Peter Krajnik

Nanofluids: Properties, Applications and Sustainability Aspects in Materials Processing Technologies

Nanofluids could be used to provide cooling and lubrication action and to control thermo-physical and tribochemical properties of material processing. It is foreseen that properly designed nanofluids could surpass conventional cutting fluids with respect to thermal conductivity, convective heat transfer coefficient, critical heat flux, viscosity, and wettability. These properties have a promising potential to lead to the development of new coolants and lubricants with applications in a wide variety of materials processing technologies. This paper analyses the developments in research on the properties of nanofluids and evaluates their potential for applications in machining, focusing on their thermal and tribological aspects. The increasing use of nanofluids leads to a need for information on their sustainability in order to recognize and avoid risks. Sustainability is discussed in view of occupational health and safety and toxicity of nanoparticles.

The use of computational fluid dynamics in the analysis of fluid flow and thermal aspects in grinding

A numerical model based on computational fluid dynamics is developed to analyze fluid flow and thermal aspects in grinding. The model uses multiphase fluid flow with heat transfer based on the volume-of-fluid method, convection, conduction in solids and a multiple reference frame model of the porous grinding zone. Fluid velocity vectors, useful flow rate, grinding temperatures and energy partition are predicted using the model. In lieu of direct measurements of these quantities, the verification relies on the indirect assessment of surface integrity. The simulation results provide adequate agreement with the measured residual stress, depth of heat-affected zone and full width at half maximum profile with respect to the grinding temperatures.

Particularities of Grinding High Speed Steel Punching Tools

A simulation model of a punch grinding process has been used to determine optimal parameters to reduce grinding cycle time and achieve a constant-temperature no-burn situation. Two basic outputs of the simulation model include arc length of contact and specific material removal rate that are both time-variant. A thermal model is included in the simulation to calculate maximum grinding temperature rise. The simulation-based optimization can help to avoid thermal damage, which includes thermal softening, residual tensile stress, and rehardening burn. The grindability of high speed steel (HSS) is presented in terms of specific grinding energy versus undeformed chip thickness and maximum temperature rise versus specific material removal rate. It is shown that for a given specific material removal rate lower temperatures are achieved when grinding fast and shallow. Higher temperatures, characteristic for slow and deep grinding, soften the material leading to a lower specific grinding energy, especially if grinding is timid. Lowest values of specific grinding energy can be achieved in fast and shallow grinding at aggressive grinding conditions.

High pressure jet assisted turning of surface hardened piston rods

An experimental study has been performed to investigate the capabilities of dry, conventional and high pressure jet assisted turning of surface hardened piston rods used in high pressure fluid power applications. The capabilities of different hard turning procedures are compared by means of chip breakability, cooling efficiency, tool wear, tool life, cutting forces and process regions of operability, i.e., technological windows. All machining experiments are performed under conventional cutting speeds using coated carbide tools.

Thermal Aspects and Grinding Aggressiveness in View of Optimizing High-Performance Grinding Operations in the Automotive Industry

This paper first reviews early contributions to modeling and analysis of thermal aspects in grinding. The role of specific energy in the determination of grinding temperatures is then discussed with respect to both chip thickness and grinding aggressiveness. The underlying modeling of cylindrical grinding is given in general terms, enabling calculation of the instantaneous geometry, kinematics and temperature for any workpiece form. The focus is on the recently developed concept of constant-temperature grinding, which entails choosing process parameters based on a thermal model for achieving a constant temperature and then optimizing the grinding process for either shorter cycle times or higher quality while applying constant temperature. Machine limitations — in terms of maximum speed, acceleration, and jerk in the headstock and wheelhead movement — are considered in the optimization. Case studies and experimental work are presented for high-performance industrial cam-lobe grinding used in the automotive industry.Copyright

Simulation of Workpiece Kinematics in Centreless Throughfeed Grinding

This paper deals with the simulation of the workpiece kinematics in centreless throughfeed grinding. The objective of this research was to develop a simulation tool that can be used to create an interactive virtual environment, to place the grinding gap elements in the defined set-up and to visualise process kinematics. The simulation is founded on an analytical 3D model, which includes a parametrical description of all grinding gap elements and their kinematics. The simulation software was programmed in C#. Its platform consists of Windows OS, .NET framework and OpenGL graphics library.

Modelling of the micro-grinding process considering the grinding tool topography

The micro topography of the grinding tool has a considerable influence on the cutting forces and temperature as well as the tool wear. This paper addresses an analytical modelling of the micro-grinding process based on the real tool topography and kinematic modelling of the cutting-edgeworkpiece interactions. An approximate shape of the abrasive grains and their distribution is obtained from the confocal images, which are taken from the tool surface - determining the grain height protrusion and the probability density function of the grains. To determine the grinding forces, a transient kinematic approach is developed. In this method, the individual grit interaction with the workpiece is extended to the whole cutting zone in the peripheral flank grinding operation. Hence a predictive model of cutting forces and surface roughness in micro grinding of titanium grade 5 is developed. Finally, the simulated forces and surface roughness are validated by the experimental results.

Wheel lift-off in creep-feed grinding: thermal damage, power surge, chip thickness and optimisation

An investigation is made into the phenomenon of early lift-off in creep-feed grinding, where the wheel lifts away from the workpiece before reaching the end of cut. In single-pass operations, early lift-off can result in thermal damage. In multi-pass operations, there is a surge in material-removal rate just before lift-off, which can result in thermal damage and excess wheel wear. This study examines the current inadequate methods of dealing with lift-off. It then develops a geometric and kinematic model for analysing the lift-off phenomenon. It finally proposes a thermal-model-based optimisation method for achieving a constant maximum surface temperature, resulting in shorter cycle times and less risk of thermal damage. The power-surge model is validated experimentally in diamond grinding of tungsten-carbide rotary tools.

Grinding of cermets with cup-wheels

Cup-wheels are frequently used to grind cermets, a difficult-to-grind material. An investigation was made into the transient geometry of the cup-wheel rim, grit dulling, wheel loading, and wheel self-sharpening with chip thickness. Tests were performed on a saw-tip grinding machine and specific energies, G-ratios and rim geometries were measured. Results showed that, like grinding of tungsten-carbide, loading is prevalent. However, unlike grinding of tungsten-carbide, grit dulling is also prevalent and wheel conditioning is of limited use. Much better results, particularly with respect to surface finish, can be obtained if the wheel is trued to a predetermined geometry. In addition, grinding parameters must be chosen to induce wheel self-sharpening. Practical recommendations are given.

Power monitoring, Fourier transforms of power, and electron microscopy in evaluating the performance of abrasives in grinding

The use of electron microscopy and power-monitoring during grinding was investigated in terms of evaluating the fracture and wear characteristics and chip-formation mechanisms of abrasive grains and bond formulations. Diamond abrasives and fused, sintered and sintered triangular-shaped aluminium-oxide abrasives were evaluated. Power was shown to be a useful tool in determining the chip-formation mechanisms and the extent of grit fracture, particularly in triangular-shaped abrasive. Conclusions were supported by electron-microscope analysis. Power was also used to evaluate low-cost diamond vs. premium diamond abrasives. Practical recommendations are given for evaluating grit, wheel and bond performance both in the laboratory and in production.