Mixing cutting fluid
A properly mixed cutting fluid forms a balanced and long-lasting emulsion that ensures excellent lubrication, cooling, and corrosion protection throughout the entire machining process. Errors made during mixing, on the other hand, can lead to instability, cloudiness, foaming, and even rapid microbial growth in the emulsion. That’s why mixing is one of the most important steps in preparing cutting fluid before use.
Mixing Order and Basic Principle
When mixing water-soluble cutting fluid, you must always follow the correct order: add the concentrate to the water—never the other way around. When the oil-based concentrate is added to water, the emulsifiers can form a uniform and stable distribution of oil droplets in the water.
If the mixing order is reversed, the opposite happens: the emulsion inverts, and only small amounts of oil disperse into the water, while water droplets remain in the oil. In this case, the emulsion can’t function as intended—its cooling and lubricating properties are reduced, and it may break down quickly during use.
Droplet Size and Emulsion Stability
The size of the droplets in an emulsion has a decisive effect on its stability and performance. A finely dispersed emulsion is always more stable and reliable than a coarse one. Small oil droplets stay evenly distributed in the water, so the fluid doesn’t separate or become cloudy easily.
Small droplets also have other advantages:
- Better lubrication, since the small droplets can more easily get between the tool and the workpiece.
- A larger combined surface area, which boosts both lubrication and heat transfer.
- Quicker clarification, as tramp oils rise to the surface and solid contaminants settle to the bottom of the tank.
Achieving a finely dispersed emulsion requires careful mixing, either by hand or preferably with a proper mixing unit that maintains a consistent flow and mixing ratio.
Temperature and Viscosity
The mixing temperature is a crucial part of successful emulsion formation. The oil’s viscosity and droplet size change with temperature: cold water makes oil more viscous and hinders the creation of a fine emulsion.
That’s why mixing should always be done with water at room temperature, not cold. The correct temperature ensures efficient emulsification and keeps droplet size small, which improves both the stability and measurement accuracy of the mixture.
Using a Refractometer and Measurement Accuracy
The quality of the emulsion and the size of the droplets directly affect the refractometer reading. A finely dispersed emulsion provides a clear, easy-to-read boundary on the refractometer scale, while a coarse emulsion makes the boundary blurry and hard to interpret.
This makes it difficult to measure the fluid’s concentration accurately, and results are at best approximate. Measurement reliability is also reduced if tramp oil gets into the system, as the refractometer cannot distinguish between actual cutting fluid and tramp oil.
The refractometer measures the total oil concentration based on the optical refractive index, but it does not differentiate between the oil in cutting fluid and tramp oil. This can lead to situations where the meter shows a seemingly correct concentration, even though the actually usable concentration of the fluid is too low.
An under-concentrated solution impairs lubrication and corrosion protection, and also increases the risk of microbial growth. This is why measurement results should always be evaluated together with a visual assessment of the fluid’s condition and the system’s behavior as a whole.
Careful Mixing – Long-Lasting Emulsion
Careful and proper mixing is the foundation for a long-lasting emulsion. By using the correct mixing order, room temperature water, and even flow, you can achieve a fine, clear, and stable emulsion that lasts through extended use while maintaining both its lubrication and cooling characteristics.
Regular measurement and monitoring ensure that the mixture stays at the right concentration and your machining process remains reliable and efficient. Properly mixed cutting fluid not only protects the tool and workpiece—it’s also essential for consistent, cost-effective production in the machine shop.
