Reducing Heat by Convection and Conduction

Heat removal includes all aspects of cooling and lubrication and has been resear­ched a lot [HEIN09b]. The basic principles for heat removal are heat convection

Fig. 7.26 Few interactions per time and short interaction time (diagram follows Fig. 7.23)

and heat conduction (Figs. 7.23, 7.27 and 7.28). It is commonly assumed, that all process energy is converted into the heat flux, qt, during grinding (Eq. 7.6) [ROWE09, MALK08]. Heat convection takes place into fluids or air, even though convection into air is neglected in many considerations and cooling lubricant is most important for heat convection in grinding. The cooling lubricant must be present in the grinding zone and have a high heat transfer coefficient and high heat capacity (Fig. 7.27).

Equation 7.11 offers one approach to calculate the heat flux into the cooling lubricant, qcooi [ROWE09, p. 376]. The temperature before boiling, rmax, estimates the maximum energy flow into the fluid. The heat transfer coefficient, hf, depends

Alternatives:

Shoe nozzle,

Coolant pressure, etc.

Fig. 7.27 Heat convection (diagram follows Fig. 7.23) on the thermal properties of the fluid as well as on the contact arc length, lc (Eqs. 7.12 and 7.13).

qcool 3 ■ hf ■ Tmax

(7.11)

hf=3 ■ ■ rc

(7.12)

bf = Vkf ■ pf ■ cf

(7.13)

qcool heat flux into the cooling lubricant hf fluid convection coefficient

Tmax temperature before boiling

Pf fluid thermal property

kf thermal conductivity

cf specific heat

Cooling lubricant can be brought into the contact zone by a high useful flow rate and high volume per time (Fig. 7.27). The air cushion around the rotating grinding wheel is particularly important for high grinding wheel speeds and can be broken by several supply systems, such as needle nozzles (Fig. 7.27).

The useful cooling lubricant flow, Qu, is defined as flow volume through the contact zone of grinding tool and workpiece [MALK08]. Morgan et al. [MORG08] estimated the achievable useful flow rate based on wheel porosity, wheel speed and empirical factors.

Qu = f • hpores • b • vs • Ф (7.14)

Qu useful cooling lubricant flow

f factor based on measurement (approximately equal to 0.5) hpores mean pore depth (roughly equal to mean grit size)

b wheel width

vs wheel speed

Ф porosity (typically 0.5 for a medium porous wheel)

Heat conduction happens into the grits, grinding wheel, and workpiece material (Eq. 7.6) (Fig. 7.28). Malkin and Guo [MALK08] defined the limit to the shear zone energy which can be carried away by the chips, qch, as the melting energy. Heat to the grinding wheel, q^, depends on the grinding wheel properties including grit, bond and structure characteristics. Wheel contact analysis and grain contact analysis are two approaches to estimate the partition ratio for qgw [ROWE09]. Grit and bond conductivity should be high as well as grit coating conductivity (Fig. 7.28). In addition, grit and bond need high heat resistance to avoid damage.

Heat flux into the workpiece material, qwp, is a main challenge for surface integrity, but forms an important transfer process especially for materials with high heat conductivity.

Updated: 24.03.2016 — 11:54