Correct cooling of the grinding process
Often machine operators ask themselves which coolant lubricant exit speed is the right one. Various institutes, scientists, university members and system providers offer different answers to this question.
To make things clearer for the user, three aspects need to be taken into account:
Coolant lubricant exit speed
Many years ago, Helmut W. Ott published a paper on the topic of jet adhesion to the grinding wheel, stating that this is best achieved if (in theory) the coolant lubricant jet velocity is increased to the circumferential speed of the grinding wheel (vcoolant lubricant = 100% vgrinding wheel) . Some universities, by contrast, have stated that the exit speed of the coolant lubricant achieves its optimum cooling effect in the grinding gap upon reaching 80% of the grinding wheel circumferential velocity. Other users and companies reduce this value to only 30-50% of the grinding wheel circumferential speed. So what is right and what is wrong?
Firstly, it can be said that every statement is correct under certain conditions. It is just important to differentiate properly between them.
Most grinding applications today use coolant lubricant exit speeds in the range of between 2-10% of the grinding wheel circumferential speed, and they mostly achieve this with larger pipe cross sections. Due to the physical correlation between the exit cross section of a coolant lubricant nozzle and the pressure-specific flow through this cross section, a correspondingly high inflow of coolant lubricant into the machine tool is given.
If we now consider the fact that the coolant lubricant fed to the machine tool needs to be filtered, pumped and, above all, cooled (electrically), there are justifiable economic limitations in industrial practice. The scientifically correct approach of accelerating the coolant lubricant to the grinding wheel circumferential speed is thus not usually feasible in an industrial context. If we were to minimize the exit cross section, this could lead to clogging of the nozzles depending on the level of contaminants in the coolant lubricant. Another option would be to use larger nozzle cross sections and increase the coolant lubricant throughput per machine to over 1800 l/min (depending on the grinding wheel width). However, to achieve this throughput, the filter system of a grinding machine would need to be more than double its size. In addition, the investment price would be three times higher and the cooling output would also multiply.
To avoid this effect, compromise solutions need to be sought. The aim of such efforts is to try and increase the exit speed to a sufficient level in order to maintain the existing coolant lubricant volume without the need to alter the associated periphery.
Modern grinding systems use Coolant Displays to show and save the coolant lubricant parameters, such as coolant lubricant exit speed, coolant lubricant volume flow and coolant lubricant pressure as control parameters.
Coolant lubricant jet quality
Another issue is coolant lubricant jet quality. Using the coolant lubricant exit speed as the parameter for describing how quickly the coolant lubricant leaves the nozzle is actually insufficient. As the coolant lubricant may fan out after exiting the coolant lubricant nozzle, thereby losing its momentum for penetrating the air cushion rotating with the grinding wheel, it is all the more important to equally take into account the jet quality. Here, it has been determined that the more form stability the jet demonstrates, the lower the speed losses. This in turn ensures that the coolant lubricant impacts the machining zone at around the same speed that it left the nozzle. If an adequate jet quality is ensured, it is possible to work with a lower coolant lubricant exit speed. This leads to lower coolant lubricant pressures in front of the coolant lubricant nozzle and thus to more cost-effective pumps and pressure supply systems.
Only a coolant lubricant jet which fans out needs to exit the nozzle at a significantly higher speed than it has when it impacts the machining zone. A jet which fans out also loses its level of effectiveness, as a majority of the coolant lubricant volume does not even reach the machining zone.
Precise nozzle positioning
Even the best nozzle and most ideal coolant lubricant exit speed lose their significance if the coolant lubricant sprays past the machining zone. Nozzle positioning systems are thus just as relevant in terms of effectively avoiding grinding burn as the above-mentioned aspects of coolant lubricant jet quality and coolant lubricant exit speed. Here, users often rely on welding wires or rulers to assist them in positioning the nozzle as exactly as possible prior to operation. During operation, however, positional accuracy can no longer be ascertained due to the high coolant lubricant spray volume. Laser-based positioning aids are also used to check and set the exact point of coolant lubricant impact.
Various opinions exist on the correct setting of coolant lubricant supply parameters, each of which may apply in certain applications. It is thus crucial to differentiate between the various options when selecting the correct parameters for your particular machining process. Basically, there are three regulating variables which are important here. These are: the coolant lubricant exit speed, the jet quality and nozzle positioning. A particular consideration when designing coolant lubricant supply is the conflict between the required exit speed and the coolant lubricant volume flow necessary for this. This generates associated costs, e.g. due to energy requirements and coolant lubricant processing.
Is your production process prone to grinding burn, or does it require a very high coolant lubricant volume flow? We can help! Grindaix GmbH is your experienced coolant lubricant system service provider. Our wealth of experience encompasses well over a thousand successful machine optimizations. When optimizing machines, we take the overall context of your coolant lubricant supply system into account, thereby achieving an efficient machining solution that is consistently free of grinding burn. Our specialists would be happy to advise you on coordinating your coolant lubricant supply and look forward to your inquiry!
Quellen:  H. W. Ott, "Schleifen wie die Profis" - Grundlagen der Schleiftechnik Ausgabe 2002, H. W. Ott & CO. Schleiftechnik CH-8330 Präfikon; H. W. Ott, "Kühlschmierstoff-Seminar - Schwerpunkt Schleiftechnik" Ausgabe 2002, H. W. Ott & CO. Schleiftechnik CH-8330 Präfikon; H. W. Ott, Dipl.-Masch.-Tech., Schleiftechnische Programme PGS I light, H. W. Ott & CO. Schleiftechnik CH-8330 Präfikon
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