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Wet or Dry?

Wet or Dry?
Issue Time:2017-06-15

Coolant selection depends on application, material, tool choice

When milling, which is better: dry or wet machining?

Throughout the world of contentious machining, the issue of whether to use coolant supply (wet) or no coolant supply (dry) is a common subject of discussion.

To further complicate the decision, near-dry or minimum quantity lubrication (MQL) cutting techniques may represent a successful compromise and therefore provide an efficient and effective answer to the troublesome question.

As in many areas of machining, making such choices is not easy, so this familiar question requires careful and informed consideration.

Machining Wet

Wet coolant, cooling mixture, cutting lubricant, cutting fluid, and coolant are all commonplace shop floor terms that are familiar to all involved in machining. Each expression refers to a fluid, which is used across multiple processes for both cooling and lubrication purposes.

All cutting activities generate friction between the surface of the tool and the workpiece. The presence of coolant helps ensure that the friction between the two surfaces is reduced, and by doing so, makes the removal of a metal layer by the tool easier.

During the machining process, the temperature in a cutting zone rises. The application of coolant lowers the cutting zone temperature and reduces the thermal load on the tool. In addition, the use of coolant contributes to improved chip evacuation and also reduces the concentration of metal dust.

Coolant supply is directly connected with several important tasks:

  • Improving machining accuracy and surface finish.
  • Increasing productivity.
  • Enhancing tool life.
  • Improving environment control.

When performing an interrupted milling process, the cutting edge of the tool comes under a cyclic thermal load, and the ambient temperature is dramatically changed when the edge enters into and then leaves the workpiece.

Cemented carbide -- today’s main tool material -- is a sintered product of powder metallurgy and is sensitive to thermal shock. When this type of tool is used, the application of a coolant supply may increase this shock and unintentionally contribute to the failure of the tool’s edge.

Extreme temperatures cause plastic deformation of the cutting edge, while varying temperatures create thermal cracks. The situation becomes even more exaggerated in high-heat milling situations, such as machining difficult-to-cut materials or making roughing passes with significant machining allowance.

In many cases, the use of an efficient coolant supply is not only reasonable but necessary. Examples include machining of materials such as titanium and high-temperature superalloys, austenitic and duplex (austenitic-ferritic) stainless steels, and special-purpose alloyed hard cast iron.

Also, the flushing effect of a coolant supply significantly improves chip evacuation and reduces recutting, particularly during milling of deep pockets or narrow slots.

Machining Dry

In cases when using coolant brings disadvantages, eliminating the fluids should result in noticeable progress.

As explained, roughing with significant stock removal results in extremely high heat generation. In this case, a coolant supply may be destructive to the tool because of thermal fluctuation. In contrast, during dry rough milling, the temperature of the insert’s cutting edge will remain high, but, if the machining data is set correctly, the tool temperature will remain at an acceptable level. The tool temperature will vary within a relatively narrow range that will not lead to thermal shock.

As for the light cuts of high-speed milling (HSM), especially for workpieces with hardness values of 45 HRC and higher, cooling by air is strongly recommended.

Other important factors to consider are cooling economy and work safety.

Typically, cutting tool investment in batch production is estimated at 3 per cent of part cost. Adding wet coolant (purchase, maintenance, filtration) can bring the investment up to 16 to 17 per cent. When no cutting fluid is used there is no need for a coolant pump, a coolant recycling system, or other machine tool accessories, further reducing total costs.

Also, prolonged exposure to wet coolant by operators may cause health problems.

Many national and international standards and published advice that relate to safety and environmental control are becoming increasingly stringent in regards to cutting fluid use.

Not Quite Dry

Another available option is milling with minimum quantity lubrication (MQL).

When this technique is employed, the tool’s cutting edge works inside a mist formed from oil and compressed air spayed directly into the cutting zone. Depending on the design of a machine tool and milling c

utter, the mist can be delivered externally or internally (through the cutter).

The main function of MQL is to lubricate the edge during the cutting action. Because of this, the machining process consumes only the necessary quantity of oil, making lubrication more effective. In addition, the resulting machined workpiece and chips are almost dry, making cleaning much easier and quicker.

MQL also increases tool life. The working area remains relatively dry, enabling various parts of the machine tool to work under better conditions and improve their effective life.

Cryogenic Cooling

Another coolant option is cryogenic machining. Using a coolant at extremely low, cryogenic temperatures drastically reduces the possibility of overheating and allows better performance and extended tool life.

Combining this principle with MQL creates a more effective minimum quantity cryogenic machining method in which low-temperature coolant (such as liquid nitrogen) is supplied directly to the cutting zone through the tool.

Alternatively, some processes propose applying carbon dioxide delivered under pressure to the cutting zone.

In both of these methods, the particles of cryogenic coolant vaporize from the tool edge and in doing so remove heat.  However, it is obvious that despite the clear benefits, cryogenic cooling is not a cheap method, and it also requires the use of specially designed machine tools.

The Tool

Dry or wet? The correct answer today continues to be dry and wet, depending on the application. Nevertheless, the manufacturers of cutting tools take into account customer requirements and provide them with tools that will ensure productive machining with the use of different cooling methods.

The majority of modern indexable mills have internal channels enabling the supply of coolant directly through the tool body. This allows more effective delivery of coolant directly to the cutting zone.

Last but not least is the indexable carbide insert itself.

Although the insert’s edge performs the cutting, how does it relate to the coolant method?

The key to understanding this relationship is the insert’s carbide grade and, more specifically, its coating, which provides a barrier for heat penetration. The coating must be resistant to the thermal shock that causes the destructive effect.

Understandably, there is no “universal” coating that is equally suitable for milling with coolant and without it. Some coatings simply are more effective for wet machining, while others provide dry machining advantages.