Prominex Article: Cutting Fluids

ARTICLE: Cutting Fluids

Metalworking processes generate heat and friction, two factors that can damage not only machine tools but the microstructure of the metals being worked. To deal with this, cutting fluids are employed. They enable the machinist to remove more metal per unit of time and improve product quality. Cutting fluids are an important factor in reducing costs and scrap while attaining optimum production and extending tool life. Development of new metals, new machining procedures and cutting tools have spurred a wider array of cutting fluids.

History

Throughout history, water has been the most common coolant used despite the drawbacks of providing little lubrication at the cutting tool-work piece interface and the fact that water promotes rust. It removes heat well but its drawbacks led to development of better fluids. Other lubricants included lard and lead mixed into lard or oil.

Functions

Cutting fluids accomplish two primary purposes: cooling and lubrication.

Machining processes generate heat. As the tool tip creates metal chips in cutting, friction of the chip sliding across the tip creates heat. This heat must be carried away from the cutting area or damage to the piece and tool will result. Without cooling, these chips can weld to the cutting surface, dulling the tool and distorting the metal being worked.

The lubricating properties reduce internal friction and heat by reducing molecular disturbance. Lubricants reduce the shear angle of the tool edge which results in thinner chips which are more easily flushed away.

Types of fluids

The three main categories of fluids are liquids, pastes/gels and mists, with liquids being by far the most common.

Liquids

Water has the best cooling properties but is a poor lubricant. It also contains bacteria which can effect metals adversely. Oil has good lubricating properties but does not remove heat well. By creating water/oil emulsions through the use of detergents, cutting oils are created that contain the benefits of both. By adjusting the proportions of water to oil, cutting fluids with different properties result. Lean oils (lower oil to water ratio) provide better cooling but less lubrication. Richer oils (higher oil to water ratios) offer greater lubrication but less cooling.

Liquid cutting fluids are mineral, semi-synthetic or synthetic in composition. Mineral oils are petroleum-based and have been in use since the 1880s. Semi-synthetic water/mineral oil emulsions first appeared in the 1930s when it was discovered that detergents enabled the suspension of oil droplets in water. In the late 1950s the first purely synthetic cutting oils appeared.

Straight cutting oils contain no water but are generally mineral oils mixed with with animal, vegetable or marine oils to improve lubricating properties. Active cutting oils contain sulfur that is released during machining and are good for use with tough low carbon and chrome-alloy steels. Inactive cutting oils contain sulfur firmly attached to the oil which isn't released during machining operations and are used with nonferrous metals like aluminum, magnesium and brass.

Semi-synthetic oils combine an oil/water emulsion with chemical fluids. They are excellent rust inhibitors but provide only moderate cooling and lubrication. For EP (extreme pressure) applications sulfur, chlorine and phosphorous are added.

Synthetics contain very little oil and mix easily with water. The chemicals enhance lubrication and bacterial control while inhibiting rust and corrosion. Coolants containing sulfur and chlorine exist for extreme machining conditions.

Pastes/Gels

Oftentimes in drilling or tapping operations where heat dissipation is not as important as lubrication, a paste is used. Various waxes, soap-like lubricants and graphites are employed. Lard is still used for tapping. These solids are applied directly to the work piece or cutting tool. Grinding wheels can have a paste impregnated into the grinding surface of the tool itself.

Mists

Sometimes a lubricant is applied in aerosol form using compressed air or inert gasses for delivery. This system often contains the serious drawback of posing health hazards due to inhalation and skin contact and precautions must be taken. Delivery systems using freon also caused environmental degradation and are now restricted.

Benefits

Proper selection and use of cutting fluids optimizes the results of any machining application. These include:

Product quality - reduced friction and heat prevents the work piece from expanding during machining which prevents size variation and closer tolerances to be attained. In addition, better finishes result because chips are removed before they can cause surface scratches.
Increased productivity - The faster heat can be removed and the more friction can be reduced, the faster cutting speeds and feeds can be increased. Faster production times lowers labor costs and costs to the consumer. As well, proper fluid use increases the life of machine tools which reduces tooling costs, downtime and maintenance.
Reduced rust and corrosion - Cutting fluids protect machine tools and work pieces from rust and corrosion. The fluids should leave a thin film of residue after water has evaporated to provide additional protection.
Reduced bacterial growth - Any water used should be pure and as bacteria free as practical. Bacteria can turn fluids rancid. Additives in the fluid help control these.

Environmental and health concerns

Cutting fluids eventually lose their effectiveness can become contaminated with bacteria or mold and must be replaced. Heat generated by machining fosters growth of micro-organisms and bacteria, fungi and molds can cause health problems or exacerbate existing conditions unless appropriate protections are taken.

Old fluids must be disposed of responsibly to minimize their environmental impact. There are EPA, state or local regulations regarding waste cutting fluids which should be consulted before disposal.

Later developments

Experience and further research have led to wider ranges of products. Modern cutting fluids have much less environmental impact as well as high compatibility with skin, making them safer for those on the shop floor. Many products are now boron free and leave no corrosion on materials. Specific fluids now exist for different applications, e.g., grinding versus milling or broaching versus turning. The benefits in terms of longer life, decreased consumption and better washing action make today's generation of cutting fluids significantly better functionally and economically than those of just a generation ago.