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What are the purposes, properties and types of cutting fluids ?
Cutting fluids, sometimes referred to as lubricants or coolants are liquids and gases applied to the tool and work piece to assist in the cutting operations.
Purpose of Cutting Fluids
Cutting fluids are used for the following
1. To cool the tool: Cooling the tool is necessary to prevent metallurgical damage and to assist in decreasing friction at the tool-chip interface and at the tool-work piece interface.
2. To cool the work piece. The role of the cutting fluid in cooling the work piece is to prevent its excessive thermal distortion.
3. To lubricate and reduce friction
4. To improve surface finish
Desirable Properties of Cutting Fluid:
1. Lubricating Qualities:
This quality reduces frictional force between work and tool. It also prevents the formation of built-up edge.
2. High Heat Carrying (Cooling) Capacity:
Cutting fluid must carry more and more heat from cutting zone quickly. Thus, reducing the temperature of work and tool. This will reduce tool wear, increase tool life and surface finish.
A small reduction in temperature can considerably increase tool life, according to the following empirical relation:
Tθn = K
Where, T= Tool life (min)
θ = Temperature at the chip tool interface (°C)
n = An exponent depends on tool form and material
K = Constant
3. Corrosion Resistant:
The cutting fluid should prevent the work material from environmental rusting or corrosion. For this purpose corrosion inhibitor like sodium nitrate is added in cutting fluid.
4. Low Viscosity:
It should have low viscosity, so that chip and dirt can settle quickly.
It should have long life, i.e., it should not get spoiled quickly, both in use and in storage.
It should be non-toxic and should not be injurious to the human skin.
It should have high flash point, and should not burn easily.
It should not smoke or foam easily.
9. Small Molecular Size:
It should have small molecular size to allow rapid diffusion and better penetration to the chip-tool interface.
10. Chemically Stable or Inert:
It should not adversely react with work material.
It should be free from undesirable odours.
Types of Cutting Fluids :
A variety of cutting fluids are available to satisfy the requirements of machining processes. Although, there is no all-purpose cutting fluid, some offer considerable versatility while some are for specific application.
The basic types of cutting fluids are following:
Water has high specific heat but is poor in lubrication. Also, it encourages rusting. It is used as cooling agent during tool grinding.
2. Soluble Oils (Emulsions):
Soluble oils or emulsifiable oils are the largest type of cutting fluids used in machining operations.
These are composed of:
i. Soluble oil.
ii. Emulsifiers (Sodium sulfonate, fatty, acid soap, esters).
iii. Additives (Corrosive resistance or coupling agents).
iv. Water (for dilution 1-20%).
Emulsifiers are chemical substances that cause suspension of tiny oil droplets iii the water. Additives are corrosive resistance chemicals or coupling agents. Coupling agents provide a white emulsion with no oil or cream separating out after mixing with water 5% dilution level being the most common dilution level. These fluids have average lubricating abilities and good cooling properties. Soluble oils are suitable for light cutting operations on general purpose machines were low metal remove rates used.
3. Mineral Oils:
Mineral oils are used for heavier cutting operations because of their good lubricating properties. They are commonly found in production machines where high metal removal rates are employed. They are most suitable for steels but should not be used on copper or its alloys since it has a corrosive effect.
4. Straight Oils (Petroleum or Vegetable Oils):
Straight oil is a petroleum or vegetable oil that is used without dilution with water. Paraffin oils, naphthenic oils, vegetable oils are some examples of straight oils. It is said that, straight oils provide excellent lubrication. For environmentally favorable requirements, vegetable oils are preferable due to their ease of biodegradation and disposal. On the other hand, they are of little use since they are liable to decompose and smell badly.
5. Synthetic Fluids:
They are water based fluids and contain no mineral oil. They have a typical particulate size of 0.003 mm. Water provides excellent cooling properties. But creates a problem of corrosion. Also, not effective as lubricant. To prevent rust formation rust inhibitors are added.
6. Semi-Synthetic Fluids:
They are mixture of soluble oils (Emulsions) and synthetics fluids (water based fluids). About 5 to 20% mineral oil is emulsified with water to produce a microemulsion. The partical size varies from 0.01 -0.1 mm. This is small enough to transmit all incidents light.
These types of fluids are used largely due to their advantages of both soluble oils and synthetics.
Some major advantages are:
i. Rapid heat dissipation.
ii. Cleanliness of the system.
iii. Bio-resistance (due to small particle size).
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