Minimum quantity lubrication - an alternative for machining?
Minimum quantity lubrication (MQL) is a machining technology for supplying a machining task with lubricant that requires considerably smaller quantities of lubricant than conventional cooling lubricant supply strategies. The savings in lubricant are so great that the name “minimum quantity lubrication” was coined for this technology.
Today, “flooding cooling” is usually established, in which the machining task is oversupplied with cooling lubricant and the high volume flow used results in high costs for the preparation of the cooling lubricant.
Advantages and disadvantages of minimum quantity lubrication
Depending on the process, minimum quantity lubrication (MQL) has a number of advantages and disadvantages that should be compared and evaluated before implementing MQL in a machine tool:
Advantages
+ Lowest lubricant consumption (a few ml per hour)
+ No need for cooling lubricant filtration and recirculation
+ Very targeted supply via the tool possible
+ No biocides and fungicides required
Lubricant quantity for minimum quantity lubrication
With minimum quantity lubrication, the lubricant is lubricated with an aerosol (cooling lubricant mist mixture) instead of a jet of coolant. The aerosol is generated using compressed air, similar to the principle of a carburetor, before being introduced into the machining zone. Due to the very low quantity of lubricant used, a number of cost-causing factors can be eliminated compared to conventional cooling lubrication strategies, as the machining process is virtually dry:
- Minimization of the amount of lubricant used
- Elimination of filtration and reconditioning
Areas of application for minimum quantity lubrication
Minimum quantity lubrication has been used in more and more machining processes step by step and as the technology has improved. Machining processes with geometrically defined cutting edges are generally better suited for the application of minimum quantity lubrication than processes with geometrically undefined cutting edges. While simple processes, such as sawing, were initially equipped with minimum quantity lubrication, it is now possible to guarantee cooling lubrication that meets the requirements for milling and turning. A key factor here is the development of tools with integrated lubricant supply. The lubricant is fed directly into the machining zone through fine channels within the tool. It is also common practice to feed the lubricant mist to the tool from the outside, which is significantly less complex.
Minimum quantity lubrication for grinding
Although minimum quantity lubrication is already being used successfully in machining processes with geometrically defined cutting edges, grinding is a machining process in which minimum quantity lubrication cannot yet be used economically. There are several reasons for this, which are due to the peculiarities of grinding:
- The lubricant aerosol cannot penetrate the air cushion rotating with the grinding wheel, is therefore deflected and does not reach the grinding gap.
- There is a particularly high thermal risk (grinding burn) when grinding, which requires reliable cooling by removing the heat from the workpiece. This is not possible or only insufficiently possible with MQL. The heat input into the workpiece can be reduced by lowering the cutting power. However, this is not economical due to the significant increase in machining time.
- It is currently not possible to equip grinding wheels with channels that bring the lubricant directly into the machining zone, similar to the tools used in processes with geometrically defined cutting edges.
Optimal cooling lubricant supply during grinding
However, the failure of minimum quantity lubrication for grinding does not mean that machining processes with geometrically indeterminate cutting edges have to be uneconomical and associated with immense cooling lubricant requirements. Sophisticated coolant supply systems can achieve both significant coolant savings and an increase in cutting performance. In addition to the actual feed, the downstream filtration and treatment of the cooling lubricant can also be optimized. The multiple use of the treated cooling lubricant eliminates a significant proportion of the replacement costs.
Grinding with the lowest possible amount of cooling lubricant
The most important thing in all cooling lubricant saving measures is the sensible coordination between the quantity of lubricant to be saved and the process parameters of the machining task. This is because the greatest cooling lubricant savings will not bring any added value in terms of process efficiency and cost-effectiveness if the cutting performance is reduced at the same time.
Process optimization by applying the exact amount of cooling lubricant required in combination with the necessary process parameters (lubricoolant outlet speed, lubricoolant volume flow) is the most suitable procedure for optimizing the lubricoolant supply during grinding. This closes the niche in lubricoolant consumption between minimum quantity lubrication and flooding cooling.
By using Grindaix nozzles, the above-mentioned process parameters can be achieved particularly reliably and are therefore also particularly suitable for optimizing the cooling lubricant system. By coordinating all components of the cooling lubricant filtration system (pumps, valves, filter principle, etc.), significant savings can be achieved despite the large quantity of cooling lubricant in circulation compared to the MQL. By operating cleaning and rinsing nozzles, cooling lubricant can be used to clean the machine of machining residues during the main/off-peak times. This significantly reduces the amount of manual cleaning that is inevitable with MQL.
Summary
Minimum quantity lubrication (MQL) is a method for supplying lubricant during metal cutting. Only very small quantities of lubricant are required, as the lubricant is introduced into the machining zone in the form of a cooling lubricant aerosol. Minimum quantity lubrication is a so-called loss lubrication, the lubricant can only be used once and is not recycled. MQL is primarily used economically in processes with geometrically defined cutting edges (e.g. milling, drilling). For processes with geometrically undefined cutting edges, however, economic application is rather questionable. Nevertheless, these processes can also be optimized with regard to conventional lubrication with cooling lubricant and significantly improved in terms of efficiency and cost-effectiveness.
Grindaix GmbH offers a comprehensive product portfolio ranging from analysis to complete machine optimization, so that machining tasks with geometrically indeterminate cutting edges are supplied with the exact quantity of lubricoolant required. This is currently much more sensible and, above all, more economical than switching to minimum quantity lubrication.
