How to penetrate the air cushion
To overcome the air cushion, various solutions commonly used in practice are applied. In all cases, the aim is to reduce or completely avoid the negative influence of the air cushion on the cooling lubricant flow. The available overcoming options depend on the nozzle design used. In principle, the following technical possibilities can be named:
1. Saturation of the air cushion with the cooling lubricant jet intended for cooling lubrication by adjusting the nozzle parameters (volume flow, pressure, jet angle).
This approach is particularly suitable for free-jet nozzles. By adjusting the cooling lubricant discharge velocity or the positioning angle, it is possible to ensure that the point of impact of the cooling lubricant 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 this then corresponds to the requirement in the influenced state. Furthermore, there is an increase in the horizontal flow force component of the cooling lubricant jet, as a result of which the jet can supply the cutting point with cooling lubricant despite being influenced.
As shown in Figure 1, by adjusting the coolant parameters in free-jet nozzles, it is possible to avoid missing the cutting point, which is accompanied by an improvement in the coolant supply. Regarding the relevance of the coolant exit speed from the coolant nozzle, Heymann cites not only the jet force development but also the necessity of the jet acceleration on the circumferential speed of the grinding wheel [3]. The exit speed is identified as an influencing variable on the kinetic energy of the coolant jet by Beck, as an important coolant fluid parameter to ensure the penetration of the air cushion. However, economic disadvantages due to the resulting increase in the cooling lubricant flow rate must also be taken into account [4].
Figure 1: Influencing the air cushion penetration through
adaptation of KSS parameters
2. Deflection of the air cushion with mechanical barriers
A common solution is to weaken or deflect the air cushion by means of fixed barriers. Air deflectors are usually used for this purpose, which are intended to deflect the air cushion before the point of impact of the cooling lubricant jet. CFD simulations and tests have been carried out by Yoshimi et al. on the flow conditions within the air cushion when deflector plates are used. Figure 2 shows the mode of action of an air baffle by means of a CFD simulation.
The decrease in the layer thickness of the air cushion directly behind the air deflector plate is clearly visible. The flow velocities, in particular those of the fast boundary layer at the grinding wheel, are briefly no longer present directly behind the air deflector plate.
It should be noted, however, that the influence of the air deflector plate can only be maintained over a very limited area of the grinding wheel. Already 90° in the direction of rotation of the wheel, the boundary layer near the surface returns to almost its original speed.
For an optimum combination of the air deflector plate with a cooling lubricant nozzle, this would have to be arranged directly behind the deflector plate so that the cooling lubricant jet can impinge in the weakened area of the air cushion. The disadvantage of this solution is the very high setup effort, as well as the necessity to readjust the barrier when the grinding wheel is worn [4]. In high-speed grinding with free-jet nozzles, the benefit of baffles is generally considered to be high. [1].
Figure 2: CFD simulation - air cushion with deflector plate [5]
However, when using an air deflector, it should be noted that the gap to the air deflector increases with radial wear of the grinding wheel lining. If the air deflector is then not readjusted in discrete time, an increasing gap between the abrasive coating and the air deflector will result. This gap aggravates the formation of an air cushion rotating with the grinding wheel in such a way that an air cushion actually increases in terms of its velocity profile compared to the non-diverted air cushion. The sheet then acts like an "orifice" and may make the situation significantly worse. The nozzle experts at team grindaix always ensure with their nozzle solutions that an air cushion is effectively weakened and deflected over the complete radius decrease of the grinding tool.
3. Use of chamber or shoe nozzles
Figure 3: Schematic representation of the function of shoe nozzles
Another possibility for deflecting the air cushion with fixed, mechanical barriers and simultaneous combination with the cooling lubricant supply, are the so-called shoe or chamber nozzles. In 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 entrained by the grinding wheel as a result of rotation and accelerated to the peripheral speed.
Due to the housing of the chamber nozzle and its very close fit to the grinding wheel, the air cushion in front of the coolant supply point is completely deflected. This means that the grinding wheel can be wetted with coolant without being affected by the air cushion.
A disadvantage of this solution is the need for a tight fit of the chamber nozzle to the grinding wheel surface. This is associated with high set-up and adjustment costs. However, the adjustment of the deflector and the coolant supply is carried out in a single operation, which can reduce the workload compared to a combined solution of deflector and coolant nozzle [2], [4].
However, for complex grinding wheel geometries, or for grinding wheels with high diameter reduction due to wear, the use of chamber nozzles is rather unsuitable [1]. In addition, it must be taken into account that due to the necessity of accelerating the cooling lubricant from the nozzle chamber, a part of the spindle drive power must be applied, which can no longer be made available to the grinding process.
4. Deflection of the air cushion by additional KSS - nozzles
Figure 4: Schematic representation of the function of double-jet nozzles according to H.W. Ott, cf. [2]
H.W. Ott introduces a cooling lubricant supply system that, in addition to supplying cooling lubricant to the grinding gap, can also be used to deflect the air cushion and, furthermore, to clean the grinding wheel. This so-called dual-jet nozzle divides the supplied cooling lubricant volume flow.
As with conventional cooling nozzles, the main part of the volume flow is directed toward the grinding gap and used for process cooling.
The comparatively smaller proportion is directed by 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 cooling lubricant jet to reach the grinding wheel for process cooling.
As a positive side effect, good cleaning properties are also achieved by flushing out machining residues from the pore spaces of the grinding wheel, which has an additional positive effect on the machining result.
The cooling lubricant volume flow used for cleaning is also almost completely available for process cooling due to the entrainment in the grinding wheel pores. 2] Figure 3 schematically shows how the nozzle system according to H.W. Ott works.
Potentials of effective penetration of the air cushion
The described technical efforts to weaken the influence of the air cushion on the cooling lubricant jet clearly show the relevance of these measures for manufacturing technology. The main reason for this is that correct penetration of the air cushion can have a very positive effect on the productivity of the grinding process.
The main priority is to ensure that the supply to the cutting point is reliably guaranteed in order to avoid thermal damage to the component and overloading of the tool. This basic requirement for the cooling lubricant supply can be considerably disturbed by the air cushion. Effective penetration of the air cushion consequently provides an increase in the robustness of the machining task.
The economic efficiency of a machine tool depends on the reliable production of many components of perfect quality within a short machining time. The decisive factor here is the achievable metal removal rate, which can only reach a level in line with requirements at high infeeds or high cutting speeds. For this purpose, sufficient cooling capacity to dissipate the heat energy generated is absolutely essential. Reliable coolant supply to the cutting point, despite the presence of the air cushion, therefore offers the potential to significantly increase the economy and efficiency of a machine tool.
In addition to process cooling, reliable grinding wheel cleaning also has a major influence on the machining process. Clogging of the grinding wheel pores with chips shortens the dressing intervals, the cooling lubricant transport in the pore spaces is impeded, the cutting performance decreases and the temperature development increases. Since the jet from flat jet nozzles, which are usually used as cleaning nozzles, is also subject to the influences of the air cushion, an adequate strategic approach must also be found here to prevent jet deflection. Reliable grinding wheel cleaning, in addition to eliminating the above-mentioned negative influences, contributes to an improvement in the finishable surface quality.
Summary
In summary, the elimination of the disturbing influence of the air cushion on the cooling lubricant jet, regardless of the type of nozzle used, can be seen as a priority challenge for production engineering in the field of grinding technology.
Do you need support from an expert in this field? Then do not hesitate to contact us. We will analyze your grinding process and offer you an individual solution.
Sources
[1] Denkena, Berend. Tönshoff, Hans Kurt. Spanen - Grundlagen, Heidelberg 2001.
[2] Meister, Markus. Vademecum des Schleifens, München 2011.
[3] Heymann, Tobias. Schleifen und Polierschleifen von wendelförmigen Spannuten von Vollhartmetallbohrwerkzeugen, Dortmund 2015.
[4] Beck, Thorsten. Berichte aus der Produktionstechnik, Kühlschmierstoffeinsatz beim Schleifen mit CBN, Aachen 2001.
[5] Yoshimi,T.; Oishi, S.; Okubo, S.; Morita, H. Development of Minimized Coolant Supply Technology in Grinding, JTEKT Engineering Journal English 2010.
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