G. Byrne

Tool Condition Monitoring (TCM) — The Status of Research and Industrial Application

Abstract The use of sensor systems for tool condition monitoring in machining and grinding is becoming more commonplace to enhance productivity. Many approaches have been proposed to accomplish tool condition monitoring and a number of these are successfully employed in industry. This paper reviews the motivation and basis for the utilization of these systems in industry, the sensors used in such systems including industrial application, new developments in signal and information processing, sensor based process optimization and control and directions for future developments. Main developments noted include the use of multiple sensors in systems for increased reliability, the development of intelligent sensors with improved signal processing and decisionmaking capability and the implementation of sensor systems in open architecture controllers for machine tool control.

Advancing Cutting Technology

This paper reviews some of the main developments in cutting technology since the foundation of CIRP over fifty years ago. Material removal processes can take place at considerably higher performance levels in the range up to Qw = 150 - 1500 cm3/min for most workpiece materials at cutting speeds up to some 8.000 m/min. Dry or near dry cutting is finding widespread application. The superhard cutting tool materi- als embody hardness levels in the range 3000 – 9000 HV with toughness levels exceeding 1000 MPa. Coated tool materials offer the opportunity to fine tune the cutting tool to the material being machined. Machining accuracies down to 10 ?m can now be achieved for conventional cutting processes with CNC machine tools, whilst ultraprecision cutting can operate in the range < 0.1?m. The main technological developments associated with the cutting tool and tool materials, the workpiece materials, the machine tool, the process conditions and the manufacturing environment which have led to this advancement are given detailed consideration in this paper. The basis for a roadmap of future development of cutting tech- nology is provided.

Material Removal Mechanisms in Lapping and Polishing

Polishing processes are critical to high value production processes such as IC manufacturing. The fundamental material removal mechanisms, howeve, are poorly understood. Technological outputs (e.g., surface finish, sub-surface damage, part shape) and throughput of lapping and polishing processes are affected by a large number of variables. Individual processes are well controlled within individual enterprises, yet there appears to be little ability to predict process performance a priori. As a first step toward improving process modeling, this paper reviews the fundamental mechanisms of material removal in lapping and polishing processes and identifies key areas where further work is required.

Laser cladding of aerospace materials

Abstract In recent years, the aerospace industry has devoted a large amount of resources to the research and development of new repair technologies for gas turbine components. Traditionally their main repair tool is tungsten inert gas (TIG) welding but a new non-traditional process is emerging called laser cladding. This paper shows that by using this process, protective coating materials can be clad onto aerospace component substrates. We have shown that it has the potential to form pore-free and crack-free coatings. Two cladding materials, in the form of fine alloy powder, were clad onto five different substrate materials. The microstructure, hardness, cracking, porosity and dilution levels were recorded in each case and compared to TIG welded samples. Some of these results are discussed here.

Environmentally Clean Machining Processes — A Strategic Approach

Summary Present day machining processes are not clean. Current trends in the manufacturing world indicate that this situation will not be acceptable in the future and that extensive research and development work is essential in order to meet the future requirements of legislation. In this paper, the various possible routes to achieve clean machining processes are analysed and discussed. It is shown that the conventional approach has its limitations and that a new and innovative approach is essential within the framework of an environmentally oriented management system and the relevant networks for data provision and analysis. Processes have to be subjected to fine analysis giving deiailed consideration to the various inputs and outputs. Combined processes, alternative processes, new technologies, energy requirements etc. have to be evaluated from the environmental perspective. A first approach to the strategy required to answer the many questions which arise in relation to clean machining processes and to an integrated environmental protection policy is presented and discussed.

Observations on chip formation and acoustic emission in machining Ti–6Al–4V alloy

Abstract Orthogonal cutting tests were undertaken to investigate the mechanisms of chip formation for a Ti–6Al–4V alloy and to assess the influences of such on acoustic emission (AE). Within the range of conditions employed (cutting speed, v c =0.25–3.0 m/s , feed, f=20–100 μm ), saw-tooth chips were produced. A transition from aperiodic to periodic saw-tooth chip formation occurring with increases in cutting speed and/or feed. Examination of chips formed shortly after the instant of tool engagement, where the undeformed chip thickness is slightly greater than the minimum undeformed chip thickness, revealed a continuous chip characterised by the presence of fine lamellae on its free surface. In agreement with the consensus that shear localisation in machining Ti and its alloys is due to the occurrence of a thermo-plastic instability, the underside of saw-tooth segments formed at relatively high cutting speeds, exhibiting evidence of ductile fracture. Chips formed at lower cutting speeds suggest that cleavage is the mechanism of catastrophic failure, at least within the upper region of the primary shear zone. An additional characteristic of machining Ti–6Al–4V alloy at high cutting speeds is the occurrence of welding between the chip and the tool. Fracture of such welds appears to be the dominant source of AE. The results are discussed with reference to the machining of hardened steels, another class of materials from which saw-tooth chips are produced.

TEM study on the surface white layer in two turned hardened steels

The structure of surface white layers was examined using transmission electron microscopy. Surface specimens were machined from a BS 817M40 steel (0.4C 1.2Cr 1.4Ni 0.2Mo) of 52 HRC and a low alloy tool steel (0.8C 1.7Cr 0.4Mo) of 58 HRC, with unworn and worn alumina/TiC composite cutting tools. Thin foil specimens were prepared such that the direction of observation was normal to the machined surface. The as-tempered microstructure of both steels was lath martensite. The structure of the machined surfaces of both steels was characterised by very fine, mis-orientated cells, less than 100 nm in size. The accompanying selected area electron diffraction patterns indicated the presence of retained austenite, the volume fraction of which increased with cutting tool wear. A refinement in the size of cementite particles was also evident. In the surface of the BS 817M40 steel machined with a worn cutting tool, there was evidence to suggest a degree of recrystallization. This may be accounted for by a transition from dynamic recovery to dynamic recrystallization during surface generation; a phenomenon which is favoured by the decrease in the work materials stacking fault energy as a result of the reverse martensite transformation.

Cutting tool wear in the machining of hardened steels: Part II: cubic boron nitride cutting tool wear

Abstract A study was undertaken to investigate the wear mechanisms of CBN/TiC cutting tools in the finish machining of BS 817M40 (AISI 4340) steel of 52 HRC. A fourfold variation in tool wear rate was observed in the machining of three different heats of this steel. One of the primary characteristics of the tool wear surfaces is the manner in which the TiC phase stands proud of the CBN phase. The relative abundance of different elements on the wear surfaces of the tools, which are present in the work material in small (Mn, Si) or very small (Al, S, O) quantities, suggests that the dominant wear mechanism of CBN/TiC cutting tools is chemical in nature. In considering the relative wear rates of the tools used to machine the different heats of steel, a reasonable correlation is noted between the work material Al and S content and the corresponding tool wear rate. Examination of built up layers at the trailing edge of the tool, however, suggests that work material Al content is rate-determining with regards to tool wear. Following these observations, a new mechanism is proposed to account for the (widely acknowledged) superior wear resistance of CBN/TiC composites in comparison to high-content CBN tools.

Pad conditioning in chemical mechanical polishing

Abstract As circuits become increasingly complex and device sizes shrink, the demands placed on manufacturing processes increases. For successful manufacture of such circuits, high levels of wafer planarity are required. Chemical mechanical polishing (CMP) is a manufacturing process used to achieve global planarity. Studies have shown that the degree of planarity achieved is influenced by the pad properties. During polishing, the properties of the pad can deteriorate causing reduced polish rates and reduced planarity. It has been shown that this is caused by plastic deformation of the pad material, leading to glazed areas on the surface. Conditioning is used to regenerate the pad surface by breaking up these areas, but in doing so induces pad wear. As some areas of the pad will experience a higher degree of glazing, varying conditioning densities are necessary to counter this effect. The conditioning profile dictates the travel of the conditioning arm over the pad and hence the conditioning density experienced at each point. The aim of such a profile is to aid planarity by creating a uniform and constant removal rate over the whole wafer. The variations in pad properties between new and worn-out pads and the effect of conditioning in changing the pad properties will be examined. By examining correlations between conditioning densities experienced and pad properties over the radius of the pad, it is hoped to further understand the effect of conditioning on the process. Such understanding is vital in improving process reliability and yields in semiconductor manufacture.

Cutting tool wear in the machining of hardened steels: Part I: alumina/TiC cutting tool wear

Abstract A study was undertaken to investigate the mechanisms of alumina/TiC cutting tool wear in the finish turning of hardened steels with particular cognisance of the work material inclusion content. A six-fold variation in tool life was observed when machining different heats of BS 817M40 steel (similar to AISI 4340) of 52 HRC. In machining steels containing Ca-bearing mixed oxide inclusions, a reaction between the alumina phase of the tool and oxide inclusionary deposits is the dominant wear mechanism. In machining steels containing very low levels of Ca or steels with a very low inclusion content, tool wear appears to be largely based on superficial plastic deformation of the tool surface. The rate of tool wear appears to be determined by the hard inclusion content or alloy carbide content of the work material. Impingement of hard particles against the tool surface are thought to result in the generation of transient localised stresses which exceed the average contact pressures and, thus, either facilitate the operation of additional slip systems or overcome the increases in the critical resolved shear stresses on active slip systems due to prior strain. The influence of saw-tooth chip formation on cutting tool wear is also considered.