Near-Dry Machining Cuts the Heat

Manufacturing Engineering, Oct 2007 by Tolinski, Michael

Systems for supplying minimum quantities of cutting fluid are slipping into shops that use traditional flood-cooling-though slowly

"Metal working fluids are not costly in themselves, but they can run up the costs of production if poorly chosen or improperly handled and controlled." This is a reasonable observation for today, even though this statement appeared nearly 60 years ago in a booklet on cutting fluids from Standard oil. The emphasis on fluid handling and control shows how conscious people were, even then, of the real costs of fluid use, and explains why they started developing the earliest methods for minimizing cutting-fluid use. The concept of near-dry machining-or more broadly, minimum quantity lubrication (MQL)-certainly isn't new.

In operations that use traditional flood-cooling methods, coolant handling creates more costs than the cost of the fluid itself. For the wet machining of aluminum castings, for example, coolant-related costs are 10-20% of total machining costs-roughly twice the costs of cutting tools. But of this, only 25% is for the lubricant itself; the rest goes towards coolant supply maintenance (42%) and operational energy costs (33%). These figures are based on a study from the Advanced Manufacturing Technology Development group of Ford Motor Co. (Dearborn, MI). (Ford is now a heavy user of near-dry machining at its Livonia, MI transmission plant, ramping up in 2005 from 41 initial MQL-enabled production machining centers to around 200 machines by next year.)

Because the small amount of cutting fluid that it uses is vaporized in the cutting process, near-dry machining can make a real dent in fluid-handling costs. Some automotive and aerospace component manufacturers have adopted MQL, but it generally has been slow to catch on in operations where it makes sense, at least in North America. But why is this, given traditional flood-cooling's liabilities of coolant degradation and disposal, worker health hazards, and shop contamination?

The simplified answer may be that for jobs that require coolant, flooding the workpiece and tool with a water/oil emulsion has always been effective, and just seems to make sense. "Change comes slow, and many engineers tend to benchmark their company's past best practices. For many companies this is flood and high-pressure coolant," says Kevin Howes of Bielomatik Inc. (New Hudson, MI).

But Howes emphasizes that other practices that are now common for fluid and chip handling were not initially popular. For example, machine guarding has been switched from fence guarding to complete dry-floor guarding, and flumes have been moved from the floor to inside the machine. "The next step in cost savings and protecting the environment is eliminating coolant wherever possible," he asserts.

Change has also been hindered by the specialized tooling and/or equipment that near-dry machining requires, says Randy Templin, vice president of Blaser Swisslube Inc. (Goshen, IN). First, a delivery system is needed that supplies a consistent, minimum quantity of lubricant at the tool/workpiece interface. MQL may require new cutting tools, spindles, and toolholders. And it typically requires new methods for handling chips and dust (even though MQL chips are free from coolant contamination). Moreover, at some companies, MQL isn't felt to be needed because their already "sound fluid-management practices utilizing high-quality flood coolants make for economical use with a limited waste stream," adds Templin.

Boeing might serve as an example of a company that is reducing its fluid use without completely adopting MQL. Boeing has reportedly approved a number of Blaser metalworking fluids that are compatible for stain-free machining of aircraft aluminum and titanium. Yet, Templin is not aware that Boeing is practicing anything more extreme than minimum quantity coolant (MQC) machining with water-miscible fluids, rather than MQL with straight oil. "MQC spray mist applications are common for face-milling long wing spars," says Templin.

But some companies have found adequate reasons for completely switching to near-dry machining, says Jeff Coffey, product engineer at Unist Inc. (Grand Rapids, MI). Justifiable cost savings is typically the first thing they seek; other issues usually have to do with improving employee safety and creating a cleaner environmental footprint. "Flood coolant is basically a mixture of water and oil. At the end of its life, it's not something you can just throw out in the backyard, any more than you can throw out an oil change in your backyard when you change the oil in your car."

The most straightforward approach to MQL is to use an external nozzle that delivers an air/oil mist to the tool/workpiece interface. Along with reducing costs, this reduces operators' exposure to flood-coolant ingredients that can cause skin dermatitis and breathing problems, adds Coffey. And even though some argue that flood cooling controls metal dust most effectively, he says flood cooling creates overall dirtier air than MQL systems that spray a mist of oil. "Most people assume it's the other way around," he observes, "but most studies show that the near-dry machining has better air quality."