March 2009 eNews

MQL opens doors for environmentally responsible metal cutting

By Wally Boelkins, CEO
Unist Inc.

While much is being done in the metalworking industry to formulate, contain, handle and dispose of cutting fluids in an environmentally responsible manner, large scale reduction in fluid usage is also being accomplished through minimum quantity lubrication (MQL), also known as Near Dry Machining (NDM).

By adding MQL systems to CNC machines and selection appropriate tooling and processes, MQL is capable of creating significant economies and process improvements in metal removal operations, helping both the environment and the bottom line.

In the same way processes are determined for flood cooling operations, there are multiple factors to be evaluated to create an optimum MQL process combination. Speeds, feeds, materials used, tool design, fluid types, and chip evacuation are among the most important considerations. There are some basic usage design characteristics which result in successful MQL integration.

The most effective type of fluid (oil) for use in MQL is vegetable oil. Because vegetable oils are grown in the presence of oxygen, they have polar properties which cause them to adhere to metal surfaces in a more organized structure than mineral oils. The friction-reducing properties of vegetable oils are about double that of mineral oils. When the oil evaporates, it also absorbs heat, similar to water.

Due to refining processes involved in the manufacture of vegetable-based cutting fluids, there can be dramatic differences in the amount of fluid consumed to accomplish the same job. The amount of oxidation can also vary from not at all to high levels which may not be acceptable for some parts. Vegetable oils have a flash point of about 400-450 degrees Fahrenheit, as compared to about 250 degrees for mineral oils.

In MQL, the hotter the chips, the better to confirm that the lubrication process is taking place as well as chip evacuation, collection, and recycling. The heat makes stringy chips more brittle. The brittleness is a good indication heat is transferred to and retained in the chips.

In optimum conditions, the chips are dry or tacky, so they fall off into a collection area and can be swept, vacuumed, conveyed, etc. Dry chips often bring higher payment from recyclers. It is sometimes necessary to modify the inside of a CNC machine to compensate for the dry falling chips, allowing them to be directed properly.

When using an MQL lubricant on a cutting tool of any type, the key is to direct the fluid to the interface between the cutting edge and the workpiece. This cannot be overstressed. In flood systems, fluid is directed in such a way that it usually reaches every surface, ensuring it reaches at least those required. In MQL, the fluid applied in small amounts is directed only to where it is needed.

In MQL, compressed air is used to convey the small amounts of fluid to the desired location. However, “atomizing,” or creating an aerosol is counterproductive to the process. Tiny particles commonly do not “wet out,” but bounce off and remain airborne. To be effective, oil must “wet out.” Also, breathing tiny particles can be harmful to the lungs. Correct application may require new or reworked tools, nozzles, or nozzle positioners.

Optimum tooling geometries and coatings need to be determined when MQL processes are utilized. Whether to apply oil onto the tool surface or use a through-the-tool type application will be determined by required operations and materials. For coolant-fed tooling there is a variety of couplings and connectors that can be used to introduce oil into the tooling. Additionally, there are MQL systems designed to feed these tools which can achieve different application requirements.

The detailed considerations that must be addressed to correctly implement MQL sometimes make companies slow to adopt these processes. However, as regulation increases and manufacturers look for new ways to control cost, the advantages of MQL to a facility’s balance sheet make its evaluation worthwhile.

While lubrication with flood cooling usually occupies 15-18 percent of total manufacturing cost, the average cost of MQL is closer to 5 percent. This is due to the elimination of many activities required within the plant for dealing with large amounts of fluids and lubricants such as the following:

• Fluid Testing
• Fluid Mixing, Treatment, Replacement
• Fluid Disposal
• Chip Cleaning
• Part Cleaning
• Fluid Handling
• Machine Cleaning
• Sump Maintenance
• Pump Maintenance

Smaller amounts of fluid require less shipping costs and occupy less floor space. They also do not contaminate the air or cause skin irritation, and do not cause safety hazards on floors and other surfaces.

There is really no cutting machine that cannot be equipped with MQL. It may be that some machines, such as many automatic screw machines are too complex, employing “wearpoint” lubrication combined with tool lubrication. Swiss CNC turning machines are good candidates, as well as many other machining centers with horizontal and vertical operations.

Saws, routers, presses, drills, many grinders, and belt sanders as well as spinning machines also present good opportunities to utilize MQL. Some manufacturers now offer MQL as optional system when ordering a machine. Most in-service machines can be retrofitted for MQL at a relatively low cost.

When environmental concerns are combined with ROI considerations, a look at MQL should be a high priority for anyone in the business of cutting metal.

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Unist Inc.

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