Set coolant concentration to 7-10% for longer tool life

Most machinists eyeball coolant concentration, but that's a gamble with tool life. The sweet spot for water-soluble coolants is typically 7-10% concentration by volume. Below 5%, you lose lubrication and corrosion protection, accelerating flank wear and built-up edge. Above 12%, the coolant becomes too slippery, causing chip welding and reducing heat transfer.

Use a refractometer to measure concentration accurately. Check weekly because evaporation and drag-out change the mix. For example, in a high-volume CNC milling operation on steel, maintaining 8% concentration reduced insert changes from every 4 hours to every 8 hours.

If you're cutting aluminum or other gummy materials, lean toward the higher end (9-10%) to prevent built-up edge. For stainless or high-temp alloys, 7-8% balances cooling and lubricity. Always follow the manufacturer's recommended range, but these guidelines give you a practical starting point.

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