Coolant performance depends heavily on water quality. Hardness, chloride content, and overall cleanliness determine how stable an emulsion becomes, how long it lasts, and how safely your machine operates. This guide explains the key water parameters and shows, step by step, how to mix coolant correctly for reliable CNC machining results.
A coolant emulsion is a mixture of concentrate and water - and water makes up more than 90% of its total volume. Because of this, water quality directly affects foam formation, corrosion risk, machine hygiene, tool life, and overall coolant stability. High-quality water leads to longer emulsion life, lower consumption, and more stable machining conditions.
Water hardness describes the amount of dissolved minerals — mainly calcium (Ca²⁺) and magnesium (Mg²⁺) — present in the water. These minerals interact with the coolant’s emulsifiers and influence foam behavior, stability, and lubrication performance.
If the water is too soft, the emulsion may foam excessively. If the water is too hard, insoluble compounds such as lime and lime soaps can form. These reduce stability, increase deposits, and accelerate corrosion.
Lime soaps arise from the reaction between anionic emulsifiers in the coolant and calcium ions in the water — directly impacting foam control and coolant efficiency.
Different hardness levels cause different behaviors in coolant systems. The ranges below help identify potential issues early:
Water hardness varies significantly across Germany. Southern regions typically have very hard water due to high mineral content. Northern regions often have softer water. Central and eastern areas show mixed hardness profiles depending on local geology.
Chloride concentrations also vary greatly and can influence corrosion behavior, coolant life, and stability. Some regions show naturally elevated chloride levels, making water treatment a consideration for certain workshops.
Because of these geographic differences, workshops should rely on local water data rather than general assumptions. Checking both hardness and chloride content ensures predictable behavior when mixing coolant.
Chloride ions (Cl⁻) play a significant role in both corrosion resistance and emulsion stability. High chloride levels increase the risk of corrosion on machine parts and workpieces. They may also disrupt the emulsion, causing it to separate over time.
Low chloride levels support longer coolant life and better protection for sensitive metals. If chloride content is high, reverse osmosis (RO) or deionized (DI) water can be used to reduce the chloride load before mixing coolant.
Even with acceptable hardness and chloride levels, contamination can destabilize coolant. Rust particles, dirt, chips, and residues from older emulsions significantly reduce coolant life and can accelerate bacterial growth. Clean tanks and mixing containers are essential for consistent performance.
Automatic mixing systems attach directly to the coolant container and use water pressure to draw in concentrate. They are ideal for workshops with higher consumption or continuous production.
Manual mixing works well for small workshops or low-volume requirements. Accuracy, however, depends entirely on the operator.
Why should concentrate always be added to water?
Because adding water into concentrate prevents proper emulsification and leads to unstable droplets.
What is the ideal water hardness?
Most coolants perform best between 8 and 14 °dH.
Why does my coolant produce foam?
Often due to soft water, contamination, incorrect concentration, pump agitation, or air leaks in compressed-air lines or pump systems.
How often should I check concentration?
Daily refractometer checks are recommended.
Can hard water cause damage?
Yes — it increases deposits, accelerates coolant degradation, and reduces corrosion protection.
Understanding water quality and mixing procedures is essential for achieving stable, long-lasting coolant emulsions. Good water, correct concentration, and clean equipment significantly reduce coolant consumption and improve machining reliability. Following these principles helps workshops avoid the most common problems and keep their CNC processes running smoothly.
