Mixing cutting fluid correctly
It's an oil-in-water emulsion, not a solution. The order matters.
by Tom Bradshaw, Technical
Always add OIL TO WATER, not water to oil. Aim for 5–10% concentration depending on the operation: 5% for general turning, 8% for milling, 10% for tougher tapping and broaching. Use a refractometer rather than guessing — under-mixed fluid foams and rusts the bed; over-mixed fluid dries out and leaves a sticky residue on the part.