Waterbased Metalworking Fluids: Proper Mixing Practices

Waterbased metalworking fluids must be properly mixed and maintained in the proper concentration range to maximise performance. When fluid concentrate and water are mixed to charge the machine's fluid reservoir, the fluid concentration must be tested to ensure that the starting concentration is correct. The concentration can be measured with a refractometer. Concentration can be expressed as a percentage or as a ratio of the amount of metalworking fluid concentrate to the total volume of mix.

A 'charge' mix ratio is the concentration used to initially fill an empty sump. The 'makeup' mix ratio is the concentration used to replace fluid losses due to evaporation and carry-off and is typically a lower concentration than the charge ratio.

There are three types of water-dilutable (miscible) metalworking fluids. They are soluble oil, semi-synthetic and synthetic fluids.

SOLUBLE OIL fluid is a combination of oil, emulsifiers and other performance additives that are supplied as a concentrate to the end user. Soluble oil concentrates generally contains 60% to 90% oil. They are diluted with water, typically at a ratio of one part concentrate to 20 parts  water or 5%. When mixed with water they have an opaque, milky appearance. They generally are considered as general purpose fluids, since they often have the capability to be used with both ferrous & non-ferrous materials in a variety of applications.

SEMI-SYNTHETIC fluids have a much lower oil content than soluble oils. The concentrate typically contains 2% to 30% oil. When mixed with water, characteristically at 5%, the blend will appear opaque to translucent These fluids usually have lubricity sufficient for applications in the moderate to heavy-duty range (Le. centreless & creep feed grinding or turning & drilling). Their wetting and cooling properties are better than soluble oils, which allow for faster speeds and feed rates.

SYNTHETIC fluids contain no mineral oil. Most synthetic fluids have a transparent appearance when mixed with water. There are some synthetic fluids that are categorised as synthetic emulsions, which contain no mineral oil but appear as an opaque, milky emulsion when mixed with water. Synthetic fluids have the capability to work in applications ranging from light (i.e. double disk grinding, surface grinding or milling) to heavy-duty (I.e. creep feed, threading & drilling) while the synthetic emulsions can perform very heavy duty operations. Synthetic fluids generally are low foaming, clean and have good cooling properties allowing for high speeds &
feeds, high production rates & good size control.

When mixing metalworking fluids, we recommend that a proportioner be used to ensure that a stable mix is formed at the correct concentration., Mix Master®11 Proportioner is drum mounted & supplies 13-18litres per minute of premix fluid for individual machine tanks.

Mixing manually is an option, but is prone to errors. Two common errors that occur which result In improper mix concentration are:

  1. Inaccurate estimate of machine tank volume (gallons) causing the wrong volume of fluid concentrate to be added to the water.
  2. Incorrect order of addition - adding water to concentrate may form an inverted emulsion (mix), affecting many metalworking fluid performance properties.

If mixing manually is the only option, it is imperative that the fluid concentrate is added to the water and not the reverse. 

When adding 'soluble oil' concentrate to water, the emulsifiers suspend the oil particles in the water & will form the stable emulsion desired, If the water is added to the concentrate, the emulsifiers 'release' part of the concentrate to 'grab' for the water, This forms an 'Inverted emulsion', causing parts of the fluid such as the lubricant package, biocide package or rust inhibitor package to be lost. The mix stability, concentration & the dye may also be affected by inverting the emulsion. The concentration of the fluid will be less, leaving the fluid a lighter colour than would be expected with a properly mixed fluid.

When mixing fluids manually, in a premix tank or a machine reservoir, fill the tank half-full with water. Next add the concentrate directly to the water. Then add the remainder of the water to create agitation and allow the fluid to mix thoroughly.

Turning on the machine tool coolant pump & Circulating the fluid will help to create a uniformly mixed product, Once the product is uniformly mixed, the concentration should be tested using a refractometer, The fluid sample used to check concentration is typically taken at the machine tool fluid delivery. It is also important to remember that each product will have its own refractometer 'multiplier' or 'factor', The factor will be needed to calculate the metalworking fluid mix concentration when using a refractometer. Refer to the Product Information Flyer (PIF) for the refractometer factor.



  • Use a proportioning device to mix and dispense fluid whenever possible.
  • Always add concentrate to water when manually mixing.
  • Increase fluid concentration by adding concentrate directly to the reservoir in a place that provides good mixing.
  • Mix fluid thoroughly before measuring concentration
  • Measure the fluid concentration with a refractometer.


  • Add water to the concentrate.
  • Mix an "unknown" concentrate Into the fluid reservoir.
  • Maintain concentration by 'sight' or 'feel',
  • Use water that is colder than 10°C. (Many emulsions are unstable at low temperatures)
  • Pour concentrate in the work area of a machine tool, and then turn on the fluid nozzles to mix the concentrate.

Metalworking fluid is 'consumed' each day as a result of evaporation, carry-off with parts and chips, reaction with hard water & splashing. It is important to recognise that metalworking fluids contain Ingredients that perform certain functions. These ingredients may be:

  • BIOCIDES to combat bacteria & mould growth which cause rancidity.
  • RUST INHIBITORS to prevent rust on the machine tool, machined part & the chips/swarf in the machine tank.
  • LUBRICANTS to improve tool life & finish or increase productivity
  • ANTIFOAM AGENTS to keep the foam from being a problem.

The product Ingredients are 'consumed' by doing their job and therefore
need to be replenished through daily makeup. This Is called 'selective
depletion'. Selective depletion can be counteracted through the process of adding daily makeup to the fluid mix.

The following operations are examples of 'selective depletion':

  • CAST IRON - depletes ferrous corrosion inhibitor quickly,
  • ALUMINIUM - depletes the lubricant package but typically won't affect the rust Inhibitor.
  • DIRTY/OILY RESERVOIR - depletes the biocide.
  • HIGH PRESSURE/ HIGH-SPEED OPERATION - depletes the antifoam ingredients,
  • HARD WATER - depletes emulsifiers that hold the product together.

The proper procedure for replenishing fluid volume and maintaining the correct fluid concentration is to add a fluid premix back to the reservoir daily. This mix is made at a lower dilution than the fluid mix used to initially charge the central system or machine reservoir, Do not add water alone, General recommendations for 'makeup mix' (target 5%) concentration.

  • SOLUBLE OILS - daily makeup should be 3-4%,
  • SEMI·SYNTHETICS - daily makeup should be at 2-3%,
  • SYNTHETICS - daily makeup should be a 1-2%,

NOTE: The values above are only guidelines & the actual makeup
concentration required may vary depending on the operation, part configuration, type of fluid, water quality, environmental conditions,
filtration system, etc. Even with proper makeup, as fluid ages, there may be an additive required (such as biocide) to extend fluid life and maintain performance.

If fluid concentration is higher than the recommended level, the concentration must be leaned out. Problems develop with high concentrations just as they do when running a fluid too low, Attempting to fix the problem by adding only water Is not recommended. Only adding water may result in key ingredients becoming too lean, potentially creating the issue such as corrosion, bacterial growth, poor tool life, broken  emulsions, etc, The best way to address high fluid concentration is to empty part of the fluid tank into a drum or other holding area & then replace this volume removed with a leaner premix. This will reduce the concentration, but still provide the needed performance properties of the fluid.

Remember that by maintaining the recommended concentration range for your metalworking fluid, you will improve the overall performance of the operation, improve productivity, extend fluid life, reduce disposal costs & improve overall operator satisfaction. It is important to start by mixing the fluid properly as recommended. Routine concentration checks, along with proper makeup are essential to good fluid management.



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