ISO 4406: The number your machine tool manufacturer uses to define acceptable coolant — and what happens when you miss it
Each increment in the ISO 4406 scale doubles the particle count. The difference between ISO 16/14/11 (clean grinding) and ISO 20/18/15 (contaminated grinding) represents a 16× increase in abrasive particle concentration at the CBN wheel interface.
At ISO 20+, the failure cascade is predictable and documented: micro-scratches accumulate → surface finish deviates → wheel glazes → cutting forces increase → thermal gradient rises → grinding burn. This typically develops over 2–4 weeks before appearing in quality data. By that point, 3–6 weeks of wheel life has been consumed at the contaminated rate.
The Molitech position: cleanliness class is not a nice-to-have. Makino publishes cleanliness targets for their grinding systems. Junker does the same. The question is not "should we filter better?" — it is "are we hitting the specification the machine was designed for?"
By Liwen Zhuang, Co-Founder · 8 min read