Tech Talk
What are the different types of cutting fluid?
There are four main types of liquid cutting fluid.
1. Straight Oil. Straight oils, as the name implies, are mostly petroleum-based hydrocarbon fluids, sometimes with additions of animal fat, vegetable oil, or marine oil to improve surface wetting and lubricity. These fluids generally have Extra Pressure (EP) additives to provide even greater lubricity under arduous machining conditions. EP additives are typically sulphur, chlorine, and phophorous. Straight oils tend to produce the best surface finishes and provide an environment in which superabrasives enjoy an extended life. Care has to be taken in choosing such fluids because some workpiece materials are very sensitive when exposed to some EP additives, particularly at elevated temperatures evident in the grain/workpiece surface interface. Often, sulphur has been the culprit in the hydrogen embrittlement of high-strength alloy steels. Also, there have been medical studies carried out on operators exposed to the atomized particles and fumes from straight oils. The results have shown a high incidence of cancer-related illnesses. Couple the medical findings with oil’s propensity to catch fire at relatively low flash points, and it is easy to see why there is great pressure on the cutting fluid research establishments to develop nonpetroleum-based cutting fluids. The fact that oil is, by far, the best lubricator, yet not necessarily a good conductor of heat, makes the task of developing new fluids a major challenge.
2. Oil/Water Emulsions. Though somewhat of a misnomer, as oil cannot be completely emulsified in water, these fluids are more than 80 percent water and the rest oil with chemical additions, which assist in the dispersion of the oil droplets in the water. These additions are soap/detergent type fluids. Oil/water emulsions are generally milky in appearance. The ratio of oil to water affects the lubricity of the fluid. These water-based fluids are more able to conduct the heat from the grinding operation than straight oils. However, rich fluids can be quite lubricative, particularly if they contain extra EP additives. A problem generally arises when the oil/water mix becomes too rich and begins to foam under the churning action of pumping and grinding with the fluid. Defoaming agents can be added to the fluid, however these agents will tend to degrade the performance of the fluid, particularly if they are added in large quantities, which will decrease the wetability of the fluid. If a defoaming agent is used, only a minima amount is required as an interim measure before a more suitable cutting fluid can be found. Deionized water will assist in extending the life of water-based fluids and will reduce the tendency of a rich mix to foam.
3. Synthetic Fluids. These too are water-based fluids, however, with no additions of oil. They are purely water solutions of liquid chemical additives. That means that the additions to the water have completely dissolved. The solutions are usually clear with a hint of color. They do not have as good rust inhibitions as fluids containing oil. Most of these fluids are inactive and do nothing more than cool the cutting zone; some, however, are active and contain chemicals which assist in the machining to form surface chemistries, which require much lower shearing force to grind. Such fluids can have an adverse effect on the machine tool itself in the forms of rusting, corroding, paint stripping, as well as attacking certain types of seals, and having no compatibility with lubricating oils whatsoever.
4. Semi-synthetic Fluids. These fluids are a hybrid of chemical solutions and the oil emulsions. Therefore, they combine the properties of each. In fact, these fluids have been developed to a point where a 10 to 15 percent solution can have almost the same lubricity of a straight oil, yet have the cooling capability of water. These are generally the premium fluids, and the most expensive, however, for grinding very sensitive materials and for producing the highest quality parts in a relatively safe environment, these are the cutting fluids of the future.
