How to Properly Prepare and Maintain Metalworking Fluids: A Step-by-Step Practical Guide

Photo: Anfal Shamsudeen / Unsplash

Metalworking fluids are an integral part of metalworking processes, where they provide cooling, lubrication, chip removal, and corrosion protection. However, their proper preparation and maintenance often determine production efficiency, tool life, and the quality of the final products. Incorrect dilution, contamination, or neglect of regular checks can lead to premature machine wear, increased maintenance costs, or even defects in the workpieces. In this article, we will guide you step by step on how to prepare metalworking fluid, maintain its optimal properties, and avoid common problems.

1. Selecting the Right Metalworking Fluid: The Key to Efficient Machining

Selecting the right machining fluid is fundamental to achieving optimal results in metalworking. Fluids are divided into several categories based on their composition and purpose: emulsions (water-dilutable concentrates), synthetic fluids (without mineral oils), and semi-synthetic mixtures. Emulsions are the most common choice due to their versatility and good cooling capacity, while synthetic fluids offer longer service life and resistance to microbial contamination. When selecting, consider the type of material being machined (steel, aluminium, alloys), the machining method (turning, milling, grinding), and the desired surface quality.

Another important factor is the compatibility of the fluid with the materials of the machining equipment and tools. Some fluids may cause corrosion or damage to seals, especially if they contain aggressive additives. Before deploying a new fluid, perform a compatibility test with the materials in your operation. Always follow the manufacturer’s recommendations regarding dilution and mixing – incorrect concentration can lead to insufficient lubrication, increased tool wear, or foam formation. For demanding applications, consider fluids with anti-wear additives or anti-corrosion properties.

2. Preparing Machining Fluid: Correct Dilution and Mixing

Preparing machining fluid starts with precise dilution of the concentrate with water. Use only demineralised or deionised water, as hard water containing minerals can cause deposits, foaming, or reduced effectiveness of additives. Check the concentration with a refractometer – the optimal value typically ranges between 5 and 10 %, depending on the type of fluid and application. When mixing, always add the concentrate to the water, never the other way around, to avoid localised over-concentration and the formation of gel-like clumps.

After dilution, thoroughly mix the liquid, ideally using a mechanical stirrer or pump circulation. Allow the mixture to stand for at least 30 minutes before use to stabilize. Check the pH value—it should be within the range of 8.5 to 9.5. A pH that is too low can lead to corrosion, while a high pH irritates the operator's skin. If the pH is outside the recommended range, adjust it by adding a pH stabilizer according to the manufacturer's instructions. Always follow hygiene principles and use protective equipment when handling concentrates.

Photo: Heber Davis / Unsplash

3. Maintenance of machining fluid during operation: Monitoring and correction

Regular maintenance of machining fluid extends its lifespan and ensures consistent performance. The foundation is daily monitoring of concentration using a refractometer and pH levels with a pH meter or test strips. The concentration should not fall below 3% or exceed 12%, otherwise there is a risk of reduced efficiency or deposit formation. Maintain the pH level within the range of 8.5–9.5 – a drop below 8 indicates a risk of microbial contamination, while values above 9.5 may cause skin irritation.

Monitoring microbial contamination is also important. Bacteria and fungi multiply rapidly in the fluid, especially at high temperatures or with insufficient circulation. Use test kits to detect microorganisms and, if contamination is found, add a biocidal additive according to the manufacturer’s recommendations. Filter the fluid to remove metal chips and impurities to prevent system clogging and tool damage. If the fluid is heavily contaminated or changes color, replace it and thoroughly clean the system.

4. Solving Common Problems: Foam, Corrosion, and Odor

Excessive foam formation is a common issue that reduces cooling and lubrication efficiency. The cause may be an overly high fluid concentration, hard water, or mechanical stress (e.g., high pump pressure). Solutions include reducing the concentration, adding a defoamer, or adjusting the machining parameters. If foam persists, check the filtration system and remove any impurities that may act as foam stabilizers.

Corrosion of machined parts or machine components indicates insufficient fluid protection. Check the pH and concentration – low pH or overly diluted fluid accelerates corrosion processes. Add an anti-corrosion additive or adjust the concentration according to the manufacturer's recommendations. Unpleasant odours are usually caused by bacteria or mould. In such cases, add a biocidal agent and increase fluid circulation. If problems persist, replace the fluid and thoroughly disinfect the system. Always follow safety and health protection principles when working with chemical additives.

Photo: Cemrecan Yurtman / Unsplash

Optimising Machining Fluid Concentration for Different Operations

The concentration of machining fluid directly affects machining quality, tool life, and workpiece protection. For roughing operations, such as turning or milling with high material removal rates, a higher concentration (8–12%) is typically recommended to ensure better lubrication and heat dissipation. Conversely, for finishing operations, where precision and surface quality are key, a lower concentration (5–7%) is sufficient to prevent excessive foaming or leaving oily residues on the workpiece.

For special applications, such as machining aluminum or titanium alloys, the concentration must be adjusted to specific requirements. Aluminum requires a lower concentration (4–6%) due to the risk of staining, while titanium alloys often need a higher content of extreme-pressure (EP) additives. Regular measurement of concentration using a refractometer or titration test is essential—even a 2% deviation can significantly reduce fluid efficiency or increase tool wear.

Cleaning and Filtration of Machining Fluid: The Key to Long Service Life

Impurities in machining fluid, such as metal chips, dust, or lubricant residues, accelerate fluid degradation and increase the risk of microbial contamination. An effective filtration system is therefore the foundation of long-term maintenance. For coarse filtration, a mechanical screen with a mesh size of 50–100 μm is sufficient to capture larger particles. Finer filtration (10–20 μm) is necessary for operations with high surface quality requirements, such as grinding or honing.

In addition to mechanical filtration, magnetic separators can be used to remove ferromagnetic particles, or centrifuges to separate oil contaminants. It is important to regularly check and clean the filtration equipment – a clogged filter reduces fluid flow and impairs cooling. For systems with fluid recirculation, it is recommended to perform a complete tank cleaning at least once a week and remove sediment from the bottom, which serves as a breeding ground for bacteria and mould.

Preventing microbial contamination: How to keep fluid free from bacteria and mould

Microbial contamination is one of the most common reasons for the premature disposal of machining fluids. Bacteria and mould multiply rapidly in aqueous solutions, especially at temperatures of 20–40 °C and in the presence of organic contaminants. The first symptom is an unpleasant odour (typically a "rotten egg" smell caused by anaerobic bacteria) or a change in fluid colour to grey or black. Prevention relies on maintaining workplace hygiene – regular cleaning of tanks, tools, and workpieces to remove residual chips and lubricants.

Biocides are an effective solution for suppressing microbial growth, but their use must be carefully dosed according to the manufacturer’s recommendations. Excessive amounts of biocide can damage the operator’s skin or reduce the fluid’s effectiveness. Alternatives include natural inhibitors, such as essential oils or enzymatic preparations, which are more environmentally friendly. It is also important to maintain the fluid’s pH within the range of 8.5–9.5 – lower values promote bacterial growth, while higher values can cause corrosion or skin irritation.

Need advice on selecting or maintaining metalworking fluids?

Every application requires an individual approach – whether it's choosing the right concentration, additives, or maintenance method. GCG Group provides detailed technical data sheets and safety data sheets (SDS) for every supplied raw material, and our experts are happy to advise you on selection and optimization for your specific needs. Contact us – or browse our catalog of over 1,300 products.