Zoltán Pálmai

Thermotechnical modelling of hard turning: A computational fluid dynamics approach

Abstract During hard turning the original hardness of the surface layer changes, as does its texture structure and stress state. Even cracks and other defects may appear if the heat effect is too intense. As experimental investigations into heat effects are difficult and expensive, simulation investigation methods based on modelling are of great significance. In this paper an example of heat modelling of hard turning is presented. The layer thickness on the workpiece surface that reaches high temperature is determined. In that layer different modifications can occur due to heat. It was found that the thickness of the layer affected by heat depends primarily on the feed and only secondarily on other cutting parameters. The cooling gradient of the layer was determined, allowing conclusions to be drawn on the re-tempering of the heated layer. The computational fluid dynamics method and a commercial software was used for the modelling and proved to be suitable for the simulation analysis of the hard turning process.

The effect of technological conditions of hard turning on the formation of white layer

In fast spreading hard turning occasionally a so-called white layer appears on the machined surface, which is mostly harmful. The formation of white layers and their composition, structure and thickness were investigated in the turning of the inner cylindrical surface of gear wheels made from 20MnCr5 case hardened steel, in order to identify to what extent the technological parameters of turning influence the white layer formation. On the basis of the measurement results it was possible to include border-line technological conditions in an empirical formula with which white layer formation can be avoided.

The Seebeck Effect on the Thermal Phenomena of Cutting and Tool Wear

It is a well-known fact that thermoelectric currents, reaching even the scale of ampere, develop during chip formation in the workpiece-tool-chip-machine system. The impact of these currents on tool wear in continuous cutting was examined with a qualitative mathematical model, in which wear is described by an autonomous non-linear differential equation.The constants of the model were tailored to wear curves determined by experiments conducted with a P20 carbide tool on a C45 quality steel workpiece. The differences among the tool isolation measurement results obtained by various researchers may be justified by simulation calculations.According to the model, the thermoelectric system behaves in a chaotic way in certain cases. Further research is necessary to decide if this is only a special characteristic of the model or the model shows the actual processes.

The Modelling of Crater Wear in Cutting with TiN Coated High Speed Steel Tool

The flow zone of the chip in contact with the tool reaches a high temperature in cutting. According to chip hardening experiments α-γ transformation may occur in steel, so the tool is in contact with a high-temperature γ phase at high pressure. The microscopic examination of worn surfaces showed that the degradation of the tool is the result of adhesive/abrasive and thermally activated processes, therefore both friction length and temperature must be taken into consideration in the modelling of crater wear. Wear rate can be described by a non-linear autonomous equation. TiN coating, which increases tool life in high speed steel, changes and slows down the wear of the tool. The activation energy of wear can be calculated from the constants of the wear equation determined by cutting experiments. The deoxidation products to be found in the workpiece in cutting may form a protective layer on the TiN layer that blocks or slows down wear.