Cleaning a CNC Machine: System Clean and Coolant Change

Where the smell comes from

An unpleasant smell at the machine is a clear sign of advanced microbial growth. When bacteria or fungi destabilise the emulsion, the trade term is that the coolant has "gone off" — on the shop floor it is simply said that the coolant stinks. At that stage a full system clean is unavoidable if process reliability and workplace health are to be restored.

The uncomfortable part first: a shop that measures pH and nitrite weekly usually never reaches this point. A sustained pH drop of more than 0.5 points is the early warning, and it arrives weeks before the smell does. The values required by German TRGS 611 are covered in The functions of coolant. Once the mix has gone off, only a complete coolant change with a system clean will help — and it runs like this:

Step 1: Start with a chemical system clean

Before the old emulsion is drained, a system cleaner has to be added. Its job is to dissolve biofilms chemically in lines and channels you cannot reach.

Calculating the amount of cleaner:
Dosage follows the formula: Quantity (litres) = tank volume / 100 * K
- Moderate contamination: K = 0.5 (0.5 % concentration)
- Heavy contamination: K = 2.0 (2.0 % concentration)
Example: a 400-litre system with heavy microbial load needs 8 litres of system cleaner.

The cleaner should circulate for 8 to 24 hours during normal operation to reach full depth of action.

Step 2: Safety and prevention

When handling microbially loaded fluids and chemical cleaners, DGUV Rule 109-003 "Working with metalworking fluids" applies in Germany:

• Liquid-tight protective gloves (nitrile/butyl)
• Safety glasses with side protection
• Respiratory protection where aerosols form
• Use of skin protection and aftercare products

A note on the gloves, because this is regularly confused: they belong to cleaning work on a machine that is switched off. During machining, gloves near rotating parts are an entanglement hazard — there, the skin protection plan protects you, not the glove.

Step 3: Draining, disposal and mechanical cleaning

After the soak time, the emulsion is fully extracted. Used water-mixed coolant is hazardous waste. In Germany it normally falls under waste code AVV 12 01 09* ("halogen-free machining emulsions and solutions"); synthetic machining oils fall under 12 01 10*. The asterisk marks it as hazardous. Disposal goes through a licensed waste contractor with the corresponding documentation — the old emulsion belongs neither in the drain nor in general waste.

Then comes the mechanical removal of lime soaps, swarf sludge and biofilm from the tank and the machine enclosure, using brushes, industrial vacuums and scrapers.

Step 4: Cleaning the system components

Critical components such as the tool changer, spindle noses and coolant nozzles are cleaned with a mildly alkaline cleaner. Drying them immediately prevents corrosion on bare machine parts.

Step 5: Flushing to neutralise

To neutralise cleaner residues, the system is filled with a 2–3 % "flushing emulsion" and circulated for around 20 minutes. This intermediate step is decisive for pH stability in the new fill.

Step 6: Preparing the new emulsion properly

Once the system is fully flushed and completely drained, the new fill can be prepared. Use suitable mixing water and mix the concentrate exactly as stated in the technical data sheet. Order of mixing, water hardness and the product-specific refractometer factor are covered in detail in Mixing coolant and water quality.

Concentration for the first fill: practice has shown it pays to set the concentration at the upper end of the manufacturer's recommendation — for turning and milling, 8 to 10 % rather than the usual 6 to 8 %. This builds a stable alkaline reserve and safely neutralises any residual moisture left in the system. A stable pH right from the start is the best insurance against fresh microbial growth. The value is checked with a refractometer; across several machines a mixing unit is worth having — emulsion mixed by hand varies in exactly the concentration that matters here.

Choosing the right fluid: pick a product that matches your material range. The EASYCUT range is built as a series with a growing material range:

• Mostly cast iron and unalloyed to low-alloy steel — BASE201.
• Mixed programme with stainless steel and some aluminium — UNI301, low-foaming and stable in the process.
• Aluminium as the main job — PRO302, the same basis as UNI301 and additionally tuned for aluminium to avoid staining.

Which type of coolant suits which operation in general is explained in Types of metalworking coolant.

---

Results: before and after

Machine enclosure and working area

---

Conclusion on process hygiene

A thorough system clean prevents the new fill from being re-infected straight away. Continuous monitoring of concentration, pH and nitrite extends emulsion service life considerably and lowers running costs — and pushes the next full coolant change a long way out.