The role of cutting fluid

First, cooling effect

The cooling effect relies on the convection heat transfer and vaporization of the

cutting fluid to remove the cutting heat from the solid (tool, workpiece and chip), 

reduce the temperature of the cutting zone, reduce workpiece deformation, 

and maintain the hardness and size of the tool. The cooling effect of cutting fluid depends 

on its thermal parameters, especially the specific heat capacity and thermal conductivity. 

In addition, liquid flow conditions and heat exchange coefficient also play an important role,

 heat exchange coefficient can be increased by changing the surface active material and the heat of vaporization. 

Water has high specific heat capacity and high thermal conductivity, so the cutting performance of water is better

 than that of oil based cutting fluid. Changing the liquid flow conditions, such as increasing the flow rate and flow

 rate can effectively improve the cooling effect of cutting fluid, especially for the cooling effect of the oil base 

cutting fluid, increasing the cutting fluid pressure and flow rate, can improve the cooling performance. 

This method is used in deep hole drilling and high speed gear hobbing. 

Spray cooling can make the liquid easy to vaporize and improve the cooling effect obviously.

The cooling effect of the cutting fluid is affected by the permeability of the cutting fluid,

 the good permeability of the cutting fluid, the cooling speed of the blade is fast, 

the permeability of the cutting fluid is related to the viscosity and infiltration of the cutting fluid. 

The permeability of low viscosity liquid is better than that of high viscosity liquid, 

the permeability of oil base cutting fluid is stronger than that of water base cutting fluid, 

and the permeability of water base cutting fluid containing surfactants is greatly improved. 

The infiltrating property of cutting fluid is related to the surface tension of cutting fluid.

 When the surface tension of the liquid is large, the liquid expands around the surface of the solid and collects into droplets,

 and the permeability of the liquid is poor. When the surface tension of the liquid is small, the liquid expands around 

the surface of the solid, and the contact Angle of the solid -- liquid -- gas is small or even zero, 

the permeability of the liquid is good. Liquid can be rapidly extended to the tool and workpiece, 

tool and chip contact gaps, can enhance the cooling effect.

The cooling effect is also related to the foam, because the inside of the foam is air, 

the thermal conductivity of the air is poor, the cutting liquid cooling effect of the foam will be reduced, 

so the general synthetic cutting fluid containing surfactants has added a small amount of emulsified silicone oil,

 play the role of defoaming. Recent studies show that ionic water-based cutting fluid can rapidly eliminate the

 electrostatic charge caused by strong friction in cutting and grinding through ionic reaction, 

and the workpiece does not produce high heat, play a good cooling effect. 

This kind of ionic cutting fluid has been widely used as a cooling lubricant for high speed grinding and strong grinding.

Two, lubrication effect

In the machining process, the friction between the cutting tool and the cutting tool and the workpiece surface, 

the cutting fluid is the lubricant to reduce this friction. Tool, because the tool in the cutting process with a back Angle,

 it and the processed material contact part than the front tool surface less contact pressure is low, 

therefore, after the tool surface friction lubrication state close to the boundary lubrication state, 

generally use strong adsorption substances, such as oil agent and extreme pressure agent shear strength reduction, 

can effectively reduce friction. The condition of the front cutter face is different from that of the back cutter face,

 the cutting of the shear zone by deformation is forced to extrude under the condition of the cutting tool extrusion,

 the contact pressure is large, the cutting is also due to plastic deformation and high temperature, after the supply of cutting fluid, 

cutting is also due to sudden cooling and shrinkage, so that the front cutter face of the tool and the chip contact length and the metal

 contact area between the cutter and the tool is reduced, At the same time, the average shear stress is reduced, which leads to the increase

 of shear Angle and the reduction of cutting force, so that the cutting performance of the workpiece material is improved. In the grinding process,

 the grinding fluid penetrates into the workpiece and the grinding fluid between the workpiece and the grinding chip to form a lubrication film.

 Because of this layer of lubrication film, the friction of these interfaces is reduced, and the friction and wear of the grinding cutting edge 

are prevented, and the workpiece surface roughness is reduced. The lubricating effect of cutting fluid, general oil base cutting fluid than 

water base cutting fluid superior oil base cutting fluid, extreme pressure additive effect is better.

Oil additives are generally long chain organic compounds with extreme pressure bases, such as advanced fatty acids, 

advanced alcohols, animal and plant oils, etc. Oil-based additives are absorbed on the surface of the metal by the polar base to

 form a layer of lubrication film, reduce the friction between the tool and the workpiece, the tool and the chip, so as to reduce the 

cutting resistance, prolong the life of the tool, reduce the workpiece surface roughness. The role of oil additives is limited to the 

condition of low temperature, when the temperature exceeds 200℃, the adsorption layer of oil agent is damaged and lose lubrication, 

so the general low speed, precision cutting using cutting fluid containing oil additives, and in high speed, heavy cutting occasions, 

should use cutting fluid containing extreme pressure additives. The so-called extreme pressure additive is some compounds containing sulfur,

 phosphorus, chlorine elements, these compounds at high temperature and metal reaction, the formation of iron sulfide, iron phosphide, 

ferric chloride, with low shear strength material, thus reducing the cutting resistance, reduce the tool and workpiece, tool and chip friction, 

making the cutting process easy to carry out. The extreme pressure additive can also inhibit the formation of chip nodules and improve

 the roughness of the workpiece surface.

Ferric chloride crystals have a layered structure, so the shear strength is lowest. Compared with iron sulfide, ferric chloride has a low melting point 

and will lose lubrication at high temperature (about 400℃). Iron phosphate is between ferric chloride and iron sulfide, iron sulfide high temperature

 resistance (700℃) is the best, in heavy load cutting and difficult cutting materials processing are generally used with sulfur extreme pressure agent 

cutting fluid. In addition to the chemical reaction with steel, iron and other black metals to produce lubrication film with low shear strength, 

the extreme pressure additive has the same effect on copper, aluminum and other non-ferrous metals. However, non-ferrous metal cutting generally 

should not use active extreme pressure additives, so as not to cause corrosion to the workpiece lubrication of cutting fluid and cutting fluid permeability,

 good permeability cutting fluid, lubricant can be timely infiltrated into the chip and tool interface and tool and workpiece interface, in the chip, 

workpiece and tool surface form lubrication film, reduce friction coefficient, reduce cutting resistance.

In addition to the above lubricating effects of cutting fluid, recent research suggests that cutting can penetrate directly into

 tiny cracks on the metal surface, changing the physical properties of the material being machined, thereby reducing cutting

 resistance and making the cutting process easier.

Three, cleaning effect

In the process of metal cutting, cutting, iron powder, grinding chips, oil and other things are easy to adhere to the workpiece surface and tool,

 grinding wheel, affecting the cutting effect, at the same time make the workpiece and machine tool dirty, not easy to clean so the cutting

 fluid must have a good cleaning effect, for oil-based cutting fluid, the lower the viscosity, the stronger the cleaning ability,

 especially containing diesel, kerosene and other light components of the cutting fluid, Penetration and cleaning performance is better. 

Water-based cutting fluid containing surfactants, cleaning effect is better surfactant on the one hand can absorb all kinds of particles,

 sludge, and form a layer of adsorption film on the workpiece surface, prevent particles and sludge adhesion on the workpiece,

 cutting tools and grinding wheel, on the other hand can penetrate into the particles and oil adhesion interface particles and oil 

separation from the interface, with the cutting fluid away, So as to play a cleaning role. The cleaning effect of cutting fluid should also be shown in the chip,

 grinding, iron powder, oil and so on have a good separation and settlement. Recycled cutting fluid can quickly make chips, iron powder, grinding chips,

 particles settling in the bottom of the container oil suspended on the surface, so as to ensure that the cutting fluid can be kept clean after repeated use,

 ensure processing quality and prolong the service cycle.

Four, rust prevention

In the process of cutting, if the workpiece and water and cutting fluid decomposition or oxidation metamorphic corrosion media contact, 

such as sulfur, sulfur dioxide, chloride ions, acid, hydrogen sulfide, alkali contact will be corroded, machine tools and cutting fluid contact

 parts will therefore produce corrosion, after the workpiece processing or process storage period, if the cutting fluid does not have a certain anti rust ability, 

the workpiece It will be eroded by the moisture in the air and the corrosive medium and produce chemical corrosion and electrochemical corrosion, 

resulting in workpiece rust, therefore, the cutting fluid must have good rust resistance, which is one of the most basic performance of the cutting fluid. 

Cutting oil generally has certain anti-rust ability. If the storage cycle between processes is not long, anti-rust additives can be added,

because adding anti-rust additives such as barium petroleum sulfonate into cutting oil will reduce the anti-wear performance of cutting oil

.