The coolant penetration in grinding with segmented wheels—part 1: mechanism and comparison with conventional wheels

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Abstract

This paper aims to study the mechanism of coolant penetration into a wheel-work contact zone when a segmented or conventional wheel is used in surface grinding. Based on the principles of fluid motion and considerations of spin-off and splash, analytical models for both conventional and segmented wheels were developed for predicting the power of pumping coolant into the contact zone. It was revealed that the pumping power of a segmented wheel increases with the increment of wheel speed, but that of a conventional wheel decreases. It was shown that coolant minimisation in surface grinding is possible when using a segmented wheel, although its efficiency depends on the wheel speed and methods of coolant supply. The model predictions were in good agreement with experimental observations.

Introduction

The chemical additives in machining coolants have raised critical concerns on environmental pollution and waste disposal cost [1]. The problem becomes considerable in grinding where a large amount of coolant is often required for high surface integrity components [2], [3]. Therefore, developing alternative cooling methods with less harmful coolant has been important to industry. Some investigations have also been conducted to try to replace coolant by cryogens [4], [5] or by using abrasive materials of higher thermal conductivity and wear resistance [6]. However, the solutions are still far from satisfaction. The difficulty in using cryogens is its low ability to penetrate into the grinding zone due to its high evaporation rate, which limits its applicability [4]. On the other hand, spin-off and splashing of coolant in grinding, usually above 95–98% of the coolant applied, make it difficult to reduce the quantity of coolant [7].

To grind difficult-to-machine materials in creep-feed mode, Suto et al. [8] introduced a segmented grinding wheel with perforated holes to allow coolant to radially flow into the wheel-work contact zone. It was reported that this type of wheel could bring about a reduction of specific energy by 36%. However, possible coolant saving was not investigated and the mechanism of coolant penetration was unclear. The present authors made a further development by introducing a pressurised fluid chamber to enhance the flow of coolant through the perforated holes [9], [10]. With such wheel system, the surface quality of ground workpieces could be improved even when the quantity of coolant applied was only 30% of that in a conventional system. Adhesion of ground chips on the wheel surface disappeared and surface tensile residual stresses caused by thermal deformation were eliminated. However, a comprehensive study of the fluid flow mechanism has not been available.

This paper aims to study the mechanisms of coolant penetration into the grinding zone associated with both the segmented and conventional grinding wheels.

Section snippets

Experimental apparatus

Fig. 1 shows the segmented grinding wheel containing 144 equally spaced CBN segments of B100P120V. An annular groove with bore holes was machined on the wheel hub to enable radial coolant flow through the space between the segments. Two different methods were used to charge the coolant into the grinding zone: (a) the free-flow method used by Suto et al. [8], where coolant was introduced by a nozzle to the wheel groove but its transportation to the grinding zone through the bore holes relied on

Modelling

The effectiveness of heat removal from a grinding zone depends largely on the amount of coolant which can be brought into the zone, and hence relies on the pumping power of a coolant supply system. If the volumetric flow rate and the pumping head of a coolant supply system are Q and H, respectively, its pumping power P is [11], [12]P=ρgQHwhere ρ is the coolant density and g is the gravitational constant.

The pumping actions of the coolant supply systems associated with the conventional and

Effect of coolant splash and spin-off

In any real surface grinding with either the conventional or segmented wheel system, coolant splash and spin-off are unavoidable. These will certainly influence the quantity and velocity of coolant to enter the control volume, and consequently, affect the pumping power. This section will discuss the splash and spin-off effects.

Performance comparison

Eqs. (21), (28) can be used to examine the variation of pumping power with different coolant supply configurations.

Conclusions

Two analytical models have been developed to provide a physical understanding of the mechanisms of coolant penetration into the grinding zone using segmented and conventional wheels. It is concluded that coolant minimisation is possible using the segmented wheel and the efficiency depends on the wheel speed and method of coolant supply. The model provides the foundation for further quantitative studies to be carried out in the second part of the series study.

Acknowledgements

This work was supported by the Australia Research Council (ARC). The authors appreciate very much Mr Y.L. Pai and Mr J. Huang at Kinik Grinding Wheel Corp. for making the wheel segments.

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