Cutting fluids spoil, leading to issues either technically, with occupational health, or often both simultaneously. So, why do the fluids actually spoil?
Water-dilutable cutting fluids are a complex mixture of various substances. Often, there are up to ten different compounds, each necessary to ensure the proper and effective function of the fluid. Due to the large number of compounds, there are also multiple contamination mechanisms. Here are some key examples:
Here are the four key examples:
Bacteria and other microbes:
Cutting fluids typically consist of organic compounds, meaning they provide both nutrients and water. At the same time, their operating temperature is around 20°C. Together, these factors create excellent conditions for bacteria and other microbes, such as yeasts and molds, to grow and multiply in the cutting fluids.
Microbes use the fluid's compounds as nutrients, essentially breaking down the fluid's components by consuming them. At the same time, they produce various compounds, such as acids, which create additional problems in the fluids.
Most bacteria are aerobic, meaning they require oxygen. When bacteria are present in fluids, they multiply rapidly, and their numbers often reach up to 10 million bacteria per milliliter of fluid. In a 1,000-liter tank, this amounts to ten trillion bacteria. No wonder the properties of the fluids deteriorate.
Most of these bacteria are so-called gram-negative bacteria. The cell walls of these bacteria are endotoxins, which pose a significant occupational health risk as they can cause various inflammatory conditions in the body.
The most significant occupational risks are associated with microbes and endotoxins in cutting fluids, as well as heavy metals and other contaminants in the fluids used during machining. (Occupational Health Institute, KAMAT database)
If the fluids begin to smell, it usually indicates the presence of anaerobic bacteria in the tank, which do not use oxygen in their metabolism. While these bacteria are almost always present in the fluids, they remain inactive as long as there is oxygen in the fluids. These bacteria not only degrade the properties of the fluids but also cause noticeable odor problems.
Often, the mechanism of spoilage starts with a significant increase in aerobic bacteria that consume oxygen from the fluid, allowing anaerobic microbes to grow, at which point the fluid begins to smell and is often already in poor condition. Fortunately, this situation can usually be prevented by keeping the fluid comprehensively maintained and responding to issues promptly.
Leakage oils
Fluids almost always contain at least some slideway oil, but often also hydraulic, spindle, and corrosion protection oils. The effects of these oils depend significantly on the type of fluid, but the most common issues include oxygen depletion due to oils accumulating on the surface (see anaerobic bacteria in the previous point), emulsification into the fluid, and microbes feeding on additives. Additionally, these oils have a significant effect on the pH value, or the acidity of the fluid.
Regardless of the fluid type or mechanism, it is essential to remove leaked oils quickly and efficiently so the fluids can function properly and last longer.
pH level
The pH level is a crucial measure for the functioning of fluids. Many of the functions of the compounds in the fluids are pH-dependent, meaning they only work correctly within a specific, often precisely defined, pH range. The pH level roughly indicates how acidic or basic the fluid is.
Most cutting fluids operate near a pH level of 9.0. The reason for specifying the first decimal place is important. As mentioned, the pH level generally needs to remain within precise limits, and "precise" means exact. If the normal pH is 9.0, a value of 8.7 already requires significant corrective actions, and at a value of 8.5, the fluid is often already spoiled.
Fluids often contain so-called buffering agents, which means that the pH level does not drop with the addition of the first drop of acid. Conversely, when the pH begins to decrease, the buffering agents are already depleted, and the decline can be rapid. Additionally, a falling pH often triggers a vicious cycle: as the pH drops, bacterial growth accelerates, microbes acidify the fluid more quickly, and the bacteria proliferate even faster.
There are several methods to prevent and correct pH level drops. Due to the mechanism of buffering agents, effective use and especially prevention require precise, preferably real-time measurements. Often, pH strips are still used, but their accuracy is unfortunately insufficient, with a best-case resolution of about 0.3 units.
Electrically charged particles
Charged atoms and molecules, or ions, often cause headaches when the fluid's lifespan starts to extend. Ions primarily accumulate in the fluid from metals and tap water. Because ions carry charges and many cutting fluid components also carry charges, they react effectively with each other. Simply put: positive and negative charges attract, combine, and form new compounds.
Many components of cutting fluids are electrically charged, meaning that metal ions circulating in the fluid will bind to them and form new compounds. In practice, this means that the compound no longer functions as it should, and for example, the oil droplet diameter can increase manyfold.
Ion accumulation in the fluid can be significantly reduced with effective filtration methods and by reducing water hardness when necessary.
Can anything be done about these?
Fortunately, yes. The most important aspect of problem-solving, and especially prevention, is having an accurate situational picture. In practice, this means measuring the condition of the fluids regularly and comprehensively.
A weekly measurement is much better than none, but the more frequently, the better, as changes can often happen very quickly. Preventing issues requires a rapid response, which is recommended because solving problems is much more difficult than preventing them.
Generally, correcting issues should begin as follows:
- Ensure that the fluid concentration remains as close as possible to the recommended values, as this helps prevent many problems.
- Adjust the fluid concentration and pH level based on measurements. Respond to changes quickly, as smaller corrective actions will suffice.
- Clean the fluids of chips and particles, even down to the micrometer scale, as quickly as possible. Ion issues are proportional to particle surface area and time, not mass. Larger chips and impurities accumulate in the tank, promoting microbial growth.
- Leaked oils (and other surface-floating compounds, which will be addressed in future writings) should be removed as quickly as possible. Use effective and reliable methods.
- Measure and ensure that the water is not too hard. If necessary, purify the water of ions, bacteria, and other impurities before adding it.
- Continuously circulate the fluid to ensure oxygenation.
- Measure, measure, and measure. Record the results and respond in time.
It is also essential to monitor the development of measurement results and understand how changes affect the cutting fluid. Based on this analysis, the correct actions can be determined to maximize the safe lifespan of the fluids.
Why do cutting fluids spoil?