• INTRODUCTION

  A cutting fluid is any liquid or gas that is applied directly to the machining operation to improve cutting performance. Cutting fluids address two main problems:

1. Heat generation at the shear zone and friction zone, and

2. Friction at the tool–chip and tool–work interfaces.

   In addition to removing heat and reducing friction, cutting fluids provide additional benefits, such as washing away chips (especially in grinding and milling), reducing the temperature of the workpart for easier handling, reducing cutting forces and power requirements, improving dimensional stability of the workpart, and improving surface finish.

• FUNCTIONS OF CUTTING FLUIDS

1. To prevent the tool from overheating, i.e. so that no temperature is reached where the tool's hardness and resistance to abrasion are reduced, thus decreasing the tool life.

2. To keep the work cool, preventing machining that results in inaccurate final dimensions.

3. To reduce power consumption, wear on the tool, and the generation of heat, by affecting the cutting process. This investigation wishes to establish a relationship between the surface chemistry of the lubricants involved and how they can accomplish reducing the contact length on the rake face of the tool where most of the heat during cutting is produced.

4. To provide a good surface finish on the work.

5. To aid in providing a satisfactory chip formation (related to contact length)

6. To wash away the chips/clear the swarf from the cutting area.

7. To prevent corrosion of the work, the tool and the machine.

• PROPERTIES OF CUTTING FLUIDS

1. High thermal conductivity for cooling .

2. Good lubricating qualities.

3. High flash point, should not entail a fire hazard.

4. Must not produce a gummy or solid precipitate at ordinary working temperatures.

5. Be stable against oxidation.

6. Must not promote corrosion or discoloration of the work material.

7. Must afford some corrosion protection to newly formed surfaces.

8. The components of the lubricant must not become rancid easily.

9. No unpleasant odor must develop from continued use.

10. Must not cause skin irritation or contamination.

11. A viscosity that will permit free flow from the work and dripping from the chips.

The water-based fluids act mainly as coolants and the neat cutting oils act mainly as lubricants. There are many variants of both types. Fatty acids are often incorporated in the neat oils. Until recently both the emulsions or soluble oils as they are also called and the neat oils, contained chlorine and sulfur additives that improved lubrication under extremely difficult conditions. Chlorine affects the skin detrimentally and its degradation products are often carcinogenic and sulfur is environmentally unacceptable. Consequently other lubrication improves under difficult conditions are searched for. Ester technology is used successfully for softer materials where high rates of metal working are needed, and where heat generation is not a major problem. These can operate at higher temperatures as they have better resistance to thermal degradation than mineral oils. They are biodegradable and do not cause dermatitis and are therefore more environmentally acceptable. In many cases phosphor and sulfur do however still form part of the cutting fluid.

For the water miscible fluids water quality has a large effect on the coolant. Hard water (high mineral content) can cause stains and corrosion of machines and work pieces. Water can be deionized to remove the impurities and minerals. Water is the best fluid for cooling. It has the best ability to carry heat away. Water, however, is a very poor lubricant and causes corrosion. Oil is excellent for lubrication but very poor for cooling, and it is also flammable. It is clear that, from a lubrication point of view water and oil have strengths but also some weaknesses. If water and oil are combined and an attempt is made to minimize the weaknesses the best properties of both may be balanced to obtain desirable end properties for the cutting fluid. Water-soluble fluids have been developed which have good lubrication, cooling ability, low-flammability and corrosion resistance. These fluids are usually mixed on site. It is crucial that the mixing directions and concentrations are followed very closely to get the maximum benefit from the coolant.

1. EMULSIONS

An emulsion is a dispersion of oil droplets in water. Soluble oils are mineral oils that contain emulsifiers. Emulsifiers are soaps or soap-like agents that allow the oil to mix with water and stay in suspension. Emulsions (soluble oils) when mixed with water produce a milky white product. Lean concentrations (more water, less oil) provide better cooling but less lubrication. Rich concentrations (less water, more oil) have better lubrication qualities but poorer cooling properties. There are different types of soluble cutting fluids available including extreme pressure soluble oils. These are used for extreme machining conditions like broaching and gear hobbing for example.

2. CHEMICAL FLUIDS

Chemical coolants are also miscible cutting fluids. Chemical cutting fluids are pre-concentrated emulsions that contain very little oil. Chemical fluids mix very easily with water to form an emulsion. The chemical components in the fluid are used to enhance the lubrication, bacterial control, and rust and corrosion characteristics. There are several types of chemical coolants available including coolants for extreme cutting conditions. Inactive chemical cutting fluids are usually clear fluids with high corrosion inhibition, high cooling, and low lubrication qualities. Active chemical fluids include wetting agents. They have excellent rust inhibition and moderate lubrication and cooling properties. Sulphur-, chlorine- and phosphorous- containing compounds are sometimes added to improve the extreme pressure characteristics. These are usually in an organic form, i.e. the sulphur, chlorine or phosphorus IS grafted onto a hydro-carbon backbone.

3. STRAIGHT CUTTING OILS

Straight cutting oils are not mixed with water. Cutting oils are generally mixtures of mineral oil and animal, vegetable or marine oils to improve the wetting and lubricating properties. Sulphur, chlorine, and phosphorous compounds are sometimes added to improve the lubrication qualities of the fluid for extreme pressure applications. There are two main types of straight oils: active and inactive.

a) Inactive Straight Cutting oils

Inactive oils contain sulphur that is very firmly attached to the oil. Mineral oils are an example of straight oils. Mineral oils provide excellent lubrication, but are not very good at heat dissipation (removing heat from the cutting tool and work piece). Mineral oils are particularly suited to non-ferrous materials such as aluminium, brass, and magnesium. Blends of mineral oils are also used in grinding operations to produce high surface finishes on ferrous and non-ferrous materials.

b) Active Straight Cutting Oils

Active oils contain sulphur that is not firmly attached to the oil, i.e. it is part of the oil molecule but is only weakly bonded to the hydro-carbon backbone. Thus the sulphur is easily released during the machining operation to react with the work piece. These oils have good lubrication and cooling properties. Special blends with higher sulphur content are available for heavy duty machining operations. They are recommended for tough low carbon and chrome-alloy steels. They are widely used in thread cutting. They are also good for grinding as they help prevent the grinding wheel from loading up. This increases the life of the grinding wheel.

4. GASES AND VAPOURS

Cutting oils and water miscible types of cutting fluids are the most widely used. Compressed air, inert gases like carbon dioxide, Freon, and Nitrogen are sometimes used. A vortex tube may be used to apply gaseous lubricants or coolants. Using this tube, it is possible to apply the gases at a very low temperature and under medium pressure thereby facilitating a higher gas density and cooling and lubrication capability. Cutting using sub-zero cold gas is known as cryogenic cutting. The gas stream also helps to blow away chips from the cutting area. The further advantage of a gaseous or vapour phase cutting fluid is that the constituents are of a far finer nature than liquids or solids, and due to the way of application, namely jet application, have far greater kinetic energy, and therefore have a greater penetrative capability. Molecular exchange is much slower when flood lubrication is used. Any adhered film is static and can be penetrated by the cutting fluid when jet type application is used. This promotes convective cooling and adds capacity for evaporative cooling. As far as vapour phase cutting fluids are concerned carbon tetra chloride (although forbidden to use as a cutting fluid) is very volatile and vaporises easily under ambient temperature conditions. Gas phase molecules do not adhere to each other as much as the molecules of liquids and are thus far more free to move.

5. PASTE AND SOLID LUBRICANTS

Waxes, pastes, soaps, graphite and molybdenum disulphide are examples falling into this category. These are generally applied directly to the work piece or tool or in some cases impregnated directly into the tool, for example the grinding wheel of a grinder. One example of a paste lubricant is lard. Many experienced journeymen recommend lard for tapping.

6. COOLANTS

The use of coolants becomes particularly essential when machining high melting point metals and alloys. Their use is most important when cutting with steel tools, but they are also used when cutting with carbide tools. Two main sources of heat in the cutting operation are - on the primary shear plane and in the flow-zone on the tool/work interface. The work done in this region is converted into heat, while the work done by sliding friction only makes a minor contribution to heating under most cutting conditions. Coolants cannot prevent the heat being generated, as they do not have direct access to the zones which are the heat sources. Cutting fluids used in more severe operations must have very good anti-weld and lubricating properties to protect the cutting tool and to ensure proper surface finish and accuracy. More severe operations are those where the cutting fluid cannot be in direct contact with the tool at the cutting point as for example during a band sawing operation or any other operation where the work-piece restricts access to the cutting point, hence turning is a less severe operation. More severe operations demand more active cutting fluids and usually also a decrease in cutting speed. This means that additives, particularly of the extreme pressure type, must be used under severe operational conditions. Cutting fluids contain a wide variety of speciality chemical additives designed to improve lubricity, surface activity, stability and anti-weld properties.