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Dry cutting is the development direction of cutting. Twenty years ago, cutting fluids were very cheap, accounting for less than 3% of the cost of most machining processes. So no one will pay more attention to this. However, nowadays, the proportion of cutting fluid in the production cost of the workshop has risen to 15%, which has caused great concern to the production operators.
Especially those oily cutting fluids have become a big expense. More importantly, its emissions pollute the environment, and foreign environmental protection departments should monitor the handling of these mixed preparations. Moreover, many countries and regions also classify them as hazardous waste, and if they contain oil and certain alloys, they must adopt more stringent control measures. In addition, many high-speed machining processes add cutting fluid to generate smoke, and the environmental protection department also limits the amount of cutting fluid smoke to be allowed. Occupational safety and employee health management departments are considering a value to reduce the allowable value of cutting fluid smoke emission. Advisory Committee’s recommendations. This includes pricing policies that set relatively high cutting fluids. As a result, more and more manufacturers are beginning to use dry cutting to avoid the expense associated with cutting fluid processing.
In the past, the use of cutting fluids in the metalworking industry has formed a "custom", so the main obstacle to the promotion of dry cutting is this habit. They believe that cutting fluids are necessary to achieve a good machined surface and improve tool life. There are also many people who think that getting wet cuts is dry cut and the cost may be higher. In fact, the two views are not correct. For most gold cuts, the dry cut should be ""standard processing environment". Drying and dry milling hardened materials at high speeds is not only possible but also more economical. The key is to know how to choose the right tool, machine and cutting method. Although cutting fluids are still needed in some cases, studies have shown that due to the great development of today's tool materials, the situation is constantly changing. New carbide grades, especially those coated, do not require cutting fluids at high speeds and temperatures, resulting in higher cutting efficiency. In fact, for intermittent cutting, the higher the temperature in the cutting zone, the less suitable the cutting fluid is.
Let's take a look at milling. Assuming that the cutting fluid can overcome the centrifugal force caused by the high-speed rotating milling cutter, it will evaporate before it reaches the cutting zone, and its cooling effect is small or even no. The application of the cutting fluid tool produces a drastic change in temperature. The milling cutter blade cools when it is cut out from the workpiece, and the temperature rises again when it is cut. Although similar heating and cooling cycles occur during dry cutting, the temperature change with the addition of cutting fluid is much greater. A sharp change in temperature creates stress in the blade, which can lead to cracks.
A similar situation can occur in turning. For example, with uncoated cemented carbide, at speeds above 130 m/min, the medium carbon steel is turned and the tip is cut into the workpiece for less than 40 seconds and then exposed to the coolant. It can clearly show the damage of thermal shock. This thermal shock accelerates crater wear and rear wear, which greatly reduces tool life. For most turning operations, dry cutting usually extends tool life.
However, for drilling, it is another situation. Cutting fluid is necessary during drilling because it provides lubrication and flushes the chips from the holes. Without cutting fluid, the chips may stick to the holes and the average surface roughness (Ra) may be twice that of wet drilling. In this case, the cutting fluid also reduces the required machine torque because the point on the edge of the drill that is in contact with the wall of the hole is lubricated. Although coated drill bits also provide lubrication similar to cutting fluids, coatings also reduce cutting forces and minimize frictional drag. From the overall effect, the cutting fluid cannot be completely replaced at present. Which type of cutting fluid is used depends on the specific situation, and the lubricative cutting fluid is preferably used for low-speed machining of difficult-to-machine materials and high surface roughness requirements. The cutting fluid with higher cooling capacity can enhance the high-speed processing performance of the free-cutting material, and can be used in the case of having a tendency to build-up chips or having strict dimensional tolerances.
However, many times the cutting fluid has been used to achieve some effect, but it requires a very high cost and also brings very harmful environmental pollution, which is not worth it. It should be noted that modern cutting tools can withstand higher cutting heat and have the performance required for high speed cutting. If necessary, compressed air can be blown away from the cutting zone by compressed air to replace the cutting fluid.
Selection of tool materials for dry cutting 1 High-speed dry cutting The best coating is nitro-aluminum. Today, the important reason why cutting fluids are usually unnecessary is coating. They slow down the impact of temperature by suppressing heat transfer from the cutting zone to the insert (tool). The coating acts like a thermal barrier because it has a much lower thermal conductivity than the tool substrate and workpiece material. As a result, these tools absorb less heat and can withstand higher cutting temperatures. Whether turning or milling, coated tools allow for more efficient cutting parameters without reducing tool life.
The coating thickness is between 2 and 18 μm and it plays an important role in tool performance. Thinner coatings perform better at temperature changes than impact coatings during impact cutting because thinner coatings have less stress and are less prone to cracking. In rapid cooling and heating, the thick coating is as easily cooled as the glass is heated and cooled very quickly. Dry cutting with thin-coated blades can extend tool life by up to 40%, which is why physical coatings are often used to coat round and milling inserts. PVD coatings tend to be thinner than chemical coatings and bond to the contours. In addition, PVD coatings can be deposited on cemented carbides at much lower temperatures, so they are more widely used for very sharp edges and large positive rake angle cutters and turning tools.
Although the coating material is titanium nitride, it accounts for 80% of all coated tools. However, in the case of high-speed dry cutting, the best PVD coating is titanium aluminum nitride (TiAlN), which is four times better than titanium nitride in continuous cutting at high temperatures, for example for high speed turning. TiAlN coatings outperform other coatings for tools that are exposed to higher thermal stresses. Such as dry milling and deep hole drilling of small diameter holes are difficult to reach.
TiAlN is harder and more thermally stable than TiN at the cutting temperature. The PVD coating utilizes its chemical wear resistance. Its hardness is as high as 3,500 °C and its operating temperature is as high as 1470 °F. Materials scientists speculate that these properties can be attributed to the amorphous alumina film, which is formed at the chip/tool ​​interface when some of the aluminum in the coating surface is oxidized at high temperatures.
This study deliberately used ultra-thin multi-layer PVD coatings, which produced a coating consisting of hundreds of layers, each layer being only a few nanometers thick. The deposit of a typical PVD coating is only a few layers of micron-thickness coating.
Despite the many advantages of PVD coatings, CVD coatings are still more popular for processing most ferrous metals. In the CVD process, the higher deposition temperature helps to increase the bonding strength and allows a higher cobalt content in the matrix, so that the toughness of the blade is good and the ability to resist plastic deformation is improved. Since CVD coatings are thicker than PVD coatings, passivation at their cutting edges is required to prevent coating flaking and to help improve the wear resistance of the tool. The feed rate is allowed to be up to 0.035 in./r (about 0.9 mm/r).
CVD is the process of depositing a layer of useful alumina on a tool, which is the most resistant and oxidation resistant coating. Alumina is a poor conductor that separates the tool from the heat generated by the deformation of the cutting, causing heat to flow into the chip. This is an excellent CVD coating material, mainly used for carbide turning tools used in dry cutting. It also protects the substrate during high-speed cutting and is the best coating for abrasive wear and crater wear.
Coated inserts have a long tool life and are more stable in dry milling than wet milling. Higher cutting speeds will further increase the cutting temperature. For example, dry machining of cast iron at cutting speeds of 14000r/min and 1575in./min (about 40m/min) can heat the cutting zone in front of the tool to 600-700 °C. Its metal removal rate is similar to milling aluminum, when the temperature produced on cast iron is higher than conventional tools.
2 The choice of cermet, ceramic, CBN, PCD cutting speed requires the tool material to be more wear-resistant, and also requires higher thermo-hardness. Cermets, cubic boron nitride and two ceramics suitable for fine processing - alumina and silicon nitride (modern term "ceramic"" contain alumina and silicon nitride, unlike the single-point alumina in the past. Their applications are becoming more and more popular. Polycrystalline diamond is another tool material used in dry cutting applications. Among all of these materials, they have high red hardness and wear resistance, and trade-offs are considered to be more brittle.
Cermet is an advanced hard alloy. Cermets can withstand higher cutting temperatures than conventional cemented carbides, but lack the impact resistance of cemented carbides, toughness during medium to heavy processing, and strength at high feed rates at low speeds. Metallic ceramics have similar blade strengths as conventional cemented carbides at small and constant loads. However, it has better high temperature and wear resistance at higher cutting speeds, longer duration, and a smoother surface of the machined workpiece. When used to process soft and viscous materials, it also has good resistance to built-up edge and good surface quality.
The preferred high temperature hardness comes from the compound of titanium added during the compounding. A cermet is a type of cemented carbide that contains a hard titanium-based compound (titanium carbide, titanium carbonitride, and titanium nitride), and the binder is nickel or nickel-molybdenum. Due to the temperature limitations of metal-type adhesives, typical cermet grades do not have sufficiently high thermohardness when the hardness of the processed material exceeds HRC40. Compared to coated and uncoated cemented carbides, cermets are more sensitive to stresses caused by fracture and feed. Therefore, it is best used for high precision workpieces and when surface quality is high. The ideal machining process is to cut those continuous surfaces.
When turning carbon steel, the upper limit of the feed rate is usually 0.025 in. / r (about 0.635 mm / r). General purpose milling can be carried out under conditions of high spindle speed and medium feed. If these conditions are met, the cermet can maintain a sharp cutting edge for a long time in mass production. If the cermet is used at the traditional cutting speed and feed rate, the tool life and surface quality can be improved compared to the cemented carbide tool, and the productivity can be improved. When the alloy steel is cut, the increase is 20% for the cutting. 50% for carbon steel, stainless steel and soft iron.
Ceramic ceramic tools are similar to cermets in that they have higher chemical stability than hard alloys and can be processed at high cutting speeds for extended periods of time. Pure alumina can withstand very high temperatures, but its strength and toughness are very low, and if the working conditions are not good, it is easy to break. In order to reduce the sensitivity of the ceramic to fracture, in order to improve its toughness and improve impact resistance, zirconia or a mixture of titanium carbide and titanium nitride is added. Despite the addition of these additives, the toughness of ceramics is much lower than that of cemented carbides.
Another way to improve the toughness of alumina ceramics is to add crystalline texture or silicon carbide whiskers to the material. These special averages are only 1 nanometer in diameter, 20 micrometers long and strong whiskers, and the ceramics are considerably increased. Toughness, strength and thermal shock resistance. In composition, whiskers can be as high as 30%.
Like alumina, silicon nitride has a higher thermohardness than cemented carbide. It also has good resistance to high temperature and mechanical shock. Its disadvantage compared to alumina ceramics is that it is not very chemically stable when processing steel. However, the gray cast iron can be processed at a speed of 1,450 ft. / min (about 440 m / min) or higher using a silicon nitride ceramic.
Although machining with ceramic knives can be very efficient, the application must be correct. For example, ceramic tools cannot be used to machine aluminum, but are particularly suitable for gray cast iron, ductile iron, hardened steel and certain unhardened steels and heat resistant alloys. However, for these materials, the success of the application depends on the preparation of the cutting edge of the tool before starting the cutting, the stability of the machine and equipment and the selection of the best processing parameters.
CBN
CBN is a very hard tool material. It is usually best used to process materials with hardness higher than RC48. It has excellent high temperature hardness - up to 2000 ° C, although it is much more brittle than hard alloy, more heat resistant than ceramic. Sexual and chemical stability is poor, but it has higher impact strength and crush resistance than ceramic knives. When cutting hardened metals, the rigidity of the machine tool can be slightly worse. In addition, some special CBN tools are able to withstand the chip load of high-power roughing, the impact of intermittent cutting and the wear and cutting heat during finishing.
For demanding parts, the equipment should be properly adjusted to increase the rigidity of the machine and equipment. The blunt edge should be large enough to prevent micro-flaking and a certain thickness of the CBN layer on the tool base, which allows the tool to operate at high speeds, heavy loads, and severe intermittent loads. These features make CBN the tool material of choice for roughing hardened steel and pearlitic grey cast iron.
The tool is relatively fragile with a thin layer of CBN, but it is also a good tool material for machining hardened ferroalloys. CBN has a low thermal conductivity and a high compressive strength, and it withstands the heat of cutting due to high cutting speed and negative rake angle. The softening of the workpiece material due to the higher temperature in the cutting zone contributes to the formation of chips. The negative geometry enhances the tool, stabilizes the cutting edge, improves tool life and allows machining at shallow depths of less than 0.010 in. (approximately 0.254).
In the case of dry turning hardened workpieces, CBN tools can be used to replace the grinding surface because it can produce a surface quality of less than 16 μin. (about 0.4 μm) and can control the accuracy of ±0.0005 in. (about ±0.0127 mm). Cutting process. CBN tools are ideal for hardened turning and high speed milling. For this range of applications, ceramics and CBNs overlap. Therefore, a cost-benefit analysis is necessary to determine which material will provide the best results.
PCD tool polycrystalline diamond is the hardest tool material and it is the most wear resistant. Its hardness and wear resistance come from the bonding of the diamond crystals without a certain orientation. The arrangement of the crystal orientations inhibits the crack propagation. When used, the PCD die is bonded to the carbide insert, which increases its strength and impact resistance, and its tool life is 100 times that of cemented carbide.
However, certain performance limits its use in many processing operations. One is the affinity of PCD for iron in ferrous metals, causing chemical reactions. This tool material can only be used to process non-ferrous parts. The second is that the PCD cannot withstand the high temperatures of the cutting zone exceeding 600 °C. Therefore, it cannot cut tough, highly ductile materials.
PCD tools are particularly suitable for the processing of non-ferrous metals, especially high-silicon aluminum alloys that are very abrasive. Efficient cutting of these materials with a sharp cutting edge and a large positive rake angle minimizes cutting pressure and built-up edge.
Edge Enhancement, Tool Geometry and Chip Removal Despite the advances in physics and application development in recent years, knives made of cermet, ceramic, CBN and PCD are still much more brittle than hard alloys and cannot withstand too much pressure. Therefore, tools made of these materials must be designed in combination with their characteristics, that is, to strengthen the support and disperse the pressure.
this point is very important. For example, in order to change the direction of the grinding force, the force is directed from the cutting edge toward the body, and the cutting edge must be machined--blade preparation. There are three kinds of cutting edge preparations and their size is also appropriate: T-shaped belt, reinforcement, T-type belt reinforcement.
The T-shaped belt is a chamfer--a narrow plane that is ground on the cutting edge to replace the weaker and sharper blade. An important task for the tool designer is to find the minimum plane width and angle that gives the blade the proper strength and longevity; because the large width and the angle of the reinforcing blade will undoubtedly increase the cutting force.
Strengthening is to round out the sharp edge. Although the reinforcement is not as angular as the T-shaped belt, the reinforcement works well for advanced blade materials used for finishing. These reinforced tools should be used for shallow depth-cutting, low-speed feeds, and to keep cutting pressures to a minimum.
The T-shaped belt reinforcement can also strengthen the T-shaped belt when reinforcing the front and rear intersections of the chamfer. In applications where small spalling occurs (as with a ceramic knife roughing steel), the reinforcement disperses the pressure at these points without enlarging the cutting edge without enlarging the chamfer.
In addition to determining the most suitable tool edge for the workpiece, the tool designer must also optimize the tool geometry and chip removal capabilities. By increasing the relief angle, the cutting force and the pressure on the tool are reduced, and the temperature in the cutting zone is also reduced. To make the positive rake angle as large as possible, the cutting force can be reduced due to better shearing action. The wide chip flute helps to eliminate chips, especially for drilling and threading.
Another way to reduce the cutting force is to cut at high speed. In order to improve efficiency, it is better to reduce the large feed rate at a high spindle speed without increasing the feed rate. In addition, today's milling cutters are much more precise than they were five years ago, and the mechanical stability and rigidity of milling machines and lathes are higher, thus eliminating possible vibrations. All of this contributes to the application of brittle, hard and wear resistant tool materials.
Another advantageous factor in the application of high temperature resistant tools is the extremely high efficiency of chip formation. For example, cutting cast iron, heat makes the material of the cutting zone a plastic body, which reduces the strength of the workpiece material in the cutting zone. The result is a threefold increase in the rate of resection of conventional roughing metals. Because the feed rate is high, the tool cuts the metal material very quickly, so that a large amount of heat stays in the chip, there is no time to transfer to the workpiece and deform it. Although the cutting temperature is very high, the temperature rise of the workpiece is small, and the accuracy of the workpiece obtained by cutting at a conventional amount is high.
Finishing with low axial forces also minimizes static deformation of workpieces, fixtures, and machine tools. Such a process requires the use of a coarse-toothed cutter, low feed and high speed of the milling cutter. Since the clamping force required to hold the workpiece is small, the fixture can be simple. There are more spacious milling channels for prismatic workpieces.
Dry Cutting Considerations When using dry machining, it is important to select the right machine and the right equipment. Because the speed is extremely fast, the material is often hard, the cutting temperature is high during dry cutting, and the machine must be rigid and horsepower.
Before performing dry cutting on the machining center, the operator should try to keep the tool extension length short, the spindle is in the best stiffness, and also consider the speed and power rating of the machine.
When it comes to the near-net shape and hardened parts of the lathe, the tool turret can be machined in the direction of the machine's rigidity, because the long guides in this direction can dissipate the cutting forces. A well-designed machine that disperses these cutting forces directly on short rails, and the tool holder consists of the fewest parts that move and support the tool. When precision is more important than flexibility, it should be considered to bolt a set of tools directly to the cross-slide to avoid the rotary indexing mechanism.
Thermal stability is critical to accuracy. Some manufacturers have adopted software to improve the accuracy of their machining centers, which compensate for the effects of temperature. However, the control temperature should begin with the effective removal of hot chips, thus eliminating the important heat sources inside the sealed work area.
Excellent machine design, there are no squats and high platforms in the machine that can gather chips. Use the chip spiral and the conveyor to discharge the chips out of the machine as quickly as possible without the cutting fluid. If there is a problem with the chip removal, replace the liquid with compressed air.
In order to protect the ball screws, guide rails, telescopic bushings, guards, seals and dust collectors are still required. If you need a dry-cut machine, you can convert a previously designed machine from a wet cutting operation to a dry cutting operation, which is usually cheaper. The dust collector and air delivery system that need to be added are slightly more expensive than the oil mist collector and cooling pump corresponding to the wet cutting process.
Operating costs with dry machining are also relatively low because it avoids coolant management and disposal costs, and secondly, compressed air consumes less power than cooling pumps. Therefore, the application of dry cutting will become more and more extensive.
Current status of dry cutting and tool materials used
Abstract: Dry cutting is the development direction of cutting. Just 20 years ago, cutting fluids were very cheap, accounting for less than 3% of the cost of most processing. However, nowadays, the proportion of cutting fluid in the production cost of the workshop has risen to 15%, which has caused great concern to the production operators. Especially those oily cutting fluids have become a big expense. In addition, many high-speed machining processes add cutting fluid to generate smoke, and the environmental protection department also limits the amount of cutting fluid smoke to be allowed. Occupational safety and employee health management departments are considering a value to reduce the allowable value of cutting fluid smoke emission. Advisory Committee’s recommendations.