workpiece is moved at a low feed speed until the first chip formation is visible with the naked eye. The precision and consistency of this feed operation depend heavily on the experience of the machine operator and generally fluctuate by at least 1–2 lm. This, however, is intolerable for demanding applications of the mi- cro-structure technique or for measuring coated sample parts. 5.4 Environmental Awareness F. Klocke, RWTH Aachen, Aachen, Germany Ecological issues are assuming increasing importance in many areas of the econo- my as a result of legislation and growing public awareness. Manufacturing, char- acterized by a chain of resource-intensive processes and by large quantities of waste materials and emissions, is frequently the focus of interest. Government-im- posed environmental regulations and increased cost pressure in conjunction with the need to prevent the production of waste materials or to dispose them appro- priately are forcing companies to introduce innovative, environmentally compati- ble manufacturing processes. In the manufacturing environment, the starting points for ecologically oriented improvement lie in the need to prevent the genera- tion of waste materials and pollutants in the first place, or to reduce the volumes produced and re-use them. The advantages which stand to be gained as a result of the application of more environmentally compatible technologies are clear: re- duced levels of energy consumption, waste, and disposal costs, together with high- er employee motivation and lower rates of absenteeism due to illness [1]. In the successful, practical application of process monitoring systems and com- ponents, monitoring- and sensor-related solutions are adapted to meet the specific requirements of the machining task concerned. This demands precise knowledge of the machining operation, ie, the manufacturing environment, the machining process, and any potential process malfunctions [2–4]. The requirements relating to the monitoring system may, however, differ considerably in terms of the objec- tives and the implementation of the monitoring system. A reduction in the quantity of cooling lubricant used in machining operations is a good example of the specific demands imposed on process monitoring and sensor systems by manufacturing processes which have been optimized in terms of environmental compatibility. On the one hand, the application of sensors is simplified since the requirements relating to the robustness and coolant resis- tance of the sensors are lower in this case. On the other hand, a reduction in the amount of cooling lubricant used frequently increases the degree of thermal load to which the parts are subjected. It therefore becomes more important to monitor any temperature-related change in dimensional and form accuracy or in the struc- ture of the material of the finished parts. The influence exerted on the structure is critical, particularly in machining operations conducted on hardened materials. Demands made on the monitoring system and on measurement engineering, which arise from the specific boundary conditions of environmentally compatible 363 Sensors in Manufacturing. Edited by H.K. Tönshoff, I. Inasaki Copyright © 2001 Wiley-VCH Verlag GmbH ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic) manufacturing processes, are analyzed and discussed below. The priorities will be the reduction in the volume of cooling lubricant used, extending to minimal lubri- cation and dry machining, as well as the working surroundings and the risk to which the machine operator is exposed by the emissions released during the ma- chining operation. Examples of some approaches to the problems posed by the need to apply metrological techniques in order to measure relevant process vari- able are presented here for machining operations conducted using both geometri- cally defined and undefined cutting edges. 5.4.1 Measurement of Emissions in the Work Environment Manufacturing is characterized by the combination of an extensive range of differ- ent types of substance and material flows. The substance flows are composed par- ticularly of emissions, which are released during the manufacture of a product, depending on the processes and materials used. In the specific case of dry ma- chining, the emissions are in the form of particles, which require the application of measurement techniques in order to gage their impact on the working environ- ment and to be able to take appropriate measures, if necessary [5, 6]. 5.4.1.1 Requirements Relating to Emission Measuring Techniques for Dry Machining The type and volume of emissions which occur in dry machining operations de- pend on the machining operation used, the process control system and the ma- chining parameters. In principle, however, a distinction can be drawn between aerosols (solid and liquid particles), gases, and vapors. Certain characteristics must be established before any conclusive data relating to the impact of emis- sions on the working environment can be released. The effects on the human or- ganism depend on the characteristics of the material in question, particle geome- try, the concentration, and the reaction time. Additionally, small particles released in the course of the cutting process can have an adverse effect on the process or can increase the level of wear sustained by machines and facilities [7–10]. 5.4.1.2 Sensor Principles One of the prerequisites for the reliable determination of emission characteristics is the selection of an appropriate measuring technique and sensor. The two fundamen- tal procedures in any measurement are sampling and analysis. The function of sam- pling is to take a sample of the air at the measuring location and to ensure that it is available for analysis. A further distinction can be drawn between continuous sam- pling without enrichment and sampling the materials which do not belong in the air, on a sample carrier. Passive sampling can be performed on gases and vapors by enrichment, diffusion, or permeation. In active sampling operations which can be conducted on gases, vapors, and aerosols, the air containing pollutants is sucked 5 Developments in Manufacturing and Their Influence on Sensors364 in and the pollutant is separated off using a sample carrier. This type of sampling operation is followed up by a chemical or physical laboratory analysis of the materi- als measured, which do not occur naturally in the air. For many areas of application for continuous sampling, there is a wide range of measuring instruments, the ma- jority of which use electrical and optical measuring principles [11–14]. There are various collections of recognized measuring operations and direc- tories listing external measuring centers which are useful sources of information to assist in the selection of suitable measuring techniques. Information and cata- logues can be obtained from national employer’s liability insurance associations and from institutions for engineering safety standards at the workplace (eg, BIA (Berufsgenossenschaftliches Institut für Arbeitssicherheit, Germany), NIOSH (National Institute for Safety and Health, USA), OSHA (Occupational Safety and Health Administration, USA)). 5.4.1.3 Description of Selected Measuring Techniques Selected analysis and monitoring techniques used for aerosols in the working en- vironment around dry machining operations are described in the following. The instruments used most frequently to measure aerosol presence are scat- tered light-measuring devices. This technique is based on the principle of direct- ing light waves away from their original direction by refraction, reflection, and dif- fraction caused by small particles. The diameter of the particles can be measured directly by evaluating the intensity, frequency, or phase of the scattered light. A distinction is drawn between instruments which detect the scattered light of a group of particles (photometer) and those which measure the particles individu- ally (optical particle counter) [13–17]. The phase Doppler anemometer technique permits the dynamics of the particle to be measured, ie, simultaneous measurement of the size, speed, and concentra- tion. As soon as small particles exceed a measured volume consisting of a system of plane-parallel interference bands, scattered light is produced which is ampli- tude modulated due to the interference phenomenon. Initially, this scattered light is used to measure the particle speed (laser Doppler anemometer). Particle size can be determined by evaluating the additional information provided by the phase position of the scattered light. The application of phase Doppler anemometers is limited largely to virtually spherical, transparent particles [18]. Numerous measuring instruments used to determine the particle size distribution exploit the principle that the aerodynamic diameter of a particle can be determined from its acceleration. A nozzle is used to accelerate the aerosol and the aerodynamic particle diameter is determined by measuring the time required by the particles to travel between two points. This technique can also be used to collect further infor- mation about other aerosol characteristics such as the distribution of the number of particles, their surface dimensions, or mass concentration [13]. The measuring techniques previously listed generally assume virtually spherical particle geometries and are unsuitable for measuring fibers. Some manufacturers also produce fiber measuring instruments capable of displaying the results imme- 5.4 Environmental Awareness 365 diately. These use the characteristics of the light signals which are scattered by the fibers. The problems in connection with the optical measurement of fiber dust arise from the random orientation of these particles in space, which causes irreproducible measurement results. The orientation of the fibers with their axis in one direction can, however, be achieved when a directed electrical field is used. However, the accuracy of the instruments currently available has been insuffi- ciently high to warrant replacing the microscopic techniques traditionally applied to evaluate fiber dust [13, 17, 19]. In comparison with the particle measuring techniques in which aerosol parti- cles are separated from a sample carrier, the optical particle measuring instru- ments all operate in non-contact mode, thus avoiding most of the faults resulting from the sampling procedure itself [16]. 5.4.1.4 Example of Application Cutting operations conducted on fiber-reinforced plastics are associated with the well-known problem of the release of dust, which is why an extraction system must normally be used during the machining process. The objective was to mea- sure the efficiency of emission recording in the case of a machine design, which had been adapted to meet the needs of a concrete machining situation (Figure 5.4-1). A measuring technique in which scattered light photometry is combined with gravimetric particle analysis permitted comparison of the progressions with time of the alveolar mass concentration with and without extraction. As demon- strated by a comparison of the caliper gage, a proportion of fine dust is recorded very rapidly and reverts to the starting level about 100 s after conclusion of the milling operation [20]. 5 Developments in Manufacturing and Their Influence on Sensors366 Fig. 5.4-1 Comparison of the aerosol concentration with and without an extraction facility 5.4.2 Dry Machining and Minimum Lubrication Cooling lubricants have come to be accepted as essential elements in production engineering. In many cases, they guarantee the quality of the machining outcome in terms of tool life, surface quality, and part accuracy. However, in recent years, the steadily increasing pressure on costs in manufacturing industries coupled with more pressing questions relating to environmental compatibility and dispo- sal of waste materials have caused many users to reexamine their use of cooling lubricant [21, 22]. Against this background, it is easy to understand why industry and research are putting so much effort into prolonging the service life of cooling lubricants and reducing the amount required to zero – dry machining – if possi- ble [23]. Generally, dry machining tends to facilitate the use of sensors to record and monitor process variables. New or at least extended tasks arise, however, in the field of temperature measurement. Process temperatures rise as a result of the elimination of the cooling lubricant functions of cooling, lubrication, and chip transport. This makes it all the more important to be able to determine the tem- peratures of tools and workpieces. 5.4.2.1 Measuring Temperatures in Dry Machining Operations The measurement of temperature during machining operations has been the sub- ject of a number of investigations over many years. A comprehensive overview of temperature measuring techniques was given by Lowack [24] and Kassbaum and Löffler [25]. Owing to the process characteristics, not all of the methods are suitable for in- process monitoring but can be used in basic investigations, eg, to determine the input quantities for modeling. The methods which are suitable for process moni- toring are presented in the following. The use of thermal converters to measure temperatures in tools and workpieces is one of the contact methods of determining temperature. This type of tempera- ture measurement shows not the temperature in the object being measured, but the temperature of the sensor. Ideally, the sensor and the object to be measured are at the same temperature. In reality, this applies only in some cases. Heat sources and heat sinks result in permanent heat transport [26]. The temperature field in tools has also been determined using a series of opti- cal temperature measuring techniques [24]. Thermal imaging is a non-contact measuring technique based on heat radiation and is therefore particularly suitable for measuring the temperature of a rotating tool [27]. The measuring point, how- ever, must be on a visible surface in order to ensure that in a drilling operation, for example, the tool temperature can be measured at the point of exit of the tip of the drill bit from a through hole. A further feature of this technique is the considerable requirement for calibra- tion. The direct correlation between the level of heat radiated and the absolute 5.4 Environmental Awareness 367 temperature applies only in the case of an ideal emitter, the so-called ‘full radia- tor’ (Stefan-Boltzmann law) [28]. Given full radiators, it is possible to deduce the absolute temperature on the basis of the level of energy emitted. When determin- ing the temperature of real parts, account must be taken of the emission factor, ie, the deviation from the behavior of an ideal full radiator. The emission factor e is defined as the ratio of the radiation energy emitted by an object at a given tem- perature to the radiation energy of a full radiator at the same temperature and de- pends on the surface structure and temperature range to be measured and also on the material concerned. The emission factor must be known if the tempera- ture of a real object is to be determined reliably. In practice, however, this is no easy matter. This severely restricts the field of application of this method to in-pro- cess measuring operations [29, 30]. 5.4.2.2 Measuring Droplets in Minimal Lubrication Mode The application of techniques of machining in the minimal lubrication mode opens up additional fields of application for sensor technology. The term minimal lubrication is used here to refer to the supply to the machining point or to the tool of minimal quantities of a lubricant mixed with compressed air. The amount of lubricant required is considerably lower than 20 mL per process hour in the case of optimally adjusted systems. Certain adjustable parameters, eg, for volume and pressure, should be capable of being monitored as a function of the tech- nique, material, and supply system concerned, in order to ensure that the systems operate smoothly in the minimal lubrication mode. It is particularly important to monitor the operability of the minimal lubrication mode system in large-scale manufacturing series in order to ensure process reliability. Care must be taken to ensure that the machining point is supplied continuously with the mixture of compressed air and lubricant and that there is no incidence of ‘intermittent supply‘. The machining outcome depends both on the orientation of the supply unit and on the spray pattern that it produces. It is important that the mixture is directed accurately to the cutting area and that excessive misting is avoided. In ex- ternal supply, dispersion tests must be conducted to examine the spray pattern of the nozzles. In these tests, a patch of lubricant produced under specified condi- tions is measured. In-process spray pattern monitoring is a further potential use for the sensor system. However, the current status of the technology does not permit the quantities listed above, such as spray pattern and droplet size, to be determined in-process during manufacturing operations; instead, they are measured primarily in the lab- oratory. The spray pattern can be assessed in general terms, eg, with regard to the development of droplets and their distribution, using high-speed cameras with a short exposure time. The laser Doppler anemometer, described in the previous section relating to the measurement of emissions within the working environment [31], is used to deter- mine the droplet speed. 5 Developments in Manufacturing and Their Influence on Sensors368 There are two methods which can be used to determine the distribution of the droplet sizes of the atomized lubricant: first, the measurement of the droplet vol- ume under the microscope, and second, comparative measurements in the range 0–16 lm using a cascade impacter. This entails dividing the area into eight size categories and selecting the particles by their size-dependent flow characteristics. 5.4.3 Turning of Hardened Materials Hard machining with a geometrically defined cutting edge has gained substan- tially in importance owing to improvements in the performance of modern cut- ting materials. The high process temperatures and large specific forces involved in hard machining result primarily in a demand for good high-temperature hard- ness of the cutting material. This requirement is met especially by ceramic and cubic boron nitride. The use of such materials often permits turning to be substi- tuted for grinding as a hard finishing process. This allows a more flexible, less cost-intensive and more environmentally compatible manufacturing process. The ecological advantages are obtained principally through the avoidance of cutting fluids coupled with smaller quantities of production waste to be disposed of. Addi- tionally, the grinding swarfs that occur in the production of gear and roller bear- ing components can be reduced substantially. 5.4.3.1 Criteria for Process and Part Quality Hard turning technology is a machining process at the end of the production chain. Process monitoring can therefore make an important contribution to ensur- ing reliable production of workpieces with the required quality. In addition to high demands on form accuracy and exacting tolerances, the surface quality and the structure of the surface zone are important criteria for the quality evaluation of hard turned parts. The surface roughness in hard turning is determined to a considerable extent and is directly influenced by the corner radius of the cutting edge and the feed rate. The occurrence of various forms of tool wear has an adverse effect on surface quality [32– 34] and generally results in higher surface roughness values (Figure 5.4-2). Tool wear also influences the structure of the surface zone (Figure 5.4-3). Owing to the low ductility of martensite, the technology of hard machining is dif- ferent from the machining of unhardened material. This results in different chip formation mechanisms, which do not rely on the formation of a shear plane or shear zone. The phenomenon is documented by the occurrence of sawtooth chips, already discussed in numerous publications [32–36]. Chip formation influ- ences the surface zone of the machined workpiece and leads to changes in the resi- dual stress state and the microstructure of the surface zone. According to Goldstein [37], mechanical stressing of the workpiece comparable to Hertzian stress occurs at the contact between the tool flank and the workpiece surface, inducing compressive residual stresses in the workpiece surface zone. Tensile residual stresses are super- 5.4 Environmental Awareness 369 imposed on these compressive residual stresses as tool wear increases. These tensile residual stresses occur as a result of thermal stress caused by flank friction and by deformation phenomena in the work material. The maximum hardness is invariably greater than that of the basic structure and depends on the level of tool wear (Figure 5.4-3). The higher temperatures with increasing wear are also responsible for the structural transformations and the occurrence of ‘white layers’. Deviations from the ideal geometric form of a component following the soft turn- ing and hardening operations result in cutting force fluctuations. These force fluc- tuations have an adverse effect on form accuracy. It is not only radial allowance fluc- tuations which lead to deviations from the ideal geometric form of the finished com- ponent. Fluctuations in allowances along the feed path cause relative displacements between the workpiece and the tool, due to differing depths of cut. These axial allow- ance fluctuations show up as inadequate straightness of the contour line. Apart from cutting forces, a second main variable affecting the form and di- mensional accuracy of hard turned parts is the change in temperature during the process. As already noted above, one great advantage of hard turning over the competing grinding process is the ability to machine without cooling lubricant. In studies of surface zone effects and surface quality in hard turning operations, the products showed neither positive nor negative effects resulting from the use of cooling lubricant [33, 38]. Macrogeometrically, however, machining without a cool- 5 Developments in Manufacturing and Their Influence on Sensors370 Fig. 5.4-2 Surface roughness as a function of wear ing lubricant has very noticeable effects. The thermal energy of the machining process heats the part and the tool and hence causes shape deviations. 5.4.3.2 Sensing and Monitoring Approaches There is a correlation between tool wear, passive force, and resulting surface struc- ture [39]. Increasing wear land at the flank face is associated with a significant rise in passive forces [34, 37] and therefore influences the form and dimensional accuracy of the workpiece. The increasing friction between tool and workpiece also leads to higher temperatures with a corresponding impact on surface struc- ture and form deviations, as described above. As a result, cutting forces and cutting temperature are the most important pro- cess quantities for monitoring. Both are influenced by tool wear and change ac- 5.4 Environmental Awareness 371 Fig. 5.4-3 Surface zone characteristics as a function of tool wear. Source: Goldstein [37] micro hardness cordingly in the course of the process. Additionally, tool wear has a direct impact on the surface roughness and is itself a target for monitoring. There are various principles for direct and indirect force measurement in cut- ting processes. Owing to the low level of cutting forces in the finishing operation, a very sensitive measuring technique is needed for application in hard turning. Force measurements, based on piezoelectric elements mounted in the main force flux, are the most promising approach to provide signals at the high level of sensi- tivity required. Since cutting force measurements at the rotating workpiece re- quire wireless signal transmission, sensor mounting at the tool site, for example in or below the cutting tool holder, is more suitable. It is impossible to identify any one standard appropriate mounting location, since this depends to a large ex- tent on the machine tool design. The possibilities for temperature measurements in cutting operations have al- ready been described in the section on dry machining. Owing to the less complex kinematics of the turning operation, it is easier to apply some of the turning tech- niques than to use the rotating tools used in milling and drilling operations. For monitoring the cutting tool wear state, the acoustic emission (AE) signal is another promising approach that has been successfully applied in various machin- ing processes for wear monitoring [40–44]. The main advantage over force mea- surements is the easier sensor mounting and the better signal-to-noise ratio with low chip thickness in precision machining operations [41, 45]. The AE signal has been found to be sufficiently sensitive to monitor wear in hard turning operations [39]. The variations in the AE signal with increasing flank wear depend on the fre- quency range under examination and require an analysis of the signal characteris- tic prior to developing a monitoring algorithm [39]. The sensing solutions described so far adopt the approach usually applied in process monitoring to measure process manifestations such as cutting forces or AE signals indirectly and to correlate the measured quantities with process distur- bances. New developments in the field of cutting tool coating and micro-system technology permit the wear and temperature at the rake face of a turning tool to be measured directly on-line. Such an approach is still restricted to the field of re- search, but has considerable potential for application in various fields [46, 47]. Sensors for temperature and wear are applied within the wear-resistant coating of a carbide cutting insert (Figure 5.4-4) and allow the thermal load and the wear ge- ometry at the cutting edge to be determined. Temperature monitoring is particu- larly beneficial to hard turning applications, since thermal load is the main indica- tor for changes in the surface zone. 5.4.4 Using Acoustic Emission to Detect Grinding Burn Much of the energy used in grinding operations is converted into heat. The rea- son for this is the friction between the grinding grit and the workpiece and the large number of kinematic cutting edges involved in the process. The heat gener- ated is dissipated via the chips, the tool, and the cooling lubricant. 5 Developments in Manufacturing and Their Influence on Sensors372 [...]... frequency spectrum thus obtained depends largely on the type of process control used and on chip formation The grinding parameters, the grinding wheel used, and the cooling lubricant all exert a particularly strong influence on the AE frequency spectrum Since the geometry, the material, and, of course, the structural condition of the workpiece affect the frequency spectrum, a comparison of the frequency spectra . the grinding wheel used, and the cooling lubricant all exert a particularly strong influence on the AE fre- quency spectrum. Since the geometry, the material,