Parameter 1: Concentration
The concentration of cutting fluid in water is the most critical and most neglected parameter. It is measured with a hand refractometer in less than 30 seconds. The correct range depends on the product and the operation: typically 5-8% for general machining, 8-12% for severe operations.
Below the minimum: loss of anti-corrosion protection, corrosion on parts and machine, bacterial proliferation. Above the maximum: foam, skin irritation in the operator, machine deposits, and higher cost without proportional benefit.
Recommended frequency: measure concentration at the start of each shift. Record the value. Adjust by adding concentrated fluid (never water alone) if below the minimum.
Parameter 2: pH
The pH of cutting fluids must be maintained between 8.5 and 9.5 to ensure anti-corrosion protection and limit bacterial growth. A pH below 8 indicates active bacterial contamination or depletion of alkaline inhibitors. Above 9.5, it can cause skin irritation and attack aluminium components.
pH is measured with indicator paper strips or a pH meter. If it drops below 8, add pH adjuster from the same fluid manufacturer (never caustic soda or ammonia directly).
Parameter 3: Bacterial contamination
Bacteria feed on the mineral oil in semi-synthetic emulsions and generate acids that lower pH, produce bad odour, and accelerate fluid degradation. Detection is done with rapid culture strips (Dip Slides) that give results in 24-48h.
If bacterial concentration exceeds 10⁶ CFU/mL, the fluid is at its limit and requires biocide treatment or replacement. Prevention is more effective than treatment: clean the tank at every change, avoid accumulation of metal fines, and maintain correct pH.
Parameter 4: Water hardness
Hard water (high concentration of calcium and magnesium) precipitates the emulsifying agents of the cutting fluid, forming calcium or magnesium soaps that deposit on the machine and reduce emulsion stability. Soft water (very demineralised) can generate foaming problems.
The optimal hardness for preparing cutting fluids is between 100 and 200 ppm (5-11 °dH). If mains water exceeds 400 ppm, consider a reverse osmosis installation or water softener.
Parameter 5: Foam
Excessive foam reduces the cooling effectiveness of the fluid, can cause tank overflow, and is a sign of a problem: excessive concentration, contamination with hydraulic oil or machine lubricant, very soft water, or excessive agitation.
Anti-foam is the last resort — the first step is to identify the cause. A correctly formulated and maintained fluid should not generate foam under normal machining conditions.
Cutting fluid maintenance requires 10 minutes per day (concentration and pH measurement) and a more complete weekly check (bacteria, foam, appearance). With that time invested, fluid life can be tripled and corrosion or surface finish rejects virtually eliminated.
Request a fluid maintenance protocol