Implementing minimum-quantity lubrication principles
In the machining process, cooling lubricants play an important role in carrying away metal chips, cooling the workpiece and lubricating the area between tool and workpiece. The lubricant usually runs in a closed circuit, but sometimes it gets carried out with the metal chips, lost because of leaks, or degraded during regular operations. Changing to minimum-quantity lubrication (MQL) can save significant resources and money.
All procedures where a geometrically defined cutting edge is used such as milling, turning and drilling can be done with MQL. It is advisable to use standard materials when first starting with MQL; grey cast-iron (GG25-GGG40) works well because the high graphite content offers additional lubrication, but all non-ferrous metals like aluminium and steel materials are also suitable. With optimal process design, almost all materials can be processed with MQL.
Using this technique, the metal chips are not carried out by the lubricants, so alternative discharge methods are needed. This can lead to higher demand for compressed air, but the savings realised by reducing the cooling lubricants typically off-set that added cost.
Tooling processes with high feed, high cutting speed and short dwell times are optimal for MQL. Nevertheless, an individual review of the specific process is recommended before introducing the new technique.
Choice of lubricant
For MQL, the lubricant must have a high-lubricating effect and be thermally resistant. Synthetic esters, which have a high viscosity and flash point, are suitable. If the separating effect is paramount, fatty alcohol can be used. Both lubricants are non-toxic, so any vapours occurring during the process do not affect staff safety.
Choice of MQL-system
There are two ways of implementing the lubricant at the point of use: either from external nozzles or internal canals in the tool itself. The external supply is the cheaper solution because you are still able to use existing tools. Retrofitting machines is easy enough but not suited to all processes. External application is better for machines with a fixed operating point. For the internal supply, it is sometimes necessary to optimise the whole system. Therefore, you should analyse existing machinery and check which components can be fitted with internal lubricant systems.
If you retrofit current cooling lubricant systems to MQL, the total investment will be about € 12 000 (€ 6000 for purchasing new components and € 6000 for converting old systems). Existing cooling lubricant systems cost about € 936 per month, whereas MQL costs only € 202 per month. That represents a payback of less than two years!