Cutting Fluids Application In Machining Processes

F.W. Taylor was the first person in the 20th century, who used water for the first time to cool the machining process and decided it extended the cutting tool life, a wide variation of cutting fluids has been used with this and different goals. Nevertheless, in the last 10 years, much has been accomplished intending to limit the practice of cutting fluids in the production, due to the expenses associated to the fluids, ecological issues, human health and so on. 

To reduce the application of cutting fluid, two methods have been intensively researched: cutting with a minimum quantity of fluid (MQF), where a very moderate volume of fluid is crushed in a stream of compressed air and cutting without any fluid (ecological machining also known as dry cutting).

When MQF is used, the steam, the mist, and the oil smoke are deemed unfit sub-products, since they cause an increment in air pollution. In Germany, the highest density of pollutant in the air in mist form is 5mg/m3 and when the contamination is in steam oil form the limit is 20 mg/m3. This requires an effective exhauster system that ensures this compulsory air contamination control. But this could be far more beneficial than dealing with difficulties of dumping used cutting fluids.

In 1992, the amount of dumped soluble oil from manufacturers were around 60% of the total volume of oils used in production processes, about 1.151.312 ton. That denotes a notable amount of money, varying from 7,5% to 17% of the production costs per part, even higher than the expenses related to tooling. Hence, even though the tooling costs may be raised by the application of either dry cutting or MQF, due to the increment in tool wear, the entire production cost may be cheaper when linked to the traditional process where

Cutting Fluids Classification
There are many ways of organizing cutting fluids and there is no regularity to install one of them within the industries. Perhaps the most common classification groups the products like the following classification: 

Air
Neat Oils:

  • Composed oils;
  • Extreme pressure oils (EP);
  • Fatty oils; 
  • Mineral oils;
  • Multiple use oils

Water Based Cutting Fluids:

  • Chemical solutions (or synthetic fluids);
  • Emulsions (soluble oil);
  • Water.

Applications Where Cutting Fluid Offers Benefits

Cutting with low strength tools, like high-speed steels, requires the application of cutting fluid. This is due to the case that the heat produced during cutting raises a lot the tool temperature, decreasing its mechanical strength and, thus, making easier the appearance of plastic deformation and total failure. In this case, cutting fluids decrease the heat, not letting the tool to lose its strength and making possible the use of moderately high cutting speeds.
Drilling, milling, threading, broaching with high-speed steel tools are common examples of these operations where the application of cutting fluids is crucial. Another significant utilization of cutting fluid is in operations where low surface roughness and/or tight dimensional thresholds are required. In these circumstances, the oil guarantees a great surface finish and the cooling fluid ensures the solid tolerances because it bypasses the thermal expansion of the workpiece.

When drilling materials that generate discontinuous chips, like grey cast iron, cutting fluid application becomes fundamental, mainly in deep drilling. In this case, the main cutting fluid function is to carry the chips away from the cutting zone, what other wise could cause chip jamming and, consequently, a possible tool breakage.

Usually continuous cutting (turning, boring, etc.) of any metal (and also several non metallic materials) made with carbide tools (with or without coating) is carried out with application of a cutting fluid. In such cases, the fluid increase tool life and, therefore it may reduce of costs. Machado et al (1997) showed how cutting fluids are important in turning AISI 8640 steel with cutting speeds up to 400 m/min and P35 carbide tools coated with three layers of TiC, Al2O3, TiN. They experimented several kinds of cutting fluids. Figure 7 shows the tool life results obtained by them. It can be seen in this figure that when dry cutting is used tool life is much shorter than when any kind of cutting fluid is used.