Selecting the right cutting fluid is critical. The choice will affect productivity and component quality, and of course there are health and safety implications. However for the user the options can appear bewildering.
The machine shop manager is faced with an array of water-based fluids, semi synthetic and synthetic oils, as well as bespoke fluids created specifically for an individual process.
Oils, particularly synthetic and semi-synthetic oils, tend to be the preferred fluid of machine tool manufacturers because they are consistent, prevent corrosion in the tooling and the machine, are non-aggressive and have excellent lubricating qualities.
Specialist companies such as Oel-Held offering a wide range of products are well positioned to give objective advice to ensure that the correct fluid is selected. But to get some perspective on the problem, it's useful to dip into the underlying technical background.
Many of the hydrocarbon-based fluids in use today are refined mineral oils produced directly from crude oil. They consist of a mixture of chain and ring hydrocarbons and contain double bonds that break apart when heated, causing the cutting fluid to age quickly and encouraging formation of dangerous compounds.
Mineral oil can however be processed with hydrogen to form fluids known as hydrocrack oils. They have a higher viscosity index and better lubricating abilities than mineral oil, and have less of a tendency to vaporise or atomise because of a more homogenous molecule size distribution. Thus emissions in the workplace are less than when using a basic mineral oil.
Carbolic acid ester is also used as a base for cutting fluids. The natural tendency of such fluids to vaporise is very low, but after they are not usually biodegradable, and are not fully compatible with elastomer and varnish - so they act aggressively when in contact with sealing and isolating materials.
Polyalphaolefines are synthetically produced lubricants with minimal evaporation rates, have a high flashpoint combined with low viscosity and a large viscosity index. They also have excellent ageing stability.
Accuracy, surface finish, cycle time and up-time can all be affected adversely by the cutting fluid, and the first question to ask is whether the fluid is suitable for the process. When cutting hard metal, for instance, it is essential that the fluid will not cause cobalt to leach out.
The second critical question is whether the oil is suitable for the machine. Most modern machine tools are designed and built to take a specific type of cutting fluid.
A different fluid will at best inhibit the machine's performance. For example, Titman Tools found that four new machine tools were proving unreliable just a few months after installation.
Investigation revealed that the coolant was attacking the machine, forming sulphides and other hostile substances including acids, which destroyed oil seals and metal components, particularly the copper wiring. The company switched to an Oel-Held cutting fluid, the problems vanished and production improved immediately.
The principal task of the cutting fluid is to remove heat from, and to lubricate, the workpiece. The correct fluid will minimise heat build-up, possibly doubling or even tripling metal removal rates.
Also - particularly if it's a synthetic oil - surface quality of the workpiece is improved substantially, and the decrease in thermal shock virtually eliminates microstructural changes and the development of hair line cracks. In conventional grinding, for instance, using a universal fluid typically 43% of the heat flows into the workpiece, 30% into the swarf and only 16% is taken up by the fluid.
Using a fluid created for the task, 54% of the heat is taken up by the fluid, 25% by the swarf and only 14% by the workpiece. Swarf removal from the working area is largely dependent on the cutting fluid's lubricating qualities; and the correct fluid is also essential to achieving the desired machine speed and feed rates.
The other major consideration is the pressure at which the cutting fluid must be used. Fluid pressure is the main cause of foaming, which creates micro-dry spots on the workpiece.
This in turn causes poor surface and inaccuracies because the temperature varies across the workpiece. Mineral oil for instance is generally only suitable for use up to 10 bar, whereas a polyalphaolefine-based synthetic such as SyntoGrind TTK from Oel-Held can be used up to 100 bar without foaming.
Cutting fluid life has cost implications. Oil based fluids, particularly semi-synthetic and synthetic oils have excellent stability and ageing properties which means that, once the machine has been filled, it may be up to ten years before the fluid needs replacing. Unlike water-based cutting fluids, oils need little management to maintain their performance.
However the filtration system must be suited to the oil - there is little point using a high specification oil if the filtration system does not remove swarf and allows the workpiece to become clogged with debris. Water-based systems need to be monitored daily to ensure that the mixture remains correct, with neither too much nor too little fluid.
The health hazards associated with cutting fluids are well known and documented - they include skin irritation (usually due to the bacteria formed beneath the sludge layer), carcinogen-risk, toxicity, misting and odours. Many fluids have been developed in a way which eliminates these problems ie using base fluids and additives which are proven unharmful to the skin, so there can have no detrimental effect on the operator - and which are not known to be carcinogenous.
In this regard look for cutting fluids free of amines, boracic acid, chlorine and heavy metals, all of which are a hazard to health. An aromatic content of less that 1% volume minimizes general health risks and problems associated with odour.
Effective filtration further minimizes health risks because it prevents the build up of bacteria in the oil that can affect the operator.
Over 60 years, Oel-Held has introduced an extensive range of metalworking fluids, including Sintogrind polyalphaolefine based fluids for grinding. Its research and development in Stuttgart is one of the most advanced anywhere in the world.
The standard range of over 100 products meeting around 90% of all typical requirements, includes specialist products that cover applications as diverse as jig grinding, deep hole drilling and honing. The company also develops application-specific fluids tailored to the individual customer need.(end)
