Application of dry cutting technology in milling

1 Introduction

Most of today's metal cutting processes are carried out in a wet manner using cutting fluids. The cutting fluid has the functions of cooling, lubrication, cleaning, chip removal, rust prevention, etc., which plays an important role in prolonging the service life of the tool and ensuring the processing quality. However, in recent years, with the increasing emphasis on environmental protection by human society, people have begun to pay attention to a series of negative effects brought by cutting fluid. On the one hand, the widespread use of cutting fluids consumes a lot of energy and resources, increasing processing costs. According to statistics calculated by many German companies, the cost of using coolant accounts for 16% of the total manufacturing cost, while the cost of cutting tools only accounts for 3-4% of the cost. On the other hand, the cutting fluid has serious environmental pollution and even harms workers' health. In order to minimize the contamination of metal cutting, the concept of "clean production" has been proposed. Dry cutting is the most effective way to eliminate cutting fluid contamination, reduce product cost, and achieve clean production. To achieve dry cutting, the tool material must be selected reasonably and the tool geometry parameters should be designed. Dry cutting technology has become one of the trends in the development of metal cutting. In recent years, its use in turning and milling has become increasingly common, with major breakthroughs in drilling, boring and hobbing. In this paper, combined with the actual work, only the application of dry cutting technology in stainless steel milling is discussed.

2 Dry cutting requirements for tools

The performance of a cutting tool depends on the tool material and the tool structure and geometry. Different machining methods have different design priorities for the tool. For dry cutting tools, the following properties must be achieved: (1) The tool should have high heat resistance and good wear resistance. (2) The coefficient of friction between the chip and the tool should be as small as possible. (3) The shape of the tool should ensure smooth chip removal and easy heat dissipation. (4) The tool should have higher strength and impact toughness. In actual production, according to the physical, mechanical properties and process characteristics of the workpiece material, the tool material and coating should be selected reasonably, the tool structure and geometric parameters should be optimized, and the matching of the tool material and the workpiece material should be paid attention to in order to design and manufacture. Dry cutting tool.

  1. Dry cutting tool material

    2. The most important tool material for dry cutting is that it must have high red hardness and high impact resistance. Current tool materials for dry machining include ultrafine grained carbide, coated cemented carbide, ceramic and cermet materials, diamond (PCD) and cubic boron nitride (CBN).
    1. Ultrafine Granular Cemented Carbide Commonly used ordinary hard alloys are brittle and easy to chip. Ultrafine particle cemented carbide can improve the toughness of ordinary cemented carbide, has good wear resistance and high temperature resistance, and can be used for milling and drilling.
    2. Coated carbide coated tools are best suited for dry machining because suitable coatings can withstand high cutting temperatures, reduce the coefficient of friction between the tool/chip and the tool/work surface, and reduce tool wear and tear. The heat also allows the tool to have a tough substrate and a cutting edge or workpiece surface that meets the cutting requirements. Therefore, a suitable matrix and coating combination and economically viable coating process technology is one of the key technologies for dry machining. Commonly used coating matrix materials are mainly cemented carbide. The coating materials mainly include TiC, TiN, TiAlN, diamond, and the like. The TiAlN coating is one of the best coatings for dry machining. For example, coated carbide inserts for dry milling developed by SECO: T150M, T25M, F20M, F40M. See Table 1.
      Table 1 Types and application range of coated cemented carbide inserts
      Knife type Coating material application Cutting fluid use
      T15OM Ti(C, N)+Al 2 O 3 Gray cast iron / ductile iron Dry cutting
      T25M TiC/Ti(C, N)+TiN Carbon steel / stainless steel Wet/dry cutting
      F20M Ti(C, N)+(Ti, Al) N+TiN stainless steel Dry cutting
      F40M Ti(C, N)+(Ti, Al) N+TiN stainless steel Wet/dry cutting
    3. Ceramic and cermet Ceramic knives are ideal for dry machining of cast iron and hardened steel due to their high heat resistance and good chemical stability. However, its toughness and easy to break have greatly limited its application in dry cutting. In order to solve this problem, some new alumina ceramic blades have been developed abroad. A ceramic insert made of fine particles (0.22 μm) and high purity (99.99%) of new alumina ceramic powder developed by Japanese scholars has excellent cutting performance in dry and dry milling carbon steel and cast iron parts.
      The cermet is a type of cemented carbide which contains a titanium-based compound and the binder is a nickel or nickel phase. The cermet tool is suitable for dry cutting. In the past, hard abrasive tools were usually ground or EDM. Now, cermet tools can be used for dry milling, which not only improves work efficiency, but also improves surface quality.
    4. CBN
      CBN materials have good toughness, high chemical stability and high hardness. The hardness is as high as 3200~4000HV, second only to diamond. Therefore, it is suitable for dry cutting cast iron and hardened steel. SECO has developed a range of CBN inserts, such as CBN20 inserts for high speed milling of hardened steels; CBN300 inserts for dry coarse and finely milled pearlite grey cast iron and ductile iron. CBN300 is an impact resistant grade obtained by changing the particle size with a special process based on CBN30, and its grain size is 22 μm. This kind of pure sintered body has high PCBN content (90%), good wear resistance, excellent thermal conductivity and impact resistance. Compared with the existing PCBN products, it is a new and more mature. PCBN products.
    5. PCD
      PCD cutters have the characteristics of high hardness, high thermal conductivity and small linear expansion coefficient. PCD cutters are suitable for dry finishing copper alloys, aluminum and aluminum alloys, composites and more. Dry machining of aluminum alloys with PCD tools results in high cutting speeds and long tool life. However, PCD tools react greatly with iron and can only be used to cut non-ferrous parts. In addition, PCD tools cannot withstand temperatures in the cutting zone exceeding 600 °C, so they cannot cut tough, highly ductile materials.
  2. Tool structure and geometry

    3. In dry cutting, the thermal load on the tip and edge material is greatly increased, and various edge damage is easily generated. Eventually the tool fails. In order to ensure the machining quality and tool life of the workpiece, it is required to consider the cutting force and friction generated during machining to be small, and the chip removal effect is better. Dry cutting tools must be optimized for tool structure and geometry. For example, 1 design makes the tool a negative rake angle or bulges the front and rear flank to delay the damage of the crater to the blade. 2 Increase the negative cutting angle and improve the cutting edge of the cutting edge and the cutting edge to improve the impact resistance and thermal shock resistance of the tool. Some tool manufacturers in Europe have developed a number of tools that are suitable for dry cutting geometries. For example, SECO's ME-type chipbreaker octagonal milling inserts use chipbreakers that effectively control chip formation and chip removal, making them ideal for dry milling.

3 Dry cutting cooling and lubrication


In some cases (such as deep hole drilling, tapping, etc.) it is difficult to achieve complete dry cutting without cutting fluid. Therefore, the application range of the complete dry cutting process is limited, and the minimum amount of lubrication can be used. MQL) quasi-dry cutting and cold air cutting.
The least amount of lubrication, also known as spray lubrication, is the use of compressed air to atomize trace amounts of vegetable oil and spray it at high speeds into the high temperature cutting zone for cooling, lubrication and chip evacuation. The amount of vegetable oil used is one ten thousandth of that of wet cutting. The same amount of heat is absorbed by the casting method 1000 times, thus greatly reducing the cost of the cutting fluid. Cold air cutting uses compressed air for cooling and chip removal, partially replacing the function of cutting fluid, but it is generally used in combination with MQL to improve lubrication.

4 Dry cutting in milling applications


In general, we are used to filling the coolant in milling, especially in the end milling process. However, adding coolant to the milling process will cause the temperature of the tool to change drastically. The milling cutter blade cools when it is cut out from the workpiece, and then cuts in. The temperature rises again. A sharp change in temperature creates stress in the blade, which can lead to cracks. Although heating and cooling cycles are also produced during dry milling, they are much smaller compared to a reasonable selection of tool materials, structures and geometric parameters. Dry cutting for milling can achieve the desired cutting results compared to wet cutting. The following are typical application examples for dry milling.
  1. Coated carbide F20M dry milling does not induce steel valve seat (work material: CF -8M, stainless acid resistant cast steel)

    2. When a factory was processing the bottom surface of a stainless steel valve seat, it used SECO's T25M (CVD coating) blade wet rough milling and finishing milling. Due to the sharp increase in production batches, the contradiction of tool life is more prominent. The mill cutter head for rough milling is R220.43-0080-07W (leaf angle 43°, diameter 80mm), the selected blade coating grade is T25M, and the blade type is OFER070405TN-ME15 (octagonal) 7mm long milling cutter). Rough milling cutting parameters: V c = 130m / min, f z = 0.1mm / z, a p = 2 ~ 3mm, a e = 80%, tool life of 4 to 6 pieces per cutting edge. Finishing cutting parameters: V c = 250 m / min, f z = 0.16 mm / z, a p = 0.7 ~ 0.8 mm, a e = 80%. Tool life per cutting edge: 12 to 16 pieces. The factory requires rough iron processing to improve tool life. After the first improvement, the same cutting parameters as the original are used. SECO's F40M (PVD physical coating) blade is used for wet rough milling, and the tool life is 9-10 pieces. This result has not yet reached the goal that the factory hopes. According to foreign counterparts, the use of dry-cutting methods to process stainless steel can greatly improve the tool life, which can exceed 20 pieces. Finally, SECO's F20M (PVD physical coating) blade dry rough milling is used, and the insert type is OFEN070405TN-D18 (the chipbreaker is D18, which is difficult to process). Cutting parameters: V c = 180 m / min, f z = 0.16 mm / z, a p = 2 ~ 3 mm. Tool life per cutting edge: 24 to 28 pieces. The result is a significant increase in tool life.
    Analysis: The relationship between cutting force and cutting temperature is a hump curve. When the cutting temperature is high enough to exceed the hump, as the cutting temperature is further increased, the cutting force is lowered, making the cutting relatively easy, so the tool life is improved. If the coolant is added at this time, the temperature in the cutting zone will decrease, resulting in an increase in cutting force, increased wear, and a decrease in tool life. On the left side of the hump in the cutting force and cutting temperature relationship, the situation is reversed. Adding coolant will cause the cutting force to decrease, making the cutting lighter. Improved tool life. However, it is a very laborious project to determine the relationship curve through cutting test, and the workpiece in this aspect needs further in-depth.
  2. Coated carbide T150M dry cutting cast iron exhaust pipe end face

    3. An auto parts processing company in Shanghai used SECO's T150M black-coated carbide insert to dry-mill the end face of the cast iron exhaust pipe while processing the exhaust pipe of the Shanghai General Motors Buick sedan. Compared with the original ceramic inserts from another company, the production efficiency is significantly improved, and the tool life exceeds 100 pieces per blade. Cutter type: R220.43-0088-S (diameter 88mm, number of teeth: z=5); blade coating grade: T150M; insert type: OFET070405TN-16; cutting parameters: n=11OOr/min, a p =1~ 1.2 mm, V f = 600 mm/min. The tool life per cutting edge is greater than 100 workpieces. The square ceramic insert has only four cutting edges and is more expensive than the hard alloy octagonal insert, and the cutting edge is only half of the octagonal blade, which is less economical.
  3. PCBN tool dry milling gray cast iron cylinder plane

    4. The CBN300 series of PCBN inserts developed by SECO has shown extraordinary success in the dry milling of gray cast iron cylinders. Its tool life is 4 times that of general PCBN inserts. For ceramic inserts, CBN300 can increase 50%. Double tool life. CBN300 has been successfully applied to the dry milling gray cast iron cylinder plane of the newly built engine flexible production line of Shanghai General Motors Co., Ltd. The milling speed can reach 1600m/min, which greatly improves the machining efficiency of the engine. Blade type SNMN090312S; cutter head type: R22033-0100-12CT (z=8), cutting parameters: a e = 80 mm, a p = 4 ~ 5 mm, f z = 0.15 mm, V c = 1600 m / min. Tool life per cutting edge: 170 pieces.

5 Conclusion

Dry machining is an ideal clean manufacturing process. The research and application of dry cutting processing in foreign countries has been extensive, and there is a big gap in China's technology compared with foreign countries. However, the above successful application examples can show that under the current technical conditions, dry cutting can be realized in China, and with the wide application in turning and milling, it will promote the application in other processing methods. Of course, the development of dry cutting technology must rely on advanced tool structures and tool materials. It is believed that with the in-depth research and popularization of dry cutting technology, China's cutting efficiency and processing quality will reach a new level.

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