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How to choose metal cutting fluid?

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How to choose metal cutting fluid?

Date of release:2017-11-28 Author: Click:

Most metal cutting requires the use of cutting fluid, even in the normal operation of dry cutting, if you use the appropriate cooling lubricant can also increase the work efficiency. As early as 1883, F.W. Taylor had proved that the cutting speed can be increased by 30% ~ 40% by rinsing knives and machined parts.


A wide range of metal cutting fluid. ASTM D2881 classifies metalworking fluids into three categories: (1) oil and oil-based fluids; (2) water-based emulsions and dispersions; and (3) chemical solutions (true solutions and colloidal solutions). The basic difference between Type 2 and Type 3 lies in the particle size and particle size distribution of the dispersion. The average particle size of the oil emulsion is greater than 1μm, the true solution and colloidal solution particle size range of 20 ~ 40nm. Colloidal emulsion (II-C) represents an intermediate state between the chemical solution and the oil-dissolved emulsion with a particle size distribution between the above two levels. This principle of partitioning is basically a theoretical distinction because there can be an infinite number of levels from the typical mineral oil to the non-oil containing chemical solutions.


In recent years, the development and changes of metal cutting fluids are mainly in the field of water-soluble liquids (Class 2,3). Because of this type of liquid water-based substrate, the heat transfer rate (water heat transfer rate of oil 2.5 times). The same amount of water to absorb a certain amount of heat, the temperature rise much slower than the oil, thereby enhancing the cooling effect, and can reduce oil mist, so the amount of water-based cutting fluid increases. In the UK, for example, water-based cutting fluids account for about 60% of the total cutting fluid market. However, water-based cutting fluid and oil compared there is poor lubrication, followed by corrosion, colloidal stability, chemical stability, biological stability, filterability, foam and other issues. These issues are crucial to Sump Life when cutting fluids are used in machine tools. Reasonable choice, application, monitoring and maintenance, the use of water-based cutting fluid is particularly important.


First, the composition and choice of metal cutting fluid


(1), according to the current market conditions in our country, the main components of cutting fluid are as follows.


Oil or Oil Based Liquids: I-A, I-B, I-C in the ASTM D 2881 classification are conventionally called cutting oils (also known as net cutting oils) and are mainly mineral oils with or without additives.


 Emulsions: Type II-A, II-B, II-C in ASTM D 2881 classification, sometimes referred to as dissolved oils. According to the mineral oil content and oil droplet size can be divided into three kinds: coarse emulsion: oil 65% to 80%, oil droplets particle size 2 ~ 10μm; microemulsion: 40% to 50% oil, oil droplets particle size <1μm; Synthetic emulsion: oil 5% to 40%, particle size of about 0.1 μm;


Synthetic liquid: oil or no oil, dissolved in water as the main polymer of organic lubricants.


Chemical solution: Oil free, belongs to Ⅲ in ASTM D 2881 classification. From the above composition point of view, cutting oil lubrication is good. Emulsions in the coarse milk, microemulsion and semi-synthetic emulsion, such as formulated when also have very good lubrication properties. The current use of coarse emulsion and microemulsion range. For heavy-duty cutting emulsion to contain extreme pressure additives. Synthetic fluid is a supplementary product of emulsions. This liquid is often used for specific purposes. Some synthetic fluids cause damage to the operator's skin and machine coating during use due to increased concentration and enhanced cleanability. The chemical solution is a mineral oil-free aqueous solution. Dilution with water before use, good flushing, cooling effect, and should be able to prevent corrosion of the contact area. This type of liquid is mainly used for grinding, the function is cleaning and cooling, no lubrication. The choice of cutting fluid, the first to avoid the use of those who machine tools, cutting tools and processing materials harmful liquids. In general, sulfur free oils containing free sulfur are suitable for processing steel and copper. Some copper alloys and high-nickel alloys, in the sulfur agent (especially with free sulfur) will produce dark spots under the mark. The composition of water-based cutting fluid is more complicated, because to take into account the stability of the emulsion system, it is necessary to consider the HLB values of the ingredients, but also to achieve the balance of various properties. Since cutting fluids are water-based, the water-soluble or water-dispersible nature of the ingredients should also be considered.


Select the cutting fluid should be fully aware of the following conditions.


(2), the nature of processing materials


The physical and chemical properties of the material to be processed vary, reflecting the new problems of ease of cutting and compatibility with cutting fluid on cutting operations. The more difficult to process the material and its compatibility with the cutting fluid are briefly described below.


 Aluminum: soft, cutting easy to stick cut. Emulsions, such as alkaline, have a chemical reaction with aluminum, causing the emulsion to delaminate. Should use a special emulsion or paraffin-based mineral oil for cooling lubricant.


Brass: a lot of fines when cutting, easy to make the emulsified oil turn green. Active sulfur-containing oil processing materials can make discoloration, such as oil selection agent to have filtering equipment.


Bronze: Significant plastic deformation before shearing, can make the emulsion green. Such as oil selection agent to have filtering equipment.


Copper: Tough, cutting fine curly chips, the emulsion can turn green, affecting the stability of the emulsion, the role of activated sulfur stains. Such as the selection of oil to be equipped with filtration equipment.


Malleable iron: a large number of fine chemically active abrasive chips are produced during cutting. These reactive fines act like filter media, weakening the activity of the emulsion, and generating iron soaps that turn the emulsion red-brown, deteriorating the stability of the emulsion. If using oil, use a centrifuge or filter to remove iron filings


Lead and its alloys: easy to cut, can generate lead soap, destroy the stability of the emulsion. Such as the use of oil, tend to thickening of the oil, to prevent the use of oil containing a large amount of fat.


Magnesium: when cutting the resulting debris, flammable. Generally do not use water-based cutting fluid, low viscosity oil can be used as a cutting fluid.


Nickel and high-nickel alloy: local heat generated during cutting, the chip may be sintered. Optional heavy duty emulsion or non-reactive sulfurized oil.


Titanium: Produce abrasive, flammable cutting, prone to work hardening phenomenon, the application of heavy-duty emulsified oil or extreme pressure oil.


Zinc: irregular cutting surface, it is difficult to obtain a good finish, and the formation of zinc soap emulsion, the emulsion separation, should be selected special emulsion


(3), processing conditions


The function of the cutter is to cut and exfoliate the processed material with strong shear in the main shear area. Between the advancing surface of the tool and the exposed fresh metal surface, the advancing surface is subjected to high stress due to the strong adhesion. Due to cutting exfoliation of debris to move through the blade to promote the formation of the shear zone. The shearing effect in the shearing area subjects the tool to friction. The same is true for lubrication and cooling. However, many types of machining belong to the metal removal, but also have their own unique conditions. It is generally believed that the main task of cutting lubricant at low speed machining (thread cutting, reaming and tooth surface cutting) is to reduce the adhesion of the propulsion surface to the crumb as a boundary lubricant. In high-speed cutting, cutting fluid's main role is to reduce friction heat, take away heat.


Those cutting fluids difficult to reach the cutting area of the processing operations, to lubrication, cooling caused great difficulties. Usually for reaming, gear cutting (especially hobbing), deep hole drilling and boring, tapping (especially blind hole), deep hole, thread machining to carefully select the appropriate cutting fluid.


(4), oil-based and water-based features


Oil-based cutting fluid refers to additive-containing mineral oil. Water-based cutting fluid refers to the emulsion, synthetic fluids and chemical solutions. Generally speaking, low-speed heavy-duty cutting needs full lubrication, usually using extreme pressure cutting oil. High-speed shallow cutting, cooling is the primary choice of water-based cutting fluid. Some extreme pressure emulsions have good lubrication and cooling, can be used for heavy-duty cutting. General grinding process, sometimes harmful but not lubricating, it can be used synthetic solution or chemical solution. Processing materials, tool material, machine tool structure is also determine the choice of oil-based liquid or water-based important basis.


Second, use and maintenance


(1), preparation (dilution) only water-based cutting fluid need to be prepared, that is, according to a certain percentage of dilution with water. Water-based cutting fluid, especially emulsified type, diluted with water should pay attention to the following aspects.


1, the water quality under normal circumstances should not be used more than 400 water hardness, due to high hardness of water contained in calcium and magnesium ions will make anionic surfactant failure, the emulsion decomposition, the emergence of insoluble metal soap. Even though emulsions are made with non-ionic surfactants, large amounts of metal ions can cause micelles to aggregate, affecting the stability of the emulsion. Too soft water should not be used. Emulsions formulated with too soft water tend to produce large amounts of foam during use.


The hardness of the water used to prepare the emulsion should be 50-200. Available deionized water and untreated industrial water mixed use. China's vast territory, a very large variety of cutting fluid, so in the purchase of water-based cutting fluid, the deployment of the local water test. It is generally forbidden to use treated sewage, chemical-containing water and secondary water to formulate emulsions. Softened water used in boilers should also be used with caution.


Hard water users can use sodium carbonate method to soften water before use. Softener dosage determined by testing. To prevent the soft water after the water's pH value is too high. Excessive use of water softener will destroy the stability of the emulsion.


2, diluted


The dilution of the cutting fluid is related to the stability of the emulsion. Cutting fluid before use, you must first determine the proportion of dilution and the required volume of emulsion. Then calculate the amount of cutting fluid (liquid) and the amount of water used.


Pick a clean container, pour all the water you need into the container, and add the cutting fluid stock solution at low speed. Preparation of emulsions, the addition of stock solution to the non-emulsified stock solution shall prevail. Cutting fluid and water addition procedures can not be reversed. Do not dispense the emulsion directly in the sump (tank) of the machine.


(2), the use of cutting fluid


The use of cutting fluid, first of all depends on the correct selection of cutting conditions suitable for processing varieties, and the rational deployment of dilution. However, the following factors are also worthy of attention.


Circulate the total amount of liquid


Cutting fluids that are recycled during machining are constantly being lost due to spatter, atomization, evaporation and the processing of materials and chips. This consumption is expressed as a (carrying value). The definition is: In order to maintain the original size of the machine tool tank cutting fluid, the amount of cutting fluid to be added each month, expressed in multiples of the original volume. For example, a = 1 for a cutting fluid circulation system means a 20 m3 liquid circulation system with V of 20 m3 per month of diluted cutting fluid (or oil). European automotive industry machining a value of 1 to 1.5. Individual cutting fluid circulation system can be as low as 0.25, that is, the original cutting fluid, assuming no additional four months will be carried over, but also up to a = 4.


Carrying value a and the shape of the processed material has a great relationship. Carrying a value is undoubtedly related to machining costs. However, the carrying value is too small will increase the cutting fluid maintenance costs. The total cost of one cubic meter of coolant per year K is:


K = k1 + k2 + k3 (yuan / m3 & # 8226; year)


In the formula, k1 is the cost of changing the coolant (stock solution + water), the waste coolant discharge cost (labor, cleaning, filling with water and downtime); k2 is the cost of carrying Loss and liquid atomization, evaporation loss);  k3 for the maintenance costs of the coolant. We can see from the above formula, carrying value is too large or too small will increase the cost.


The gradual consumption of coolant, the liquid circulation system to reduce the liquid temperature rise or even overheating, reducing cooling effectiveness. Decline in cooling efficiency will affect the accuracy of the workpiece, and the tool hardness decreased. Cutting fluid temperature will exacerbate liquid atomization and evaporation, polluting the workshop environment, thereby increasing liquid consumption, the formation of a vicious cycle. Usually machine tank (tank) in the half full state can not play the proper function of the liquid, and liquid perishable.


When using oil cutting fluid (net cutting oil) temperature is too high, the more serious harm. Net cutting oil cooling capacity is low, and more used in those difficult to process, the heat of the cutting. Excessive temperature of the net cutting oil in the oil sump not only poses the aforementioned hazards, but it may also lead to the decomposition of additives (decomposition may produce harmful substances), damaging machine tools, processing materials and tools, and worsening the environment. Especially with a large number of chlorinated additives, cutting oil, mostly used for harsh machining, the heat generated. At this time, the oil tank should increase the oil content to increase the heat capacity.


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Cutting fluid flow


General machining should ensure that the pressure, high flow. Boring deep hole and hollow rod tools can use high pressure jet coolant, in order to facilitate flushing out the chip. Some medium and low carbon steel and titanium drilling process using pulsed injection coolant is more favorable, but pay attention to the performance of the pump. Harsh processing of chlorine used in cutting oil, to increase the flow.


The size of the flow rate can be expressed by the cycle factor f. The definition is that the amount of circulation per hour is a multiple of the total capacity. Cutting fluid circulation system temperature, foam, pollutant content have an impact on f.


Grease shape


The shape of the nozzle should be adapted to the shape and size of the workpiece, as well as the type of tool and operating procedures. A good nozzle should be cutting fluid has been to maintain a smooth flow, so that all parts of the processing of the bath fully in the liquid. The shape of the nozzle should be adjusted according to the actual effect. The basic requirement is to make enough coolant for cooling and lubrication.


Bubble


Water-based cutting fluid and cutting oil in the use of foam will occur too much problem. Pumping speed causes turbulence in the fluid, or hose dragging creates a foam that increases liquid. In particular, the water-based cutting fluid foam is one of its main performance indicators.


Different nature of the cutting fluid mixture (such as the net cutting oil and emulsion mixed) will also increase the foam. Before the machine changes the cutting fluid to wash the tank and the circulation line. In addition, the preparation of emulsions to avoid intense mixing and air mixing.


Over-softened water and alkali-containing water increase the foam of the emulsion. Fluid circulation pump seal lax will increase the liquid foam. The hazards of foam make the cooling lubricant failure and tank volume waste. Serious coolant can cause damage to machine tools, tools and workpieces.


Machine sealing


Always check the machine tool shaft seal (especially with emulsion as a coolant) to prevent cutting fluid from entering the machine gear box, headstock, or other sealed transmission. If the emulsion into the mineral oil lubrication system will wear the machine. Net cutting oil containing extreme pressure agent into the machine tool transmission or hydraulic system, less harmful.


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