Intensive research and development in universities and in the industry has supported the broad industrial application of high-performance machining with minimal quantity lubrication (MQL). The benefits of MQL include typical increases in productivity of up to 30 percent and reduced service costs compared with conventional full-flood cooling lubrication. The purchase of an MQL system requires significantly less than that of a cooling lubricant system.
Until recently, cooling lubricants were essential to manufacturing operations for certain functions of the machining process, such as cooling, lubricating and chip removal. These processes are now under review as cost pressures (on average 10 percent of workpiece-related costs are cooling lubricant costs) combine with growing environmental awareness and the need to improve employee satisfaction through cleaner work areas.
The alternative: the use of minimal quantity lubrication.
The SKF LubriLean systems fully satisfy current requirements for reliable manufacturing, such as the timely and adequate availability of lubricant at the cutting unit. The targeted dispensing of lubricant directly at the cutting site lubricates the friction points between the tool, workpiece and chip (fig. 1). The metering of the lubricant generally amounts to much less than 50 ml/h, depending upon the cutting method, the material to be cut and the selected machine settings.
Subjectively, this low quantity of lubricant produces dry workpieces as well as dry chip. This results in considerable cost savings, since it is no longer necessary to clean the workpiece and/or chip.
In the one-channel aerosol technology, a fine aerosol with a homogeneous droplet size of approximately 0.5 micron is produced from lubricant and compressed air in a container using a special nozzle system. This technology also enables reliable lubrication of the tool blade, even with rotational speeds of more than 20,000 r/min, or through the turret of turning centres of automatic lathes. This is achieved without the presence of excess lubricant – as is customary with two-channel systems – and the associated contamination of the machining room. The VarioSuper and DigitalSuper version of SKF LubriLean systems have been developed for this purpose. With these systems, the "oil quantity" and "compressed air" parameters can be adjusted directly through the machine tool control during the automatic tool change (Fig. 2).
A leader in the field of system monitoring
Until very recently, MQL systems could only be monitored indirectly and incompletely. It was not possible to monitor the aerosol flow directly. Compressed air, fill level and oil flow sensors were used to
determine the state of the MQL system. Now, SKF provides a system that not only generates the required amount of high-quality aerosol but also monitors the supply of this aerosol to the lubrication point. The heart of this monitoring system is the AM 1000 aerosol monitor (Fig. 3) that is positioned in the aerosol path as close to the machining point as possible and uses an optical measuring procedure to determine the number of oil droplets proportionate to air volume.
An industry standard interface can be used to transfer this analogue measured value, for example, to the machine tool control unit for analysis. As an alternative, the aerosol monitor can be connected to the DigitalSuper version of the SKF LubriLean system via CAN bus.
During start-up of the MQL machining process, the measured values can be stored as reference values in the machine control unit. If other values are subsequently measured during production, this indicates changes in the overall MQL system, which can then be investigated before they have a negative effect on production quality. This significantly increases process safety because relevant deviations have thus far only been detected very late, based on defective workpiece surface quality or even tool breakage.
High-performance MQL systems generate aerosols with droplet diameters of less than one micron to prevent most of the droplets from being separated from the air by centrifugal force when the aerosol passes through the rotating spindle.
Modern MQL devices do not control the individual valves for aerosol production, but enable selection of program numbers. Thus the only thing the customer has to do to vary the produced aerosol volume is convey a program number to the MQL device. Figure 4 clearly shows the change in aerosol density as a response to changing the program numbers.
SKF Lubrication Systems subjects its MQL systems to a continuous endurance test under a rigorous three-shift operation at its own facilities. Feedback from the production area is immediately conveyed to the Engineering and Development department, where actions make sure that the MQL device technology satisfies user requirements – the highest level of performance with the simplest operation.
The SKF lubrication systems service professionals around the world are trained on a regular basis. Practical training by SKF specialists is carried out primarily at the SKF Lubrication Systems plant in Berlin, Germany. Here, 20 machine tools are equipped with diverse LubriLean MQL systems. The training programme includes consultations with the SKF lubrication systems machining specialists, conducting practical machining trials to ascertain tool-related settings for the different MQL systems to achieve optimum efficiency.
One-channel MQL technology has been proven to provide reliable processing. The savings in investments and operating expenses are significant compared with a standard cooling lubricant system and have become indispensable in machine tools for use in the automotive, aircraft, tool- and mould-making industries. Tool design is almost standardized, especially when it comes to outside and inside geometry, as well as the design of the interfaces between tool holder and tool. It is now possible to meet the user demand for MQL system monitoring.
The use of MQL machining will increase as this technology is implemented whenever possible in new production lines, especially in the automotive industry. It is estimated that the market share of minimal quantity lubrication will be an average of 27 percent in large-scale serial production five years from now.
Almost 1 million tonnes of oil are currently used for conventional cooling and lubrication of machining processes in the machining industry in Germany alone (Fig. 5).
The use of MQL technology can significantly reduce the consumption of these enormous quantities of oil, thereby considerably reducing the impact on the environment. In addition to this, a considerable increase in productivity can be achieved through better cutting results as well as in extended service life of the tools.
Use of MQL at SKF
Within the scope of the integration project "Introducing MQL in SKF plants," a joint campaign with SKF VOGEL, the SKF Process Development Department and the SKF Manufacturing Development Centre (MDC) in Gothenburg, pilot projects for the introduction of MQL were set up.
Two pilot projects are examples of sustainability and improved productivity. One project is the lathing of cages during bearing cage production at SKF in Gothenburg; the second project is the lathing and parting of rollers in roller channel 2, also in Gothenburg.
For cage production, four lathes had to be converted from cooling lubricant to MQL. The reason for this project was the fast ageing of the cooling lubricant, which was accompanied by very unpleasant odours, and, as a result, frequent changing of the cooling lubricant together with the associated costs.
For the tests, one of the four lathes was equipped with a LubriLean system VarioPlus and separated from the cooling lubricant supply.
The MQL system was integrated into the machine control of the machine tool. This way, the MQL supply could be switched on at the requested time, as was done previously with the cooling lubricant. In addition to improving the surface quality, another goal was to adjust the MQL system in such a way that the required coolant quantity was used up completely during the process so that no residues remained on the workpiece. Thus, no add-itional washing procedure was required.
The project successfully demonstrated a reduction in cooling lubricant. With conventional cooling lubricants, the average consumption per year was generally 400 litres; using MQL, consumption could be reduced to 10 litres per year. This meant a cost reduction of 97 percent, making a significant contribution to environmental protection. The return on investment was only 0.67 years. As a consequence of these convincing results, the three other lathes were also changed over to MQL.
An additional, very important impact was the satisfaction of the respective machine operators. The conversion to MQL provided them with relief from unpleasant odours.
The second SKF roller production project involved upgrading a machining centre, which was already operating with minimal quantity lubrication from another manufacturer, with a LubriLean MQL system. The objective was to increase productivity, improve surface quality and significantly reduce lubricant consumption.
For the test a special MQL tool holder for parting of rollers was developed and implemented in collaboration with the tool manufacturer Sandvik. For minimal quantity lubrication, a LubriLean VarioSuper system was used. The tests returned the following results:
Because of these positive experiences, all the other machines in roller channel 2 will be upgraded.
These projects clearly indicate the savings potential that can be tapped by introducing this now fully developed technology from SKF.
For comprehensive information pertaining to SKF LubriLean minimal quantity lubrication systems, visit: www.skf.com/lubrication.