From dermatitis to decreased tool life, clean coolant is simply better and makes for happier machinists and a more productive shop floor.

Dirty metalworking fluid can cause a host of problems for your CNC machining operation and can be harmful to machinists—if not cared for properly. Here are the top reasons for keeping your coolant and filters clean and maintained—with advice on how to manage it.

Most machinists would agree that a properly mixed, high-quality cutting fluid does smell quite pleasant. But if it is not maintained or cleaned, after awhile, cutting fluid can stink. Give it a couple of weeks of neglect and the Monday morning stench is enough to make even the most olfactory-challenged machinist yearn for the weekend. If the assault on your nose isn’t bad enough, however, rancid coolant is hard on other things too, starting with your health—and can negatively affect tool and machine life, finishes and a host of other key metalworking processes and parts.

Here are a few pointers you should keep in mind if you’re to optimize productivity while protecting the shop’s most important assets: its workers.

Machining Coolant Can Cause Rashes and Dermatitis

To be fair, even freshly mixed cutting fluids contain a host of chemical additives such as biocides, emulsifiers, corrosion inhibitors, anti-foaming agents and more, any of which can cause skin irritation, rashes and dermatitis. When cutting fluid is neglected, the concentration of these chemicals may reach unsafe conditions. Also, an improperly maintained machine tool sump becomes a perfect breeding ground for microbes.

“At low concentrations bacteria can grow unimpeded in the sump and cause pH of the coolant to drop and cause rust,” says John Treese, director of global training at Master Fluid Solutions. “Low concentration [of fluid] can grow bacteria that can aggravate skin issues including simple cuts and scratches. High concentration adds too much chemical that can do the same.”

Aside from creating noxious odors and degrading coolant, these microorganisms—when combined with tiny skin abrasions—can hasten the chance of infection. “Microabrasions,” as they are known in the industry, happen when extremely fine chips and swarf reside in dirty coolant, not so much the larger chips we see flying off the workpiece during the cutting process, explains Jim Brumgard, an application engineer in Castrol’s industrial division.

“Of course, big chips can cause cuts, but that type of cut usually requires first aid,”  says Brumgard. “The skin abrasions occur when there is coolant splashed onto the skin and then the operator wipes away the dirty coolant … These often microscopic tiny fines will cause the scratches as the towel or rag is wiping them across the skin.” 

They are usually recognized as irritated skin, but what has happened is these tiny cuts have allowed the coolant to cause even more irritation due to broken skin. Bottom line: The dirty coolant has become abrasive to the skin and leaves it more susceptible to irritation, Brumgard asserts. 

That’s why coolant should be treated with respect. Wash hands regularly with a mild, nonabrasive soap. Use a moisturizer to create a protective skin barrier that slows the absorption of chemical additives. Wear thin nitrile gloves if practical (though never wear heavy gloves around machinery). Keep clothing clean and dry. And avoid using solvents to clean parts, as these eliminate the natural oils on our skin, leaving it unprotected.


Fine Mists Can Be Harmful to the Respiratory System

Though less noticeable, the delicate tissue lining our lungs is even more susceptible to irritation than our skin. This can be especially problematic at higher spindle speeds, where fine mists are generated and easily inhaled. Unfortunately, the negative health effects connected to mist might not be noticed until years later. No one wants to wear a face mask at work, but if your lathe or machining center isn’t equipped with a properly sized, high-performing air purifier and mist collector, doing so would be an excellent idea, even if you do perform routine cutting fluid maintenance. Clean coolant might smell darned nice, but that doesn’t mean you should inhale it. 

Already dealt with the health issues but need some more direct advice on coolant maintenance and disposal? Read “4 Tips to Optimize Machine Fluid Maintenance and Coolant Disposal.”

Be Clean with Machining Coolants And Be Clean with Your Machine

As a rule, water-soluble or “miscible” cutting fluids are fairly alkaline, with a pH of around 8.6 or higher. This helps to prevent corrosion of metal surfaces, keeps microorganisms in check and is a bit easier on human skin. It does, however, tend to eat paint and even rubber seals, especially on older machines not designed with modern cutting fluids in mind. To minimize this, it’s a good idea to wipe equipment down before going home at the end of each shift. Regularly inspect seals and wipers for wear and replace as needed. Install a skimmer on each machine to eliminate tramp oil, bacteria’s favorite food.

“Tramp oil will slowly dissolve into the coolant, causing to it to get thicker and stickier with time,” Treese explains. “Bacteria can feed easily on this emulsified tramp oil and then can attack the coolants’ oils as well. It is also harder to maintain the proper concentration when the sump is full of chips.”

The chips and sludge are displacing coolant, so the volume is ever changing. Keeping the sump clean makes concentration control easier. Monitor pH with a meter (preferred) or test strips weekly—if pH is falling, there’s a good chance that something nasty is growing down there. Time to give the machine a thorough cleaning and install some fresh coolant, preferably using a sump doc. 

Dirty Coolant Hurts Machine Tool Life and Other Parts of the Machine

Cutting fluids provide lubricity and remove heat from the work zone. As coolant breaks down, the sulfur, chlorine and other extreme-pressure compounds contained within become ineffective. This leads to poor tool and machine life as well as problems with accuracy and part quality.

“Machine tool life suffers when concentrations drop below the coolant manufacturers minimum concentration suggestions and can cause rust on the machine tool,” says Treese. “There’s just not enough chemical to protect the machine tool.”

“Machine life suffers from dirty coolant: That’s a true statement,” says Brumgard. “Fluid will attach itself to the workpieces, but also to the chips and the dirt, and so you have to monitor coolant for all these things.”

But there’s more damage that can occur to the machine itself—and to filters and pumps.

“The dirt in your coolant will be abrasive, it will scratch paint, and it will scratch the glazing on a machine’s windows inside a newer machine with a cabinet around the workzone,” says Brumgard. “The other thing it can hurt and wear out are pumps, especially high-pressure pumps, even when they have extra filtration to protect the pump from wear.”

So that means there’s a cost of changing filters and making sure the filter is changed and not bypassed or ignored, Brumgard explains. Some might think to remove the filter, but then you start to really wearing out costly components. And with through-the-tool coolant delivery holders, you have to pay attention to the rotary coupling’s wear, to avoid leakage, Brumgard points out.

It’s better to focus on maintaining a clean coolant operation. Here are some ways: Aside from skimming and filtration, cutting fluid concentration should be checked with a refractometer at least weekly, refreshing tired coolant as necessary with properly mixed makeup fluid. One way to accomplish this is with an automatic proportioning unit that sits on the drum, allowing the operator to dial in whatever percentage of fluid is needed to bring the sump back to the correct concentration. If you don’t have such a unit, remember the acronym OIL, short for “oil in last,” when mixing any cutting fluid. Your cutting tools will thank you.

“Machine life suffers from dirty coolant: That’s a true statement. Fluid will attach itself to the workpieces, but also to the chips and the dirt, and so you have to monitor coolant for all these things.”
Jim Brumgard
Application Engineer, Castrol Industrial

Fluid Maintenance and Disposal Matters to the Bottom Line

Cutting fluid and its disposal are a necessary expense. Though these costs don’t compare to the expense of an idle CNC machine tool, it’s a good idea to maximize coolant life whenever possible. Shops with a few dozen machine tools might consider a centralized cutting fluid system. In lieu of this, a coolant recycling center is probably the best bet, and use a Shop-Vac or sump doc to bring old cutting fluid back to the reservoir for processing.

Here again, maintenance is key. The mantra is: Skim constantly, clean regularly and check religiously. Watch fluid concentrations, using no more concentrate than is necessary but without skimping—as a rule, 10 percent is a good starting point for general-purpose work, although difficult materials such as titanium and superalloys may call for a slightly higher percentage, while aluminum generally cuts best with a leaner mix.

But be careful, warns Treese: “If a manufacturer of coolant recommends a 10 percent concentration in the sump, running at 20 percent actually doubles the amount of chemical in the sump—it does not add 10 percent more chemical as some think.”

The water is also important—invest in a deionizer or reverse osmosis system where possible, but do not use softened water. Like Goldilocks, machining operations benefit from cutting fluids that are not too soft, not too hard, but just right.

What Challenges Do You Face When Trying to Maintain Machining Coolant?

Dirty metalworking fluids can cause a host of problems for your CNC machining operations, and they can be harmful to machinists if not cared for properly.

But keeping your coolant and filters clean and maintained does present challenges.

Share your insights on this issue by taking our poll.

What challenges do you face when trying to maintain machining coolant?

Do you maintain clean coolant? If not, what challenges do you face in keeping it clean regularly? Share your experience.

Talk to Us!

You must mention the use of Centrifugal Pump Carts , they put the coolant thru a pump that separates the way oil from the coolant, then it filters the coolant, and now you have really clean coolant. Skimmers are ok, but they never get all the oil, plus they always suck up good coolant. Also, the use of a Homogenizer can make the coolant last a long time.
I was on an interview at a shop in Virginia, over 160 cnc machines, they constantly run coolant thru centrifugal pump carts and homogenizers, and they claim to have never disposed of old coolant, because they are always renewing the coolant. Some shops run those little goldfish tank aerators/filters in the sump on the weekends to continually circulate the coolant. Zebra Products sells a similar device that is better suited for coolant/oil. If you keep your coolant clean, it is one less reason for a worker to quit the job,
I've seen more filthy shops than I care to tell, places that never cleaned out sumps, workers spitting chewing tobacco into the machines (and they wondered why the coolant was Green/Brown/Black, the stink was awful !! ), my last job, no working oil skimmers, they never changed coolant in over 20 years, ( the manager would not allow the time for cleaning/maintenance ) they kept topping up with more coolant, ignoring the stench and build-up of bacteria and rancid way oil everywhere....this was one of the reasons why i quit... Don't be that shop!! Clean and maintain your coolant, get with the 21st century !!!


Fantastic insight, Phil. Thank you for sharing!


They never change coolant at b and r machine 305 moody St. Ludlow MA. This shop is infected all of us that work there sad


Perhaps you can share this article and hope for a change.


nice i learn someting new.


We're so happy you shared that. Thanks Marcelino!


My name is John Mathew Am willing to make some purchase of some items with your company and ship to Belize. Before we proceed can you please answer the following question below ;

1. Do you accept master and visa credit card payment ?
2. Do you accept private pick up by our forwarder ?
3. Can you send me your price sheet or catalog?

I will be waiting for your quick response


Here is a link to our e-commerce site where there is a digital catalog you can look through:
Thanks for your interest


Hi, should I change my coolant if there is a high bacteria count or is there something else I can do, the same for Yeast ?

Regards Shaun


Hello Saun,
when you recharge the machine sump make sure you use Whamex by masterfluid solutions . it will clean the machine thuroughly.
first run a skimmer
Second thing is to run an aerator in the sump as far away from the skimmer as you can. the bubbles push trampoil to the skimmer
third coolant should be mixed with dionized water for the best results
and if uyou still have an issue , use a biocide anti bactia solution. I would suspect you would not need to if you follow all the steps


It makes sense that your machine might get suffer from rust damages if you do not keep your coolant clean. My uncle is thinking about becoming an industrial worker so that he can make enough money to pay off his car, and he wanted to learn about the importance of coolant in machines. I'll let him know that keeping your coolant clean will ensure that his machines are properly functioning.


Great to hear, Stefan! We wish your uncle the best in his new venture.

If you'd like to learn more about coolants, we have several articles here on the Better MRO site: (

And if you have any specific questions, please feel free to post on our forum and our experts will be happy to help! ( Thank you for visiting Better MRO and sharing your experience!


My company has 5 mid size Haas machines and I am on the safety committee struggling to learn how to properly collect and dispose of the constant run off. The operators currently have large plastic Foldgers (yep the coffee) cans they have trimmed out a spot for the hose to be fed into. They then dump these can into buckets out back with an aerator to evaporate. Neither container is labeled (OSHA Violation) and I am concerned how to manage. What are some common industry practices for this?
Thanks in advance!


Hi Jeremy,
Refer to the SDS sheet for proper handling and storage and purchase an approved container that will be able to capture and store the coolant. MSC has multiple choices for this purpose (even beyond this link)- The containers should also be labeled and we suggest you place sorbent pads below them to avoid spillage or leaking, making sure the proper containment is used based on the volume of the potential spill ( ).
You can create your own labels here-
Also, due to the slip and fall potential, the collection areas should be blocked off. One way to do this would be to use barricades.


To..Better MRO admin..

Yes, Very good initiatives on the subject of maintenance of coolant systems, but still lot of issues like blockage of coolant pump, total coolant line, hoses, Tool disc blockage, reducing coolant pressure, tool life, part finish, like somany things also can be taken as a subject.

So that everyone will aware of cost saving through proper usage and maitenance of coolant system.

R.Murali,Product Head, CNC lathe division, BFW Limited, India.


I can understand how a business could really benefit from making sure that their machines are maintained correctly. It was interesting to learn about how they can optimize productivity by cleaning their equipment, and sump at the end of the day. It could be really useful for them to get the best technical support from a professional to help the machines be more effective.


Unfortunately I work in a dirty company and they don't change the coolant for a year or more. 


My husbands wasn't cleaned either. Eventually they started cleaning his machine every six months. Few people at my husbands job had lung issues. Then when the mask mandate went in he was working 10hrs a day with coolant sitting on his mask since his machine sprayed him consistently. Within two months of the mask mandate he broke out with a rash that looked like 2nd degree burns & developed rapid ILD & I almost lost him. The coolant definitely caused the rash & I suspect the ILD also. Unfortunately he can no longer work & currently trying to get on the transplant list for a double lung transplant. Please be careful with this shit. There are studies out of the UK Thats says this shit can cause skin conditions & lung disease. 


Thanks for sharing your story. We wish you and your husband the best.


 Hi! info is great. we have been having issues with our coolant for a couple of years. We had the coolant company come in to help us figure it out. The problems are a strong stench, dirty socks smell we can't get rid of. Burning sinus and possible skin iritants. Coolant caking on top layer turning brownish with white and black spores on surfaces. They tested ph it was good. they took samples from differnt machines. They said no bacteria or microbes/ mold. Gave us bacticide to add anyways. With no changing to the problems. In one machine we switched coolant to hocut 795-H after running cleaner in the machine provided by the supplier. Previous coolant was hocut ws 8800. Which is still in other machines. Good for a few weeks smell came back. We installed disk skimmers on the cnc machines. It's better but still having build up on the surface. Coolant is white or milky when fresh. Changing to light brown grey on the surface. In the other machines coolant is green when fresh. It has been changing black on the surface. Coming out of the skimmer it's black and oily texture. In the new white coolant of the skimmer is light to really dark grey. We have gotten another coolant valcool vp 800 pm to posibily try next. Running low on solutions. The  coolant tank has a chip seperator so this area is not circulating from the skimmer. Not sure if the fish pump is the way to go. Do you have any recomendations? Concentrations are all over the place. Were trying to create a maintence plan and we have ph strips to check weekly. Just started today. Please help!!!


   Did you let the coolant tank dry after taking out the coolant?

You need to let the tank dry; then thoroughly clean the tank and system with cleaner. then add new coolant. Do you use filters and a centrifuge or skimmer? Check the PH and concentration regularly, and provide regular maintenance.



I have issues with fine swarf in the coolant system coming back through and creating a fine build up on the machine surfaces and Tooling any ideas to remedy this?




There are several reasons to keep machines clean, and these reasons make it more evident why to keep the machining coolant clean. Dirty coolant reduces the effectiveness, which reduces the quality of the machined parts


Great article. Thanks for the insights.

i would also like to share my experience from shop. We run about a dozen machining centers and some time back we sourced coolant quality continuous monitoring sensors from a company, SciLogic. Since we had some dimensions which would fire if coolant concentrations are off.  The system has been sending alarms for concentration drops and trends on ph, temperatures etc. and recommending additions. It has worked wonders. As an owner Didn’t focus on coolant as a contributor to rejections before.


This message if for Kip.

We make a water-based product from Alpha Biologix called "Quantum Pure" That kills the bacteria in any coolant. We've tested in CNC shops across the country, and in our CNC's running the same coolant for 2 plus years. Coolant goes airborne, and that bacterium is inhaled, gets on the skin and cloths throughout the building. We've treated large systems and small (50) gallons.... Non-chemical, we're just getting started with the sales and marketing efforts. Machinists love it, no more stinky cloths or infections...

Mark K 



Leave a reply

Your email address will not be published. Required fields are marked *


Signing into Better MRO is easy. Use your username / password, or register to create an account. We’ll bring you back here as soon as you’re done.

Redirecting you in 5 seconds