B. Mills

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.

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.

On the formation mechanism of adherent layers on a cutting tool

Abstract Experimental data has shown that calcium containing resulphurised free machining austenitic stainless steel has much better machinability than resulphurised free machining austenitic stainless steel, especially in terms of tool life. A thick adhering layer has been found on the rake and flank faces of a WCTiCCo cutting tool when machining calcium containing resulphurised austenitic stainless steel. The mechanism by which this layer is formed and the effect of this layer on tool life is discussed. The adhering layer has been characterised in terms of composition, thickness and shape using scanning electron microscopy and energy-dispersive X-ray analysis. A theory for adhering layer formation is presented in which layer formation is described in four stages: (1) Extrusion of viscous non-metallic inclusions onto the tool surfaces; (2) Adhesion of the coating on the tool; (3) Hardening and growth in thickness of the coating; and (4) Formation of a stable adhered layer.

Analysis and simulation of the grinding process. Part IV: Effects of wheel wear

A method of simulating dressing and grinding was described in Parts I and II of this paper. In Part IV, the effects of wheel wear and wheel characteristics on grinding performance are simulated and compared with experimental results. The results show that grinding performance is strongly affected by dressing conditions immediately after dressing. As grinding continues, the grinding power, and also the surface roughness, tends to converge towards similar values for all dressing conditions when the same grinding conditions are employed. Results from the simulation show that the influence of wheel wear is affected by the wheel fracture characteristics. The convergence of the grinding behaviour shown in the simulation and experiments suggests that stable grinding performance in a wheel redress life cycle may be achieved by selecting dressing conditions, taking account of the grinding behaviour.

Formation of a transfer layer at the tool-chip interface during machining

Abstract The formation of tribo-layers during machining is very common, especially when machining ‘free machining’ steels. Previous flow zone models of metal cutting have not included the shear strain rate distribution at the tool-chip interface, although the distribution directly affects contact behaviour. A new flow zone model is developed, based on the concept of the cutting interface, which occurs where the shear strain rate of chip deformation reaches a maximum and not where the speed of the chip is zero. The model is a dynamic model and it explains the dynamic contact behaviour between chip and the tool. The model enables changes and accumulation of changes in micro-machining to be related to tool wear and workpiece surface integrity. It will be able to produce information on changes in micro cutting conditions and the effect of change and the accumulation of such change on the tool wear and the surface integrity of the workpiece machined. The new flow zone model is used to interpret the tool wear processes occurring when machining three grades of austenitic stainless steel.

Analysis and simulation of the grinding process. Part III: Comparison with experiment

A method of simulating dressing and grinding was described in Parts I and II of this three-part series. In Part III, the effects on grinding performance of varying the dressing conditions are simulated and compared with experimental results. The results show that a coarse dressing condition leads to low grinding force and grinding power but a high workpiece surface roughness. The grinding performance of the wheel in the dwell period for “spark-out” is simulated. Simulated and experimental results both show that grinding power in the dwell period decreases following an exponential decay function, however the reduction of surface roughness does not follow an exponential decay.

A Grinding Power Model for Selection of Dressing and Grinding Conditions

The grinding power is often used as a parameter for monitoring the grinding process. The power may also be used to monitor the effects of dressing. Empirical models are required to guide the selection of the dressing and grinding conditions. In this paper, the effects of dressing conditions and grinding conditions on grinding force and grinding power are reviewed. The effects of grinding conditions and dressing conditions on grinding force and grinding power are related to the shape of the idealized chip thickness. It is found that the grinding force and grinding power can be related to the dressing operation by considering the effective density of the cutting edges on the wheel surface. The semi-empirical model developed in this paper can be used to predict the variation of the grinding power during the wheel redress life cycle. Therefore the model can be used to guide the selection of dressing and grinding conditions. The potential use of the model for adaptive control of the grinding process is also described.

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.

An Intelligent Multiagent Approach for Selection of Grinding Conditions

The advantages of a multi-agent approach are presented for the selection of grinding conditions. The agents consist of case based reasoning, neural network reasoning and rule based reasoning. Case based reasoning is employed as the main problem-solving agent to select combinations of the grinding wheel and values of control parameters. Rule based reasoning is employed where relevant data are not available in the case base. A neural network is employed to select a grinding wheel if required. The operator makes the final decision about the wheel or the values of control parameters. The multi-agent approach combines the strengths of the different agents employed, to generate hybrid solutions and overcomes the limitations of any single approach. A blackboard method was used as the means of integrating the multi-agent system. The system works as expected and demonstrates the potential of using artificial intelligence for selection of grinding conditions, as well as the capability to develop a powerful database by learning from experience.

A generic intelligent control system for grinding

Abstract The achievement of accuracy in precision machining depends on the ability to take account of system deflection. The application of intelligent control to grinding processes is a way to achieve a desired workpiece size and surface integrity. A generic intelligent control system for grinding has been developed based and implemented on a commercially available CNC grinding machine. The implementation has been shown to be successful. The paper describes the object-oriented development of the generic intelligent control system for grinding based on the proposed modular conceptual framework for adaptive grinding control, reviews previous work towards intelligent grinding control, summarises the previously used adaptive strategies and introduces the structure and operation of the generic intelligent control system. Finally, the problems encountered during the system development are discussed.