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Types Of Coolants

Cutting fluids have been used extensively in metal cutting operations for the last 200 years. In the beginning, cutting fluids consisted of simple oils applied with brushes to lubricate the machine tool. Occasionally, lard, animal fat or whale oil was added to improve the oil’s lubricity. As cutting operations became more severe, cutting fluid formulations became more complex. Today’s cutting fluids are special blends of chemical additives, lubricants and water formulated to meet the performance demands of the metalworking industry.

There are now several types of cutting fluids on the market, the most common of which can be broadly categorized as cutting oils or water-miscible fluids. Water-miscible fluids, including soluble oils, synthetics and semi synthetics, are now used in approximately 80 to 90 percent of all applications. Although straight cutting oils are less popular than they were in the past, they are still the fluid of choice for certain metalworking applications

Essentially there is water, water with oil and synthetic coolants. Adding oil or synthetic materials to the water does several things. It adds rust preventative, it helps carry away the heat, it more effectively removes particles, it helps prevent corrosion and it retards or prevents bacteria and fungus build up.

Oil is superior in applications where heat transfer is critical. The most common, extreme example of this is tapping deep holes. It is extremely difficult to get a good flow of material across the work face in this application so the coolant must provide maximum lubricity and heat transfer. In this application it is also very easy to control the spray and to keep the coolant away from the operator since all the work takes place in a deep, sealed hole.

Synthetic coolants are superior in applications such as grinding where a strong flow across the workface is both possible and desirable. The synthetic coolants do not transfer heat as well as oils, but the flow possible in grinding applications more than makes up for that. In addition, open grinding such as flat, exposed surfaces throws a great deal of coolant into the air in various size droplets. Oil based coolants are more likely to trap and contain metals and particles. In addition, there is the problem in breathing petroleum products.

Specific synthetic lubricants can be superior in terms of lubricity and heat transfer to petroleum based products. Perhaps the most common example is the synthetic products available for automotive applications. In high performance automobile engines such as CorvetteÒ engines the dealers strongly recommend synthetic lubricants over natural, petroleum lubricants because of the extreme requirement for lubrication and cooling.

Some coolants are a lot easier to test than others. Some coolants show a color change as the amount of dissolved coolant gets greater. Some coolants trap particles and float them in suspension a lot better than others. Some coolants are safer than others and some work better for grinding. Some are also much safer to breathe and dispose of. For the purposes of easy testing we recommend clear coolants.

The most common metalworking fluids used today belong to one of two categories based on their oil content:

  • Oil Based Fluids including straight oils and soluble oils
  • Chemical Fluids including synthetics and semi synthetics

Fluids vary in suitability for metalworking operations. For example, petroleum – based cutting oils are frequently used for drilling and tapping operations due to their excellent lubricity while water miscible fluids provide the cooling properties required for most turning and grinding operations.

Oil Based Cutting Fluids

Straight Oils – 100% Petroleum Oil

Straight oils, so called because they do not contain water, are basically petroleum or mineral oils. They may have additives designed to improve specific properties [1,3]. Generally additives are not required for the easiest tasks such as light-duty machining of ferrous and nonferrous metals. For more severe applications, straight oils may contain wetting agents (typically up to 20% fatty oils) and extreme pressure (EP) additives such as sulfur, chlorine, or phosphorus compounds. These additives improve the oil’s wettability; that is, the ability of the oil to coat the cutting tool, work piece and metal fines. They also enhance lubrication, improve the oil’s ability to handle large amounts of metal fines, and help guard against microscopic welding in heavy duty machining. For extreme conditions, additives (primarily with chlorine and sulfurized fatty oils) may exceed 20%. These additives strongly enhance the antiwelding properties of the product.


The major advantage of straight oils is the excellent lubricity or “cushioning” effect they provide between the work piece and cutting tool. This is particularly useful for low speed, low clearance operations requiring high quality surface finishes. Although their cost is high, they provide the longest tool life for a number of applications. Highly compounded straight oils are still preferred for severe cutting operations such as crush grinding, severe broaching and tapping, deep-hole drilling, and for the more difficult to cut metals such as certain stainless steels and superalloys. They are also the fluid of choice for most honing operations due to their high lubricating qualities.

Straight oils offer good rust protection, extended sump life, easy maintenance, and are less likely to cause problems if misused. They also resist rancidity, since bacteria cannot thrive unless water contaminates the oil.


Disadvantages of straight oils include poor heat dissipating properties and increased fire risk. They may also create a mist or smoke that results in an unsafe work environment for the machine operator, particularly when machines have inadequate shielding or when shops have poor ventilation systems. Straight oils are usually limited to low temperature, low speed operations. The oily film left on the work piece makes cleaning more difficult, often requiring the use of cleaning solvents.

Straight oil products of different viscosities are available for each duty class. Viscosity can be thought of as a lubricant factor.  The higher the oil’s viscosity, the greater its lubricity. Highly viscous fluids tend to cling to the work piece and tool. This causes increased cutting fluid loss by dragout and necessitates lengthier, more costly cleanup procedures. It can be more efficient to choose low viscosity oil that has been compounded to provide the same lubricity as a highly viscous one.

Soluble Oils – 60% to 90% Petroleum Oil

Soluble oils (also referred to as emulsions, emulsifiable oils or water soluble oils) are generally comprised of 60 – 90 percent petroleum or mineral oil, emulsifiers and other additives. A concentrate is mixed with water to form the metalworking fluid. When mixed, emulsifiers, (a soap like material) cause the oil to disperse in water forming a stable “oil in water” emulsion. They also cause the oils to cling to the work piece during machining. Emulsifier particles refract light, giving the fluid a milky, opaque appearance.


Soluble oils offer improved cooling capabilities and good lubrication due to the blending of oil and water. They also tend to leave a protective oil film on moving components of machine tools and resist emulsification of greases and slideway oils.

Soluble oils are a general-purpose product suitable for light and medium duty operations involving a variety of ferrous and nonferrous applications. Although they do not match the lubricity offered by straight oils, wetting agents and EP additives (such as chlorine, phosphorus or sulfur compounds) can extend their machining application range to include heavy-duty operations. Most cutting operations handled by straight oils (such as broaching, trepanning, and tapping) may be accomplished using heavy-duty soluble oils.


The presence of water makes soluble oils more susceptible to rust control problems, bacterial growth and rancidity, tramp oil contamination, and evaporation losses. Soluble oils are usually formulated with additives to provide additional corrosion protection and resistance to microbial degradation. Maintenance costs to retain the desired characteristics of soluble oil are relatively high. Other disadvantages of soluble oils include the following:

When mixed with hard water, soluble oils tend to form precipitates on parts, machines and filters.

Due to their high oil content, they may be the most difficult of the water miscible fluids to clean from the work piece.

Misting of soluble oils may produce a dirty and unsafe work environment, through slippery surfaces and inhalation hazards.

As a result of these disadvantages, soluble oils have been replaced in most operations with chemical cutting fluids.

Chemical Cutting Fluids

Chemical cutting fluids, called synthetic or semi synthetic fluids, have been widely accepted since they were first introduced in about 1945. They are stable, preformed emulsions, which contain very little oil and mix easily with water. Chemical cutting fluids rely on chemical agents for lubrication and friction reduction. These additives also improve wettability. At temperatures above approximately 390OF (2000C), these additives become ineffective and EP lubricant additives (chlorine, phosphorus and sulfur compounds) are utilized.

These compounds react with freshly machined metal to form chemical layers, which act as a solid lubricant and guard against welding during heavy-duty machining operations. Fluids containing EP lubricants significantly reduce the heat generated during cutting and grinding operations.

Synthetics – No Petroleum Oils

Synthetic fluids contain no petroleum or mineral oil. They were introduced in the late 1950’s and generally consist of chemical lubricants and rust inhibitors dissolved in water. Like soluble oils, synthetics are provided as a concentrate, which is mixed with water to form the metalworking fluid. These fluids are designed for high cooling capacity, lubricity, corrosion prevention, and easy maintenance. Due to their higher cooling capacity, synthetics tend to be preferred for high heat, high velocity turning operations such as surface grinding. They are also desirable when clarity or low foam characteristics are required. Heavy-duty synthetics, introduced during the last few years, are now capable of handling most machining operations.

Synthetic fluids can be further classified as simple, complex or emulsifiable synthetics based on their composition. Simple synthetic concentrates (also referred to as true solutions) are primarily used for light duty grinding operations. Complex synthetics contain synthetic lubricants and may be used for moderate to heavy duty machining operations. Machining may also be performed at higher speeds and feeds when using complex synthetics. Both simple and complex synthetics form transparent solutions when mixed in a coolant sump, allowing machine operators to see the work piece.

Emulsifiable synthetics contain additional compounds to create lubrication properties similar to soluble oils, allowing these fluids to double as a lubricant and coolant during heavy-duty machining applications. Due to their wettability, good cooling and lubricity, emulsifiable synthetics are capable of handling heavy-duty grinding and cutting operations on tough, difficult to machine and high temperature alloys. The appearance of emulsifiable synthetic fluids ranges from translucent to opaque.

Chemical agents found in most synthetic fluids include:
       Amines and nitrites for rust prevention
       Nitrates for nitrite stabilization
Phosphates and borates for water softening
Soaps and wetting agents for lubrication
Phosphorus, chlorine, and sulfur compounds for chemical lubrication
Glycols to act as blending agents
Biocides to control bacterial growth.


Synthetics provide excellent microbial control and resistance to rancidity. They are largely nonflammable and nonsmoking with good corrosion control and superior cooling qualities. Synthetics have greater stability when mixed with hard water. In addition they reduce misting problems and foaming problems.

Synthetics are easily separated from the work piece and chips, allowing for easy cleaning and handling of these materials. In addition, since the amount of fluid clinging to the work piece and chips is reduced, less makeup fluid is needed to replace coolant lost to dragout.

Good settling properties allow fine particulates to readily drop out of suspension, preventing them from recirculating and clogging the machine cooling system. Overall, synthetics are easier to maintain due to their cleanliness, they offer long service life if properly maintained and can be used for a variety of machining operations.


Although synthetics are less susceptible to problems associated with oil based fluids, moderate to high agitation conditions may still cause them to foam or generate fine mists. A number of health and safety concerns, such as misting and dermatitis, also remain with the use of synthetics in the shop. Ingredients added to enhance the lubricity and wettability of emulsifiable synthetics may increase the tendency of these fluids to emulsify tramp oil, foam and leave semi crystalline to gummy residues on machine systems (particularly when mixed with hard water).

Synthetic fluids are easily contaminated by other machine fluids such as lubricating oils and need to be monitored and maintained to be used effectively.

Semi synthetics – 2% to 30% Petroleum Oil

As the name implies, semi synthetics (also referred to as semi-chemical fluids) are essentially a hybrid of soluble oils and synthetics. They contain small dispersions of mineral oil, typically 2 to

30 percent, in a water dilatable concentrate. The remaining portion of a semi synthetic concentrate consists mainly of emulsifiers and water. Wetting agents, corrosion inhibitors and biocide additives are also present. Semi synthetics are often referred to as chemical emulsions or preformed chemical emulsions since the concentrate already contains water and the emulsification of oil and water occurs during its production.

The high emulsifier content of semi synthetics tends to keep suspended oil globules small in size, decreasing the amount of light refracted by the fluid. Semi synthetics are normally translucent but can vary from almost transparent (having only a slight haze) to opaque. Most semi synthetics are also heat sensitive. Oil molecules in semi synthetics tend to gather around the cutting tool and provide more lubricity. As the solution cools, the molecules redisperse.


Like synthetics, semi synthetics are suitable for use in a wide range of machining applications and are substantially easier to maintain than soluble oils. They provide good lubricity for moderate to heavy-duty applications. They also have better cooling and wetting properties than soluble oils, allowing users to cut at higher speeds and faster feed rates. Their viscosity is also less than that of a soluble oil, providing better settling and cleaning properties. Semi synthetics provide better control over rancidity and bacterial growth, generate less smoke and oil mist (because they contain less oil than straight or soluble oils), have greater longevity, and good corrosion protection.


Water hardness affects the stability of semi synthetics and may result in the formation of hard water scum deposits. Semi synthetics also foam easily because of their cleaning additives and generally offer less lubrication than soluble oils.

Cutting Fluid Types Advantages vs. Disadvantages

Straight Oils
Advantages Disadvantages
Excellent lubricity Poor heat dissipation
Good rust protection Increased risk of fire
Good sump life Increased risk of smoking
Easy maintenance Increased risk of misting
Rancid resistant Oil film on work piece
Limited to low – speed, severe cutting operations
Soluble Oils
Advantages Disadvantages
Good lubrication More    susceptible       to             rust problems
Improved cooling capabilities Susceptible to bacterial growth
Good rust protection Susceptible      to         tramp             oil contamination
General-purpose product for light to heavyduty operations Susceptible      to             evaporation losses
Increased maintenance costs
May     form     precipitates     on
Oily film on work piece
Advantages Disadvantages
Excellent microbial control Reduced lubrication
Resistance to rancidity May cause misting
Relatively nontoxic May cause foaming
Transparent May cause dermatitis
Nonflammable / nonsmoking May emulsify tramp oil
Good corrosion control May form residues
Superior cooling qualities Easily contaminated machine fluids by other
Reduced misting / foaming
Easily separated from work/ chips
Good settling / cleaning properties
Easy maintenance
Long service life

Long service life


Used for a wide range of machining applications
Semi – synthetics
Advantages Disadvantages
Good microbial control Water hardness affects stability
Resistance to rancidity May cause misting
Relatively nontoxic May cause foaming
Nonflammable / nonsmoking May cause dermatitis
Good corrosion control May emulsify tramp oil
Good cooling May form residues
Good lubrication Easily contaminated machine fluids by other
Reduced misting / foaming
Easily separated from work/ chips
Good settling / cleaning properties
Easy maintenance
Long service life
Used for a wide range of machining applications