The method of using cutting fluid has a great impact on tool life and processing quality. Even the best cutting fluid cannot play its due role if it cannot be effectively delivered to the cutting area. Therefore, when using a cutting fluid mainly based on lubrication (such as cutting oil), it should be transported to a location where an oil film can be generated on the friction surface.
On the contrary, if the selected cutting fluid is mainly cooling (such as water-based cutting fluid), the cutting fluid should be close to the cutting edge of the tool. Under such conditions, pressure method is usually used to force the cutting fluid into the cutting area, thereby taking away the heat generated by friction and deformation of the tool, workpiece, and chips.
Continuous application of cutting fluid is better than intermittent application of cutting fluid. Intermittent application of cutting fluid can produce thermal cycles, which can cause cracks and chipping in hard and brittle tool materials (such as carbide tools). In addition to shortening tool life, intermittent use of cutting fluid can also make the work surface rough and uneven.
Another benefit of using cutting fluid correctly is the effective removal of chips, which also helps extend tool life. Proper placement of cutting fluid nozzles can prevent the chip flutes of milling cutters and drill bits from being blocked by chips or causing poor chip discharge. For the processing of some large workpieces, or strong cutting and grinding with large feed rates, two or more rows of coolant nozzles are used to fully cool them, which is beneficial to improving processing efficiency and ensuring processing quality.
1, manual oiling method
Solid or paste lubricant can be applied or dripped onto the tool or workpiece with a brush or brush (mainly when tapping threads and die sets). Recently, a hand-held liquid supply device has been developed, which atomizes lubricant through pressurization and sprays it onto the cutting tools and workpieces.
On machine tools without metering cooling systems, manual oiling is an effective method if the number of holes or threads to be drilled is small. When two different processes are to be completed on the same machine tool, manual oil can be used in conjunction with the overflow cooling system on the machine tool.
2, overflow method
The most common method of using cutting fluid is the overflow method. Use a low-pressure pump to drive the cutting fluid into the pipeline, and then flow out from the nozzle through the valve. The nozzle is installed close to the cutting area. The cutting fluid flows through the cutting area and then flows to different parts of the machine tool, and then collects into the oil collection pan, and then flows back from the oil collection pan to the cutting fluid tank for recycling. Therefore, the cutting fluid tank should have sufficient volume to allow the cutting fluid time to cool and allow fine chips and abrasive particles to settle.
Depending on the machining process, cutting-fluid tank capacity is approximately 20-200L. Some processes need larger tanks: deep-hole drilling and heavy grinding may require 500-1000L or more. The Metalworking WeChat account is worth following. Fit a coarse filter in the collection tray to keep large chips out of the tank, and a fine filter at the pump suction. Machines for grinding, honing, deep-hole drilling and deep-hole boring require high workpiece surface quality, so finer wear debris, grinding-wheel particles and chips must be removed. For example, gun-drilling deep holes requires filtration through 10um filter paper.
The use of filtration equipment can avoid excessive contaminants or excessive metal particles in the cutting fluid, helping to keep the cutting fluid clean and extend the service life of the cutting fluid. Modern automated machine tools are generally equipped with cutting fluid filtration, separation, and purification devices.
The overflow method allows the cutting fluid to continuously flow to the cutting area and wash away the chips. The flow rate of the cutting fluid must be larger so that the tool and workpiece can be submerged by the cutting fluid. In addition to providing appropriate cutting fluid to the cutting area, there must be enough cutting fluid to prevent abnormal temperature rise. In deep hole drilling, if the cutting fluid tank is too small, the temperature of the cutting fluid will rise quickly. When the oil temperature exceeds 60°C, cutting cannot continue. Therefore, deep hole drilling machines are generally equipped with larger cooling oil tanks.
The distribution pattern of cutting fluid flow directly affects the efficiency of cutting fluid. The nozzle should be placed so that the cutting fluid will not be thrown away from the tool or workpiece due to centrifugal force. It is best to use two or more nozzles, one to deliver the cutting fluid to the cutting area, while the others are used to assist cooling and flush away chips.
When turning and boring, the cutting fluid is required to be sent directly to the cutting area so that the cutting fluid covers the cutting edge of the tool and the workpiece to provide a good cooling effect. Practical experience has proven that the inner diameter of the cutting fluid nozzle is at least equivalent to three-quarters of the width of the turning tool.
For heavy-duty turning and boring operations, a second nozzle is required to supply cutting fluid along the underside of the tool. The cutting fluid supplied by the lower nozzle can be smoothly delivered between the tool and the workpiece without being blocked by cutting, which helps to lubricate at low speeds.
For horizontal drilling and reaming, cutting fluid is best delivered to the cutting zone through a hollow tool bore, ensuring sufficient fluid at the cutting edge and flushing chips out of the hole. Because drill flutes intended for chip evacuation also carry fluid away from the cutting zone, little fluid reaches that zone even on a vertical drill; a hollow drill can address this problem. The Metalworking WeChat account is worth following. Most drilling in China currently uses twist drills. Fluid enters in the opposite direction to chip evacuation, making it difficult to reach the cutting edge. This weakens cooling and lubrication, making drills prone to burning, severe wear and short tool life.
How to improve the cutting fluid supply method is an issue worth studying.
When milling, it is best to have two nozzles to deliver cutting fluid to the inlet and outlet sides of the milling cutter. The cutting fluid flowing out of one nozzle is sent to the cutting area by the milling cutter teeth, and the cutting fluid flowing out of the other nozzle flushes the chips out of the tool. Narrow milling cutters can use a standard round nozzle, and wide tools should use a flat nozzle with a width of at least 3/4 of the tool width to achieve good coverage.
For face milling, an annular sprayer made from a tube with many small holes is better. In this way, the cutting fluid can be sent to each cutting edge, so that the tool is completely immersed in the cutting fluid and has a uniform cooling effect. If you frequently use a face mill of a particular size, it is best to have a circular injector with a scalloped shape and a curve at the opening that matches the radius of the cutter.
Using low-pressure and high-flow grinding fluid during grinding can generally achieve good results. However, when the flow rate is too large, unnecessary splashing will occur, especially for synthetic cutting fluids with poor defoaming properties, which are more likely to cause the grinding fluid to overflow. This can be solved by installing a splash guard and adding a defoaming agent.
The effect of using the general spraying method during grinding is very poor. The grinding fluid can hardly take away any heat before the grinding heat spreads to the entire workpiece. This is because the surface speed of the grinding wheel is very high, and there is always an air film on the surface surrounding the grinding wheel, which hinders the penetration of the cutting fluid into the cutting area. A special nozzle should be designed to force the cutting fluid to be sent to the grinding wheel through the air film. This nozzle should be as close as possible to the workpiece to prevent the cutting fluid from being completely lost due to the centrifugal force of the grinding wheel.
Another method to overcome the air film generated on the grinding wheel is to install a baffle close to the nozzle to block the air flow. This can form a partial vacuum between the interface between the grinding wheel and the workpiece to suck in the grinding fluid.
3, high pressure method
Certain machining operations, such as deep-hole drilling and trepanning, commonly use high-pressure cutting-fluid systems at pressures of 0.69–13.79 MPa. Deep-hole drilling uses a single-edge drill and is similar to boring, except that the drill contains internal cutting-fluid passages. Trepanning produces a cylindrical hole in the workpiece while leaving a solid cylindrical core. As the tool enters the workpiece, the solid core passes through the hollow cylindrical cutting head. A pressure pump delivers cutting fluid around the tool, forcing chips to flow out through the tool's center.
The cutting fluid used for trepanning must have good extreme pressure properties and anti-sintering properties. The viscosity should be very low so that it can flow freely around the tool. It should also have good oil properties to reduce the friction coefficient between the tool and the workpiece, and between the tool and the chips.
The main problem in deep hole drilling is how to maintain sufficient cutting fluid flow in the cutting area. One way is to use the cutting flute as a passage for cutting fluid. The cutting fluid pressure is 0.35-0.69MPa. It flows into the drill bit through the rotating sealing sleeve, and then directly enters the cutting area. The cutting fluid flowing out of the hole helps remove chips. When drilling deep holes, the use of oil hole drilling is a big improvement compared with the overflow method, and the drill bit life and productivity are greatly improved.
The high-pressure method helps the cutting fluid reach the cutting area and is sometimes used on other machine tools. Grinding enables high-pressure nozzles to facilitate cleaning of the grinding wheel.
4, spray method
Cutting fluid can be sprayed onto the tool and workpiece in the form of airborne oil mist. The cutting fluid passes through a small nozzle and uses compressed air with a pressure of 0.069-0.552MPa to disperse the cutting fluid into very small droplets and spray them into the cutting area. In this case, water-based cutting fluid is better than oil-based cutting fluid, because the oil mist of oil-based cutting fluid pollutes the environment, is harmful to health, and is easy to integrate larger oil droplets. The spray method is best suited for machining with high cutting speeds and low cutting areas (such as end milling).
Choose a cutting fluid with good cooling performance. When small droplets come into contact with hot tools, workpieces or chips, they can quickly evaporate and take away the heat. Spray cooling does not require a splash guard, oil collection pan and oil return pipe. It only uses a small ball shape, and the workpiece is dry. Even if there is a little oil, it can be easily wiped dry.
The spray method has the following advantages: 1) The tool life is longer than dry cutting; 2) When the overflow system is not available or inappropriate, it can be used to provide cooling; 3) The cutting fluid can reach places that are inaccessible by other methods; 4) Between the workpiece and the tool, the flow rate of the cutting fluid is higher than that of the overflow method, and the cooling efficiency is calculated based on the same volume of cutting fluid, which is many times higher than the overflow method; 5) Costs can be reduced under certain conditions; 6) The workpiece being cut can be seen.
Disadvantages of the spray method are limited cooling capacity and the need for ventilation.
There are three types of spray systems. 1) Suction type: This works on the same principle as a household sprayer, mainly using the Venturi principle. Compressed air draws cutting fluid out of the tank and mixes and atomizes it in the air stream. One tube carries compressed air and another siphons the cutting fluid, with both connected to a mixing fitting. It is suitable for spraying low-viscosity cutting oils and emulsions. 2) Pneumatic type (pressurization method): Cutting fluid is placed in a sealed tank and pressurized with compressed air at 0.2–0.4 MPa. When the solenoid valve opens, the cutting fluid is forced out and mixes with the compressed-air stream through a mixing valve for atomization.
Metal processing WeChat is worth paying attention to. This device is suitable for spraying water-based synthetic liquids and emulsions, but the aqueous solutions and emulsions must not contain fatty oils or suspended solid matter. The atomization mixing ratio can be adjusted by the mixing valve and pressure regulating valve. 3) injection type, its principle is to use a gear pump to pressurize the cutting fluid, and directly spray it into the compressed air flow through the mixing valve to atomize it. This device is suitable for atomizing transparent cooling water and low-viscosity cutting oil.
Spraying can be used in end milling, turning, automatic machine tool processing, and CNC machine tool processing. The spray device with solenoid valve control is suitable for tapping and reaming holes on CNC machine tools.
5, Refrigeration liquid cooling method
There are many types of refrigeration liquid cooling methods. Gases such as nitrogen, argon, and carbon dioxide can be compressed into liquids and placed in cylinders. Freon gas can be compressed into liquids by mechanical devices and released during use. After passing through the regulating valve, it is directly injected from the nozzle into the cutting area, and the tools, workpieces and chips are cooled by gasification and heat absorption. This method has a very good cooling effect and is suitable for cutting difficult-to-machine materials such as stainless steel, heat-resistant steel, and high-strength alloy steel. It can greatly improve the durability of the tool.
6, centralized cutting fluid supply system
For large and medium-sized machining plants, when possible, they should consider using a centralized circulation system to supply cutting fluid to multiple machine tools, but each machine tool must use the same cutting fluid. Several grinding machines can handle grinding chips using a linked conveyor system. Centralized processing of fine chips and grinding chips moistened by cutting fluid can reduce manual processing and improve working conditions.
The centralized cutting fluid supply system allows factories to better maintain cutting fluids. The cutting fluid is concentrated in a large pool. Through regular sampling inspection, the original solution or water is regularly replenished according to the inspection results to facilitate the control of the concentration of the cutting fluid. It can reduce the number of sampling inspections and conduct inspections on more items to ensure the quality of the cutting fluid during its service life. Compared with many separate multiple cutting fluid supply systems set up separately, the cost is also relatively reduced because the maintenance work of cutting fluid is reduced.
A central supply system's main advantage is that centrifugation can effectively remove tramp oil and metal particles from cutting fluid, also removing approximately half the bacteria because they readily grow at the interface between floating oil and metal particles. Continuous removal of these contaminants, regular quality checks and planned addition of additives or concentrate based on the results are important reasons central systems can greatly extend cutting-fluid life.
This also reduces waste disposal of water-soluble cutting fluids.
(Source: Internet)