The cooling lubricant supply system is required to accomplish several different tasks during the machining process as well as during auxiliary process time or even during the off-state of the grinding machine. First of all, it has to provide an uninterrupted flow of cooling lubricant to the active zone. Moreover it is required to store and transport the cooling lubricant maintaining a constant quality and temperature and with a sufficient quantity to execute the job of cooling, lubricating, flushing, and chip transport. In addition to economic requirements related to investment costs or maintenance cost, a number of further requirements must be met including operating safety especially when using oil as cooling lubricant [Konig et al. 1993, Brucher 1996, Konig and Klocke 1996].
In an industrial environment, it is a common approach to install a central or group circulation system that supplies a number of machine tools using the same cooling lubricant. These systems require the specification of a single cooling lubricant for all processes supplied but reduce the complexity for cleaning, cooling, controlling and supply of the fluid, and, in addition, reduce the circulating volume of the cooling lubricant [Brucher 1996].
Centralized systems are composed of components transporting the fluid to the process (pumps, pipes, nozzles, measurement and control devices, mixing devices), a return system (channels, pipes, pumps), maintenance devices (filters, reservoirs, monitoring devices), and equipment for swarf treatment (conveyor, chip crusher, centrifuges, cleaning nozzles). Design of a cooling lubricant supply system strongly depends on the required flow and pressure of the fluid leaving the nozzle at the contact zone. By applying a particular nozzle form, its positioning and the required fluid pressure determine the total volume of cooling lubricant to be supplied. Additionally, the volume stored in the feed and return pipes, the volume contained in filter and tempering devices, a minimum reserve volume, and, if necessary, an additional volume for foam discharging have to be taken into account [VDI-Richtlinie 3035 1997].
Although cooling lubricant supply systems are often designed for either water-miscible or water-immiscible cooling lubricants, the application of both types and many different specifications of cooling lubricants during the life span of a machine has become an increasing demand by industry. This needs to be incorporated into the material choice for a cooling lubricant supply system where it is generally recommended to avoid zinc-plated steel pipes or nonferrous fittings. In order to prevent the degradation of cooling lubricant or corrosion of machine components, the compatibility of all materials used has to be ensured especially in terms of a fluid that changes its physical and chemical properties over the course of time. Tanks for fresh and used fluid are required to store the entire cooling lubricant volume of the supply system in case of a machine stoppage that leads to the fluid all flowing back into those tanks due to a gravitation-controlled piping system. Because of long distances between machine tools and central tanks, it is often necessary to install a separate backflow tank at each machine. Furthermore, the design of each tank should prevent the deposition of any solid residuals or backflow of fluid and offer a facility to empty the tank completely [VDI-Richtlinie 3035 1997].
Depending on volume flow, fluid pressure, and contamination of the fluid, a variety of different pumps can be used. A crucial feature of all types is the sealing of the pump shaft against the contaminated cooling lubricant, which is presently done by axial face seals made of tungsten carbide including special CVD or PVD coatings. To minimize pressure drop, the supply pump should be placed as close as possible to the delivery nozzles. The cross section of the connected piping system must be adjusted to the particular flow conditions in order to avoid cavitation, which normally limits the flow velocity in suction pipes to 1.5 m/s and in pressure pipes to 2.5 m/s [VDI-Richtlinie 3035 1997].
Grinding swarf contaminates the grinding fluid and degrades the grinding operation itself and the lifetime of the fluid if accumulated in the fluid. Cleaning and conditioning of the fluid is accomplished by a number of different methods and principles. Chemical, biological, and, above all, mechanical contamination can be eliminated by sedimentation, filtering, centrifugation, and magnetic tape separators depending on the required degree of purity. Sedimentation is a reasonable method for coarse cleaning of the fluid but in finish grinding, a stage cleaning process using several different filter principles is recommended. The most common filters used are tape filters and candle filters, both requiring a change and disposal of the filter within a fixed period of time. In addition to a very clean cooling lubricant, it is crucial for high-speed grinding applications and finish grinding operations to have the fluid supplied at a precisely controlled temperature. The grinding heat needs to be dissipated while passing through the fluid supply system. The fluid needs to dwell long enough in the supply system to allow heat dissipation or must be removed in an extra cooling system [Tawakoli 1990, Brucher 1996, Konig and Klocke 1996, VDI-Richtlinie 3035 1997].
It is essential in all grinding applications to control the fluid pressure or volume flow. With regard to safety of the grinding process, an automatic machine stop is essential to cope with an unplanned pressure drop or significant flow reduction. Furthermore, it is desirable to check and control the quality of the supplied fluid by measuring the pH-value or electrical conductivity. In grinding with water-immiscible cooling lubricants, the mist generated in combination with high process temperatures and glowing chips flying away from the contact zone poses an explosion hazard. Therefore, safety devices such as blowback flaps and fire extinguishers need to be installed. For grinding with minimum lubrication systems, separate technical regulations take effect [Tawakoli 1990, VDI-Richtlinie 3035 1997].