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Glossary of Metalworking Fluids, or Coolants, Terms and Terminology

Aeration or oxygenation is the process of injecting air into your coolant sump. Just as every aquarium has something making bubbles, so should every machine sump. And the reasons are the same. Aerobic bacteria consume oxygen (O2) so that the levels of O2 will decrease, especially under floating oils. As the O2 level drops, anaerobic bacterial activity rises. And it is these anaerobic bacteria that eat your coolant. Aquarium scientists say the optimal bubble diameter for gas-liquid transfer is 1/8” to 1/4”. The air-diffuser we use on our oxygenators creates bubbles of just this size. Using shop air to blow big bubbles in your coolant is not as effective as many small holes. We suggest using holes about 40 microns (about 2 mil) in diameter to get the best results. We encourage everyone to aerate their coolant at all times, in any way possible. Some say their coolants don’t ‘like’ to have air injected into them. We consider this unlikely, for two reasons. First, air is something we breathe and are always exposed to. Second, the very activity of a machine and coolant pump agitate and aerate coolant, why not continue the process when the machine is idle?

Anti-emulsification agents are ingredients that keep foreign or external oils from emulsifying in your coolant. This is why you see oils floating on top of new coolant the instant they are introduced, appearing nice and clear and in a coherent mass. As coolant gets older, these anti-emulsion agents are consumedby anaerobic bacteria. You may notice that introduced oils take longer and longer to separate out from your coolant. Once these anti-emulsion agents are totally consumed by anaerobic bacteria, these external oils can be emulsified into the water just as the original coolant was. This is why older coolants will ‘eat’ oil, and never appear to reject them, unless you let the sump sit calmly for days. Even then, the brownish mass that appears on the surface is no longer true free-floating oil, but an emulsion that can be impossible to skim and separate from your coolant.

Bacteria comes in two types as far as coolant is concerned. Both are always present in our environment. Aerobic bacteria consume oxygen and organic materials. Anaerobic bacteria hate oxygen, and love to consume hydrocarbon based materials,- like rust inhibitors and anti-emulsification agents in coolant. Bacterial Rafts are visible colonies of anaerobic bacteria that appear to be floating oils on your coolant. But they differ from real oil in that they are mostly solid, opaque, and only appear when your coolant is about dead. At this point in life your coolant probably has those Monday morning smells. (See Monday Morning Smells).

Coolant is a soup of ingredients that have been specially blended by your coolant supplier. Two of the ingredients are rust inhibitors and anti-emulsion agents. It has been particularly designed to work with pure water at a specific concentration. It is, in fact, a type of oil. In this way your machine has the best of both worlds; the lubricity of oil plus the cooling qualities of water (see oils below). It also contains emulsification agents, among other things. Coolant is also called industrial cutting fluid.

Coolant Sump is where the flood coolant is kept in your machine. The name sump comes from a time when this tank collected all the unwanted oils and garbage from the machine. Today we think of the coolant sump as a kind of ‘aquarium,’ where oxygen depletion and anaerobic activity go hand-in-hand. A place where we must encourage the aerobic bacteria and discourage the anaerobic bacteria. A tank that must be kept at a set coolant concentration using pure water, where floating oils are eliminated and O2 levels are kept as high as possible.

Emulsion Agents are ingredients that keeps the coolant compound fully emulsified when properly mixed with water. Some coolants can be ‘cracked’ (separated from the water) in various ways because of extremes in water hardness, temperature, or by exposure to certain metals.

Floating Oils are bad for coolant, at least that is what we are taught. The fact is that the floating oils are not bad for coolant in themselves, but really create oxygen-dead zones where anaerobic bacteria love to grow (see bacterial rafts and oil). Measuring must be done with a reliable refractometer. Since there is no real coolant mix scale, it is advisable that you create a reference solution before doing anything else. Then start measuring from day one. Measure your newly mixed coolant. Measure each machine sump as often as possible. And keep measuring. The closer you can keep you coolant mix on the target recommended by the manufacturer/ blender, the longer it will last and the better it will perform.

Mixing is the process of combining your coolant concentrate with water. The most effective method of emulsifying coolant into water is using the venturi effect. This is a simple device you will find on anyfertilizer sprayer you attach to the end of a garden hose. The flow of water over an orifice pulls the coolant concentrate up into the water stream. That point of contact is extremely turbulent, and is where all the mixing gets done. The coolant has been designed to emulsify easily under these conditions. Any venturitype mixer will do an equivalent job, whether it be a $5 garden variety or a brand name $1000 version. The only other variable to consider are precision of controlling the mixing ratio, long term maintenance and reliability.

Monday Morning Smells are caused by large numbers of anaerobic bacteria decaying. Because they consume hydrocarbon- based compounds, some of their decay bi-products are HCl and H2S (hydrochloric acid and hydrogen sulfide). The HCl can appear as a greenish cloud when a machine is started after a long time of sitting idle. It usually dissipates from the sump into the air as soon as the coolant is agitated, and can cause skin irritation. H2S is not harmful in itself, but also dissipates into the air quickly once the sump is agitated, and is the source of the ‘rotten-egg’ smell.

Oils are necessary for making your machine work well. On the other hand, they are also a prime culprit in the killing of coolant. The oils come from three sources: way oils, hydraulic-coils from leaking seals, and unknown ‘tramp’ oils carried into your machine by the parts you are machining. Oils in general are not immiscible in water (won’t mix no matter how much you shake them) but there is another oil in your machine we are not usually aware of. And that oil is the coolant itself. It may be mostly water, but coolant contains emulsion agents and anti-emulsification agents. Most importantly, oil can be eaten by anaerobic bacteria.

Sump Lifetime is the amount of time it takes for the brand new coolant in your machine to ‘die’ (by whatever criteria you have for coolant death). Many continue to use coolant even after it has ceased functioning, because some components are still effective (lubricity, tool cooling). But eventually, where it is the smell or skin irritation, something will get you to suck all the coolant from the sump, shovel out the chips, scrub the sides of the tank, and then fill it up with clean new coolant. Taking care of it from that day on should mean that you don’t have to be concerned with that machine for another year. This doesn’t apply very well to machines that lose coolant during the machining process by flinging coolant on the floor or being carried off by chips.

Water is possibly the most critical component of your coolant. Ideally you should use pure distilled water to mix all your coolant, as this is probably what the chemist used when designing the coolant to begin with. In real life, we use the water from our city or well. Over time as water evaporates from the sump, there is a build up of minerals. In extreme cases we have seen water ‘crack’ coolants into their constituent components. In others we have seen perfectly mixed coolant have the consistency of thin custard. The importance of water quality can’t be overstated. There are some test kits available to measure water hardness and pH. There are also supplies of distilled water and reverse osmosis symptoms that are expensive at first, but can pay for themselves in the long run in terms of improved coolant quality and performance.

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