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AVOIDING THERMAL DAMAGE IN GRINDING

by Professor Trevor Howes, Director
The Center for Grinding Research and Development, and Dr. Hemant Gupta, Department of Metallurgy, The University of Connecticut

© Abrasive Engineering Society 1991
The following is an excerpt from a technical paper presented at an AES conference. A complete copy is available from AES

Fluid Film Boiling

In grinding, stock removal rate is often limited by thermal damage to the ground component. A significant portion of the energy used in removing stock transmits to the workpiece as the heat which causes damage to the ground surface. In order to avoid thermal damage, the amount of heat entering the workpiece must be controlled.

The use of grinding fluids is effective in preventing thermal damage since fluids not only remove heat from the grinding zone but also lubricate to reduce the friction between the wheel and the workpiece. There are, however, limitations on the ability of the fluid to cool. Shafto et. at. (1975) have shown that for creep-feed grinding, water-based fluids cool effectively only when the workpiece surface temperature is below a critical value of about 130°C. Once this temperature is exceeded, the fluid loses its ability to cool, and the workpiece temperature rises steedly to a temperature which could be expected under dry grinding conditions (see fig. 1). Depending on this temperature, various degrees of thermal damage may result. Shafto and coworkers have attributed the transition from wet to dry grinding conditions to a change in the boiling mechanism from nudeate to film boiling.

Types of Damage

Following the onset of film boiling, the workpiece surface temperature may rise up to 1000°C or even more under certain conditions. The magnitude of this maximum temperature determines the extent of thermal damage. In creep feed grinding, the damage may be so severe that it can be detected by the naked eye. Generally, however, thermal damage occurs on microscopic or submicroscopic levels and is not readily apparent. Thermal damage is detrimental to the ground part and may reduce its fatigue life and wear resistance dramatically. As a result, the component may fail long before its expected life. A brief description of the most frequently occurring forms of thermal damage, rehardening burn, temper burn, and residual stresses, follows....

..... (sections and figures omitted).....

Avoiding Thermal Damage

To meet accepted standards of quality a ground part must retain good wear resistance while preserving its expected life. Remedies to thermal damage-which comprorases wear resistance and part lif&-must be sought. An effective remedy is to regulate conditions to avoid film boiling. In the following section, several methods of controlling temperature are discussed.

Heat-producing grinding force and grinding power can he reduced by using sharp wheels, soft wheel grades, and frequent dressing (King and Hahn, 1986). Another method of controlling temperature is to employ Cubic Boron Nitride (CBN) wheels. The lower maximum temperature produced by CBN wheels, as compared to conventional alumina wheels, is said to be due to the good thermal conductivity of CBN, i.e. the CBN itself removes a large fraction of the heat from the grinding zone (Neailley, 1988). The comparison of maximum temperature as a Function of depth of cut for conventional alumina and CBN wheels is shown in fig.4. The heat generated also depends upon the time of contact between the wheel and the workpiece. Hence by increasing the work speed, the contact time may be reduced which, in turn, results in less heat evolution and less heat entering the workpiece.

King and Hahn (1986) have discussed the influence of wheel speed on grinding power and have determined experimentally that very low or very high wheel speed may require high grinding power. Their results show that optimum wheel speed in most cases is in the range of 8000-10,000 f.p.m.; at higher or lower speeds, thermal damage is more likely to occur.

The introduction of slots in the grinding wheel has beneficial effects on the critical heat flux for burning. It can be seen in figure 5 that the critical heat flux, obtained in simulation tests where the grinding heat flux is provided by an electrically heated element fused when film boiling occurs, increased by approximately 25% when 32 slots were introduced into the wheel (Salter, 1985). It should be noted that the higher the critical heat flux, the lower the chances are that film boiling or burnout develops. Figure 5 also shows the effect of fine dressing vs. coarse dressing. In these results, coarser dressing leads to higher values of critical heat flux..

Proper choice of fluid type and application method plays an important role in avoiding thermal damage. The choice whether to use a neat oil or a water-based fluid is significant. It is widely accepted that oil provides good lubrication but poor cooling. Use of oil reduces the friction in the grinding zone and will result in lower grinding power (King and Hahn, 1986). Despite its poor cooling properties, the film boiling phenomenon is fairly uncommon for oil because the film boiling temperature for oil (= 3OO°C) is high. See figure 6a and 6b (Salter, 1985). Water-based fluids have good cooling properties, but they lubricate less well and have a much lower film boiling temperature

In most cases, the use of an oil reduces the likelihood of the onset of fluid film boiling.

In some grinding applications, the use of an oil is impractical and a water-based fluid must be chosen. when this is the case, other measures can be taken to reduce the likelihood of thermal damage. The temperature of the grinding fluid, for example, has a pronounced influence on its heat removal capability. Salmon (1979) has shown that as the temperature of the bulk fluid decreases, the amount of heat removal and the power flux at bum increase (see fig. 7a and 7b). It is apparent from figure 7 that by reducing the fluid temperature from 40° to 20°C, the critical power flux could be raised by a factor of 2. (Similar results were obtained in simulated grinding tests by Powell et. al., 1978) Similarly, the supply pressure to a fluid jet affects the removal of heat from the grinding zone. Powell's studies have established a relationship between critical heat flux and coolant pressure as shown in figure 8. It can be concluded that by increasing the pressure to some optimum level, more heat can be removed, and film boiling can be avoided. A useful guide is to make the fluid application velocity equal to wheel peripheral speed.

Conclusion

The 52100 bearing steel is susceptible to several forms of thermal damage during grinding: rehardening burn, temper burn, and residual stresses. These forms of damage result from fluid film boiling in the grinding zone. In order to avoid completely the possibility of thermal damage, fluid film boiling must be avoided. This can be accomplished either by reducing the heat generated in or by improving heat removal from the grinding zone.

    A number of methods are suggested to avoid thermal damage. These are:
  1. Use a sharp wheel, soft wheel grades, frequent dressing.
  2. Use high work speed and optimum wheel speed.
  3. lntroduce an optimum number of slots into the grinding wheel.
  4. Use coarse dressing rather than fine dressing.
  5. When possible, use neat oil as a grinding fluid.
  6. If using water-based fluid is necessary, maintain proper pressure and temperature of fluid.
  7. Use a CBN wheel.

REFERENCES

  1. Howes, ID., Neailley, K., and Harrison, A.J., 1987, "fluid Film Boiling in Shallow Cut Grinding," Annals of the CJRP, Vol. 36, No. 1, pp. 223226.
  2. King, R.l., and Hahn, R.S., 1986, Modern Grinding Technology, Chapman and Hall, New York, London
  3. Neailley, K., 1988, "Surface Integrity of Machined Components -- Microstructural Aspects," Metals and Materials, Vol. 4, No. 2., pp.93-96.
  4. Powell, J.W., and Howes, T.D., 1978, "A Study of the Heat flux at which Bum Occurs in Creep Feed Grinding," Proceedings of the 19th International MTDR Conference, Manchester, pp. 629A36.
  5. Salmon, S.C., 1979, "Creep Feed Surface Grinding",Ph.D. Thesis, University of Bristol.
  6. Salter, N., 1985, "Creep-feed Grinding of Cylindrical Forms," Ph.D Thesis, University of Bristol, UK. Shafto, G.R., Howes, ID., and Andrew, C., 1975, "Thermal Aspects of Creep-feed Grinding," Proceedings of the 16th International MTDR Conference, Manchester UK.
  7. Andrew, C., Howes, ID., and Pearce, T.R.A., 1985, Creep Feed Grinding, Industrial Press, New York.
  8. Field, M., and Koster, W., 1978, "Optimizing Grinding Parameters to Combine High Productivity with High Surface Integrity," Annals of the CIRP, Vol. 27, No. 1, 523-526.

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    2/25/03