Based on the phenomenon of boundary layer adhesion described in the magazine article, Interfering influences on the cooling lubricant supply during grinding - Part II - Paragraph g) Air cushion, additional studies on the form and characteristics of the grinding wheel air cushion are available in the literature. In addition to FEM (Finite Element Methods) and CFD (Computational Fluid Dynamics) analyses, optical measurement methods with a laser Doppler anemometer are also used. In order to be able to compare and relate the characteristics of the air cushion with the cutting speed vc of the grinding wheel, the flow velocity of the air cushion in m/s is to be regarded as a significant measured variable.
Figure 1: Radial progression of air cushion flow velocity [Wu/Morgan]
Figure 1 clearly shows the fast air layer near the grinding wheel surface. In addition, the decrease in velocity with increasing radial distance from the grinding wheel can be seen. According to Wu/Morgan, the decrease in air velocity in the air cushion is described as exponentially decreasing. Air velocity measurement by means of a laser Doppler anemometer shows this relationship for various circumferential velocities vc between 20 - 40 m/s.
The velocity profile of the air cushion can be characterized by the measured values. The highest flow velocities of the air cushion occur close to the grinding wheel surface. These correspond analytically, at a distance of 0 mm, to the circumferential speed of the grinding wheel. At the measuring point of the smallest radial distance, a clear decrease in the flow velocity can already be seen, which continues to decrease quadratically with increasing distance.
In detailed investigations of the Rheinische Fachhochschule der Stadt Köln under Prof.-Dr. Ing. Wilfried Saxler and Mr. M.Sc. Roman Stabbauer on the subject of air cushion characterization during grinding, further informative findings were obtained. To characterize the air cushion, the flow velocity and the dynamic pressure in the radial and axial direction of the grinding wheel were measured using a Prandtl tube.
First, the flow velocity of the air cushion in radial direction to the grinding wheel was investigated. Here, the measuring point of the Prandtl tube was positioned exactly in the center under the grinding wheel (open-pored stainless corundum, grit size 60) and the flow velocity was measured at different distances in the radial direction (Figure 2). In addition, the flow velocity of the air cushion was measured in the axial direction, i.e. over the grinding wheel width (Figure 6 right). Here, the distance of the Prandtl tube was as close as possible to the grinding wheel. Furthermore, the cutting speed of the grinding wheel was varied between 20 and 60 m/s in both test series.
Figure 2: Measurement of the flow velocity of the air cushion with different distances in radial direction at different velocities [Sax/Stab]
At a cutting speed of 20 m/s, the flow velocity at the first measuring point of 1 mm distance is approx. 9 m/s. With increasing radial distance, the flow velocity drops exponentially and is already only just above 2 m/s at a distance of 50 mm. At a distance of 100 mm, the flow velocity is already so low that the measuring instrument can hardly detect any difference to the measurement without rotating grinding wheel. When the peripheral speed of the wheel is increased up to 60 m/s, the same curve can be observed. However, it is noticeable that the flow velocity of the air cushion just above the wheel surface is less than 50% of the set wheel circumferential speed. At 60 m/s, only about 30 m/s flow velocity can be measured. When looking at the measurements of the air cushion across the grinding wheel width, it can be observed that the air cushion does not build up evenly across the wheel width. Towards the edges of the grinding wheel, the air cushion increases and the higher the wheel peripheral speed, the stronger this effect is observed.
The same investigations were additionally carried out with a finer and denser sintered corundum grinding wheel, a very fine and closed diamond wheel and a smooth steel wheel. It became clear that the flow velocity, and thus also the influence of the air cushion, decreases with a finer grit size or a smoother wheel surface. A coarse and open-pored grinding wheel can carry more air with it and accelerate it more. A diamond grinding wheel with a dense bond has no porosity and can be compared to a smooth steel wheel in terms of air cushion. Hardly any difference can be observed here, although the surface roughnesses differ significantly. The porosity thus has a greater influence than the topography of the surface.
If the peripheral speed of the grinding wheel and the corresponding flow velocity of the air cushion of different grinding wheels are plotted on a graph (Figure 4), an almost linear curve can be observed. The course of the graph of the open-pore and coarser alumina grinding wheel has a steeper slope than the graph of the denser and finer sintered alumina grinding wheel. Compared to this, the graph of the closed diamond grinding wheel with a very fine surface topography has a much flatter slope.
By defining a slope μL for various types of grinding wheels, it is thus possible to describe the formation of the air cushion at different wheel circumferential speeds. To analyze the interactions between the coolant supply and the air cushion, series of tests were carried out with a high-speed camera. Here, for example, it was observed how the coolant jet, which hits the disk tangentially, behaves when the circumferential speed is increased or when the exit speed is increased.
Figure 5: Airflow velocity at different radial distances [Wu/Morgan]
The change in the air cushion velocity over the grinding wheel width in the axial direction was also evaluated using a velocity profile by Wu and Morgan in 2009 (see Figure 5). Here it was also recognized and mentioned that at the flanks of the grinding wheel, in relation to the grinding wheel center, lower flow velocities of the air cushion are present. Between the flanks, a stationary velocity range can be observed as a function of the radial distance from the grinding wheel surface and the axial position. This finding from the investigation using a laser Doppler anemometer is in contradiction to other investigations, which report a maximum of the flow velocity in the center of the grinding wheel, or two clear velocity increases to the left and right of the center position [Wu/Morgan].
In summary, the grinding wheel air cushion can be reduced in terms of the occurring flow velocities to a vertical and a horizontal flow force component, which arises from the internal friction of the air and from boundary layer adhesion between air and grinding wheel. According to Wu/Morgan, these can be converted into an x and y coordinate direction. By using optical measurement techniques (laser Doppler anemometer), the velocity profile of the air cushion can be determined. For the development of the velocity as a function of the radial distance, a quadratically decreasing course can be determined. When the velocity profile is viewed axially, the flank areas of the grinding wheel exhibit significantly lower flow velocities compared with the center.
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