How to properly replace and use coolant?

What is a water-miscible metalworking fluid?

Water-miscible metalworking fluids are concentrates that are diluted with water before use. Depending on the formulation, they can form an emulsion, a micro-emulsion or a true solution.

Formulations can also contain corrosion inhibitors, lubricity additives, anti-foam agents, stabilisers and other components. Some systems use biocides, while others are designed around different microbiological-control strategies. The exact composition and permitted treatment methods should be checked in the product documentation.

Why do water-mix coolants deteriorate?

Fluid life is affected by the whole machining system rather than by the concentrate alone. Common causes of premature deterioration include:

• incorrect working concentration,
• poor make-up water quality,
• bacterial or fungal contamination,
• excess tramp oil,
• high sump temperature,
• metal fines and process contamination,
• poor circulation or stagnant zones,
• adding incompatible chemicals or cleaners,
• filling fresh fluid into a contaminated sump,
• using a coolant that does not match the material or machining operation.

How to prepare a fresh coolant mix correctly

FUCHS recommends preparing the solution outside the machine where practical and adding the coolant concentrate to water. This helps the emulsifiers form the intended mixture and reduces the risk of creating an unstable inverted emulsion.

Step 1. Check the required concentration

Use the product TDS or supplier recommendation for the specific machining operation and material. Do not copy a concentration from another coolant simply because the products look similar.

Step 2. Check the make-up water

Water hardness, pH, dissolved salts and microbiological quality can influence foam, corrosion protection and fluid stability. HSE recommends checking input water quality and consulting the metalworking-fluid supplier about the appropriate water requirements.

Step 3. Add concentrate to water

For manual mixing, start with the required amount of water and add concentrate while mixing. A proportioning mixer is preferable for repeatable large-volume preparation.

Step 4. Verify the actual concentration

Use a refractometer and the product-specific refractometer factor where applicable. The refractometer reading is not automatically equal to the true percentage concentration for every product.

Do not top up with plain water by default

Coolant is lost from the sump in different ways: water evaporates, while fluid is also carried out on parts, swarf and filters. Because these mechanisms change concentration differently, make-up fluid normally needs a controlled concentration rather than simply adding water or full-strength fresh mix.

FUCHS notes that make-up concentration is often lower than the initial fill concentration, but the correct value must be determined for the actual system. Track the working concentration and adjust the make-up mix accordingly.

How to change coolant and clean the sump

Fresh fluid should not be introduced into a dirty or biologically contaminated system. HSE’s current guidance for water-mix sumps emphasises draining and cleaning the system before refilling and following the fluid supplier’s instructions for cleaning chemicals, dosage and circulation time.

1. Assess the existing fluid. Record concentration, pH, bacterial result, tramp oil and visible contamination before draining where this is useful for troubleshooting.
2. Drain the old fluid safely. Use suitable equipment to minimise splashing and skin exposure.
3. Remove swarf, fines, sludge and deposits. Do not remove sharp metal debris by hand.
4. Clean the sump and circulation system. Use the cleaner, concentration and circulation procedure recommended for the coolant system.
5. Control aerosol generation. Avoid high-pressure water hoses where practicable because they can generate mist and disturb contaminated biofilm.
6. Do not use plain water as the default final wash. HSE warns that plain-water washing can de-passivate metal surfaces and increase corrosion risk.
7. Refill with correctly prepared fresh coolant. Verify concentration after the fluid has circulated and stabilised.

The older version of this article gave a universal fresh-emulsion flush concentration and routine hot-water detergent rinse. Those have been removed as general rules. The correct cleaning solution, concentration and rinsing procedure depend on the coolant chemistry, machine materials, degree of contamination and supplier instructions.

How often should coolant be checked?

The correct frequency depends on sump size, machining severity, system history and the fluid supplier’s requirements. HSE’s MW5 guidance provides a useful baseline for water-mix systems.

Some coolant suppliers recommend daily concentration checks, particularly in demanding or highly variable processes. A plant can therefore use a stricter schedule than the HSE baseline where that improves process control.

How to interpret bacterial contamination

HSE provides the following action levels for conventional water-mix metalworking fluids monitored with dipslides:

• below 10⁴ CFU/ml: good control; continue regular monitoring and maintenance,
• 10⁴ to below 10⁶ CFU/ml: review fluid quality and correct concentration, pH and other operating parameters,
• 10⁶ CFU/ml or above: poor control requiring immediate action; draining and cleaning is normally required.

Do not use biocide as the first response

HSE recommends correcting good-practice factors such as concentration, pH, tramp oil, metal contamination, temperature, agitation and flow before adding biocide. If bacterial growth continues, use only a compatible biocide at the dose recommended by the supplier.

Health risks: why fluid maintenance matters to operators

Metalworking fluids can affect workers through skin contact and inhalation of mist. HSE identifies risks including dermatitis, occupational asthma and occupational hypersensitivity pneumonitis.

Good fluid maintenance is therefore only one part of exposure control. Depending on the machining process, the workplace may also require:

• machine enclosure,
• effective local exhaust ventilation or mist extraction,
• control of compressed-air cleaning,
• skin-exposure controls and suitable PPE,
• training and supervision,
• health surveillance where exposure cannot be prevented.

An unpleasant odour is a useful warning sign but should not be the only criterion for action. A fluid can have excessive bacterial contamination before a strong smell develops, so direct microbiological monitoring is important.

How to extend coolant service life

Coolant life can often be extended by controlling the causes of degradation rather than repeatedly adding treatment chemicals. Useful measures include:

• keeping concentration within the supplier’s range,
• using suitable make-up water,
• removing tramp oil,
• maintaining filtration and swarf removal,
• avoiding food, drink, cigarette waste and other contamination in the sump,
• maintaining circulation and avoiding stagnant zones,
• controlling temperature,
• using condition monitoring to identify trends early.

Specialised filtration, skimming and regeneration systems can be useful in larger installations, but their suitability should be assessed for the actual fluid and machine system. The older claim that UV treatment will generally extend coolant life “several times” has not been retained because that result is highly system-dependent.

How to choose a water-miscible coolant

There is no universal coolant for every machine and material. Selection should include:

• machining operation and severity,
• workpiece material,
• tool material and cutting parameters,
• water quality,
• machine compatibility,
• foam tendency and coolant pressure,
• corrosion-protection requirements,
• filtration and central-system design,
• operator exposure and workplace controls.

The current English Melkib range is available in Cooling lubricants.

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Practical coolant-management checklist

1. Confirm the correct coolant and working concentration for the process.
2. Check make-up water quality.
3. Prepare coolant by adding concentrate to water.
4. Verify concentration with a refractometer and the correct product factor.
5. Record pH, concentration, bacterial results and corrective actions.
6. Remove tramp oil and metal contamination before they accumulate.
7. Investigate unusual odour, foam, corrosion or instability promptly.
8. Use biocide only when justified and according to supplier instructions.
9. Never refill a heavily contaminated system without cleaning it first.
10. Review mist control, skin protection and health surveillance together with fluid quality.

FAQ – water-mix metalworking fluids

Should coolant concentrate be added to water or water to concentrate?

Add the concentrate to water. This is the standard recommendation for water-miscible machining fluids and helps the mixture form correctly.

How often should concentration and pH be checked?

HSE uses weekly checks as a baseline for fluid-quality management, while some suppliers recommend more frequent checks, including daily concentration monitoring. Use the stricter schedule required by the process, supplier and risk assessment.

Should coolant be topped up with plain water?

Not as a routine rule. The correct make-up concentration depends on how water and coolant are being lost from the system. Measure the working concentration and prepare an appropriate make-up mixture.

Can a pressure washer be used to clean the sump?

HSE recommends avoiding high-pressure water hoses where practicable because they generate mist and can disturb contaminated deposits or biofilm. If such cleaning is necessary, it requires an appropriate exposure and safety assessment.

When should a contaminated coolant be drained?

For conventional systems, HSE regards bacterial results at or above 10⁶ CFU/ml as poor control requiring immediate action; normally the system should be drained and cleaned.

Does bad smell always mean the coolant must be replaced?

Odour is a warning sign, not a complete diagnostic method. Check concentration, pH, bacterial contamination, tramp oil and the supplier’s condition criteria before deciding on corrective action.