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When to Change Your CNC Coolant

Knowing when to change your CNC coolant is essential to maintain optimal performance and prevent potential health hazards. Proper coolant maintenance is crucial for the smooth operation of your CNC machinery. Coolants play a vital role in lubricating tools, dissipating heat, and preventing corrosion during the machining process. However, over time, coolants can degrade, leading to various issues that can compromise the efficiency and safety of your operation. 

While each coolant has a recommended service life, several factors can accelerate its degradation, necessitating an earlier change. One of the primary indicators that your coolant needs replacement is the presence of visible changes, such as discoloration, excessive foaming, or a rancid odor. These signs often indicate bacterial growth, chemical breakdown, or contamination, which can lead to decreased lubricity, reduced heat transfer, and potential health risks for operators.

Additionally, if you notice an increase in tool wear, surface finish issues, or residue buildup on your machine components, it could be a sign that your coolant has lost its effectiveness. These problems can lead to subpar product quality, increased maintenance costs, and potential safety hazards.

Beyond visual cues, regular coolant testing can provide valuable insights into the fluid's condition. Monitoring factors such as pH levels, concentration, and bacterial counts can help you make informed decisions about when to replace your coolant, ensuring optimal performance and minimizing potential health risks.

What factors must you take into consideration when it comes to changing your CNC machine coolant?

There are many signs that a fluid is at the end of it’s life and you should change your coolant. If one or more of the following changes has happened, the fluid should be evaluated to see if it is safe for continued use or if it should be replaced.

Low sump level – It May be Time to Change Your Coolant

Check the sump level at the start of the shift. This can easily be done when you use the Zebra Dazzle sump level and concentration management system. A low sump level (based 30% below the full mark) shows metalworking fluid loss or water evaporation. This increases the concentration of chemicals present in your metalworking fluids (MWF).

Check the concentration with a refractometer. If it is too strong, add clean water to reach your desired concentration. If the concentration is correct, then fluid was lost due to drag-out. You should add fluid at an appropriate dilution. If pre-diluted fluid is not available, water and concentrate can be added.

All systems should be monitored carefully and fluid additions should be maintained on a regular basis to maintain a constant working concentration. The correct concentration should then be verified with a refractometer when you are finished.

Abnormal Fluid Appearance – It May be Time to Change Your Coolant

Determine if the fluid color in your sumps looks normal. Synthetic fluids are usually clean and clear when it is in good condition. Semi-synthetics are often transparent to milky. Soluble oil looks milky white with no free oil layer.

If the fluid in your tanks is a deep gray or black, then bacteria are often present. If the fluid picks up a yellow or brown tint, tramp oil may be present. Dye fading may indicate that a fluid is aging. This can also effect the lubrication for your tooling. When fluids smell bad, it usually means that there is uncontrolled microbial growth. This issue can be brought under control by changing the sump pH using odor control tablets as a treatment.

It may be possible to cover up the odor, but that is not a great idea. It’s best to address the cause. Microorganisms in the fluid can grow and be aerosolized into the air as part of the mist. This may cause adverse health effects to exposed employees.

If the fluid has a strong and “locker room” odor, it likely has biological growth. Your metalworking fluid should be treated with biocide and evaluated. If need be, the fluid should then be discarded, the sump properly cleaned, and the fluid replaced.

Floating Matter on the Fluid – It May be Time to Change Your Coolant

If the fluid has floating material like chips, swarf, or mold growth, this is not normal. Try to remove as much as possible with a skimmer. The level of dirt (total suspended solids) in the fluid is a measure of the efficiency of the filtering system.

Periodic checks and maintenance of the filtration system and oil skimmer are necessary. This way you know that they are functioning as designed.

Sometimes you will have tramp oil floating on the surface. When you are working with water-diluted fluids, check to see if the sump is completely covered with oil. If the machinist cannot swish the tramp oil out of the way for more than 5 to 8 seconds before the sump is covered again, there is too much tramp oil present.

An easy solution to remove the surface oil is with a skimmer.

Tramp oil is one of the main causes of dermatitis. These fluids are not developed with repeated skin contact in mind. Some chemical components of these machine lubricants are highly irritating to the skin. Unemulsified (tramp) oils can also be a significant carrier of metallic fines. These can be deposited on the skin and cause mechanical irritation. These fines, suspended by tramp oil, are a major cause of dermatitis.

A solution to this is could be to use natural oil machinists soap.

Excessive Foam – It May be Time to Change Your Coolant

A lot of foam may be caused by soft water with some products. The coolants may also be too highly concentrated, or it may be contaminated by cleaners. There also may be an imbalance in the fluid surfactants.

Another possibility is that you could have an undersized system, excessive flow rates, or the fluid may not be at rest long enough to allow air to escape. It can be hard to tell.

Check if the level of cutting fluid in the reservoir is low, causing air to be drawn into the pump. Dirty machines or trenches can also cause excessive foaming and bubbles. This could mean that the emulsion is becoming unstable. The cleaners in the fluid could be depleted as well. Another possibility is that the contaminants are being deposited from the fluid or that there is filter failure. Keep up with your housekeeping to monitor these issues.

If employees have skin irritation, this could mean that the fluid has one or more of the following properties:

  • too high a concentration
  • high alkalinity
  • metal contamination
  • an unstable emulsion
  • contamination from work-piece coatings

Of course, skin irritation can also be due to causes not directly related to metalworking fluids, such as changes in the weather, poor personal hygiene, poor work habits, the use of harsh hand soaps, wearing contaminated clothing, or prolonged exposure to the fluid.

Employees Have Respiratory Irritation – It May be Time to Change Your Coolant

Exposure to MWF aerosols can also lead to complaints of irritation and tightness in the chest.

Factors that can contribute to irritation could be:

  • improper delivery of fluid to the cutting zone
  • improper use of additives
  • a high coolant concentration
  • a heavy concentration of machines in a small area
  • inadequate or poorly designed enclosures and mist collectors
  • loss of microbial control
  • poor general ventilation of the shop
  • insufficient fresh air make-up rates
  • high mist concentrations

Other problems might also be fluid-related and that should be investigated as well. Check to see if the fluid is failing and may no longer be safe to use.

Some examples are:

  • rust or corrosion of the machine tool or of the part produced
  • staining of the metals machined or machine tool
  • tool failure due to the loss of performance additives
  • growth of bacterial or fungal growth that also blocks fluid flow
  • change of fluid viscosity (thinner or thicker)
  • accumulation of water at the bottom of the oil sump drain, in straight oils
  • dirt and grit suspended in the fluid
  • failure at the workpiece-tool interface (ex: burning of a ground part because of excessive heat build-up)

Source: OSHA

Case Study

Coolant Maintenance Sucks!

Chip works for a large aviation plant, and had serious problems removing tramp oil and cutting oil from his large mills.

Large fluctuations in fluid levels and the quantity of tramp oil in his metalworking fluids (coolant), was making coolant maintenance difficult. He tried floating skimmers from Keller, Abanaki, and ITW, but they didn’t work.

For his situation a belt skimmer, disk skimmer, and even a tube skimmer were not appropriate.

For that reason he went with a Zebra Skimmers coolant coalescer, the Zebra Muscle, using the Hammerhead floating skimmer.

Chip installed the Zebra Muscle coalescer easily, and the unit worked perfectly, at first. Then one day he calls us with a problem.

“It stopped sucking” he claimed!

We didn’t understand why it would fail.

Engineering took over and had him go over every element of the oil collection system in detail.

When they got to the Hammerhead, he confessed that, “I sucked on the tube to try and prime it but nothing comes through!”

“You’re sucking on the tube?” We didn’t believe our ears.


“With all that coolant, metal, and also biology in there?”


What a guy, we thought!

Then we had an inspiration.

“Chip, which way do the Hammerhead floats point?”

“They’re on the bottom.”

“Chip,” we said carefully, “turn the Hammerhead over, turn the pump on and tell us how it works. Don’t suck on it, okay?”

He came back to us a few moments later.

“Hey, it works great again! Thanks!”

The moral:

Don’t EVER suck on machine coolant!

And give us a call at 1-888-249-4855 if anything goes wrong – we’ve seen it all!

Click here to contact us about you coolant issues because we can help guide you to your best solution!




About Zebra Skimmers:

Our mission has always been simple. It is to provide the metal working industry with the oil skimmers and metalworking fluid management systems they need to complete jobs quickly and efficiently. Innovative and affordable solutions have gained us many accolades. Our continued growth is due in no small part to our excellent customer service… Not to mention our satisfaction guarantees and the performance of our tools.

Among our diverse options of oil skimmers and metal working fluid management systems, you can expect to find: