CNC coolant concentration troubleshooting on the shop floor

I learned to stop treating coolant as background noise after one boring bar job kept giving us a dull finish on the last two parts of every batch. The inserts looked fine, offsets were steady, and the program had run for months. The real change was in the sump. The operator was topping up with premix, but the refractometer reading was all over the place because tramp oil was floating on top and…