When guide rail oil meets cutting fluid, keep them apart!

We know that the primary functions of cutting fluids—cooling, lubrication, rust prevention, and cleaning—are achieved through various additives with distinct roles.

Some provide lubrication, others prevent rust, while others act as biocides or foam suppressants (without defoamers, your machine tools would be taking bubble baths daily). Other additives exist, but we won’t cover them all here.

Unfortunately, while these additives are crucial, many of their components are oil-based and highly temperamental. Some are mutually incompatible, while others are insoluble in water.

Newly purchased cutting fluids come as concentrates that must be diluted with water before use.

Therefore, we must highlight the following additive: without it, emulsion-type concentrates cannot emulsify stably with water to form effective cutting fluids, rendering all other properties meaningless.

The Emulsifier: The Enabler Behind Water-Soluble Cutting Fluids

This crucial component is called the “emulsifier.”

The emulsifier’s role is to enable those components that are difficult to dissolve or insoluble in water to blend harmoniously, much like milk and water.

It ensures all additives are uniformly and stably distributed throughout the cutting fluid, creating the dilutable solution we require.

Guideway Oil: Why Anti-Emulsification Is Non-Negotiable

Now let’s discuss machine tool guideway oil. Guideway oil must possess excellent lubricity, rust prevention, and anti-wear properties (meaning the lubricating oil film resists being squeezed out or ruptured under heavy loads). Another crucial property is anti-emulsification capability.

We know cutting fluids contain emulsifiers that fully emulsify their components.

This brings us to today’s core issue: one requires emulsification, while the other demands anti-emulsification.

When Cutting Fluid Meets Guideway Oil: A Built-In Conflict

When cutting fluid encounters guideway oil, the active components in the guideway oil get emulsified by the cutting fluid’s emulsifiers.

Your guideways then face a “triple-deficit state”—deprived of protection, lubrication, and rust prevention.

What should you do? Incidentally, the emulsifiers in cutting fluids have adverse effects not only on guideway oil but also on other machine tool fluids like hydraulic oil, and even painted surfaces.

The wear, corrosion, loss of precision, and even damage in many machine tools can be traced back to the influence of emulsifiers.

If your machine tool guideways operate in a fully enclosed environment, you need not read further. However, to the best of our knowledge, 99% of machine tools cannot completely seal their guideways.

Therefore, you should carefully review the following information and share it with relevant colleagues—they will undoubtedly appreciate your assistance.

Why Separability Determines Precision and Fluid Life

In modern machining workshops, selecting the right guideway oil is critical. Guideway oil impacts machining precision and the lifespan of metalworking fluids, directly affecting machine tool productivity.

An ideal guideway oil must not only provide excellent friction control but also maintain outstanding separability from water-soluble cutting fluids commonly used in metalworking processes.

If the selected guideway oil fails to separate completely from the cutting fluid, the oil will emulsify or the cutting fluid’s performance will deteriorate. These are the two primary causes of guideway corrosion and poor lubrication in modern machine tools.

In machining, when guideway oil encounters cutting fluid, there is only one imperative: keep them apart!

Therefore, when selecting guideway oil and cutting fluid, their separability must be effectively evaluated and tested.

Accurately assessing and measuring their separability prevents losses during actual machining processes, playing a crucial role in ensuring the precise operation of mechanical processing equipment.

Here are six simple yet practical methods: one trick to spot issues, two tests to verify, and three maintenance steps to easily resolve separability problems between guideway oil and cutting fluid:

One Trick to Spot Issues: Symptoms of Poor Separability

If guideway oil emulsifies and fails, your machine tool may experience the following problems:

• Reduced lubrication effectiveness and increased friction
• Potential for higher energy consumption
• Wear on material surfaces or coatings in contact with the guideways
• Corrosion of the machine and components

Alternatively, if your cutting fluid is contaminated by guideway oil, you may encounter issues such as:

•  Changes in cutting fluid concentration, making performance difficult to control
•  Deteriorated lubrication, severe tool wear, and reduced machining surface quality
•  Increased risk of bacterial growth and unpleasant odors
• Lowering the cutting fluid’s pH value, potentially causing corrosion
• Excessive foaming in the cutting fluid

Two Standard Tests to Verify Oil–Fluid Compatibility

The cost of treating cutting fluid contaminated by lubricating oil is high. Therefore, it is better to prevent problems than to remedy them after symptoms appear.

Machining companies can quickly determine the separability of candidate guide rail oils and cutting fluids through two simple standard tests.

TOYODA Anti-Emulsification Test

The TOYODA test simulates the phenomenon of guide rail oil contaminating cutting fluids. In the test, 90 milliliters of cutting fluid and 10 milliliters of guide rail oil are poured into a container.

They are stirred vertically for 15 seconds.

After 16 hours, the condition of the liquid inside the container is observed, and the solvent volume (in milliliters) is measured in three sections: the separated guide rail oil (top), the “mixture,” and the separated cutting fluid (bottom).

If the recorded test result is 90/0/10 (90 ml cutting fluid, 0 ml mixture, 10 ml guide rail oil), this indicates “complete separation” between the oil and cutting fluid.

If the result is 98/2/0 (98 ml cutting fluid, 2 ml mixture, 0 ml guide rail oil), it indicates emulsification has occurred, and the cutting fluid and guide rail oil have not separated effectively.

SKC Cutting Fluid Separability Test

This test simulates the contamination of guide rail oil by water-soluble cutting fluids. Guide rail oil (8 mL) is mixed with various conventional cutting fluids (2 mL) at an 80/20 ratio.

The mixture is stirred at 1500 rpm for 1 minute, then visually assessed at 1 hour, 1 day, and 7 days. The mixture’s state is graded on a 1-6 scale based on the following criteria:

• 1 = Complete separation
• 2 = Partial separation
• 3 = Oil + intermediate mixture
• 4 = Oil + intermediate mixture (+ cutting fluid)
• 5 = Intermediate mixture + cutting fluid
• 6 = Entirely intermediate mixture

Experiments demonstrate that using cutting fluids and guideway lubricants from the same supplier facilitates better separation between the two.

Using actual products as examples, when Mobil Veda™ Digital Series guideway and slideway lubricant and Mobil Cutt™ water-soluble cutting fluid were mixed at oil/cutting fluid ratios of 80/20 and 10/90 respectively, and subjected to the two tests above:

Mobil Veda™ Digital Series readily separated from the cutting fluid;

Mobil Cut™ cutting fluid retained a layer of lubricant on top, which was easily removed, with only minimal mixture observed (data provided by ExxonMobil Research and Engineering Company).

Illustration: MobilVeda™ Digital Series guide rail and slideway lubricant demonstrates significantly superior separation from cutting fluid, producing only minimal mixture. [(Top image) 80/20 oil/cutting fluid ratio; (Bottom image) 10/90 oil/cutting fluid ratio]

Maintenance Practices That Preserve Separability in Real Production

Three Maintenance Tips: Key to Ensuring Efficient Shop Floor Operations

Of course, confirming the separation properties between guideway oil and cutting fluid does not guarantee permanent results.

During equipment operation, numerous uncontrollable factors can still impact the performance of both guideway oil and cutting fluid—factors that must not be overlooked.

Routine maintenance and upkeep remain critical factors in ensuring efficient workshop operations.

Proper Maintenance: Beyond guideway oil, other machine tool lubricants—such as hydraulic oil and gear oil—can also cause contamination.

Therefore, regular maintenance is essential to prevent contamination caused by cutting fluid coming into contact with different machine oils, inhibit the proliferation of anaerobic bacteria within the cutting fluid, thereby maintaining its performance, extending its service life, and reducing odor generation.

Cutting Fluid Performance Monitoring: Regularly monitor the concentration of the cutting fluid using a refractometer. Under normal conditions, a distinct, fine line will appear on the refractometer.

However, if the cutting fluid contains a significant amount of emulsified guideway oil, the line on the refractometer becomes blurred, indicating a high level of floating oil.

Alternatively, the concentration of the cutting fluid can be measured using titration. Comparing the results with the concentration of fresh cutting fluid also reveals the degree of oil emulsification.

Removing Floating Oil: Many modern machine tools are equipped with automatic floating oil separators, or separators can be added separately to the equipment.

In large systems, filters and centrifuges are typically used to remove floating oil and other contaminants. Additionally, tools such as industrial vacuum cleaners can be used for manual removal of floating oil.

Conclusion

In modern machining environments, the interaction between guideway oil and cutting fluid is not a minor compatibility issue but a decisive factor in machine reliability, precision, and operating cost.

Cutting fluid emulsifiers, while essential for stable performance, can quickly undermine guideway lubrication if separability is overlooked, leading to wear, corrosion, and fluid degradation.

By selecting compatible fluids, verifying separability through practical testing, and maintaining disciplined monitoring and oil removal practices, manufacturers can effectively prevent emulsification-related failures.

Ultimately, keeping guideway oil and cutting fluid separate is fundamental to protecting machine tools, extending fluid life, and sustaining stable, high-quality machining performance.