Most machine shop operators think of Metalworking Fluid Maintenance (MWF, or coolant) as a necessary nuisance, something that blows chips off the part. In fact, the most important jobs for all MWF are keeping the tool cool so it lasts longer which maximizing cutting, and lubricating the tool edge so that it makes a faster and cleaner cut.
Today’s MWF are a sophisticated soup of chemicals that all try to do the same thing, blend the best properties of oil into the best properties of water. Along the way the compounders who make these MWF try for other goals as well: rust inhibition, tolerance of a wide range of water hardness, ability to work with many metals, and even environmental safety.
Metalworking fluid is used to cool and lubricate the work surface. Depending on the type used, it will also contain, in part, components that aid in rust protection, bio-defense, and solution stability. Here’s a breakdown of the types of metalworking fluids and what their constituent components are for the cutting process.
All MWF types have components which aid in the extreme pressure generated by the metalworking process:
- a boundary lubricant to keep molecules slippery
- corrosion protection inhibitors to prevent rust
- defoamers to control foam
- metal deactivators to deactivate the ions that can degrade the fluid
- a dye for aesthetics
All but the straight oils contain some type of biocide to control bacteria, pH buffers to keep the pH consistent as possible, and a stability agent to keep the mixture in solution. Oil-soluble and semi-synthetics contain mineral oils as their bases, and incorporate emulsifiers to maintain their emulsions when mixed with water.
A full-synthetic contains no oils, but a wetting agent to handle lubricity needs. Each type, however, has its own attributes, and these should be considered when selecting a particular blend for the metalworking operation.
When your fluid starts to fail, tank-side additives can be added to control the problem.
Check with your coolant supplier to find out more information on what additives they recommend to use with your particular coolant blend for each failure type.
Three major forces are making us take a better look at our metalworking fluid maintenance processes.
- Coolants have become expensive
- The increasing cost of labor means they are more expensive to take care of
- The cost of disposal has increased
In order to keep our coolants working properly for years we have to understand how they work, and why they go bad. The rest of these basics will cover the hows and whys behind cutting fluid maintenance. An understanding of what is happening with your coolant can lead to positive outcomes such as maximizing cutting performance for longer lasting coolant before it will need changing or major maintenance.
The three factors affecting metalworking fluid maintenance
Chemistry, is where metalworking fluid maintenance starts.
MWF are a complex blend of many chemicals, whether organic or fully synthetic. The basic function is to allow oil to emulsify into water. They also must be able to:
- resist corrosion of steel
- not alter the surface of aluminum
- maintain pH stability around 9 to 10
- resist breakdown from the extreme heat of the tool tip
Metalworking fluids all start on the formulator’s chemistry lab bench.
This is where chemists blend together various compounds, much like a kitchen recipe. Most of these ingredients are publicly known and governmentally approved as being safe.
A few ingredients are proprietary. Both will be listed on the fluid’s Material Safety Data Sheet, or MSDS. Just knowing the ingredient, however, is not enough to make the coolant yourself. You still need to know how it is used, at what stage of the recipe, and exactly how it is added are also critical to its function.
The main property the chemist will impart is the ability for the coolant to emulsify into water (except straight oils) and remain stable as an emulsion over a wide temperature range. Other properties are for the coolant to prevent rust on parts and machines, reject tramp oils which cause biofilms and excessive mist, provide the right amount of lubricity so tools and metals do not weld together, and to prevent foaming.
Biocides will be added to the recipe as well, in safe amounts, to prevent unwanted biological activity.
The aging of coolant revolves primarily upon chemical activity.
Over time, acids are introduced into the coolant system via bacterial activity. The bacteria comes mainly from the water that is used. This is why it is important, when topping off your sump, to add a reduced coolant mix rather than straight water.
The acids that accumulate in your sump are hydrochloric acid (HCl) and hydrogen sulfide (H2S). These acids will eventually bring down the sump’s pH, which increases rust and corrosion, bad odors, and possible skin reactions.
For those who have swimming pools, the concept called pH buffering is used. Buffering means that there is some compound, usually a carbonate of some form (like baking soda) that remains in solution but does not fully dissolve. Coolant formulators add these buffering components to their formula. But as more acids accumulate, they will combine with these reserved carbonates and neutralize.
Once all the reserves are used, additional accumulation of more acid will impact the pH of the sump.
Once the pH goes too low, it is likely that they will lose their performance qualities. Therefore, they are no longer a useful tool. It will lose its anti-rust capabilities, bacteria will grow worse, and the emulsion itself may split out into its constituent components.
A split emulsion can also be caused by water hardness, if in excess of 450ppm of carbonates. Hardness can cause coolant to foam excessively. It also helps the tramp oils form a layer of residues on the coolant surfaces, which is difficult for removal.
The chemical cancentrates of the coolant can also be affected by the metals that are introduced during the machining process. Ionic activity between the positive atoms of the metals and negative atoms in the coolant can cause premature failure as the emulsion starts to break. The amount of metal chips in the coolant system, as well as their size, can affect the emulsion in as little as 15 minutes (on a grinding application).
Biology, is where metalworking fluid maintenance all end, because microbes are the real enemy.
Whether you fight infections with an antibiotic, or enjoy beer with your cheese, you have dealt with a microbe in some form. Given the right environment and a little bit of time, your pristine sump can go from having only a few hundred microbes in every milliliter (about ten drops) to over a million.
Biological activity within a coolant system needs to be kept to a bare minimum for your coolant to function properly.
Most bacteria enter the system through the water supply, but can enter via debris that is dumped into chip hoppers, conveyors, and even the sump itself. They also enter off the material, and build up in any coolant-related equipment, including those for coolant delivery.
Bacteria feed off the oil layer and consume carbons, hydrogen, nitrogen, sulfur, and phospherous. A population of 8 bacterium can multiply into 1,032,000,000 in a 12 hour period,. A visible colony is composed of at least 2 billion bacteria!
Communication among your entire team will be important for awareness of how bacteria enter the system, develop into a biomass, and eventually degrade your coolant, plugging or clogging filters, cause corrosion, and health and safety issues.
If your coolant system keeps having problems with bacteria or even fungi, these failure factors must be addressed. Remedy actions must be taken, to solve the problem.
Mechanics, is what makes the cutting fluid work, and what keeps them working.
The pump forces fluids through pipes and onto the work-piece. The cutting fluids then splash down into the collection sump.
Oils from the ways, leaking seals, and from other sources ends up in the sump as well, eventually floating on top of the MWF. Chips and dirt from the machine also find their way into the sump. This collection of fluids and particles presents a very different problem of coolant maintenance for the longevity of every sump.
We usually don’t think of coolant maintenance as mechanical, but this is the most important aspect, which keeps the the chemistry and biology in proper order. There is a direct link between what we do (or don’t do) at the mechanical level to what goes on at the microscopic level.
By mechanics we are specifically referring to keeping the fluids moving and taking out foreign contaminants. These include tramp oils, emulsified oils, particulates created from the machining process, and any other thing that may end up in the sump.
Remove the oil layer for better metalworking fluid maintenance
This layer is what seals off the air supply to the sump and provides life for the bacteria waste. By keeping oxygen from the coolant, anaerobic bacteria will thrive. These are more dangerous to the chemical components of the coolant than any other types.
The oil acts as their food source. They combine with it to create a biofilm, which forms a skin on the surface. These biofilms are difficult to remove. They lead to the contamination of the entire machine in your workplace once they are formed.
The simplest existing method to remove floating oils are a variety of mechanical skimmers, which mount to the sump, pull off the oil layer with some type of media (disk, belt, or tube), and drain it to a collection container.
Some also have a mini separation tank so there is no wasted coolant which increases its effectiveness. This is because it may be picked up during the process since most coolant contains some mineral oil.
There are also coalescers, which pull the oil off via a collection apparatus. Then they transfer it to a drum or tank for separation. The de-oiled coolant then is introduced back the sump. This type of unit is the best type, since it requires very little maintenance and keeps the fluid in circulation. Some of this variety also incorporates other aeration devices and can filter small floating objects.
For the best metalworking fluid maintenance, you have to keep the fluids moving
Circulation of the sump in your shops is as important as any other mechanical method since it keeps things moving, forcing oxygen into the system. Oils have a harder time pooling on the surface, which helps to prevent bacterial biofilms.
If you have any doubt about this strategy, think about how an aquarium works. Why do ponds can go stagnant? It may not be possible to leave the coolant pump on at all times. A small centrifugal pump can be introduced to the sump to keep the coolant from remaining in a static state though.
All baffles should be agitated if the machine sump is divided. Small aerators can also perform this function if the sump holds 50 gallons or less.
Keep in mind any coolant that pools in the lines is affected by the same forces in play. Lines are usually overlooked when machine sumps are emptied, cleaned, and recharged. A percent of the bacterial contamination still exists within the system.
Don’t forget to remove the particulates
Finally, remove both floating and sinking chips. This way, they do not interact with the chemical components of the fluid to cause premature failure. Although most of the material sinks the bottom, they will interact with any fluids that they touch. This also happens even if a conveyor is in operation.
Use a shovel or industrial vacuum to remove the larger, sinking chips. Also, use a general filtration system on a usual basis to remove all others. You can filter some floating chips with a coalescing unit that has this ability. However, very fine chips, such as 5-25µ, need to be removed.
Contact your rep with questions about how to properly service your metalworking fluids. We will also be happy to assist with any questions you may have for your shops.
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