B.F. von Turkovich

New observations on the mechanism of chip formation when machining titanium alloys

Abstract Titanium and other aerospace structural superalloys are extremely difficult to machine except at low cutting speeds because of rapid tool wear. To increase productivity it is necessary to understand the mechanics of chip formation when machining these alloys. In this paper we report some new findings towards that goal.

Tool wear in titanium machining

Summary The mechanism controlling the crater wear of cutting tool materials in the machining of titanium alloys has been shown to be fundamentally different than that in the machining of steel and nickel-based alloys. It is suggested that tool wear is greatly reduced when adhesion occurs between the tool and the chip, preventing relative sliding at the tool/chip interface. This adhesion is promoted by chemical reaction at the interface. The thickness of the reaction layer is determined by the balance between the diffusion flux of tool constituents through the layer and the removal of tool constituents through chemical dissolution at the interface between the reaction layer and the titanium chip. In this way, for given cutting conditions, a characteristic thickness of the reaction layer is maintained and tool wear is limited by the rate of dissolution of the reaction layer into the titanium. The existence of a stable reaction layer of Tic on diamond and WC-based tools (the two most wear-resistant tool materials) has been demonstrated, and the estimated diffusion flux correlates well with the observed wear rate.

An Experimental Investigation of Air Quality in Wet and Dry Turning

Abstract Environmental and industrial hygiene issues are receiving increased attention by manufacturers. As a result a number of important issues have arisen, one of which is air quality and its effect on worker health. This paper presents the results of experiments focused on characterizing the aerosol emissions associated with wet and dry turning. Air quality is characterized by measuring the mass concentration and particle size distribution of the dust and mist created during a set of machining experiments. The relative importance of vaporization/condensation and atomization as mist generating mechanisms is also explored.

The effect of lattice defects on hydrogen solubility in palladium: I. Experimentally observed solubility enhancements and thermodynamics of absorption

Abstract Measurable, and reproducible solubility enhancements have been observed for hydrogen in cold-worked palladium in the low content α-phase (273–363 K). It is shown that the solubility of hydrogen is unaffected in palladium samples which had been quenched from near their melting points. It is concluded that the stress field about the dislocation array is the principal cause of the solubility enhancement. Absorption of hydrogen at the cores of dislocations is believed to be insignificant. From absorption isotherms measured at various temperatures, the relative partial molar enthalpies and entropies of solution of hydrogen into deformed palladium have been determined. The heat of solution of hydrogen by 78% deformed palladium is approximately 880 J/H more exothermic than for well-annealed palladium in the low hydrogen content α-phase.

A Comprehensive Tool Wear Model

The mechanisms that control the wear rates of tool materials nay be separated into three regimes depending on the cutting temperature and the properties of the tool and workpiece materials. In the low temperature regime, mechanical wear processes such as abrasion are rate-controlling and the wear of the tool material is determined primarily by its hardness. At higher cutting speeds, corresponding to higher cutting temperatures, the solid solubility of the tool material in the workpiece becomes significant and the chemical stability of the tool material determines the wear resistance. In the machining of steel with titanium carbide, chemical dissolution wear predominates above a cutting temperature of approximately 600 C. The machining of steel with cubic boron nitride or the machining of superalloys with CBN or silicon nitride-based tool materials is performed above 1200 C, at which temperature the wear rate becomes diffusion-limited and both the chemical stability and the chemical diffusivity control the wear rate. diffusivity control the wear rate. While these phenomena are well understood qualitatively, a quantitative model of tool wear is needed as part of an expert system to select tooling, forecast tool wear and set tool inventories in CIMS. The abrasive wear model of Rabinowicz has been integrated with the chemical dissolution wear model of Kramer to provide an algorithm that predicts the wear rates of hard coating throughout the speed range of application of high speed steel and cemented tungsten carbide tooling.

Thermal Aspects of Grinding: The Effect of Heat Generation at the Shear Planes

Summary The model of heat transfer in the grinding zone which had been previously developed by the author has been modified to account for the fact that some of the grinding energy is generated at the shear planes. Examples are given to evaluate how much error was incurred in making the earlier simplification that all the heat is generated at the wear flat areas. For grinding with aluminum oxide wheels, it is found that the error is typically not huge, but may be significant, depending on the grinding conditions. For grinding with CBN wheels, however, the error is very important.

The Bauschinger Effect of Sheet Metal Under Cyclic Reverse Pure Bending

Abstract In some sheet metal forming operations, metal flow involves a series of bending and unbending steps. In any such bending sequence the occurrence of the Bauschinger effect is likely. Correct process modelling of such forming operations therefore requires inclusion of the Bauschinger effect. This paper presents results of an experimental procedure developed for the determination of the Bauschinger effect. Sheet strips were subjected to a series of bending and reverse bending steps by means of a pure moment bending device. Results show the Bauschinger effect in steel and aluminum sheet as a function of strain amplitude, prestrain, and sheet thickness.

Environmental-Based Systems Planning for Machining

Abstract An approach for incorporating multi-endpoint environmental effects in manufacturing systems planning is presented. This approach combines unit process models, hazard evaluation and systems simulation to develop a predictive capability for energy consumption, waste flows and exposure risks over a planning horizon. Case studies for a machining transfer line are presented to show a hierarchy of decisions supported using this method, including effects of production rate constraints, catalyst selection and resource allocation.

A New Technique of Dressing and Conditioning Resin Bonded Superabrasive Grinding Wheels

Summary A new technique of dressing and conditioning a resin bonded superabrasive (diamond or cubic boron nitride) grinding wheel is presented. It comprises contacting the surface of the wheel with a wetting liquid (in this case a suitable lubricant), forming a film on the surface of the resin, and using the resulting wetted wheel surface to grind a hot pressed silicon carbide or silicon nitride ceramic alongside the workpiece of interest. The resulting thin ceramic chips (whose surfaces are extremely active as they are freshly generated) adhere to the wetted resin face forming a thin slurry layer. Such a layer if rejuvinated protects the resin bonding material from thermal and/or mechanical degradation or damage and the superabrasive from premature pullout, thus increasing the grinding wheel life.

Visibility Map and Its Application to Numerical Control

Summary The paper presents a novel technique for computing the optimal number of set-ups for machining complex surfaces. Akin to the Gaussian map, the visibility map of a parametric surface represents the feasible directions along which a cutter can reach the surface. When maps overlap, they can be machined in one set- up. Mapping of 3-, 4, and 5- axis numerical control machines to a unit sphere and algorithms dealing with spherical maps are given.