A slightly different concept of lubricant distribution is the supply of cooling lubricant from the interior of the grinding wheel or the grinding layer. The fluid is fed into a chamber of the wheel body allowing the centrifugal force to distribute it through radial channels to the grinding layer. Through pores or gaps in the grinding layer, the cooling lubricant is directly provided to the active zone. The technical complexity of this solution has so far prevented a broad application. Alternatively, a porous grinding wheel can be infiltrated by fluid supplied from a conventional external cooling nozzle, which will leave the wheel inside the contact zone due to the centrifugal force [Tawakoli 1990, Heinzel 1999].
10.9.3 Minimum Quantity Lubrication Nozzles
Minimum quantity lubrication (MQL) is aimed at reducing the amount of lubricant used for a grinding application. MQL nozzles have been the topic of several research projects. With the assistance of pressurized air, a mist of cooling lubricant is sprayed onto the surface of the grinding wheel. Ideally, only a thin film of fluid covers the wheel surface prior to its entry into the contact zone. Investigations concerning MQL report an increase in grinding force, wheel wear, workpiece surface roughness, and onset of grinding burn at lower material removal rates in contrast to
conventional fluid supply systems. Furthermore, secondary functions of the fluid such as chip transport or cooling of the grinding machine have to be carried out through additional devices. However, certain grinding operations using a minimum quantity lubrication show some potential for applications in an industrial environment such as rough grinding with plated metal bond cubic boron nitride (CBN) wheels [Heinzel 1999, Weinert 1999].
10.9.6 Auxiliary Nozzles
In addition to the nozzle supplying cooling lubricant to the contact zone, it is recommended to use auxiliary nozzles in radial direction to the grinding wheel. Their task is to remove chips and other loading from the wheel surface as well as to extinguish sparks or glowing chip particles. The effectiveness of these measures depends more on the fluid pressure than on the volume flow. It could be seen that there is a significant reduction in surface roughness of the ground workpieces by applying two more of those cleaning nozzles [Vits 1985, Konig and Klocke 1996].