Despite overwhelming evidence that Minimal Quantity Lubrication (MQL) systems bring both financial and environmental benefits to metal cutting operations many organisations have been either reluctant or slow to adopt the technology.  Apart from questioning the initial outlay for such a system perhaps these organisations also have unjustified questions regarding quality of component finish.

The cost of purchasing, maintaining and disposing of cutting fluids has become a significant factor when evaluating a machine’s operating cost – a survey conducted on the production of the European automotive industry revealed that the expense of cooling lubricant comprises nearly 20% of the total manufacturing cost.  Obviously these are large scale operations but even on individual machines by introducing MQL systems that can deliver smaller, measured quantities of lubricant to the cutting tip there are initial cost savings to be made – which can easily outweigh the purchase of the system.

MQL is becoming the number one solution in helping organisations curb expenditure on cutting fluid since MQL uses waterless lubricants that are far more effective and give a finish which is the same, or better than wet machining

Results of a recent time-trial comparing wet cutting with MQL fed machining operations on the same components highlight the improved productivity efficiencies, nearly 200 and  300% for some operations.

Application	wet cutting	2-channel MQL system
Rough milling	134 sec	113 sec
Finish milling	150 sec	120 sec
Rough boring	93 sec	62 sec
Drilling	30 sec	19 sec
Reaming	106 sec	45 sec

 

What of the other benefits, typically there is an environmental advantage, since lubricant is used in a more controlled manner, and in very much reduced quantities [as low as 10ml per hour to prevent tool damage] there is little or no oil contaminant on the swarf.  This brings about very real cost savings in terms of recycling since there is less cleaning to be carried out.

Lubricate or Not?
Generally, lubricants are used to evacuate swarf from the surface, to cool the cutting tool, reduce thermal cracking and to cool the workpiece.  And there are instances where common industrial practice for machining of hardened steel parts is using a dry cut, swarf removal controlled by air blasts and vacuum.  Traditional beliefs indicate that completely dry cutting, as compared to flood cutting, lowers the required cutting force and power on the part of the machine tool as a result of increased cutting temperature.  Unfortunately achievable tool life and part finish often suffer under completely dry conditions.  Also the permissible feed and depth of cut have to be restricted.

Under these considerations MQL presents itself as a real solution in hard turning, achieving slower tool wear while maintaining cutting forces/power at reasonable levels, provided that the MQL parameters can be strategically tuned.

For alloys such as aluminium coolant is mainly used to stop the swarf plasticising under the extreme cutting temperature; when it plasticises, it adheres to the cutting material causing a breakdown in cutting performance.  The cutting fluid for machining aluminium using traditional coolant-based methods is typically more costly than for other materials because the coolant must have a higher concentration of oil in the mixture. Again a MQL system is a real alternative bringing with it all the cost and environmental benefits.

How Do MQL Systems Operate?

MQL involves lubricating the workpiece with a very fine mist of coolant which can cool ten-times more quickly than when delivered in liquid form. The coolant is typically a mixture of vegetable oil and some additives and can include rust inhibitors when required.

This lubricant is atomised into micron sized particles which stay suspended in the air; the particles preventing oil sticking inside the delivery pipes.

One of the more popular forms of MQL systems currently being accepted by metal cutting organisations is a two channel design where oil and air are delivered separately to the tool holder before being mixed and sent to the cutting tool.

In one popular design the lubricant (see attached schematic diagram) is transported via a pressurised ring-main to a rotary transmission unit, whereby a ‘quick valve’ controls the exact dose of oil. The quantity of oil is supplied through the lance which is located in the centre of the spindle, and on into the pipe nozzle in the tool holder. Compressed air is also fed to the mixing chamber via the rotary transmission through the second channel of the lance. This process produces a fine air-oil mixture which is then delivered directly to the cutting tip of the tool.

This particular two-channel design is engineered for high-speed, high productivity schedules, the use of higher viscosity oils giving better lubrication for longer tool life, creating an inherent reduction in service and maintenance of the cutting machine. It also enables quick tool changes to take place due to its minimal switch on/off time of only 0.1s.

Other key advantages include precise control and adjustment of the oil flow, no overdosing during tool changes [complies with EPA requirements], oil mist does not get into the environment; immediate surroundings stay clean as does the workpiece. It also results in swarf that is more easily and more cheaply disposed off, since no cleaning is required. For a machine designer MQL eliminates large-bore coolant delivery pipes, fire protection control and fluid-handling recovery.

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For further information contact:

Jim Hughes - Bielomatik
Tel: 01480 408500 Fax: 01480 403808
Email: jim.hughes@bielomatik.co.uk