Sunday, August 4, 2013
Minimum Quantity Lubrication Systems (MQL) – How to reduce energy and consumable costs without comprising machining performance.
MQL as an acronym (Minimum Quantity Lubrication) is becoming slowly more understood in the machine tool industry as its clear technical and cost benefits start to be delivered for users looking to reduce consumption costs and improve technical and environmental factors for their machining processes. Previously MQL had limited applications and was frequently associated with specialist processes such as turning or specialist mould and die finishing with ball end-mill applications. This article aims to explain the basic principles of the system and offers some recommendations on specific savings which may be achieved by users of MQL compared to conventional coolant systems. MQL utilises a soluble and/or bio-degradable cutting oil delivered in a fine mist to the machining area via either externally delivered nozzles or via integrated tooling to the work piece. It is used as an alternative to either dry machining where tool life is reduced or in place of high pressure flood coolant. Firstly, the disadvantages of flood coolant are numerous such as the high running cost of coolant pumps, maintenance of coolant systems, cleaning of the work area of the machine tool and particularly in summer or warm conditions the unpleasant odour sometimes emitted by poorly maintained coolant systems. It is probably a common viewpoint in the machining industry that a frequent observation of machine shops using flood coolant require a high level of maintenance not only in monitoring coolant condition but also clearing up spills from leaks and so on that present a health and safety hazard in the work place. While therefore it would be ideal to implement dry machining the effect on tool life, heat management and process performance means that this is not possible in most cases. Where dry machining is successfully employed, the process performance can be inhibited by the need to balance productivity against surface finish and tool life and maintenance costs. Therefore even where users have a defined dry cutting process, MQL can potentially assist with extending tool life and increase machining performance in terms of surface finish, cutting forces and process capability. To analyse the cost benefits of MQL, it is firstly worthwhile to study the typical categories of costs associated with the running of a conventional machine tool employing flood coolant in a production environment. When most people are asked about their opinion on which element contributes to the most electricity consumption of a typical CNC machine tool, most people may guess that it is either the main machine spindle (Either work holding or tool holding) or an axis servo motor. In reality these elements are much less, according to a study made by Kuroda shown in Figure 1, which identifies for mid-sized CNC machines that the main spindle only consumes about 12% of the total power consumption and around 7% for main axes feed from the servo motors. Comparatively however, the coolant system including chiller, pumps, and so on can contribute over 60% of the total machine power consumption costs. While this figure sounds relatively high, consider that in reality the machine axes are moving not all the time particularly if point to point positioning is being employed and modern servo motor technology actually produces very efficient motors from a power consumption perspective when not under load. Consider also that a spindle will not always be active either only during actual cutting operations ideally. Therefore a coolant pumping system with associated chillers needs to be ran fairly consistently despite the machine not actually cutting or machining and perhaps also off shift if coolant temperature needs to be maintained for optimum machining performance during actual shift hours to reduce idling time. Imagine thenn also larger users of CNC machine tool maybe also with centralised cooling system and you can start to visualise the potential cost implications of flood coolant systems in the overall consumption costs of machining operations.