CNC Coolant: Concentration, Pressure and Upkeep
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Coolant cools, lubricates and clears chips. Neglected, it does none of those well, and it starts to smell, stain parts and rust machines. Most problems come back to concentration.
Concentration
Water-soluble coolants are mixed with water, often somewhere between 5 and 10 percent concentrate for machining, depending on the product and the material. Check the maker's recommendation for your work.
- Too lean: poor lubrication, shorter tool life, rust on parts and machines, and faster bacterial growth.
- Too rich: wasted concentrate, foam, sticky residue and skin irritation.
Check it with a refractometer: put a drop on the prism, read the scale, and multiply by the refractometer factor on the coolant's data sheet. Water evaporates and concentrate doesn't, so the mix tends to get richer over time. Top up with a premixed lean mix, not plain water or straight concentrate.
pH, tramp oil and bacteria
- pH: many water-soluble coolants run around 9. A falling pH usually means bacteria are winning.
- Tramp oil: way lube and hydraulic oil leak into the sump and float on top, feeding bacteria and shortening coolant life. Skim it off.
- Bacteria: the rotten-egg smell on Monday mornings. Keep coolant circulating, remove chips and tramp oil, and keep the concentration up.
Flood vs through-spindle
Flood coolant from nozzles is fine for most milling. Through-spindle coolant goes through the tool to the cutting edge, which matters for deep drilling, deep pockets and tapping, where chips have nowhere else to go. Common through-spindle systems run around 20 bar (300 psi); high-pressure systems run 70 bar (1,000 psi) and more and can break chips that would otherwise wrap around the tool. High pressure needs coolant-fed tools and good filtration.
When not to flood
Some work runs better dry or with an air blast. Carbon fiber and other composites are usually machined dry with dust extraction, and some hard steels and cast irons are cut dry with suitable tools. In interrupted cuts, coolant that only reaches the edge part of the time can crack carbide through thermal shock.
Change it before it gets bad
Even well-kept coolant wears out. When it smells despite treatment, foams, or stains parts, clean the sump and machine properly and recharge it. New coolant poured into a dirty sump is reinfected quickly.
For more on machine care, see spindle bearings and signs of a worn ball screw.