KC-N15 amplifier and the computer (Fig. 1). The actual mounting of the bevel gear inside the Teflon tooling is shown in Fig. 3(b).
3. Simulation of the medium behavior The working medium is a homogeneous mixture of a highly viscous natural polymer (viscosity ¼730 Pa s) and SiC abrasive particles (average size: 74 mm) in a predefined ratio. The medium acts as a continuously deforming tool and enables the tiny cutting points (the abrasives) scratch the surface through which it is made to flow. Consequently, characteristics of the medium flow influence the machining in the AFM process. In the UAAFM process, an additional effect in terms of the externally applied vibration is superimposed along with the classical action of the AFM. Thus, it is important to know the behavior of medium in the UAAFM process.
Fig. 3. (a) UAAFM setup and (b) details of the Teflon tooling.
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Accordingly, a numerical model of the abrasive medium was developed using the finite element modeling (FEM) technique while flowing through (i.e., in physical contact with) the workpiece. The model was used to estimate pressure, velocity and the temperature developed while machining through the AFM and UAAFM. Computations were performed using the ANSYS FLUENT CFD tool (version-14). The Navier–Stokes equation (Eq. (1)) was used for fluid flow simulation in order to define the fluid flow characteristics.
⎛ ∂u ⎞ ρ⎜ + u . ∇u⎟ = − ∇p + ∇. T + fi ⎝ ∂t ⎠
3,552,428 Hybrid (tetrahedral, hexahedral and cubic) 10 μm 40,000 10 4
the factor Dρ /Dt becomes zero. Hence Eq. (3) reduces to
(1)
3.1. Continuity equation The law of conservation of mass, can be stated as,
⇒
(2)
∂uy ∂ux ∂uz + + =0 ∂x ∂y ∂z
(4)
Similarly, for the y-axis and z-axis, the corresponding relationships get modified. 3.2. Momentum equation According to the law of conservation of linear momentum, the rate of change of linear momentum Is identical to the sum of the external forces acting on the region. the concept of law of conservation of momentum has been used for analysis of medium in other variant of the AFM process [10]. Applying this for the UAAFM process gives
ρ
Du = ∇. σ + ρFr Dt
(5)
where, Fr ¼Radial force vector due to applied vibration measured in per unit mass, N/kg and r ¼Cauchy stress tensor, N/m2 Then, Eq. (5) can be written as
where,
D ∂ = + u. ∇ Dt ∂t
Number of cells Mesh (element type) Element size Minimum number of iterations Stopping criteria
∇. u = 0
where u is the velocity vector, m/s, ρ is the fluid density, kg/m3, p is the pressure, Pa, T is the component of total stress tensor, N/m2, fi is the body forces (per unit volume) acting on the fluid, N/m3, and ∇ is the vector differential operator. The Navier–stokes equations are strictly a statement of the conservation of momentum. in order to understand and describe the fluid flow on the outer wall of the specimen, conservation of mass, conservation of energy, and/or an equation of state were invoked in the simulation as briefly described in the following sections.
Dρ + ρ∇ . u = 0 Dt
Table 1 Details of the mesh model used in the simulation studies.
(3)
The density of carrier medium was measured and consequently assumed to be constant throughout the machining process as well as in simulation. The polymer medium used is incompressible, and therefore, u (x, t) satisfies the incompressibility constraint. Hence,
⎛ ∂u ∂σij ∂u ⎞ ρ⎜⎜ i + ui i ⎟⎟ = + ρFr ∂xj ⎠ ∂xj ⎝ ∂t
(6)
The tooling, medium inlet and outlet flow conditions and workpiece dimensions were modeled exactly to replicate the existing setup
Fig. 4. Model of the tooling used for finishing the bevel gears: (a) configuration in classical AFM, (b) specially designed tooling for UAAFM and direction of medium flow and (c) a mesh model of the medium and the tooling.
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Fig. 5. Simulation results for the distribution of medium flow properties for classic AFM: (a) pressure distribution (b) temperature distribution (c) velocity distribution and (d) velocity flow vectors at AFM conditions. Inset: zoomed view of the velocity vectors.
conditions. The same conditions were also included for performing the experimental trials. In simulation, the following processing conditions were used: working pressure of the abrasive flow¼30 105 Pa; initial density of the abrasive medium¼1260 kg/m3. The properties of the medium were: temperature¼300 K, thermal conductivity¼ 0.8 W m 1 K 1, specific heat coefficient¼ 1800 J/kg K, medium viscosity¼ 730 Pa s. The cross section of the half thickness of the bevel gear of EN8 steel material was used to demonstrate the simulated results. The half section was used to reduce the computational time. The tooling used for positioning the bevel gear in the setup and the arrangements to facilitate the medium flow through the gear teeth in AFM and UAAFM are illustrated in Fig. 4(a) and (b). The positioning of the piezo-actuator and its coupling with the bevel gear is also shown in Fig. 4(b). However, the actuator arrangement was excluded in the simulation model, instead, only micro-motion at 19 kHz and 10 mm was considered as one of the input conditions for the UAAFM mode. The sinusoidal micro-motion, applied orthogonal to the gear axis, was simulated through a user defined function. The figure also illustrates the movement of the medium through the bevel gear. The fixture was specially designed considering the constraint of medium entry into the work space owing to unequal diameters of the gear at the two ends. Accordingly, six small medium entry holes as shown in the left half of the tooling (Fig. 4) were provided and the direction of flow has been indicated. The medium, however, flows from both the directions during the operation. The simulation results were obtained for the medium flow from the large end of the gear towards the smaller end. A mesh model of the working medium while flowing through the outer wall of the workpiece bevel gear is shown in Fig. 4(c). The element sizes in the mesh at different zones of the model were varied depending on the importance of the data points and to minimize the computation time.
In the present work, the medium was discretized using hybrid elements (Fig. 4(c)). The element size was kept fine at the workpiece zone targeting higher accuracy while course mesh was used at the outer parts. The optimized grid size was selected based on the convergent time and accuracy in solution. The details of the mesh model are given in Table 1. In order to investigate the appropriate element/cell number, a grid independency study was performed by proceeding with a small element number. The optimum number of element was determined based on the consistency in the outputs (pressure, velocity and temperature distribution). The simulation was stopped when the error converged to 10 4. The output values of interest had reached steady state at this stage of iterations. The simulation results for both the machining cases (AFM and UAAFM) are shown in Figs. 5 and 6. The pressure, velocity, temperature and the direction of velocity vector profiles while flowing through the workpiece in AFM are presented in Fig. 5(a–d). The corresponding simulation results for the UAAFM process have been illustrated in Fig. 6(a–d). The results reveal that, in both AFM and UAAFM conditions, the pressure decreases with increase in length of medium flow travel (Figs. 5(a) and 6(a)). The velocity profiles, on the other hand, show that the medium velocity increases along the length of travel of the medium (Figs. 5(b) and 6 (b)). The medium pressure gets marginally dropped due to the increased effective volume once it passes along the teeth of the bevel gear and there is a corresponding increase in the flow velocity. Similar observations in AFM processing were also claimed by other author [10]. However, the changes in the values of pressure and velocity are much higher in case of the UAAFM. The maximum increase in medium velocity was recorded as 18.9%. The increased velocity, attributed to the application of the external ultrasonic vibration, helps in easy removal of the asperity peaks on
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Fig. 6. Simulation results for the distribution of medium flow properties for UAAFM: (a) pressure distribution (b) temperature distribution (c) velocity distribution and (d) velocity flow vectors at UAAFM conditions. Inset: zoomed view of the velocity vectors.
the workpiece surface as already explained in the Section 2 (Fig. 2). This also makes the finishing process reasonably faster. The simulation results also showed that (Figs. 5(c) and 6(c)) temperature of the working medium in both AFM and UAAFM was increasing along the length of travel of the medium. The maximum temperature value was computed at the end of the maximum travel of the medium (UAAFM: 308 K; AFM: 302 K), i.e. in this case, at the smaller end of the workpiece. This is due to the friction between the medium and the workpiece wall as well as the rapid cutting action by the SiC abrasives on the asperity peaks of the work surface. However, the rise in temperature of the medium is well below the thermal degradation temperature of the medium as confirmed by the Thermo Gravimetric Analysis (TGA) [18]. Therefore, the applied ultrasonic assistance will have insignificant effect on the rheological properties of the medium. Further, it is interesting to note that the velocity vectors in case of UAAFM were found traveling at an angle ‘θ’ with respect to the axis of the workpiece (Fig. 6(d), Inset); whereas no such angular deviation of the velocity vectors was observed in AFM (Fig. 5(d)). This observed deviation is attributed to the applied ultrasonic assistance along with the medium flow velocity as discussed in the Section 2 (Fig. 2). The change in the direction of the velocity vectors in the UAAFM is expected to help the tiny tool points (abrasives) in the cutting (shearing) process as explained already. The experimental results are presented in the following sections. 4. Experimental procedure Fig. 7. A view of EN8 bevel gear workpieces prior to finishing.
A horizontal type double acting multi variant abrasive flow machining setup was used for the experimental trials. A natural
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Table 2 Chemical composition of the workpiece gear. Element Weight%
C 0.1690
Si 0.1740
Mn 0.6640
P 0.0280
S 0.0220
Cr 0.0110
Al 0.0350
V 0.011
Fe Balance
Table 3 Experimental conditions used in UAAFM and AFM. Type
Applied vibration (sinewave-continuous mode)
Frequency (kHz) UAAFM 19 kHz AFM 0
Amplitude (mm) 10 um 0
Extrusion pressure ( 105Pa)
Processing time (min)
Media flow rate (cm3/min)
Abrasive size (mesh)
Abrasive concentration (wt%)
Media viscosity (Pa s)
30 30
5-10-15 5-10-15
560 560
220 220
60 60
730 730
material removal were also calculated using the following equations (Eqs. (7) and (8)). The surface roughness values of the workpiece surfaces before and after finishing were measured using an optical profiler (Model: Veeco NT1100). Measurements were taken at five different positions on the sample and average of those values was considered. The amount of material removed was quantified by measuring the mass of workpiece before and after the process using a digital weighing machine (Shimadzu, model: AUW220D, accuracy: 0.01 mg). A scanning electron microscope (FE-SEM, Model: FEL Quanta 200) was used for examining the surface topography.
SF =
Intial Ra − final Ra × 100% intial Ra
MR = [Intial weight − final weight]
Fig. 8. Effect of processing time on improvement on surface finish.
(7)
(8)
5. Results and discussions The experimental results have been presented in the following sections. Some correlations with the simulation results have presented. 5.1. Surface roughness and material removal
Fig. 9. Improvement on material removal as a function of processing time.
polymer medium (prepared as in [18]) with SiC abrasives in the ratio of 40:60 was used as the working medium. Fig. 7 shows a few EN8 bevel gear workpieces. The chemical composition of the bevel gears as presented in Table 2 were observed through a BAIRD made atomic absorption spectroscopy instrument. The trial conditions used for the finishing operations are presented in Table 3. The percentage improvement in surface roughness values and
Trials were conducted to study the effects of applied frequency on finishing of the workpieces along with other important parameters such as extrusion pressure, media flow rate, abrasive size and processing time. Surface finish improvement rate (SF) and material removal (MR) on the bevel gears were monitored as the process responses. Results were also compared with those obtained through simple AFM process. Corresponding results are presented in Figs. 8 and 9. It may be noted that SF and MR are significantly higher in UAAFM process as compared to classical AFM process. The observed low finishing rate as shown in the UAAFM while compared to AFM for the observed periods may be attributed to higher material removal and hence higher surface finish attained during the initial few cycles (o 5 min of machining) itself (Fig. 8). Machining beyond 5 min in case of UAAFM become a fine finishing operation in which rate of improvement gets reduced (Ψ2 o Ψ1, Ψ4 o Ψ3, Fig. 8) while better finish is resulted. On the other hand, in the conventional AFM, the radial force acting on the abrasive particles is negligible (Fr ¼0) leading to relatively less interaction of the abrasives with the workpiece surface. In the UAAFM process, the workpiece is in dynamic condition, which vibrates radially at a high frequency and is subjected to additional radial forces (Fr) along with the conventional AFM process (Fig. 2 (b)). The radial velocity of the workpiece gets added to the velocity of the abrasives in the axial direction and contributes towards
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Fig. 10. Topography of the machined surfaces obtained through optical profilometer: (a) prefinished, (b) AFM and (c) UAAFM.
Fig. 11. Typical SEM micrographs of gear tooth surface: before UAAFM at (a) magnification: 1000, (b) magnification: 5000; after UAAFM at (c) magnification: 1000 and (d) magnification: 5000.
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observed enhancement in surface finish improvement rate and material removal. Figs. 8 and 9 clearly show that rate of improvement in surface finish and MR in UAAFM pertaining to 5 min of machining is even higher than the values after 15 min of machining through conventional AFM although the initial surface finish in both the cases were similar (Ra ¼1.8 mm). This further clarifies that the rate of removal and surface finish improvement during the initial cycles in UAAFM are significantly higher than that of AFM. After 5 and 15 min of UAAFM, the improvements in SF and MR are 73.12% and 24 mg respectively. Initially, the as received surfaces contain high asperity peaks. Thus, while removing those peaks during the initial cycles, the average Ra value reduces relatively faster. Further processing is generally used for achieving fineness; removal of peaks becomes gradually insignificant. Continued machining thus, results in reduced improvement rate compared to initial cycle (Ψ4 o Ψ2, Fig. 8). The trend of surface finish improvement with increase in processing time was analyzed and can be correlated with the FEM velocity results (Figs. 5(a) and 6(b)) where the velocity increases with additional vibration effect thus makes the UAAFM process rapid when compared to AFM. Typical material removal characteristics observed during the UAAFM and AFM with silicon abrasive carrier while finishing the bevel gears is shown in Fig. 9. It is observed that during the initial abrasion cycles (say up to 10 min of machining), material removal (MR) is very high which however, decreases with the increase in the processing time (Ø3 { Ø1). This is due to the reduced effectiveness of the dull abrasives to further scratch the already glazed surface. The corresponding improvements in surface roughness also gets reduced significantly (Ψ3 { Ψ1, Fig. 8). Owing to the presence of radial component (Fr, Fig. 2(b)) in UAAFM, however even the dull abrasives during the later stages of machining (say beyond 10 minutes) are capable of removing micro-chips from the workpiece surfaces as indicated by a significantly high rate of MR (Ø4 4Ø3, Fig. 9). 5.2. Morphology of machined surface The finished surfaces of the EN8 steel bevel gears using UAAFM were also investigated by examining the surface texture and microstructure. The effect of the processing parameters on the surface quality was evaluated using a scanning electron microscopy and an optical profilometer. Fig. 10(a–c) illustrate the surface topographies of the workpiece surface before finishing, after finishing through conventional AFM and UAAFM processed surfaces. It is clear from the figures that the surface texture of the gears finished by the UAAFM appears glazed indicating significant improvement in surface finish. It was observed that there was a 55% rate of improvement in the surface of the gears finished through classical AFM, while an improvement rate of 73.12% was recorded after finishing the part through UAAFM for similar machining interval. The typical profiles presented show significantly reduced peaks in case the UAAFMed surface. The observed deep feed marks got swallowed reasonably during AFM, while they appear almost removed in the profile corresponding to the UAAFM. Thus, the effectiveness of the application of the additional effect in terms of the external ultrasonic vibration could be realised. The SEM analysis was also conducted on UAAFM finished bevel gear workpieces to investigate the morphology as well as to study the mechanism of UAAFM process. Fig. 11(a–b) clearly show that before UAAFM process, the work surface of the as received gear tooth contains scratches, micro-cracks, pits etc. Micro-graphs of UAAFMed surface morphology are shown in the Fig. 11(c–d). The UAAFM finished surface presents a glazed appearance, while very light feed marks (due to gear hobbing) and scratches by the medium abrasives can be identified. The asperity heights were removed during the UAAFM operation as discussed in Section 2
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(Fig. 2) and accordingly, the heights of the hobbing feed marks also got significantly reduced. It is observed from the figure that the abrasives hit the target surface with some inclination, say angle ‘θ’ (as shown schematically in Fig. 2(b) and also evident from simulation results presented at Fig. 6(d)). The angular scratching by the abrasives in the UAAFM process could be confirmed by the observed scratches in the magnified view of the UAAFMED surface (Fig. 11(d)). This is due to the applied vibrational (ultrasonic assistance) at high frequency and low amplitude. The inactive sliding and rolling abrasives now hit the target surface causing increased removal of the asperity peaks. On continued machining, the ultrasonic assistance contributes significantly on surface finish as evidenced by the glazed surface (Fig. 11(c) and (d)).
6. Conclusion In the present work, simulations as well as experimental studies of the UAAFM process on bevel gears were carried out for better understanding of the process. Results have been presented with suitable illustrations. The study reveals that the UAAFM is one of the best alternative methods for finishing gears. The following major conclusions could be drawn from the study. i. The abrasives in the UAAFM process scratch the asperity peaks of the workpiece surface at a higher velocity than in the AFM in the corresponding machining conditions. This facilitates easy and faster removal of materials which enhances the performance of the process. Finishing of bevel gears using UAAFM is more effective than conventional AFM process in terms of finishing time and improvement in surface roughness. ii. The simulation studies confirm significant changes in the medium behavior in the UAAFM process with reference to the classical AFM in terms of velocity and pressure profiles. iii. The additional ultrasonic vibration causes the abrasives to interact with the workpiece asperities to an angular shift ‘θ ’. This causes increase in the active abrasive grain density and reduction in inactive sliding and rolling of the abrasives. iv. Improvement in surface quality is rapid during the initial phases of finishing, while the material removal rate remains almost unchanged. v. Gear tooth surface morphology gets significantly improved resulting in glazed texture owing to enhanced density of active abrasive grains in a given processing time.
Acknowledgment The authors are thankful to the Department of Science and Technology, Government of India for providing financial assistance to this work through DST Project Grant no. DST-384-MID.
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