Selection of cutting fluid under different metal cutting conditions

In mechanical cutting, when cutting with CNC machine tools, it is divided according to the cutting method, mainly including turning, milling, drilling and boring; according to the amount of material removed, there are roughing, semi-finishing and finishing; Cutting tool materials are divided into high-speed steel, hard alloy, ceramic, diamond and cubic boron nitride. According to the machine tool, there are mainly machine tools that require oil-based cutting fluid, no required machine tools, and single-piece small batch production types. Machine tools as well as batch production or automatic production lines (flexible manufacturing systems).

For different types of cutting, the cutting characteristics of the same metal are different, and the change of processing difficulty is also obvious. For example, the same Q235 carbon steel material is used, and it is difficult to machine other threads on the surface roughness quality index; it is difficult to finish the rough machining. The harder cutting process also requires higher cutting fluids. Proper use of cutting fluids ensures effective machining quality, extended tool life and improved machining efficiency.

Roughing

When the CNC machine tool needs to be separated by coarse and fine machining or the workpiece is not completed by one machine, the cutting fluid can be selected according to the characteristics of coarse and fine machining. When roughing, the amount of back-feeding knife and the feed amount are large, resulting in large cutting resistance, thus generating a large amount of cutting heat , and the heat transferred to the workpiece and the tool is correspondingly increased, so that the thermal deformation of the workpiece and the wear of the tool are intensified. Water-based cutting fluids that are mainly cooling and have a certain degree of lubrication cleaning and rust prevention should be selected , and a large flow rate should be continuously poured . When turning or the like is continuous processing or roughing allowance is uniform, the cutting heat is an important consideration, and the cooling effect of the cutting fluid is the first indicator.

When milling or machining irregular shapes, uneven margins and intermittent machining, the cutting speed is lower than continuous uniform machining. The influence of cutting heat is less than the impact of impact and vibration on the tool and workpiece, and the lubrication and cooling of the cutting fluid The role should be balanced. When the machine conditions permit, you can use the inner hole car with the internal liquid supply hole, the boring tool and the grooving knife in the hole machining and cutting, or use the pressure supply liquid, and use the spray liquid supply when roughing the difficult-to-machine materials. Play better.

The rough-worked workpiece generally has a machining allowance. At the same time, when processing difficult-to-machine materials and non-ferrous materials, the surface roughness accuracy index is not high. Therefore, when machining difficult-to-machine materials and non-ferrous materials, rough machining is in the cutting fluid. The chemical composition requirements are not high, and water-based extreme pressure emulsions can be used.

When roughing cast iron and brittle non-ferrous metals, the common feature of these materials when cutting is that the chips are broken, and the fine chips are flowing under the impact of the cutting fluid, which is large when flowing through the cutting fluid tank as the cutting fluid circulates. Part of it will deposit, and some will flow with the cutting fluid. Gathering in the small part of the cutting fluid conveying pipe will block the cooling nozzle and cause the chips to adhere to the moving parts of the machine tool (such as the rail moving pair). At the same time, the cutting fluid chemically reacts with certain components in the cast iron, causing the cutting fluid to deteriorate and causing the performance of the cutting fluid to decrease. Since the use of cutting fluids brings these problems, cutting fluids are generally not used. In order to reduce the effects of dust and cutting heat, when the conditions are met, you can consider using a dust extraction device to absorb dust, fine chips and a part of heat. If cutting fluid is used, water-based cutting fluid is easy to use, and the cutting fluid must be filtered and purified to prevent deterioration of cutting fluid and concentration. When roughing, the concentration of the cutting fluid is lower than that of finishing.

2. Finishing

Finishing is divided by cutting speed, with high-speed finishing and low-speed finishing . In high-speed finishing, the use of oil-based cutting fluid can ensure the surface quality of the workpiece and prolong the service life of the tool. However, the mineral oil contained in the oil has low viscosity and flash point, often producing soot, oil mist and oil vapor, and decomposing toxic components. It affects the production environment and impairs the physical and mental health of the operator. Therefore, when finishing high-speed cutting, the emulsion or microemulsion containing the extreme pressure additive in the water-based cutting fluid should be used as much as possible, and the concentration should be higher than the concentration of the rough processing. At low speed finishing, the cutting temperature is low and the above problems are not easily caused. In order to ensure the machining accuracy of the workpiece, oil-based cutting fluid should be used.

Compared with roughing, the cutting speed during finishing is high, the cutting heat generated by chip deformation is lower than the cutting heat generated by the tool flank friction, and the cutting force is small. Even at high speed cutting, the cutting heat does not thermally deform the workpiece. Rough machining is obvious, the reason why the cutter uses cutting fluid is to reduce the wear of the flank of the tool. When the tool is finished, the relative cutting speed is high, and the temperature at which the cutting of the chip is removed from the workpiece is very high. Even if the cutting fluid is used, if the cutting fluid has poor permeability or insufficient pressure, the cutting fluid does not penetrate sufficiently. The cutting zone makes the tool extremely prone to flank wear. Even if the tool can still be used, the surface roughness quality of the workpiece is not up to standard and the tool needs to be replaced. Therefore, when finishing, it is necessary to use a cutting fluid with remarkable lubrication effect and good permeability to prolong the service life of the tool and ensure the machining accuracy and surface roughness quality requirements of the workpiece .

For low-speed finishing of ferrous materials, active extreme pressure cutting oil can be used; when low-speed finishing of non-ferrous metals, non-active extreme pressure cutting oil should be used; when high-speed finishing of non-ferrous metals, emulsions of inactive extreme pressure additives can be used.

3. Hole processing

In CNC machining, hole machining mainly refers to drilling, reaming, turning the inner hole on a lathe and boring on a trampoline. Due to the difficulty in chip removal and heat dissipation, the rigidity of the tool (rod) is poor, and the cutting speed of the hole machining is lower than that of the outer contour machining, and the difficulty is also higher than the outer contour machining.

When drilling, the general twist drill is used for drilling. It is rough machining. It is difficult to remove chips during drilling. It is not easy to dissipate cutting heat, which often causes the blade to anneal, which affects the service life and processing efficiency of the drill bit. The use of a good cutting fluid can significantly extend the life of the drill bit and increase the productivity. Extreme pressure emulsion or extreme pressure synthetic cutting fluid is generally used. Extreme pressure synthetic cutting fluid has low surface tension, good permeability and can cool the drill bit in time, which is very effective for prolonging tool life and improving machining efficiency. For difficult-to-cut materials such as stainless steel and heat-resistant alloys, low-pressure extreme pressure cutting oils can be used.

When drilling, whether it is ordinary drilling or deep hole drilling, the heat dissipation condition is poor, and a large amount of cutting heat is generated. The discharge direction of the chips is opposite to the feeding direction of the drill bit, and the cutting fluid needs to penetrate into the cutting edge of the drill to be lubricated, cooled and For auxiliary chip removal, the cutting fluid should first have good permeability, and the liquid supply mode, flow rate and pressure should also meet the requirements.

In the improvement of the bit life index, the oil-based cutting fluid is generally better than the water-based cutting fluid. The extreme pressure micro-emulsion in the water-based cutting fluid is the best. The low-viscosity active vulcanized oil in the oil-based cutting fluid is the most comprehensive in all cutting fluids. Ok.

Since reaming is a low-speed thin-chip cutting process in finishing, the reaming should focus on the dimensional accuracy and surface roughness accuracy of the reaming and the service life of the reaming accuracy.

In the control pore size index, the oil-based cutting fluid enlarges the pore size, the mineral oil enlarges the pore size, the extreme pressure cutting oil is small, the active sulfurized and chlorinated oil are the smallest; the water-based cutting fluid reduces the pore size and contains sulfur. The extreme pressure microemulsion and microemulsion minimize the pore size, the emulsion is centered, and the synthetic cutting fluid is the smallest. In view of this, in order to control the size of the reaming hole, when a new reamer is used, the water-based cutting fluid is used to make the hole difficult to expand, and when the reamer is worn to a certain extent, the oil-based cutting fluid can be used to slightly enlarge the aperture.

In reducing the surface roughness of the reaming, the use of water-based cutting fluids is better than the use of oil-based cutting fluids. In the oil-based cutting fluid, the active sulfurized chlorinated oil has the best effect, followed by the chlorine-containing extreme pressure oil and the mixed mineral oil, and the pure mineral oil is the worst; in the water-based cutting fluid, the emulsion, the micro-emulsion, the sulfur-containing extreme pressure The effect of the microemulsion is basically the same, and the synthetic cutting fluid is the worst.

In the control of the life of the reamer, the inactive extreme pressure cutting oil and the antifriction cutting oil in the oil-based cutting fluid have the best performance; in the water-based cutting fluid, the synthetic cutting fluid is the worst.

In addition to the floating boring tool, the turning hole and boring are single-edge cutting, the heat dissipation condition is worse than the outer circle, and the cutting fluid is used in the same way as the drilling hole and the reaming hole, and the flow rate and pressure should be appropriately increased.

4. Thread processing

Thread processing belongs to forming processing. When attacking and threading, it belongs to multi-blade low-speed cutting. The cutting edge is surrounded by cutting material. The cutting torque is large, the chip removal is difficult, the heat can not be taken away by the chip in time, the chip debris is congested and the vibration is easy to generate. The tool is easy to wear. In particular, the cutting conditions are more severe when tapping the thread: the chip space is small, the chips are not easy to break and flow out, and the tapping and chip removing generate large cutting force and friction force, which easily leads to tapping and breaking of the tap. The cutting fluid is required to have both a low coefficient of friction and a good permeability to reduce the frictional resistance of the tool and prolong the service life of the tool.

When cutting black metal materials, oil-based cutting fluids with high sulfur, chlorine additive components, low viscosity and good permeability should generally be used.

When using high speed steel for low speed turning, the cutting fluid is the same as the tapping and threading. The use of carbide tools to turn the thread, its cutting speed is fast, the impact force is high, the cutting temperature is high, the cutting edge area is small, the cutting force is large, the thread tooth type precision is high, and it is not easy to cause the built-up edge and scales to affect the thread. The problem of surface quality has high requirements on the service life of the tool. It requires the cooling fluid, lubrication and permeability of the cutting fluid to be provided at the same time. It is advisable to use water-based cutting fluid containing extreme pressure additives.

5. Different tool materials

In many cases, in order to avoid damage caused by uneven heating of the tool, it is customary to use dry cutting when cutting with a carbide tool. However, on CNC machine tools, it is best to use cutting fluids in terms of efficiency, overall cost, and processing quality. When machining with carbide tools, when the cutting speed is relatively high, water-based cutting fluid is generally used. It should be used continuously at a large flow rate and be observed to ensure that the nozzle is always aligned with the cutting position. When cutting with high speed steel, the cutting speed is low. Generally, oil-based cutting fluids based on lubrication are used.

The use of ceramic tools, cermet tools, diamond and cubic boron nitride tools is mainly suitable for high-speed cutting, dry cutting and hard cutting, without the use of cutting fluid. Sometimes, in order to avoid excessive cutting temperatures, the use of these tools is often used in many cases. Due to the high cutting speed, it is advisable to use a water-based cutting fluid.

6. Conclusion

In the CNC cutting process, the rational use of cutting fluid plays an important role in improving machining efficiency, improving machining quality, extending tool life and reducing overall cost. However, the cutting fluid will cause health hazards to the processor during use; waste treatment and discharge will also pollute the environment. In response to the proposal of green manufacturing, processors should explore the need to use cutting fluids or use less cutting fluid to complete the processing tasks. Relevant enterprises and scientific research institutions should also continuously develop cutting fluids with better performance, green environmental protection, good versatility and good economy. Meet the requirements of cutting fluid users. Machine tool managers, process technicians and machine tool operators should track and pay attention in a timely manner, and actively promote and use new environmentally friendly cutting fluids and new cooling lubrication technologies, so that more suitable cutting fluids and technologies can be used in production practice.