Introduction on metalworking fluids

In the process of machining, the metalworking fluid thus plays a crucial role beyond pure lubrication. While conventional lubrication techniques focus on reducing wear, heat generation and friction, the fluid has an entirely different task in metalworking. Here, the aim is not only to minimise wear, dissipate heat and reduce friction, but above all to facilitate the intended shape change during machining. This subtle nuance emphasises facilitating the desired shape change, which marks a fundamental difference between lubrication and metalworking.

Choosing the right metalworking fluid

Selecting the most appropriate fluid for a specific application is a complex matter. Numerous factors play a role in this choice. While laboratory tests can initially help in evaluating environmental impact, health aspects, corrosion resistance and wear limitation, thorough field testing is essential to ensure that all specified requirements are met. However, setting up an effective field test is challenging as controlling all influencing factors, including human interaction, is a complex task.

The various machining processes have similarities but also significant differences, both in the degree of machining intensity and in the cooling and lubrication requirements. Different product types are chosen for each process. The role of metalworking fluids also differs for each process. These machining processes can be divided into four distinct groups:

Turning and milling, boring and drilling, planing and sawing are some of the machining processes where the use of water-dilutable fluids such as emulsions, micro-emulsions or synthetic fluids is common. These fluids play a crucial role in temperature control, particularly to limit heat generation from the cutting tool.

For operations such as threading or tapping, pure oil with strong anti-wear and EP (Extreme Pressure) properties is often used. The purpose of this fluid is to prevent adhesion of work material to the many cutting edges and prevent localised welding between the tool and the workpiece.

When grinding, a water-dilutable fluid is usually used, such as an emulsion, micro-emulsion or synthetic fluid. Sometimes pure mineral oil is also used. The purpose of these fluids is to flush out fallen grit and serve to cool the workpiece surface to prevent unwanted structural changes.

Pure mineral oil is often used in honing, but sometimes a water-dilutable fluid is also used. The function of this fluid is to promote mixed lubrication between the cutting tool and the workpiece, thus achieving the desired surface profile.

Microbiology of metalworking fluids

When emulsifiable products are used, the growth of microorganisms can occur at the interface between "entrained oil" and emulsion, leading to unpleasant odour problems, blockages and reduced quality of operations. To minimise this, it is crucial to minimise emulsion contamination with lubricating oil. Floating oil should be removed thoroughly, while a continuous circulation system with aeration is preferred to avoid an anaerobic environment.

Efficient cleaning, limiting contamination with chip material and avoiding waste in the reservoir are essential steps to prevent micro-organism growth. In extreme cases, the use of biocides can help, but this should be done in moderation to avoid resistance. Maintaining a clean system by continuous removal of contaminants, including thorough cleaning when changing or switching fluids, is key to minimising this issue.