Metal Working Fluids (MWFs): At the Cutting Edge
If lubricants oil the interface between moving and sliding parts of a machine to keep it running effortlessly, cutting fluids wax the cutting frontier between the tool and the workpiece to keep the machining operations running smoothly.
Coolants or Cutting Fluids or Metal Working Fluids (MWFs) cost only 1-2% of the part being machined. However, their influence on the manufacturing cost of the part is a whopping 95% because soiled MWFs cause about 70-90% of machine tool wear and failures.
While application primarily dictates the choice of the MWF, safety issues also influence this selection. Human contact with coolants is inevitable. And it is the contaminants inside MWFs, not the constituents, which cause health ailments such as dermatitis and respiratory disorders.
Types of Metal Working Fluids
Chemical composition is the basis for classifying MWFs. Types include:
- Straight / Neat Oils: are not mixed with water at all
- Soluble / Emulsifiable / Water Miscible Oils: are macro emulsions or micro emulsions with more than 30% oil
- Chemical Cutting Fluids can be:
- Semi Synthetics: are micro emulsions with under 30% oil and with the size of the droplet of this oil being under 1 micron
- Full Synthetics: are true solutions without any oil
While some MWFs are multipurpose, some are good only for limited applications. Additives include emulsifiers, anti-oxidants, stabilizers, anti-microbial pesticides, and defoamers.
Straight Oils are good lubricants, but not very good coolants. This restricts their use to low-speed, heavy-duty applications. These have not evolved much since their introduction around the year 1900 and chemical synthetics have replaced them in many areas.
They comprise petroleum or mineral oils without a trace of water. Makers add 20% chlorine, fatty oils, sulphur, or phosphorous additives for imparting specific properties. The additive percentage jumps for more strenuous applications.
Soluble Oils contain 60-90% (generally, over 30%) petroleum oil with additional water soluble / emulsifiable oils that stick to the workpiece. These are compatible with low to medium duty applications such as trepanning, broaching, and tapping.
Being water based, they are better coolants than straight oils although not as good with lubrication. The water content makes them prone to microbial infestation and they tend to corrode machine tools and workpieces. You have to use additives and spend more on maintenance.
Chemical MWFs were first used in 1945 and have steadily commanded broadening acceptance since then. Full or semi synthetic, they derive their cooling and lubricating properties from chemicals and as such contain negligible quantities of petroleum oils.
These are stable and contain preformed emulsions that easily mix with water. Some contain extreme pressure (EP) lubricants that sizably cut down heat and friction associated with cutting and grinding operations.
Full Synthetics contain 0% petroleum oil and are easy to maintain. They provide high levels of cooling, lubrication, and rust prevention but may cause skin ailments such as dermatitis. Use them where you need clear / transparent MWFs in low quantities.
- chlorine, phosphorous, and sulphur compounds boost chemical lubrication
- wetting agents and soaps improve lubrication
- nitrites and amines control rusting
- nitrates stabilize nitrites
- germicides check bacterial growth
- borates and phosphates soften water
- glycols facilitate blending
This makes them excellent coolants and lubricants that control rust and rancidity while being durable. They promote seamless separation of chips from the workpiece, do not clog the cooling system, and are non-toxic, non-smoky, and non-flammable.
Semi Synthetics contain 2-30% petroleum oil in an organic water-soluble base. These are almost completely transparent and provide better lubricity at higher fluid temperatures.
In turning and milling, you rapidly remove large quantities of material. Use semi synthetic MWFs or soluble oil MWFs for such applications because these MWFs offer the synergic cooling and lubricating effect of the oil-water combination.
As compared to oil-based MWFs, chemical MWFs offer better economy, cooling, hygiene, and rust prevention with lesser inclination to foam. Semi synthetics produce 30% less drag vis-à-vis soluble oils while flowing in the coolant system tubes. For full synthetics, this figure jumps to 50%.
Chemical MWFs offer superior recyclability and weld-through characteristics while minimizing maintenance and residues. They last longer, provide longer sump life, do not smoke while welding (because they do not contain oil), and wash well in aqueous cleaning systems.
Straight oils however offer better lubrication and are less likely to cause problems when misapplied. Plus, they hold their ground in case of heavy-duty applications such as:
- cutting tough-to-cut metals viz. super alloys and certain stainless steels
- severe broaching and tapping
- crush grinding
- deep hole drilling
But then, straight oils are most likely to cause health and safety hazards. They can also precipitate the perils of fire, toxic oil mist, and slippery floors.
Disposal of Cutting Fluids
No matter how well you maintain the cutting fluid, eventually it will run out of steam and you have to dispose it. This, you have to do according to the law or face steep penalties.
Although the goal of waste disposal regulations is to completely eliminate pollution, this is rarely possible given the enormous costs and complexities of compliance. Practically, such legislation ensures a minimally polluted environment.
Now, the Resource Conservation and Recovery Act (RCRA) defines wastes in terms of their properties that can prove hazardous. It does not define them in terms of their state (solid, liquid, or gas) or composition.
Then, the Federal Environmental Protection Agency (EPA) identifies four hazardous characteristics for solid, liquid, or gaseous wastes. A waste is hazardous if it is any one of the following four:
- Toxic and leaches lethal doses into groundwater if managed improperly
- Inflammable and can create fires during storage, transport, or disposal
- Corrosive as identified by its pH value, for wastes with too low or too high pH values can affect human health and environment while reacting lethally with other wastes
- Reactive or Unstable that can provoke explosions or emissions
As a machine shop owner, you are responsible for the safe disposal of wastes. Depending on the composition and volume of wastes, your budget, and the availability of treatment-disposal options, alternatives include:
- Ultra-Filtration: handles wastewater generated in metalworking by neutralizing the hazards of waste MWF, part-washer cleaners, detergents, and other oily wastewater
Such systems with 100-300 gallons a day capacity cost about $5,000-$13,000 that is more economical than contract hauling. These are more effective than chemical treatment, are space-efficient, and easy-to-operate
- Chemical Treatment alters the composition of wastes. This is not very viable for treating metalworking wastes and small machine shop owners can afford it only for pH control
- Contract Hauling is expensive. Employ it only if your shop generates less than 200 gallons of wastes or if these wastes are highly toxic or exceptionally complex
- Evaporators do not eliminate wastes, but only lower their volumes and, thereby, the associated disposal costs. They are easy-to-operate, space-efficient, and work well with all coolants
Plus, you do not need a great deal of chemical knowledge to operate them. You do need a lot of power to operate these and a lot of labor to clean them. Use them only when you run out of options
- Centrifuges separate only solids. Furthermore, they are expensive making them good only for pre-treatment
You can dispose waste MWFs directly in the municipal sewer (with the prior sanction of the appropriate authority) only if such fluid:
- dissolves in water
- is non-toxic
- has pH between 6.0 and 9.0
- holds tramp oil below 100 mg/l concentration
- has non-toxic concentrations of heavy metal ions
- is regularly treated with biocides
- free of chips and fines
Once, MWFs were prohibitively expensive for small shop owners. One look at the exacting regulations and penalties for unlawful discharge and you understand why their use is rising by the day.