W.B. Rowe

Experimental investigation of heat transfer in grinding

Abstract New findings are presented for temperatures, heat flux distribution and the implications for workpiece damage and partition ratio. Workpiece temperatures were measured using a 25 u.m single pole thermocouple assembly. It was found that the critical temperature for the onset of temper colours for ferrous materials lies within the range 450 to 500 deg.C. Measured temperature distributions in the contact zone compared best with theory assuming a square law heat flux. The effective contact length for vitrified CBN and alumina wheels was confirmed to be greater than the geometric value. Substantially lower partition ratios were found with CBN compared to alumina.

Temperatures in High Efficiency Deep Grinding (HEDG)

Abstract A thermal model is presented for deep grinding with particular relevance to the HEDG process. HEDG is defined as deep grinding at high workspeeds and very high removal rates. The contact between the workpiece and wheel is represented as a circular surface. It is found that the contact angle and also the Peclet number (widely used in heat transfer and defined below) have strong effects on the grinding zone temperatures. Experiments were carried out to demonstrate the high removal rates achievable and to measure the resulting contact temperature. It was found that high removal rates and absence of thermal damage could be achieved as predicted. The new model is shown to provide a good estimation of contact temperatures. It is also confirmed that HEDG can achieve low specific grinding energy compared with shallow grinding and creep grinding. The chips take away a substantial proportion of the heat generated in the grinding process. As in creep grinding, burn-out of the coolant causes a steep rise in contact temperature of the workpiece.

Avoidance of Thermal Damage in Grinding and Prediction of the Damage Threshold

To achieve control of the grinding process it is necessary to predict the conditions at which thermal damage to the work-piece surface occurs. The objective of this research was to develop an improved thermal model which would accurately predict the position of the burn boundary. The main advance, compared to previous methods of thermal modelling, was the partitioning of the heat flux between the grinding wheel and the workpiece. This allows more realistic values of heat flux to he employed in the model. Expressions for upper and lower bound solutions have been developed which predict the critical specific energy for the onset of burn.

The Effect of Deformation on the Contact Area in Grinding

Grinding efficiency and workpiece surface integrity are greatly affected by deflections that occur within the grinding contact zone. This paper is concerned with the effect deflections have on the real length of contact. A new relationship for the contact between the grinding wheel and the surface of the workpiece is introduced based on contact mechanics. The real contact length between the grinding wheel and the workpiece has been modelled based on the theory for cylinders in contact including the effect of the surface roughness of the contact faces. A second formulation is presented which takes account of the contact area at the micro level of the grains. The new model more accurately describes the mechanics of grinding contact than previous contact models. Application of the new model to published experimental data for plunge surface grinding operations explains why measured contact Application length can be 50% - 200% greater than the geometric contact length.

A Simplified Approach to Control of Thermal Damage in Grinding

The critical factors for the control of thermal damage in grinding at conventional workspeeds have been established with reference to experimental and previously published work. For ferrous materials, significant damage occurs above a maximum workpiece background temperature of 475°C. It is also known that the energy entering the workpiece is reduced due to conduction into the grinding wheel. It has been found that the partitioning of energy between the grinding wheel and the workpiece remains approximately constant. However, the overall partition ratio to the workpiece, which takes account of energy transfer to the chips as well as energy transfer to the wheel, is variable. The effective thermal properties of the grinding wheel may be established by correlating theory with grinding experiments. An effective coefficient for the temperature equation can be obtained corresponding to the use of the geometric contact length in the equation. Using these conclusions, a simplified approach has been developed for control of thermal damage.

An Advance in the Modelling of Thermal Effects in the Grinding Process

The proportion of the grinding energy entering the workpiece may be analysed either for the whole grinding wheel-workpiece contact zone or for the average grain contact zone which is two orders of magnitude smaller. An analysis which does not clearly apply to one zone or the other introduces conceptual difficulties since the relative speed seen by the workpiece is very different for the two cases. At the grinding zone level the workpiece sees a relative speed of vw whereas at the grain level the workpiece sees a relative speed of vs +/- vw. An analysis has been presented for partitioning at the grain level which overcomes these conceptual problems. Results from the new model are compared with results from other models.

Validation of Thermal Properties in Grinding.

Abstract Thermal properties of the abrasive material are required for energy partitioning and prediction of temperatures in grinding. Alumina and cubic boron nitride wheels are investigated by several methods. A novel sensor was designed to measure bulk thermal property. It is shown that the effective thermal properties exhibited in the grinding process are lower than the values measured directly. It is therefore concluded that a grain model is more appropriate than a bulk property model. A case study based on grinding AISI 52100 with cbn and alumina is used to illustrate the sensitivity of the most significant parameters for fine grinding.

Temperature case studies in grinding including an inclined heat source model

Abstract Previous analytical models of heat transfer in grinding have been based on the sliding heat source analysed by Jaeger and Carslaw in 1942. It is now shown that for deep grinding processes, and particularly for high efficiency deep grinding (HEDG), the sliding model overestimates the temperatures experienced by the finished workpiece surface. These situations are re-analysed using sliding sources and inclined sources to estimate contact surface temperatures and subsurface temperatures in grinding. Convection in the abrasive contact region to the process fluid, to the grinding wheel and to the chip material removed is taken into account. It is shown that each of these effects can predominate under different process conditions. Case studies illustrate the importance of thermal processes in achieving efficient material removal. The results for HEDG are particularly interesting and suggest that under the right conditions specific energy may be self-limiting. This is offered as a possible explanation for the efficiency of the process.

Intelligent CNC for Grinding

The aim of this research was to improve productivity in grinding by bringing together process technology and microprocessor technology in an adaptive CNC system. The main objective was to produce parts satisfying specified geometrical, metallurgical and surface finish requirements. A secondary objective was to perform grinding under optimal conditions at a maximum production rate. In order to facilitate the application of a flexible range of strategies for control of the grinding process it was decided to formulate a modular conceptual framework. A variety of strategies and process models were incorporated within the framework according to the requirements of the process in question. This philosophy can be extended to a range of processes. The system was implemented using an OSAI A-B 8600 controller to control a Cincinnati centreless grinding machine. Initial production trials proved satisfactory in that the system ‘learned’ the properties of the parts produced, produced parts within specification and reduced the floor-to-floor time.

Temperature control in CBN grinding

The main advantage of CBN grinding wheels is the long wheel life owing to the hardness of the CBN abrasive. Recent research has confirmed another advantage of CBN, which is cooler grinding. The new research allows the temperature in grinding to be predicted based on experimentally validated CBN thermal properties. This work also provides for in-process prevention of thermal damage in grinding. A well-documented feature of CBN grinding is the reduced risk of thermal damage to the workpiece. This advantage can allow a marked increase in removal rate whilst maintaining surface quality of the component compared to grinding with conventional abrasives such as aluminium oxide. The reduced risk of thermal damage in CBN grinding is sometimes attributed to the lower grinding specific energies. While lower specific energies when achieved are an advantage, this explanation ignores a fundamental advantage of the CBN abrasive. The experimental investigation has shown that a major advantage of CBN grinding is that a substantially lower proportion of the total grinding energy enters the workpiece compared to grinding with alumina wheels. The results further indicate that the effective thermal conductivity of CBN grains is considerably lower than its reported theoretical value of 1300 W(mK)−1.