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There are three main functions of metalworking coolant in CNC machining:

The main functions of coolant

Cooling

Cooling lowers the temperature in the cutting zone, which gives a better surface finish (no burning, for example) and longer tool life. Effective cooling also reduces the energy needed for the cut, limits built-up edge and improves chip removal – a decisive factor for stable CNC processes.

Lubrication

Lubrication reduces friction between workpiece and cutting tool, allows higher cutting speeds and improves surface roughness. The benefit is clearest on tough, ductile materials. On brittle materials such as cast iron the effect is far less pronounced.

Flushing

Reliably flushing chips out of the cutting zone and the machine enclosure prevents surface damage such as scoring. Flushing also carries a technological function of its own — in drilling, in deep pockets on a mill, or when boring on a lathe. Effective chip removal reduces tool wear and improves machining quality overall.

One caution: in high-speed machining with carbide tooling, coolant supply must be continuous and sufficient. Interruptions cause thermal shock and micro-cracks, and eventually tool failure.

Further functions of coolant

  • Corrosion protection for workpiece, tool and machine
  • Material compatibility with seals, hoses and machine components
  • Foam control for a stable process and a clean machine environment
  • Emulsion stability and resistance to tramp oils and greases
  • Control of microbial load through proper maintenance and monitoring

How well a given coolant performs these duties depends on its product group — semi-synthetic, ester-based, synthetic or polymer-based. The comparison is in our article Types of metalworking coolant.

Occupational safety: what TRGS 611 actually requires

With water-mixed coolants, occupational safety is not an appendix — it is part of fluid maintenance. In Germany the governing documents are TRGS 611 and DGUV Rule 109-003 "Working with metalworking fluids". The core concern of TRGS 611: in water-mixed fluids, nitrite and secondary amines can form carcinogenic N-nitrosamines. So it is not the product alone that is assessed, but the condition of the fluid in the tank.

The figures that actually apply on the shop floor:

  • Nitrite in the used fluid: 20 mg/l. Above that, action is required — immediately, not at the next service.
  • Measure weekly as a rule. Only after three consecutive readings below 10 mg/l may the interval be stretched to up to four weeks. If the value rises above 10 mg/l again, it goes back to weekly.
  • pH weekly as well. The warning sign is not an absolute value but a sustained drop of more than 0.5 pH points against the freshly prepared mix. That is usually the first visible indication of bacterial growth.
  • Limits in the product itself: NDELA 5 mg/kg, NMOR 1 mg/kg. These figures are in the safety data sheet — for our products it is on the relevant product page as a download.
  • Workplace air: 0.2 µg/m³ for carcinogenic N-nitrosamines is regarded as state of the art.

In practice that means one nitrite strip and one pH strip per machine per week, documented. That is the whole effort. A shop that skips it notices a fluid going off by the smell — and by then the tank has to be dumped rather than corrected.

Airways and skin – the two real exposures

Two paths lead from the coolant to the people around it, and both can be controlled.

Airways. The exposure comes not from the liquid but from aerosols and vapour generated at high spindle speeds and with high-pressure coolant. What works: enclosed machining areas, functioning extraction, enough extraction run-on time before the door is opened, and a fluid that is not microbially loaded — a spoiled emulsion distributes germs through the room as aerosol. Concentration plays its part too: mixing too rich puts more active substance into the air without improving cooling.

Skin. The most common finding is not a true allergy but irritant contact dermatitis — the skin is damaged by repeated contact and by degreasing. Genuine sensitisation to individual components does occur, but it is the rarer case. The distinction matters because the countermeasures differ: irritation is answered with a skin protection plan (barrier cream before work, gentle cleaning, care afterwards), whereas confirmed sensitisation means the product has to be changed.

One point that is regularly got wrong in practice: gloves are not a solution near rotating parts, they are an entanglement hazard. At a running machine the skin protection plan replaces the glove, not the other way round.

And the uncomfortable truth behind both: most complaints originate not in the product but in the state of the mix. A correctly set, regularly checked emulsion is a far smaller burden than a tank nobody has measured for months.

Maintenance: what the monitoring actually runs on

The TRGS 611 requirements can only be met if the concentration is right in the first place. That takes two things in the shop:

  • A refractometer for the weekly concentration check — the Brix reading has to be multiplied by the product-specific factor, otherwise the number is wrong.
  • A mixing unit, as soon as more than one machine is supplied. Emulsion mixed by hand varies considerably in concentration, and that variation is behind many skin and tool-life problems.

Both are under Fluid Management. How water hardness and concentration interact, and in which order to mix, is covered in Mixing coolant and water quality.

The coolant in use must always match the operation and the workpiece material. Water-mixed fluids have to hold their chemical stability and physical properties over time, must leave no corrosion, and should work efficiently and with as little environmental impact as possible.

Coolant is a full and indispensable part of the process chain in CNC machining. It has a direct effect on productivity, tool life and machine availability — and, as TRGS 611 makes plain, on the health of the people standing at the machine.

Frequently asked questions

Explore the EASYCUT coolant range at CNCmarket to improve precision, efficiency and tool life in your production.

More technical articles on metalworking fluids are here.