Grinding - a Finishing Process

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- Tempering of the workpiece: If the grinding temperature is excessive, tempering and softening of the workpiece will occur

- Burn: Excessive grinding temperatures can discolour the surface of the workpiece. While this may not be a problem in itself, it can indicate that grinding temperatures have been of an order, which can leave micro-cracks in the workpiece surface or change its properties by reducing surface ductility and toughness.

- Residual stress: This is usually caused by deformation of the material or where the material has been subject to localized heating and cooling

Advantages of grinding

- Good dimensional accuracy

- Good surface texture, low Ra, i.e. 0.25 µm.

- Good surface integrity, i.e. less risk of thermal damage.

- Three times as fast as grinding.

- Requires fewer operations than grinding, i.e. workpiece does not need to be reset in another machine as can be finished in-situ.

- Less energy used (approximately 5 times less).

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Discussion

Types of Damage

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. However, thermal damage occurs on microscopic or sub-microscopic 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.

Avoiding Thermal Damage

To meet accepted standards of quality a ground part must retain good wear resistance while preserving its expected life. Seek remedies to thermal damage which comprorases wear resistance and part life must be sought. An effective remedy is to regulate conditions to avoid film boiling. Several methods of controlling temperature are discussed below.

- 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. 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 shows 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.

- Proper choice of fluid type and application method plays an important role in avoiding thermal damage. The choice whether to use 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 uncommon for oil because the film boiling temperature for oil (3OO°C) is high. Water-based fluids have good cooling properties, but they lubricate less well and have a much lower film boiling temperature

- In some grinding applications, the use of 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.

- 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. 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.

Reference

- Harvey, James A. Machine Shop Trade Secrets

- A.K.Hajra Choudhury. Elements of WORKSHOP TECHNOLOGY.

- R.K.Gupta. Manufacturing Engineering.

Conclusion

The steel is susceptible to several forms of thermal damage during grinding: re-hardening 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:

- Use a sharp wheel, soft wheel grades, frequent dressing.

- Use high work speed and optimum wheel speed.

- Introduce an optimum number of slots into the grinding wheel.

- Use coarse dressing rather than fine dressing.

- When possible, use neat oil as a grinding fluid.

- If using water-based fluid is necessary, maintain proper pressure and temperature of fluid.

- Use a CBN wheel.