Cutting fluid concentration drift causes 30-40% of tool life problems. A $200 refractometer prevents this.
Why It Matters
- Too dilute (less than 4%): BUE, rust, poor lubricity
- Too concentrated (more than 10%): foaming, residue, wasted money
- Right (5-8% typical): optimal
Measuring Concentration
Refractometer Method
- Calibrate with distilled water (set to 0)
- Take sample from cutting fluid
- Place 2-3 drops on prism
- Read scale through eyepiece
- Multiply by manufacturer factor (1.0-1.5)
Mixing New Coolant
- Use clean water (distilled or RO ideal)
- Add water to mixing tank FIRST
- Slowly add concentrate WHILE STIRRING
- Test concentration with refractometer
- Adjust by adding water or concentrate
Top-Up Procedure
As coolant evaporates, water leaves but oil stays. Pure water replacement makes mix MORE concentrated.
Correct top-up: mix new coolant at 50% target concentration (e.g., 4% for 8% target system) and add to existing tank.
Daily Inspection
- Visual: foam, oil slicks, debris
- Smell: healthy faint oil; rotten = bacteria
- pH: 8.5-9.5 (test weekly)
- Concentration: refractometer 5-8%
When to Refresh
- pH drops below 8.0
- Concentration unstable (more than 1% drift/day)
- Foaming despite anti-foam
- Strong rotten/sulfur smell
- Visible bacterial growth
Recommended Equipment
- Refractometer: $200-500 (Atago, Misco)
- pH test strips: $30/100
- Belt skimmer: $400-1500
See also titanium coolant guide and coolant decision guide.
