Why machine dry?

Chips generated during near-dry machining are not soaked with lubricant and coolants reducing the cleaning necessary for recycling.

Foundries cast metal products into final shapes, but these are often released as raw, rough, or intermediate products for further downstream processing.

Because the machining of these pieces generates a lot of heat, standard machine shop operation involves flooding the tooling and workpieces with high quantities of fluid to lubricate the surfaces, cool down the workpieces, and remove chips.

Near-dry machining or minimum quantity lubrication (MQL) uses either no fluids at all or only a small amount, applied directly to the cutting surface to reduce friction.

Studies show that flood coolant uses up to 60,000 ml of fluid per hour, while MQL uses less than 500 ml per hour.

Improvements production costs, occupational health, and environmental sustainability are all possible by moving away from flooded coolants to near-dry technologies, given the right circumstances.

How MQL Works

The first step in successfully machining a casting is choosing a metal foundry that casts high-quality input material. The quality of casting surfaces, as well as the homogeneity of their internal structures, will have measured effects on the quality of machining possible.

Alongside the choice of source material is the selection of tooling. Tool selection is a very technical process that requires an understanding of material compatibility (between the tool and the workpiece) as well as knowledge about coatings, cutting data, and lubricant types.

Once materials and tools are selected, machining can begin. Dry machining is the process of machining a workpiece without any fluids. Near-dry or MQL machining applies a tiny amount of fluid to reduce friction. Sometimes this fluid is delivered via a mist spray through the tool itself, while at other times it can be dripped onto the surface of the workpiece.

Certain materials, like magnesium, always have been processed by dry machining because they react with water, eliminating any possibility of using coolants. Special tool designs, materials, and coatings have therefore been developed to machine magnesium, and this approach has now been applied to other materials.

Scientists and tool manufacturers have researched the tool/workpiece interface by studying the compatibility of different metals, the generation and removal of heat, the effect of speed of rotation on friction, and surface finish. Investigations have also been carried out on lubricating fluids that are most effective for near-dry applications. Even entire machines have been redesigned so that metal shavings naturally fall away from the work site, taking heat with them. Compressed air and liquid nitrogen also are now being used as alternative means to provide cooling.

Contrary to expectations, running machines faster and dry with new-generation tools has actually prolonged tool life and reduced heat generation in the workzone. The rapid temperature shocks that can result from flood cooling can even damage these tools.

Tooling Advances Air Near-Dry Machining

Specific advances have been made in tool manufacture including the materials used, coatings applied, and other design considerations. Some advances are:

• Tungsten carbide, a robust tool material that tolerates heat and can be run at high speeds.
• Titanium nitride, a hard surface coating that provides thermal insulation.
• Edge honing, a technique used to strengthen the cutting edge of tools so that they do not crumble in the absence of lubrication.
• Tool geometries, which have been optimized to reduce friction.

So what are the benefits of switching to near-dry machining?

The primary drivers for a switch from flood cooling to near-dry machining are cost, health, and environmental factors.

Cost Benefits. Costs of flood coolant and lubricant are on average 5 to 15 per cent of the total cost of production. As manufacturers seek to be competitive in the marketplace, moving to near-dry machining makes economic sense.

Occupational Health Benefits. Large-scale use of chemicals for machining adds occupational risk to the manufacturing environment. Eliminating these chemicals reduces the potential for spills and injuries as well as health hazards from long-term exposure to these materials.

Environmental Benefits. Regulations regarding the safe disposal of used chemicals and metal shavings that have been washed with chemicals are making it more expensive for manufacturers too. Not only is the cost of disposal high, but the risk of fines and adverse environmental consequences for accidental violation also adds to the pressure manufacturers feel to consider near-dry machining.

Not only is the volume of lubricating oil reduced significantly in near-dry machining, but the type of oil is also shifting to a more environmentally friendly alternative. Most near-dry oils are plant- or ester-based, which do not have the negative environmental impacts that hydrocarbon products do.

Also, the metal chips and shavings generated from machining are not soaked with lubricant and coolants. This reduces the processing required to clean the discarded metal for recycling. Cleaning waste metal produces more liquid waste, and eliminating the cleaning step minimizes the total waste generated from machining.

From a practical perspective, flooded lubricant systems tend to splash liquid beyond the limits of equipment being used. Workshop spills must be carefully managed to prevent chemicals from being washed away into clean water systems. The move to near-dry machining creates a workshop environment that is clean and controlled and significantly reduces the risk of accidental chemical discharges.

Operational Benefits. Other unexpected advantages are related to the machining process itself. Surprisingly, the advances in near-dry technology have led to an improved surface finish of products and longer tool life.

Multinational companies with extensive machining applications in their manufacturing chain are using near-dry machining throughout their organizations to make a step change to their cost structure. Ford Motor Co. is one such organization that has implemented near-dry machining in six of its facilities. There is much to be gained for mass producers, which seem to be at the forefront of adopting the new technology.

With every new technology comes a period of observation and evaluation from industry players who have used the old technology for many years to good effect. However, the adoption of near-dry machining by larger organizations like Ford is causing other companies to take notice.

Without a doubt, near-dry machining is a trend that will grow in influence over the machining sector. The tangible cost, health, and environmental benefits are just too substantial to ignore. Eventually these outcomes will coax machine shop out of their comfort with the status quo and into a new era of environmentally friendly and cheaper machining.

Early adopters will have an advantage over others as they gain experience with the new technologies. Companies that are already transitioning to near-dry machining can set their schedules, choose their priorities, and evaluate vendor relationships. Early adopters of these technologies and practices might enjoy a competitive advantage; now might be the time for a thoughtful and deliberate transition to near-dry machining.