Cutting fluids, Coolants and Lubricants

During metal cutting process, metal is removed in the form of chips.
• The friction between the tool and the chip causes to rise in temperature.
• This temperature is so high and leads to welding of chips on to the tool face and work piece, thus reducing the tool life and efficiency of machining.
• Also the rise in temperature causes deformation of work piece and leads to change in grain structure. Thereby varying properties of work piece.
• To overcome the above it is a must to have some means to reduce friction.
• Thus increasing the machining efficiency , tool life and provide free movement between mating parts.
• This is achieved by the use of cutting fluids, coolants and lubricants.
Definition
• Any substance that is applied to the cutting zone during metal machining operation is called CUTTING FLUID.
• Term ‘Cutting Fluid’ denotes to those coolants and lubricants that are used in metal machining
FUNCTIONS OF CUTTING FLUIDS
• To obtain best surface finish and dimensional accuracy
• To carry away the heat that is generated during machining
• Cool the tool and the work piece
• To improve the machinability
• To prevent adhesion of chips to the tool and work piece.
• To flush away chips from the cutting zone
• To reduce the friction at chip-tool and work-tool interface
PROPERTIES OF CUTTING FLUIDS :
To get the optimum benefits the cutting fluid should posses the following qualities :
• Possess good lubricating properties
• High heat absorption capacity
• No fire or accidental hazards
• Not emit toxic odors and vapours harmful to the operator
• Should be non-poisonous
• Low viscosity
• Prevent rusting of the machine and working surfaces
• Should not produce fog, smoke during use
• Suitable for a variety of cutting operations
• Should not discolor the finished work piece
• Chemically stable
• Should impart anti welding properties
• High flash point
• Should not deteriorate in storage
TYPES OF CUTTING FLUIDS
The cutting fluids are classified into five main groups
• Water soluble oils or compounds.
• Straight oils (mineral oils and fatty oils)
• Chemical-additive oils (sulphurised oils and chlorinated oils)
• Chemical compounds
• Solid lubricants
SELECTION OF CUTTING FLUIDS
The selection of a proper cutting fluids any particular application depends on

Cutting tool material,
Work piece material,
Operation being performed,
Cost factor,
Service expected the fluid,
Cutting condition.
Merits of cutting fluids and coolants
• Carry away chips
• Drive away scale and dirt
• Improve surface finish
• Protect the finished surface from corrosion
• Increase the tool life
• Improve machinability
• Reduce machining forces
• Provide lubrication
• Provides cushioning effect between the tool and work piece
FLOOD METHOD
A reciprocating pump is used to force the coolant over the work piece and tool
The out let of the pump is connected to a nozzle through flexible pipe.
The nozzle can be adjusted to direct the stream of the fluid at cutting zone,
This is the most common method of application of cutting fluid.
JET METHOD
1) A small jet of fluid at high speed is directed at the point of metal supporting from the underneath. As shown in the figure
2) In high speed jet system gear or vane type pump is used to maintain high pressure.
MIST METHOD
1) The fluid is passed through a specially designed nozzle to form very fine droplets (mist).
2) This mist is directed at the cutting zone at high speed.
3) It has better cooling effect than flood or jet method.
Lubricants :
• A cutting fluid used for the purpose of reducing friction
between the contacting surfaces in the cutting zone is
called lubricants
• Lubricants help in reducing the heat generated during the machining processes
• Machine tools such as lathes, milling machines etc., can only perform satisfactorily if they are properly maintained and lubricated.
• The parts of machine tools which need lubrication are slide ways, gear box, spindle, bearings, mating parts etc.
Purpose of lubricants :
• To reduce the friction between a cutting tool and the material being cut.
• To minimize the heat generated during operation.
• To prevent rise in temperature.
• To reduce wear.
• To increase tool life.
• Provide cushioning effect against vibrations.
• To drive away chips, scale and dirt between the mating parts.
• Protect finished surfaces from corrosion.
Properties of lubricants :
• Viscosity
• Oiliness
• Flash and Fire Points
• Pour (freezing) point
• Chemical stability
• Carbon residue
VISCOSITY :
• The measure of resistance of the oil to flow is called viscosity
• The oil used as lubricants must have sufficient viscosity that they are not squeezed out from two surfaces.
OILINESS :
• The ability of the oil to wet the metallic surface is called oiliness.
• Lubricants with good oiliness prevent the break down of the oil film and reduce the wear.
Flash and Fire point :
• Flash point is the lowest temperature at which an oil gives off sufficient vapour to support momentary flame.
• Fire point is the temperature at which an oil gives vapour to burn it continuously when ignited.
• Flash and Fire point should be high for lubricants.
Pour (freezing) Point
• This is the temperature at which an oil will cease to flow when cooled.
• It indicates the suitability of lubricants for use in cold conditions such as in ice-making machines, refrigerators etc.
• Lubricants used in such conditions should possess low pour point.
Chemical Stability
• The lubricant must be chemically stable and must not oxidize easily.
• It should not be vaporize under working conditions and should resist corrosion.
Carbon Residue :
• At high temperature lubricating oil decompose and deposit free carbon on the surface.
• A good lubricating oil should deposit small amount of carbon i.e. low carbon residue at high temperature
Lubricants are classified as following
Liquid lubricants
a) Animal oils b) Vegetable oils
c) Mineral oils d) Synthetic oils
Semi-solid lubricants (Grease)
Solid lubricants (Grease)
Waxes.
Liquid lubricants
Animal oil (tallow oil, lard oil)
• They are obtained from animal fats. Tallow oil is used in cold drawing
Vegetable oils (castor oils, olive oils, rapeseed oil)
• These are extracted from seeds and trees. Castor oil is used in automobiles
• Animal oils and vegetable oils are not widely used in machine tools, since they get oxidized easily.
Mineral oil :
• These oils are petroleum products and are most commonly use because of their cheapness and stability
• They do not form emulsions with water
Synthetic oils :
• Synthetic lubricant (polyalkylene glycols and silicon fluids) possess good lubricating properties.
• But their high cost limits their applications
Semi-solid lubricants
• Grease forms an important semi-solid lubricants, used when the fluidity of oil is undesirable.
• It is an oil which has been thickened by soap.
• It offers simplest method of lubrication with additional advantages of preventing the entry of dirt.
• In automobiles they are used where high temperatures
are encountered.
Advantages
• It is particularly suited to bearings at slow speed and large clearances.
• It provides seal against dust grits and moisture.
• It provides constant lubrication for a longer period.
• In ball and roller bearings grease lubrication is invariably
used.
Disadvantages
• The grease provides very little lubrication until the bearing temperature rises sufficiently to melt it.
Solid lubricants
• Solid lubricants are used where oil film can not be maintained due to high pressure or temperature .
• Graphite is the best known of the solid lubricants.
• These are used in high temperature services.
• However , these are not suitable for ball and roller bearings.
Waxes
• Derived from petroleum.
• They are compounded with fatty acids and soaps.
• They are used under high pressure application.
• Used during drawing, rolling, extrusion, etc.
Lubrication of machine tools: Purpose
• Machine tool gives satisfactory performance if they are properly cleaned and lubricated.
• Slide ways , gear box and spindle bearings must be lubricated to minimize friction.
• Lubrication of slide ways protects the surface from wear.
• If the mating parts are lubricated with correct lubricant the movement of slides will be precisely controlled.
• Ball and roller bearings are lubricated to minimize the friction and to protect from rust and wear.
Methods of lubrication
• Lubricating oil is generally applied to sliding parts with help of oil-can.
• Lubricants are also applied by the following systems .
1) Gravity feed system
2) Pressure feed system
Gravity feed system
• In this system lubricant is applied to the sliding and mating parts through oil cups .
• The oil cups are located at the higher levels of sliding and mating parts .
• In this system oil flows to the parts due to gravity
Pressure feed system
• This system is used generally in automobiles.
• In this system lubricants are applied under pressure by using oil pumps.
• In this system oil pump is situated below the engine.