DEVELOPMENT OF CHILLED AIR COOLING TECHNOLOGY FOR PRECISION GRINDING: AN ECO-FRIENDLY ALTERNATIVE H. Huang*+, K. Ramesh+, L. Yin+ and A. Yui++ +
Singapore Institute of Manufacturing Technology, Singapore 638075 ++ National Defense Academy, Yokosuka, Japan * email:
[email protected]
ABSTRACT The presence of hazardous chemical additives in a liquid-base coolant causes environmental problems. Increasingly, stringent government legislations are being imposed on coolant use and disposal, which could result in an expense of up to 20% of the total machining cost. This paper reports on the development of an ecologically friendly cooling technology that uses chilled air with a biodegradable oil and air mist to replace the conventional liquid-base coolant. It is found that when the material removal rate is below a critical value, the chilled air/oil mist can replace the liquid-base coolant in terms of grinding surface quality. INTRODUCTION A liquid-base coolant is often used in a grinding process to avoid thermal damage and to achieve better surface integrity and longer tool life. However, the presence of chemical additives, such as sulfur, phosphorous and chlorine, in the coolant introduces a health hazard to operators [1]. Disposal of a used chemical coolant involves incineration, which partially contributes to global warming [2]. As a result, stringent government legislations are being imposed on coolant use and disposal. It has been reported by the German automobile manufacturer's association that about 17% of the manufacturing related cost is due to coolant related issues and 94.5% of coolant related cost is attributed to the maintenance disposal and plant investment [3]. In view of this, studies of “green” machining have gained increasing significance. This paper reports on the development of an ecologically friendly grinding technology that uses chilled air with a biodegradable oil and air mist to replace the conventional liquid-base coolant. CHILLED AIR COOLING SYSTEM The chilled air/oil mist (or chilled air, in short) cooling system consists of an air compressor, an air chiller, an oil-air mist generator and a nozzle, as shown in Figure 1. The air is first compressed and then released to the chilling and mixing systems. During air chilling, a two-
stage vapor compression refrigeration cycle is adopted to produce a chilled air with a temperature of -30 °C, a pressure of 0.3 MPa and a flow rate of 0.4 m3/min. The chilled air and the oil-air mist are supplied through two separate paths and then combined using a specially designed twin compartment nozzle (Figure 2b). The twin compartment nozzle has circular holes in the middle as an outlet for the chilled air and square holes surrounding the circular holes for the oil-air mist. Air/Oil Mixer
Air
Nozzle
Air Compressor
Wheel Air Chiller (Air Dryer, Condenser, Compressor, Heat Exchanger) Workpiece
Figure 1: Illustration of the chilled air cooling system.
(b) (a) Figure 2: Sectional view of (a) conventional nozzle and (b) twin compartment nozzle. 20 10 o
Air Temperature ( C)
Coming out from the twin compartment nozzle, the oil-air mist functions as a carrier for maximizing the chilled air penetration into the grinding zone and preventing the air temperature from rising. It also behaves as a lubricant for reducing the wheel-workpiece friction. Figure 3 shows the effect of the air travelling distance on the air temperature for the two nozzles. Compared with the conventional nozzle (Figure 2a), the twin compartment nozzle has a much better performance in terms of cooling effect.
0 -10 -20 Twin Compartment Conventional
-30 -40 0
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Distance from Nozzle Outlet (mm)
Figure 3: Effect of air travel distance on air temperature.
GRINDING EXPERIMENTS Grinding experiments were carried out to evaluate the performance of the chilled air cooling on an Okamoto surface grinder, by comparing with the water-base coolant. S45C and SS304 steels were used as workpiece materials. A vitrified CBN wheel (B91N100V) was employed. Grinding velocity and table feed were fixed at 33 m/s and 17.35 m/min., respectively. Depth of cut was varied from 1 to 20 µm. The twin compartment nozzle was used to supply a chilled air with a temperature of -30 °C, a pressure of 0.3 MPa and a flow rate of 0.4 m3/min. wrapped by an oil-air mist. The water-base coolant flow rate was approximately 4 liter/min. Grinding force measurements revealed that when the specific material removal rate (MRR) was below a critical value the chilled air grinding of either S45C or SS304 steel resulted in a grinding force with a similar magnitude to that caused using the water-base coolant, as shown in Figure 4. When specific MRR was above the critical value, the increase in grinding force for the chilled air was more significant than for the water-base coolant. This indicates that the cooling effect of the chilled air might be insufficient to remove the grinding heat when the specific MRR was relatively high. From Figure 4, it can be estimated that the critical values for S45C or SS304 are 2 and 1.5, respectively. Increasing the chilled air pressure would result in a better cooling effect, which could enable an increase in the specific MRR. 10 (a)
Ft - coolant Ft - chilled air Fn - coolant Fn - chilled air
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Grinding Force (N)
Grinding Force (N/mm)
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Figure 4: Effect of specific MRR on grinding forces for (a) S45C and (b) SS 304 steels. A simplified thermal energy equilibrium model that relates to grinding energy, heat transfer and internal energy rise can be used to predict the critical specific MRR. In the model, it is assumed that all the grinding power (per grain), Eg, is transformed into grinding heat, Qg. The grinding heat is transferred through the cooling media only via convection and evaporation. Eg is computed as (1) Eg = Ft.v/n where Ft is the tangential grinding force, v the wheel velocity and n the number of active cutting grains. Ft and normal grinding force, Fn, can be computed using the following equations [4],
Ft = Cp.f.d.b / v + µ.Fn (2) Fn = Cp.π.f.d.b.tanα / (2v) (3) where Cp is the specific grinding energy, f the table feed, d the depth of cut, µ the friction coefficient, b the wheel width and α the semi-included angle of a grain. The convective heat transfer, Qc, is determined by [5] Qc = h.A.∆T (4) 2 where h is the convection heat transfer coefficient (kW/ Km ), A the contact surface area of the cooling media and ∆T the temperature difference between the cooling media and the average grinding zone temperature. The heat transfer due to evaporation, Qe, is written as Qe= ε.gm.wm.ρ.u.L (5) where ε is the evaporation factor, ρ the cooling media density, gm the abrasive grit depth of cut, wm the distance between the two active cutting edges, u the cooling media velocity and L the latent heat of evaporation.
0.8
Grinding Power/grain (watts) Heat Trasfer/grain (watts)
Qg is the sum of Qc and Qe and can be computed using Equations (4) and (5) under the same experimental conditions. Eg can also be calculated using Equations (1) to (3). Values of the coefficients and properties of the chilled air, the water-base coolant and the workpiece materials in the equations can be found elsewhere [4-5]. Using S45C as an example, values of Qg and Eg were calculated and are plotted as a function of the specific MRR in Figure 5. The cross points of Qg and Eg indicate the equilibrium status for using the two different cooling media. The critical specific MRR for the chilled air is 2.2 mm3/mm s, which is in agreement with the experimental result in Figure 4a. When the MRR is greater than 2, the measured force for the chilled air deviates from that for the water- base coolant.
Q - water-base coolant g
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Q - chilled air/oli mist
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Specific MRR (mm /mm.s)
Figure 5: Computed grinding energy per grain and heat transfer per grain for waterbase coolant and chilled air/ as a function of the specific material removal rate.
As can be seen in Figure 5 using the water-base coolant the critical specific MRR can reach 9. Apparently, the current chilled air cooling system is competitive to the water-base coolant only at a low specific MRR. To achieve a larger MRR, a greater air pressure or lower air temperature should be supplied to increase the cooling effect. This requires a larger capacity of air chiller or air compressor.
The surface roughness measurements showed that the chilled air grinding led to a similar surface finish for both S45C and SS304 steels, as shown in Figure 6. 0.8
1
µm) Surface Roughness R (µ a
µm) Surface Roughness R (µ a
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Chilled air/oil mist
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Specific MRR (mm /mm.s)
Figure 6: Surface roughness of ground specimens of (a) S45C and (b) SS 304 steels. However, detailed SEM examinations revealed that the SS304 steel surface generated by the wet grinding (Figure 7c) exhibits slightly more deformed grinding grooves than those caused by the chilled air grinding (Figure 7d), but the ground surface characteristics are similar for S45C (Figure 7a and 7b). The possible explanation is because the air/oil mist which penetrated into the grinding zone significantly reduced the friction between the wheel and the workpiece. Therefore, a smaller work hardening resulted when using the air/oil mist than using the waterbase coolant. These effects could have more impact on the SS304 steel than the S45C steel possibly because the SS304 steel is much softer. Further investigations are still needed to verify this conjecture. (a)
(b)
(c)
(d)
Figure 7: SEM photos (x1000) of ground surfaces for (a) wet grinding of S45C, (b) chilled air grinding of S45C, (c) wet grinding of SS304 and (d) chilled air grinding of SS304.
500 chilled air - parallel to grinding marks water-base - parallel to grinding marks
400
Residual Stress (MPa)
The residual stress of a ground SS304 surface was measured using x-ray at three different locations and the data are plotted in Figure 8. For either the water-base coolant or the chilled air used, the residual stresses left on the ground surface measured along the grinding marks are tensile and those measured perpendicular to the grinding marks are compressive. There are little differences between the two cooling methods except for the largest MRR used. In this case, the chilled air grinding produced a greater compressive stress and a smaller tensile stress than the water-base coolant. Studies to clarify this point are still under way.
300 200 100 0 -100 -200 -300 chiiled air - perpendicular to grinding marks water-base - perpendicular to grinding marks
-400 -500 0
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Specific MRR (mm /mm.s)
Figure 8: Residual stresses left on the ground SS304 workpiece.
The average surface temperature rise in the grinding zone was also measured using thermal couples for SS304. Using the chilled air cooling, the measured temperature rise varied from 15 °C to 70 °C for the specific material removal rates ranging from 0.15 to 0.9 mm3/mm.s, which was only slightly higher than those using the water-base coolant (from 10°C to 50°C). CONCLUSIONS An eco-friendly cooling system was established. This system produced a similar cooling effect to the water-base coolant when the material removal rate is not high. This was explained using a thermal energy equilibrium model. The current system could be applied in fine grinding processes, which require small material removals. To apply the chilled air cooling to grinding processes with large material removals, the chilled air temperature must be further lowered and the air pressure needs to be increased. ACKNOWLEDGEMENT: The authors would like to thank Ms. Y.C. Liu & Ms. P.L. Teo for experimental assistance. REFERENCES: [1] H.K. Tonshoff, B. Karpuschewski and T. Glatzel, Particle emission and immision in dry grinding, Annals of the CIRP, 46(2) 1997, p693-695. [2] F. Klocke, A. Schultz, K. Gerschwiler and M. Rehse, Clean manufacturing technology – The competitive edge of tomorrow, Inter. J. of Manufact. Sci. and Tech., 1(2) 1998, p77-86. [3] F. Klocke, Dry cutting, Annals of the CIRP, 46(2) 1997, p519-526. [4] S. Matsui,K. Shoji, Estimation of wear process of grinding wheels, JSPE, 35(4), 1969, p235-240. [5] A. Bejan, Convection Heat Transfer, Wiley Publications, New York, 1984, p351-354.
