Technical possibilities for influencing air cushions

This approach is particularly suitable for free-jet nozzles. By adjusting the coolant exit speed or the positioning angle, it can initially be achieved that the point of impact of the coolant jet is above the cutting point despite the jet deflection. The point of impact is brought forward by the amount of the deflection, so that it then meets the requirement in the influenced state. Furthermore, there is an increase in the horizontal flow force component of the cooling lubricant jet, which means that the jet can supply the cutting point with cooling lubricant despite the influence.

As shown in Figure 1, adjusting the coolant parameters for free-jet nozzles can prevent the coolant from missing the cutting point, thus improving the coolant supply. With regard to the relevance of the cooling lubricant exit speed from the cooling lubricant nozzle, Heymann cites not only the development of the jet force but also the necessity of accelerating the jet to the peripheral speed of the grinding wheel [3]. The exit speed is identified by Beck as an influencing factor on the kinetic energy of the cooling lubricant jet and as an important cooling lubricant parameter to ensure penetration of the air cushion. However, economic disadvantages due to the resulting increase in cooling lubricant volume flow must also be taken into account [4].

Another option for deflecting the air cushion with fixed, mechanical barriers and simultaneous combination with the cooling lubricant supply is the so-called shoe or chamber nozzles. With this nozzle design, a certain segment of the grinding wheel surface is covered by the chamber nozzle. This chamber is filled with cooling lubricant, which is carried along by the grinding wheel due to its rotation and accelerated to the circumferential speed.

Due to the housing of the chamber nozzle and its very close fit to the grinding wheel, the air cushion is completely deflected in front of the coolant supply point. This means that the grinding wheel can be wetted with coolant without being affected by the air cushion.

The disadvantage of this solution is the necessity of a tight fit of the chamber nozzle to the grinding wheel surface. This is associated with a high level of setup and adjustment effort. However, the adjustment of the discharge element and the cooling lubricant supply is carried out in a single operation, which can further reduce the labor required compared to a combination solution consisting of a discharge sheet and cooling lubricant nozzle [2], [4].
However, the use of chamber nozzles is rather unsuitable for complex grinding wheel geometries or for grinding wheels with a high wear-related diameter reduction [1]. It should also be noted that, due to the necessity of accelerating the cooling lubricant from the nozzle chamber, part of the spindle drive power must be applied, which can no longer be provided to the grinding process as a result.

H.W. Ott introduces a coolant supply system that can be used not only to supply coolant into the grinding gap and to deflect the air cushion, but also for grinding wheel cleaning. This so-called two-stream nozzle divides the supplied coolant flow rate. The main part of the flow rate is directed to the grinding gap, as with conventional cooling nozzles, and used for process cooling.

The comparatively smaller part is directed through flat-spray nozzles at a 90° angle, directly onto the grinding wheel surface. This creates a barrier for the air cushion, which should make it easier for the coolant jet to reach the grinding wheel for process cooling.
As a positive side effect, good cleaning properties are also achieved by flushing machining residues out of the pore spaces of the grinding wheel, which additionally positively influences the machining result. The coolant volume flow used for cleaning is also almost completely available for process cooling due to entrainment in the grinding wheel pores. [2] Figure 3 shows the schematic functioning of the nozzle system according to H.W. Ott.