With the continuous development of science and technology, machining technology has entered a new development stage of “high speed, high efficiency, intelligence, compound and environmental protectionâ€. Many high-speed and high-efficiency tools have emerged: in the field of hole processing, large diameter flat drill, large The diameter composite drill replaces the traditional drilling and expansion two-step machining; high-precision and high-efficiency cutters such as large-diameter spiral groove taps and content chip taps are also widely used. In the steam turbine industry, large hole nesting tools also have the effect of efficient machining.
Processing status of spherical washers
The spherical washer is an indispensable adjusting component for the assembly of the upper and lower cylinders of the steam turbine. The material is 2Cr12NiMo1W1V high-strength heat-resistant alloy steel, which has the characteristics of poor thermal conductivity, high toughness, easy work hardening, easy to produce built-up edge, etc. A steel grade with poor machinability.
The traditional machining process of commonly used spherical washers is to first drill with a drill bit, and then use a bore cutter to cut a few knives to the required inner diameter. Due to the particularity of the material itself, the shortcomings of the spherical washer are low, the tool wears faster, the auxiliary time is long, and the processing quality is unstable. Moreover, since most of the material (in the middle of the bar) becomes iron filings, a large amount of raw materials are wasted.
In view of the processing characteristics of such parts, comprehensive consideration of improving the processing efficiency of the workpiece, reducing the tool cost, improving the material utilization, etc., we have developed a large-diameter nesting knife, which effectively solves the processing problems of such parts.
Development and application of large diameter nesting knife
The tool is mainly developed for spherical washers with inner holes of Φ70~Φ136.
Determination of structural parameters
Whether the structural parameters of the nesting tool are determined is reasonable, directly related to its technicality, cutting efficiency, processing quality and service life.
Structural determination
The large-diameter nesting knife adopts the Mohs No. 5 taper shank, which can be applied to ordinary drilling machines and lathes. The cutter head adopts a replaceable structure, so that only different cutting heads can be replaced when processing different types of parts, which reduces auxiliary processing time and improves Machining efficiency; the number of teeth of the nesting tool is 4 teeth, the widths of the two symmetrical cutting edges are different, and the axial heights are different by the distance C. When machining, the two high teeth first contact the workpiece, and then the two teeth are processed to the rest. Such a cutter structure can facilitate chipping and reduce cutting resistance; the blade has a 2° inverted cone, and the rest is cylindrical, which increases the stability of the cutting of the tool; in order to make the tool have good wear resistance and toughness, the blade The quenching hardness is 62-65HRC, and the rest is 35~40HRC.
Determination of geometric parameters
Large diameter nesting knives are mainly used for large hole machining of turbine spherical washers. After a large number of tests, it is proved that the tool rake angle γ0 of the 2Cr12NiMo1W1V material nesting is preferably 16°-18°. The rake angle is the main factor affecting the sharpness of the tool and the strength of the tool body. The front angle is too small, the cutting resistance is large, the tool wear is aggravated, and the tool life is reduced. The front angle is too large, the tool tip strength is low, the heat dissipation condition is poor, and the machine is easy to produce. Wear and chipping phenomenon. The back angle α0 of the main and auxiliary cutting edges is 4°, which not only maintains the machining dimensional accuracy of the tool and increases the number of regrinds, but also improves the stability during cutting, making it less prone to chipping. The tool helix angle ω0 is preferably 12° to 15°.
Material selection
Due to the large diameter of the tool and the characteristics of the material to be processed, the tool is required to have high red hardness and toughness. Generally, high-speed steel (W6Mo5Cr4V2, W9Mo3Cr4V) has low performance due to heat treatment and should not be used. Therefore, it has high strength and toughness. High-performance high-speed steel (W2Mo9Cr4VCo8) with good edge and sharp edge is preferred.
Selection of cutting amount
There are many factors affecting the amount of cutting, such as workpiece material, tool material, tool structure and geometric parameters, processing steps and specific conditions of use, such as machine tools, clamping, cutting fluid and many other factors. Through a large number of cutting tests, the speed n=80~100r/min is determined, and the feed rate f=0.20~0.30mm is optimal. During machining, the cooling of the tool is also critical. It has a negligible effect on the quality of the machining, the cutting efficiency and the service life of the tool. Therefore, the coolant must be used during machining and the tool must be cooled sufficiently.
Tool surface treatment
The material used for the large-diameter nesting knife is M42 high-performance high-speed steel, which has high material cost and processing difficulty. In order to improve the strength of the tool and prolong the service life of the tool, the surface of the cutting edge of the tool can be surface-coated. The experimental analysis shows that the surface coating with TiAlN and AlTiN has the best effect. After surface treatment, the durability of the nesting knife can be increased by 2~3 times.
Processing example
Large-diameter nesting knives have achieved good processing results when processing spherical washers for steam turbine supercritical and ultra-supercritical units.
The parts are machined on a common C630 lathe and processed on five different sizes of spherical washers. According to the specifications of the spherical washer, the cutting amount is n=60～90r/min, the feed rate f=0.15～0.25mm, and the cooling method uses oil-based coolant. Each large-diameter nesting knife can process more than 100 spherical washers, and the flank wear of the tool is only 0.1mm~0.2mm, and the tool can be repeatedly repaired after being worn, which greatly improves the processing efficiency and reduces the tool cost. .
Conclusion
In the steam turbine manufacturing industry, spherical washers are in great demand every year and have a wide variety. The large-diameter nesting knife is used for roughing, the cutting is stable and the processing is stable. It replaces the two processes of drilling and boring (expanding), which improves the cutting efficiency by more than 5 times, reduces the tool cost by 50%, and increases the tool life by 3 times. The above saves more than 50% of raw materials and reduces the labor intensity of workers. The raw materials that are put out can be used as small-sized parts, which improves the utilization of raw materials and reduces the cost of raw materials. This large diameter nesting knife can be applied to similarly shaped parts.
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