10 Rules for Working with Cutting Fluids

10 Rules for Working with Metalworking Fluids (MWF)

Choosing the right metalworking fluid (MWF) is only part of the job. Its performance also depends on water quality, concentration, preparation procedure, equipment cleanliness, filtration, circulation, and how make-up fluid is managed.
Even a correctly selected MWF may become unstable if fresh fluid is introduced into a contaminated system, prepared incorrectly, or left without regular monitoring during operation.
Below are 10 practical rules for working with water-miscible MWFs, together with recommendations for initial troubleshooting of common problems.

1. Select the MWF for the specific machining operation

There is no single universal MWF that performs equally well on all equipment, with all metals, and under all machining conditions.
When selecting a product, consider the workpiece material, machining process, operating loads, lubrication and cooling requirements, corrosion protection, biostability, equipment design, fluid delivery system, and water quality.
For example, high biostability does not automatically mean high lubricity. The MWF should be selected according to the complete set of requirements for the actual machining process.

2. Check the water used to prepare the MWF

In a water-miscible MWF, water makes up most of the working solution, so its characteristics can have a major effect on fluid behavior.
The most important parameters include:

• hardness;
• conductivity and total dissolved salts;
• pH;
• mechanical impurities;
• when necessary, the microbiological condition of the water.

Microbiological condition of the water means, among other things, the presence of bacteria, fungi, and other microorganisms that can enter the MWF system with the water and contribute to biological contamination of the working fluid.
Water from wells, open reservoirs, and other sources with unstable characteristics requires particular attention. Such water may contain not only elevated levels of dissolved salts or suspended solids, but also significant microbiological contamination.
For this reason, water from a well, river, lake, pond, or another natural source should not be used to prepare MWF without prior testing and, where necessary, suitable water treatment.
Using water directly from a river or an industrial open reservoir is particularly risky because of unstable composition, suspended solids, and possible biological contamination.
For some MWFs, very soft water may promote foaming, while elevated salt levels may affect fluid stability and corrosion risk.
At the same time, there is no single “ideal” hardness, pH, or conductivity value for every MWF. Water parameters should be matched to the requirements of the specific product and its operating conditions.

3. Prepare the working solution correctly

The basic rule for water-miscible MWFs is:
add the concentrate to the water, not the water to the concentrate.
This is especially important for emulsion-type products, where incorrect mixing order may interfere with proper emulsion formation.
Synthetic MWFs may behave differently during mixing, but in plant practice it is useful to apply one consistent rule for water-miscible products: concentrate into water.

Recommended preparation procedure

1. Prepare a clean container intended specifically for MWF preparation.
2. Add the required amount of water.
3. Gradually add the calculated amount of concentrate.
4. Mix thoroughly and evenly.
5. Check the final concentration.
6. Only then use the prepared fluid to fill the system.

Do not prepare MWF in dirty or unidentified containers. Prevent contamination of the concentrate or prepared solution by other liquids, oils, or dirt.

4. Keep the concentration within the recommended range

More concentrate does not automatically mean better MWF performance.
Too low a concentration may reduce lubricating, corrosion-protection, and other functional properties of the specific product.
Excessively high concentration is also undesirable: it increases concentrate consumption and may change the behavior of the working fluid.
Use the recommended concentration range for the specific MWF, taking into account the machining process, material, and operating conditions.
Do not estimate concentration by eye.

5. Monitor concentration regularly

A refractometer is commonly used for routine concentration control of many water-miscible MWFs.
The refractometer reading does not always directly equal the MWF concentration in percent. A product-specific refractometer factor may be required, so concentration calculations should follow the data for the particular fluid.
For routine checks, you can use a refractometer for MWF .
It is good practice to record the readings in a log. This helps track not only the current concentration but also how it changes over time.
If the boundary line on the refractometer scale becomes unclear, the reading should be treated with caution. Contamination by tramp oil and other substances may interfere with measurement.

6. Organize controlled make-up fluid preparation

A common organizational mistake is allowing each machine operator to decide independently how much water or concentrate to add to a machine.
A better approach is to organize centralized preparation of make-up fluid.
A separate process tank can be used for this purpose, for example an IBC container or another suitable vessel. The make-up fluid is prepared under the supervision of a responsible specialist who controls its concentration and condition. Operators then receive already prepared fluid for topping up individual machines.
If the plant has a centralized MWF preparation and supply system, make-up can be organized centrally as well.
It is not recommended to allow each operator to prepare make-up fluid independently according to personal judgment. This makes concentration control more difficult and can lead to significant differences even between identical machines.

What concentration should be used for make-up

The make-up fluid does not necessarily need to have the same concentration as the initial working solution.
In practical operation, many MWF systems are topped up with a lower-concentration solution, often approximately 2–3%, provided this is suitable for the specific product and operating conditions.
This is because water and MWF components are not lost from the system in exactly the same proportion.
A practical sequence is:
measure the concentration in the system → determine the required correction → add prepared make-up fluid → check the concentration again.
Do not automatically top up only with water, only with concentrate, or always use the original working concentration.

7. Clean the system before replacing the MWF

Changing MWF is not simply a matter of draining the old fluid and filling the system with fresh solution.
Chips, sludge, tramp oil, deposits, old MWF residues, and microbiological contamination may remain in the tank, pipework, and difficult-to-reach areas.
If fresh MWF is added to a heavily contaminated system, many of the existing problems may quickly transfer to the new working fluid.
Before replacement, drain the old fluid as completely as possible, mechanically remove chips, sludge, and deposits, and clean accessible surfaces.

If there are signs of biological contamination

If there is a strong unpleasant odor and other signs of significant biological contamination, Novohim practice includes flushing the system with a 10% BIOL Lemon MWF solution .
BIOL Lemon was developed with a strong focus on biostability and is used, in particular, where circulating MWF systems have significant microbiological contamination.
Equipment may be operated on a 10% BIOL Lemon solution during the cleaning process. However, this product has limited lubricating properties, so this mode should not be treated as a universal substitute for the normal operating MWF in processes with higher lubrication requirements.
Appropriate personal protective equipment should be used, including protective gloves.

Additional system cleaning

For additional cleaning, Novohim also recommends MWF BIOL Eco.
BIOL Eco can be used as a cleaning working solution at the normal concentration recommended for the product. This can help flush the tank, pipelines, and circulation loop before fresh MWF is introduced.
After flushing, the contaminated solution should be drained, sludge and other residues should be removed again, and only then should freshly prepared MWF be added.
The exact cleaning procedure should be matched to the condition of the system and equipment. Do not add arbitrary biocides, alkalis, or other chemicals without understanding the cause of the problem and their compatibility with the specific MWF.

8. Remove chips, sludge, and tramp oil

MWF inevitably becomes contaminated during operation. The rate at which contamination accumulates depends heavily on equipment condition and maintenance practices.
Monitor:

• metal chips and fine sludge;
• leakage of hydraulic, slideway, and other oils;
• filter condition;
• tank cleanliness;
• skimmers and other cleaning equipment, where installed.

Tramp oil on the MWF surface is not only a visual issue. Heavy contamination can affect the condition of the working fluid and make control more difficult.
It is important not only to remove tramp oil, but also to identify and eliminate the source of the leak.

9. Monitor the condition of the MWF regularly

Do not wait for strong odor, corrosion, or excessive foam to appear.
During operation, it is advisable to monitor:

• fluid level;
• concentration;
• appearance;
• odor;
• foaming;
• sediment and contamination;
• tramp oil;
• signs of corrosion;
• pH when necessary;
• microbiological condition if biological contamination is suspected.

Results should preferably be recorded in a log.
For each machine or central system, a simple record can be used:
date → concentration → pH if required → appearance and odor → identified problems → corrective action taken.
Trend monitoring is more useful than relying on a single isolated measurement.
Where possible, MWF control should be handled by a designated responsible specialist rather than by each operator using an individual method.

10. If a problem occurs, find the cause first

Foam, odor, corrosion, color change, or MWF instability does not automatically mean that a defoamer, biocide, extra concentrate, or another additive should be added immediately.
The same symptom can have several different causes.
A better sequence is:
symptom → parameter check → possible causes → cause verification → corrective action.
If the cause is not obvious, check the system in a logical order: water quality, preparation procedure, actual concentration, make-up procedure, tank and system cleanliness, tramp oil and other contamination, circulation and filtration, appearance and odor, pH and other control parameters, and—where relevant—the microbiological condition of the fluid.

Common MWF problems: possible causes and what to check

The MWF is foaming heavily

Foaming does not necessarily mean the MWF itself is defective.
Check water quality and hardness, concentration, fluid level, circulation intensity, possible air entrainment at the pump, filter condition, fluid return to the tank, tramp oil, and the overall condition of the working fluid.
Do not begin by adding defoamer without first identifying the source of the foaming.

An unpleasant odor has appeared

A noticeable change in odor may be one sign of microbiological contamination, especially if the color, pH, or stability of the working fluid has also changed.
Check tank cleanliness, sludge accumulation, tramp oil, concentration, circulation, and whether the equipment has been idle for extended periods.
Where necessary, microbiological condition can be checked using suitable test methods.
If the system is heavily biologically contaminated, simply correcting concentration may not be enough. The system may need to be cleaned and the working fluid replaced. In Novohim practice, a 10% BIOL Lemon solution may be used for flushing systems with significant biological contamination.

Corrosion appears on parts or equipment

Possible causes include:

• concentration below the recommended range;
• unsuitable water characteristics;
• accumulation of salts in the working fluid;
• MWF contamination;
• deterioration of the working fluid condition;
• incompatibility of the specific MWF with the material or machining conditions.

Check concentration and water first, then evaluate the other factors.
Do not automatically conclude that any corrosion means the MWF is “poor quality”.

The MWF separates or becomes non-uniform

For emulsion-type MWFs, check:

• preparation procedure;
• water quality;
• actual concentration;
• external contamination;
• temperature;
• overall fluid condition.

It is also important to distinguish between separation of the concentrate in its container and instability of the working emulsion in the machine. These are different situations and may have different causes.

Fresh MWF deteriorates quickly after replacement

If a fresh working solution quickly changes odor, color, or other characteristics, check more than just the product itself.
Possible causes include:

• insufficiently cleaned tank;
• contaminated pipework;
• residues of the old MWF;
• sludge and deposits;
• tramp oil;
• unsuitable water;
• incorrect concentration;
• stagnant areas in the system.

This is why preparing the equipment before filling with fresh MWF is just as important as preparing the working solution correctly.

What should be monitored during MWF operation

Concentration	Keeps the MWF within the recommended operating range
Fluid level	Supports controlled make-up management
Water quality	Affects the behavior of the working solution
Chips and sludge	Helps prevent excessive system contamination
Tramp oil	Helps identify leaks and contamination
Foam	Can indicate water, circulation, or fluid-condition problems
Color and odor	Helps detect changes in fluid condition
pH	Additional indicator of changes in MWF condition
Microbiological condition	Checked when biological contamination is suspected

How to organize MWF management at a plant

The best results come from a consistent management system rather than occasional checks.
It is advisable to appoint a person responsible for MWF preparation and control, define a monitoring schedule, keep records, use a dedicated controlled tank for make-up fluid preparation, maintain filtration and circulation equipment, eliminate tramp-oil leaks, and clean tanks and systems in a timely manner.
Operators should not be allowed to change concentration independently without control.
This is particularly important in plants with many machines. The same MWF can behave very differently on different machines if operators prepare, monitor, and top up the fluid differently.

Storing MWF concentrate

Store the concentrate in closed containers according to the conditions specified for the particular product.
Prevent water and dirt from entering the concentrate, avoid prolonged storage in open containers, do not use unsuitable vessels, and follow the product-specific storage temperature requirements.
If the appearance of the concentrate changes during storage or separation is observed, check the recommendations for that specific product. Do not automatically assume that any separation is normal—or that it means the product is unusable.

When should the MWF be completely replaced

Not every problem can be corrected by topping up or adjusting concentration.
A complete replacement should be considered if the working fluid is heavily contaminated, has lost stability, shows severe biological contamination, or cannot be restored to acceptable operating condition.
It is important not only to drain the old MWF but also to identify the cause of the problem and prepare the system before introducing fresh fluid. Otherwise, the same problem may return quickly.

FAQ

What should be added first: water or MWF concentrate?
Add the concentrate to the water. It is good practice to establish this as a standard procedure for water-miscible MWF preparation.
Can well water be used to prepare MWF?
Yes, but only after its suitability has been assessed. Well water may have high hardness, elevated dissolved salts, mechanical impurities, or microbiological contamination. Testing is advisable before it is used on a regular basis.
Can river water or water from an open reservoir be used?
Using such water without prior analysis and suitable treatment is not recommended. Its composition may be unstable and it may contain suspended solids and significant levels of microorganisms.
Can I top up a machine with water only?
Not automatically. First check the actual concentration and determine what correction is required.
What concentration should be used for make-up fluid?
In many practical MWF systems, a lower-concentration make-up fluid is used, often approximately 2–3%. The actual value depends on the product, its working concentration, and the condition of the fluid in the system.
Who should prepare MWF at the plant?
Ideally, preparation and control should be centralized and handled by a responsible specialist. Each operator should not determine the water-to-concentrate ratio independently.
Why is a refractometer needed?
A refractometer provides a fast way to monitor the concentration of many water-miscible MWFs. Correct interpretation may require the product-specific refractometer factor.
What should be done if the system has severe biological contamination?
Assess the condition of the fluid and system, remove chips, sludge, and contamination, and clean the system. In Novohim practice, a 10% BIOL Lemon solution is used for flushing systems with significant biological contamination. For additional cleaning, BIOL Eco may be used at its normal recommended concentration.
Why has the MWF started foaming?
Possible causes include water characteristics, concentration, circulation, air entrainment, filtration, tramp oil, contamination, or a change in the condition of the MWF itself. Identify the cause before adding defoamer.
Should the tank be cleaned before adding fresh MWF?
Yes. Thorough cleaning is especially important after operation with heavily contaminated or biologically affected working fluid.

The key rule

Stable MWF operation does not depend on a single parameter.
Correctly selected MWF + suitable water + correct preparation + concentration control + organized make-up management + a clean system + effective filtration and circulation + regular monitoring should be treated as one operating system.
If a problem occurs, do not start by randomly adding concentrate, biocide, defoamer, or other corrective chemicals. First determine what has changed in the working fluid or equipment, and only then decide on the corrective action.