Getting the Most from MQL

Here's a broad view of minimum quantity lubrication along with a closer look at using MQL in turning applications.

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Machining generates heat, and flood coolant is the common method to alleviate this heat. But flood coolant is a brute force solution that raises a host of issues of its own. Large quantities of coolant require specialized equipment to circulate, filter and test, and the purchase, maintenance and operation of this equipment can be expensive. The use of coolant may also cause thermal shock to carbide tools, resulting in microscopic cracks that weaken the tool and can ultimately lead to tool failure. Water-based coolants are also susceptible to bacterial growth. Biocides help with this issue, but who wants to breathe these biocides as they and the coolant are projected into the air during the cutting process? The fluid also often ends up in the wrong places, such as on floors, equipment and machine operators.

What if there was another way? While flooding the cutting zone with coolant may eliminate the symptoms of the problem (the heat), it does not address one of the main causes of the heat: friction.

Another Option

Minimum quantity lubrication (MQL) takes a different approach to dealing with heat in the machining process. Instead of using gallons of coolant (as flood coolant typically does through the course of an eight-hour shift) MQL uses only a few ounces of a high quality lubricant delivered in an atomized spray, reducing friction and forming a thin thermal layer between the tool and the chip. Most of the heat of cutting (as much as 95 percent) goes into the chip. The oil layer between the chip and rake face reduces the friction of the chip riding up the face, therefore reducing the heat generated. It also helps keep the chip from transferring its heat to the tool. Since the oil is consumed in the process, clean, dry chips are produced. The chips are worth more when recycled, and no fluid management or disposal strategy is necessary.

Weighing In

MQL brings some distinct advantages. According to Tim Walker, senior vice president at Unist, Inc., the method’s benefits fall into three main categories: environmental (good for the planet), enterprise (good for the shop and employees) and economic (good for the bottom line). While often the right choice, MQL is more sensitive to process and requires more care when setting up and using than does the use of flood coolant.

MQL provides a more environmentally friendly approach than traditional flood cooling for several reasons. The better MQL oils are non-toxic and biodegradable. Bacteria do not grow in oils, so no biocides are needed. Beyond that, it uses less than 1/10,000 the amount of fluid as does flood coolant. There is simply less—a lot less. There is nothing to be disposed of or recycled. There is no waste fluid. Recyclers and pumps are not needed, which reduces energy consumption. In comparison with flood cooling, MQL’s environmental footprint is invisible.

The reduced fluid quantities have a direct impact on the work environment as well. Floors stay cleaner (and safer). Equipment is not coated with layers of coolant that decreases the life and uptime of the machines. Surface finish of parts is usually improved, and because of reduced thermal shock, tool life increases. Shopfloor employees usually realize the added benefit of fewer skin irritation issues related to interacting with metalworking fluids, and they do not have to breathe the coolant and its additives.

While the price per ounce of MQL oils can be significantly higher than general flood coolants, because so much less is used, the overall price of the fluid per part is usually less. When combined with the savings from other areas (such as improved tool life, elimination of the need for equipment that maintains the fluids, and energy savings), MQL can have a strong positive impact on the bottom line.

Unfortunately, MQL does not come without its drawbacks. The application of flood coolant is much easier to get right. Because MQL uses much less fluid, external nozzles must be put and kept in just the right spot for it to work well. Through-tool application lessens these concerns, but it requires additional setup and expense. Another important consideration is chip evacuation, since MQL does not provide a stream of fluid to wash the chips away. A common solution to this is to use an air blow-off nozzle to clear the chips. Finally, workpiece materials must be considered, as some have proven to work with MQL more effectively than others.

MQL in Turning

Implementing MQL in a turning operation can provide numerous benefits, but proper nozzle placement can present some challenges. Where the nozzle head needs to be directed is dependent on the type of tool being used. When using a cutoff tool, lubricant should be sprayed from underneath the cutting tool up into the cut. Spraying on the top side of the tool does not work because the entire cutting edge is covered by the chip being formed.

When turning the end of the material or ODs, where the cutting tool is not entirely covered by the material, it is better to spray on the top side of the tool. The spray should be directed so that it hits the cutting edge and some lubricant lands on the material being machined. As the cutting edge moves through lubricant film on the material it will help lubricate the cutting edge.

The existing coolant ports on a turning machine are used to route the MQL fluids to the toolholder on the turret. All but one of the ports are blocked off, and a nozzle designated for the specific holder is created. As with any external MQL nozzle, the MQL fluids need to be applied to the cutting tool/workpiece interface, so this approach allows the customization and positioning needed for each tool to be done independently. When the nozzle is not positioned correctly, lubricant often ends up applied to the cut chips, resulting in smoking and premature tool wear because the tools are not getting the necessary lubrication.

Like all MQL applications, proper steps need to be taken to ensure that the end results are successful. Paying attention to and addressing all critical factors will help ensure the success of any MQL system. 

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