Dry or Wet Machining?
When milling, a major question is “Which is better: dry or wet machining?”
To further complicate the decision, new-to-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, and therefore, this familiar question requires careful and informed consideration.
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 in across multiple processes for both cooling and lubrication purposes.
All cutting activities generate unwelcome friction between the surfaces of the tool being used and the workpiece it is in contact with. The presence of coolant ensures that the friction between the two surfaces is reduced and by doing so makes the removal of a metal layer by the tool, a great deal easier (lubrication).
During the machining process, the temperature in a cutting zone becomes extremely high. The application of coolant lowers the cutting zone temperature and reduces the thermal load on the tool (cooling). In addition, the use of coolant contributes to improved chip evacuation and also reduces the concentration of metal dust in the area of a manufacturing unit. Therefore, the coolant supply is directly connected with several important tasks:
When performing an interrupted cutting, milling process, the cutting edge of the tool comes under a cyclic thermal load; the ambient temperature is dramatically changed when the edge enters into, then leaves the workpiece. The tool’s cutting edge is exposed to severe heat stress comparable to repeatable thermal shock. Cemented carbide, today’s main tool material, is a sintered product of powder metallurgy, and it is sensitivity to thermal shock load which destroys cutting edges. When using this type of tool, the application of a coolant supply may increase such “shock treatment” and unintentionally contribute to the failure of the tool’s edge. Extreme temperatures result in plastic deformation of the cutting edge, whilst the presence of temperature differences leads to thermal cracks. This situation becomes even more exaggerated in high-heat generation milling situations, such as machining difficult-to-cut materials or making rough passes with significant machining allowance. As explained, although wet cooling delivers undoubted benefits, it also has the capacity to produce several major disadvantages within the milling process.
In many cases the use of an efficient coolant supply is not only reasonable but it is absolutely necessary, as (in many cases without coolant) productive milling would be impossible. This is seen when machining materials such as titanium and high-temperature super alloys, austenitic and duplex (austenitic-ferritic) stainless steels, or even special-purpose alloyed hard cast iron, where friction and heat generation are considerable. The flushing effect of a coolant supply significantly improves chip evacuation and reduces re-cutting, particularly when milling deep pockets or narrow slots.
Intensive heat generation, when using traditional wet cooling, produces a vapour film in the cutting zone that intensifies heat transfer. An HPC jet, directed exactly to the cutting zone, effectively penetrates this film and overcomes the unwelcome obstacle. It also improves the cutting action by changing the shear-plane angle and creating thin, manageable chips. Taking advantage of HPC techniques is only possible only when using appropriate machine tools, or by modernizing existed machines.
Dry Machining and Other Options
Ignoring cases where the use of cutting fluid is essential, machine operators must appreciate that if the use of wet cooling brings disadvantages, the elimination of coolant will result in noticeable progress.
In these cases, dry machining offers promising opportunities. As previously explained, rough milling with significant stock removal results in extremely high-heat generation. In this case, a coolant supply may be destructive due to critical thermal stress. In contrast, when dry rough milling, the temperature of the insert’s cutting edge will remain high, although 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 light cuts of high speed milling (HSM), especially for workpieces with hardness values of HRC 45 and above, cooling by air is strongly recommended. In the above examples, the absence of wet coolant also considerably increased tool life.
Other important factors to consider are cooling economy and work safety. If cutting tool investment in batch production is estimated at 3% of a part cost, the share connected with wet coolant (purchasing, maintaining, filtration, etc.) according to a variety of sources can reach 16-17%.
Furthermore, prolonged exposure to wet coolant by operating personnel may cause health problems and industrial illnesses. Many national and international standards and published advice related to safety and environmental control, make increasingly tougher demands related to cutting fluids.
Another available option is milling with minimum quantity lubrication (MQL), sometimes called “near-to-dry”. When using this technique, the tool’s cutting edge works inside a mist formed from oil and compressed air that is spayed directly into the cutting zone. Depending on the design of a machine tool and milling cutter, the mist can be delivered externally or internally (via 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, and therefore the lubrication is more effective. In addition, the resulting machined workpiece and chips are almost dry (“near-to-dry”), making their cleaning much easier and quicker. Moreover, the working area of the machine tool also remains relatively dry, enabling various parts of the machine tool to work under better conditions and improving their effective life.
So – dry or wet?
The answer is, it depends on the specific application (a workpiece material, operation, etc.) and available machining tools. 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 vast majority of modern indexable mills have internal channels enabling the supply of coolant directly via the tool body. This allows more effective delivery of the coolant directly to the cutting zone. For face mills of previous generations without coolant channels, ISCAR proposes a clamping screw with an adjustable nozzle – in many cases it not only improves coolant supply but also contributes to better chip evacuation.
When exploring milling cutters intended for HPC and cryogenic machining, the body of the cutter should be designed accordingly.
The shape of the internal channels, their size and sealing elements (if necessary) should ensure the maximal free flow of coolant without any disturbance. The most important elements are the nozzles that are mounted in the outlets of the channels, as they optimize the effect of the high-velocity coolant jet and direct it exactly to the necessary area.
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, which is equally suitable for productive milling with coolant and without it. Some coatings are more effective for wet machining, whilst others provide dry machining advantages.
Although indexable carbide inserts are available with coatings to suit all applications, the field of insert coating layers is so complex it is worth an entirely separate discussion.