MQL: Lean and Green: Drilling Performance

The benefits of minimum-quantity lubrication—which is suitable for many types of machine tools—are numerous

While flood coolants have traditionally been used almost exclusively in machining operations, the concept of minimum-quantity lubrication has been steadily gaining momentum during the last several years. MQL, also known as near-dry machining, replaces flood coolant with a minute amount of high-efficiency lubricant applied precisely to the cutting tool/workpiece interface. The process has proven effective for machining all nonferrous metals and most grades of steel.

All images: Unist

MQL uses precise fluid application coupled with a high-quality lubricant, such as in this step-drilling operation.

The most common lubricants for this process are biodegradable vegetable oil derivatives, which, because of their polarity, are known for their extensive friction-reducing properties. Unlike mineral-based and synthetic fluids, vegetable-based fluids are developed in the presence of oxygen, which causes them to organize and align better as they bond with the surface of metals, providing superior lubricity. These fluids, when properly applied, significantly reduce the friction and corresponding heat created when cut metal slides over the cutting tool before breaking off and becoming a chip. Reducing that friction not only extends tool life, but also eliminates the need for cooling the tool with flood coolant. Additionally, the amount of fluid used is often less than 8 oz. per shift, which is either consumed in the cut or remains as a light film on the part and chips.

The Benefits of MQL

The benefits of MQL are almost all a direct result of eliminating flood coolant, including:

• Significantly longer cutting tool life from reduced friction, ranging from 25 to 500 percent.

• Reduced production times due to the higher feed rates MQL allows. Depending on materials and processes, feeds can be 25 to 100 percent faster.

• Clean, nearly dry chips that require no post processing and generate high prices for recycling.

• No treating and replacing old or rancid coolant

• No disposal costs for old coolant

• No coolant skimmers required

• No coolant testing required

• Cleaner floors and a safer work area

• Clean finished parts with a light film of protective oil

• The entire process is environmentally friendly, i.e., there is no fluid to treat, recycle or dispose of. The fluid itself is 100% biodegradable, and is consumed in the process.

There are many ancillary costs associated with the use of flood coolants—typically 15 to 18 percent of the total manufacturing cost of machined parts. Switching to MQL can typically reduce that number to around 5 percent, while increasing tool life and feed rates (see items one and two above).

Clean, near-dry chips from MQL applications can yield higher prices for recycling than flood-coolant applications.

Because it is different than traditional cooling and lubricating methods, MQL has been slow to gain acceptance. Machine builders as well as machinists are accustomed to the way things have always been done. As a result, the initial markets to readily accept MQL were applications where flood coolant simply wasn’t feasible. Although this is changing somewhat, there are still industry reservations.

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