Mixing Cutting Fluid
Properly mixed metalworking fluid forms a balanced, long-lasting emulsion that ensures good lubrication, cooling, and corrosion protection throughout the machining process. However, mistakes during mixing can lead to emulsion instability, cloudiness, foaming, and even rapid microbial growth. This is why mixing is one of the most important steps in preparing metalworking fluid before use.
Mixing Order and Basic Principle
When mixing a water-miscible metalworking fluid, the correct order must always be followed: add the concentrate to the water—never the other way around. When the oil-based concentrate is added to water, the emulsifiers can form an even and stable distribution of oil droplets throughout the water.
The opposite occurs when the mixing order is incorrect: the emulsion inverts, resulting in only small amounts of oil in the water while water droplets remain in the oil. In this case, the emulsion no longer performs as intended—its cooling and lubrication properties are reduced, and the fluid may break down quickly during use.
Droplet Size and Emulsion Stability
The droplet size of an emulsion has a decisive effect on its stability and performance. A fine emulsion is always more stable and reliable than a coarse one. Small oil droplets remain evenly distributed in the water, preventing the fluid from separating or becoming cloudy as easily.
Small droplets also offer other advantages:
- Better lubricating ability, because small droplets can more easily fit between the tool and the workpiece.
- A greater total surface area, which improves lubrication and heat transfer.
- Faster clarification, as tramp oil rises to the surface and solid contaminants settle to the bottom of the tank.
Achieving a fine emulsion requires thorough mixing, either by hand or, preferably, with a proper mixing unit that maintains a consistent flow and mixing ratio.
Temperature and Viscosity
Mixing temperature is an essential part of successful emulsion formation. Oil viscosity and droplet size change with temperature: cold water makes the oil more viscous and makes it more difficult to form a fine emulsion.
For this reason, mixing must always be carried out using room-temperature water, not cold water. The correct temperature ensures that emulsification works efficiently and that the droplet size remains small, improving the stability of the mixture and the accuracy of measurements.
Using a Refractometer and Measurement Accuracy
Emulsion quality and droplet size also directly affect the refractometer reading. A fine emulsion produces a clear, easily readable boundary on the refractometer scale, while a coarse-droplet emulsion makes the boundary indistinct and difficult to interpret.
In this situation, measuring the fluid concentration becomes uncertain, and the results are at best approximate. The reliability of the measurement also decreases if tramp oil has entered the system, as a refractometer cannot distinguish it from the actual metalworking fluid.
A refractometer measures the total oil concentration based on the optical refractive index, but it cannot distinguish between the oil in the metalworking fluid and tramp oil. This can result in the meter showing an apparently correct concentration even though the actual usable concentration of the fluid is too low.
Excessively low concentration reduces lubrication and corrosion protection while increasing the risk of microbial growth. Therefore, measurement results should always be evaluated together with the visual condition of the fluid and the behavior of the system.
Thorough Mixing—A Long-Lasting Emulsion
Careful and correctly performed mixing is the foundation of a long-lasting emulsion. Using the correct mixing order, room-temperature water, and a consistent flow produces a fine, clear, and stable emulsion that withstands extended use while retaining its lubricating and cooling properties.
Regular measurement and monitoring ensure that the mixture remains at the correct concentration and that the machining process stays reliable and efficient. Properly mixed cutting fluid not only protects the tool and workpiece—it is also essential for consistent, cost-effective production in the machine shop.
