Cutting Fluid Concentration and Mixing: Measurement, Water Quality and Make-Up

The single most important variable determining performance in a water-miscible cutting fluid is concentration. Even when the right product has been chosen, a fluid mixed at the wrong ratio gives a worse result than a wrongly chosen product. This guide covers why concentration is critical, how it is measured, in what sequence the mix is prepared and how the make-up mixture is calculated. A comparison of product types is dealt with separately in the cutting fluid types guide.

1. Why concentration is critical

In a water-miscible cutting fluid the water provides cooling, while the concentrate provides lubrication, corrosion protection and microbiological balance. Concentration is the volumetric proportion of concentrate in the mix, and deviation in either direction produces its own problem.

The health side is not a heading that can be neglected. For metalworking fluid aerosol, NIOSH has published a recommended exposure limit (REL) of 0.4 mg/m³ thoracic particulate matter, or 0.5 mg/m³ measured as total particulate matter, as a time-weighted average for work of up to 10 hours a day and 40 hours a week. OSHA has no dedicated permissible exposure limit (PEL) for metalworking fluid aerosol; in practice the 8-hour PEL values of 5 mg/m³ for mineral oil mist and 15 mg/m³ for particulates not otherwise regulated are taken as the basis. An over-concentrated mix increases both the aerosol load and the risk of skin contact. The subject is covered in full under occupational health and safety in metalworking fluids.

Critical point: A concentration problem cannot be assessed on its own. Low concentration triggers bacterial growth, bacterial growth lowers the pH, and low pH breaks down corrosion protection. The first link in the chain is almost always an unmeasured concentration.

2. Typical working concentration ranges

Concentration is determined by the severity of the operation. As the metal removal rate, contact pressure and the adhesion tendency of the material increase, the need for lubrication increases; in operations where cooling dominates, on the other hand, a more dilute mix is sufficient.

These ranges are values in common industrial use; the binding reference is the product's technical data sheet. It is normal for the band recommended by different manufacturers for the same operation to shift by a few points, because the proportion of active substances in the concentrate differs markedly between products.

The effect of the material group also has to be taken into account. In ISO S group titanium and nickel-based alloys, and in ISO M stainless steels, moving towards the upper band is normal; the ISO material groups guide sets out this distinction in detail.

3. Measurement methods

3.1 Hand refractometer and the Brix reading

The most widespread method on the shop floor is the hand refractometer. The light refraction ratio of the few drops of mix placed on the prism is read, and the result is given on the Brix scale. This reading is not directly the concentration.

The true concentration is found with the following relation:

C (%) = Brix reading × refractometer factor

The refractometer factor is product-specific and is stated in the technical data sheet. In emulsions based on neat soluble oil the factor is close to 1.0, meaning the reading can be taken directly as the concentration. In semi-synthetic and synthetic products the factor is typically in the 1.5–2.5 band; in some synthetic products values of up to 3.0 are seen in manufacturer data sheets.

Worked example: If the refractometer reads 3.2 Brix and the product factor is 1.8, the true concentration is 3.2 × 1.8 = 5.76%. The same reading means 3.2% for a product whose factor is 1.0. Omitting the factor leads to a mix assumed to be 5.8% actually being 3.2%; this difference is enough for corrosion and bacterial problems to begin.

A few conditions are required for a correct reading: the sample being at room temperature rather than tank temperature, the prism being zeroed with distilled water before every measurement, the sample being taken from the circulating fluid rather than from the sludge at the bottom of the tank, and, where the boundary line is blurred, the reading being taken from the middle of the line.

3.2 The limits of the refractometer

A refractometer sees not only the concentrate but every dissolved and dispersed substance in the fluid. Hydraulic and slideway oil that enters the system (tramp oil), dissolved salts and fine metal particles raise the reading. It is a frequently encountered situation for a refractometer to show 7% in an aged tank while the true concentrate proportion is markedly lower. The refractometer is therefore a trend instrument; where there is doubt, it has to be confirmed by a second method. The source of tramp oil and its removal are dealt with in the cutting fluid maintenance guide.

3.3 Titration, acid split and conductivity

In acid titration a sample of known volume is titrated with a dilute hydrochloric acid solution in the presence of an indicator, and the concentration is calculated from the volume of acid consumed. In the acid split method the sample is acidified and held at high temperature so that the oil phase separates; the separated volume is multiplied by the product-specific split factor and converted to concentration. Since there is no oil phase to separate in synthetic products, this method is not applied to them; titration or conductivity is preferred for those.

Automatic dosing units measure the concentration online and dose concentrate and water separately, keeping the tank within the target band. They are preferred in multi-tank workshops because they eliminate measurement error and the human factor; manufacturer platforms offer these units together with a recording and reporting function.

4. Mixing sequence

A water-miscible cutting fluid is an oil-in-water emulsion in which water is the continuous phase. For this structure to form, the concentrate has to be added to the water. Pouring water onto the concentrate creates an inverted emulsion in which the continuous phase is oil (water-in-oil instead of oil-in-water). An inverted emulsion is unstable; it produces separation after mixing, an oil layer on the surface, residue and poor cooling performance.

In industry this rule is known by the abbreviation "OIL": Oil In Last. The sequence in practice is as follows:

1. The clean tank is filled with the water for the target volume; the amount of water is calculated beforehand.
2. The concentrate is added to the water in a thin, continuous stream while mixing continues. It is not emptied in all at once.
3. Mixing continues until the mix looks homogeneous.
4. Verification with a refractometer is carried out and the result is recorded.

In manual mixing, using a mixer or a circulation pump gives a markedly more stable emulsion than stirring by hand in a bucket. Venturi-type mixers draw in the concentrate with the vacuum created by the water flow and mix it in the turbulent zone; they are fast and cheap. Positive displacement dosing pumps, on the other hand, inject the concentrate in proportion to the water flow rate and hold the ratio even when the water pressure fluctuates. In both arrangements the concentrate is fed into the water stream, so the mixing sequence is set up correctly of its own accord.

Very cold water makes emulsion formation difficult and slows the dispersion of the concentrate. Preparing the mix with water at room temperature is preferred.

5. Mixing water quality

More than 90% of the mix by volume is water; water is therefore the single largest component of the mix and its quality directly determines the result. The hardness, chloride content and sulphate content of the water are the three headings that have to be monitored.

Conversion between hardness units is made with the relation 1 °dH ≈ 17.8 ppm CaCO₃. The band above is the comfort range frequently given in manufacturer publications; sources do not overlap, particularly at the upper limit, and some of them state a markedly narrower range. For this reason the water quality table in the product's own technical data sheet is taken as the binding value.

There is a balance to be aware of on the subject of reverse osmosis (RO) and deionised (DI) water. Because these waters are stripped of all salts, they look like the "cleanest" option; however, hardness close to zero increases the tendency to foam, and foam that is hard to control can form in the tank. Against that, the same waters prevent chloride and salt accumulation, so they are an advantage where mains water has a high chloride content and when preparing the make-up mixture; some products require soft or directly treated water for long-term stability. In practice, treated water is either blended with mains water or brought into the target band with a calcium salt based hardening additive. In very hard water, softening with an ion exchanger is preferred. Which route is chosen depends on the water quality requirement of the product.

Critical point: Evaporation of water concentrates hardness in the tank. In a tank continuously topped up with hard mains water, hardness rises within weeks to multiples of its initial value; calcium soap formation, filter clogging and corrosion are the consequences of this accumulation.

6. Tank filling and make-up calculation

6.1 Initial fill

At the initial fill the target concentration is applied directly. If a 300 litre tank is being filled to a 6% target:

Vconcentrate = Vtank × Ctarget = 300 × 0.06 = 18 L

Vwater = 300 − 18 = 282 L

That is, 18 litres of concentrate is added to 282 litres of water. Calculating on the total of the volumes removes the difference between "18 litres into 282 litres of water" and "18 litres into 300 litres of water"; the second gives 5.7%.

6.2 Why the make-up mixture is different

Losses from the tank occur in two ways. Water evaporates and the concentrate is left behind; the fluid carried out on chips and components, on the other hand, is the whole mix. Evaporation is typically dominant over drag-out, so the concentration in the tank rises over time. Making up the loss with mix at the working concentration raises the concentration further still. This is why the make-up mixture is always more dilute.

The required make-up concentration is found by measurement:

Cmake-up = (Vtank × Ctarget − Vremaining × Cmeasured) / Vmake-up

Worked example: The target is 6% in a 300 litre tank. After a week the level has fallen to 250 litres, the refractometer reads 4.4 Brix and the product factor is 1.5.

• Measured concentration: 4.4 × 1.5 = 6.6%
• Concentrate currently in the tank: 250 × 0.066 = 16.5 L
• Concentrate required for 6% in 300 litres: 300 × 0.06 = 18.0 L
• Missing concentrate: 18.0 − 16.5 = 1.5 L
• Volume to be added: 300 − 250 = 50 L
• Make-up concentration: 1.5 / 50 = 0.03 → 3.0%

That is, the 50 litre make-up is prepared at 3.0% with 48.5 litres of water and 1.5 litres of concentrate. With a working concentration of 6%, preparing the make-up mixture at half that ratio is the natural result of this calculation.

Manufacturer publications give different bands for the make-up ratio at a 5% working concentration, ranging between 1–2% and 2.5–3%. At this point, where the sources diverge, carrying out the above calculation with a measurement result at every make-up is more reliable than adopting a fixed ratio. The evaporation and drag-out rate varies with machine type, tank volume, chip conveyor and workshop temperature; a single fixed ratio is not correct for every tank.

7. Measurement frequency and record keeping

The value of measurement comes from its being carried out regularly and recorded. UK HSE practice takes the following scheme as its basis for metalworking fluids.

Dip slide results are divided by HSE into three bands: below 10⁴ colony forming units per millilitre (CFU/mL) is defined as good control, between 10⁴ and 10⁶ CFU/mL as reasonable control requiring review and intervention, and 10⁶ CFU/mL and above as the level at which control has been lost and the tank has to be emptied and cleaned.

The record form has to contain at least the following fields: date, machine/tank number, level, Brix reading, refractometer factor, calculated concentration, pH, the amount of water and concentrate added, the action taken and the person who carried out the measurement. These records both make trend analysis possible and are used as evidence of control in occupational health and safety inspections. HSE states that the measurement frequency can be reduced where an effective record history can be demonstrated.

In Türkiye, the supply, labelling and safety data sheets of metalworking fluids fall under the Kimyasalların Kaydı, Değerlendirilmesi, İzni ve Kısıtlanması Hakkında Yönetmelik (KKDİK, the Turkish REACH equivalent) and the Maddelerin ve Karışımların Sınıflandırılması, Etiketlenmesi ve Ambalajlanması Hakkında Yönetmelik (SEA, the Turkish CLP equivalent). Management of exposure in the workplace is subject to the provisions of Occupational Health and Safety Law No. 6331 (6331 sayılı İş Sağlığı ve Güvenliği Kanunu) and the Kimyasal Maddelerle Çalışmalarda Sağlık ve Güvenlik Önlemleri Hakkında Yönetmelik (the Turkish regulation on health and safety in work with chemical agents). Disposal of used fluid is carried out within the framework of the Atık Yönetimi Yönetmeliği (the Turkish waste management regulation), through licensed disposal facilities.

8. Common mistakes

• Omitting the refractometer factor and taking the Brix reading directly as the concentration.
• Using the factor of a different product; the factor is product-specific, and similarity of type is not sufficient.
• Making up with water alone or with mix at the working concentration alone.
• Pouring water onto the concentrate.
• Not zeroing the prism with distilled water and measuring with a hot sample.
• Taking the sample from the bottom of the tank, from the settled sludge zone.
• Carrying out the measurement without recording it; without a record no trend can be seen.

The correct concentration is reflected directly in machining cost through its effect on tool life and surface quality. Assessing it together with the cutting speed, spindle speed and feed calculation when determining cutting parameters ensures that the contribution expected from the fluid is realised. Semi-synthetic coolant options are listed in the chemicals category.

Sources

• NIOSH — Criteria for a Recommended Standard: Occupational Exposure to Metalworking Fluids, DHHS (NIOSH) Publication No. 98-102 — cdc.gov/niosh/docs/98-102
• OSHA — Metalworking Fluids: Safety and Health Best Practices Manual — osha.gov/metalworking-fluids/manual
• HSE (UK) — Metalworking fluids: bacterial contamination and fluid quality checks — hse.gov.uk/metalworking
• UKLA / HSE — Good Practice Guide for Safe Handling and Disposal of Metalworking Fluids — ukla.org.uk
• Blaser Swisslube — Water quality is a key factor — blaser.com
• Master Fluid Solutions — Coolant Concentration Facts and Terminology (technical bulletin) and the refractometer factor definitions in the product data sheets — masterfluids.com
• FUCHS — Cutting Fluids Manual: Collected Knowledge on Cutting Fluids for Metalworking — fuchs.com
• DIN 51385 — Schmierstoffe – Bearbeitungsmedien für die Umformung und Zerspanung von Werkstoffen – Begriffe (definitions of water-miscible and neat cutting fluids)