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machining of turbine rotor with cbn inserts

The turbine rotor, also known as the generator shaft is the mechanical core of the turbine generator. This component is responsible for converting the kinetic energy of the main shaft into electrical energy within the generator. Typically, it is produced from alloy steel and machined from a forged shaft. The alloy steel shaft endures great tensile and compressive stress during operation so it must be well balanced to prevent vibration.

Instead of the conventional high speed steel tooling, most turbine rotors are machined with polycrystalline cbn inserts. These inserts overcome the speed and temperature limitations of high speed steel and allow the stainless steel material to be machined at about five times faster. The following are the main machining processes that are used to create the desired shape and surface finish as well as the polycrystalline cbn inserts used for machining.

Turning of the Rotor Shaft
The stainless steel material used to produce the turbine rotor requires strong and thick inserts for roughing, semi-finishing and finishing operations. Heavy rough turning can be done using the SNMM insert. The SNMM insert provides indexable, multiple cutting edges which make it very easy to have a continuous operation without re-grinding the cutting edges. The round RCMG insert can also be used when there is a need to remove a large amount of metal at a high cutting speed and feed rate.

Other inserts that may equally perform as well as the SNMM insert include the CNMM, SNMG and SNMQ inserts. For semi-finishing and finishing operations, the CNMM, TNMM inserts may be used. Note that for fine finishing operations it is advisable to use an insert that has the wiper feature which creates a more accurate surface finish.

Milling of the Turbine Rotor
Milling the ends and faces of various cylindrical sections of the turbine rotor is a vital machining operation that usually occurs after turning. Polycrystalline cbn inserts with strong negative geometry may be used with robust face milling cutters for facing, or with helical mills for pocket openings. In most cases, only semi-finishing and fine finishing operations are required. For semi-finishing milling operations on the turbine rotor, requiring a medium depth of cut of about 12 mm, use the SNHM or SNHT insert.

When there are many sections to mill on the rotor shaft or the demand on the CNC machine is high, a hexagonal insert that offers six indexable cutting edges may be used to save time and eliminate the need to re-grind the inserts. Hexagonal cbn inserts like the HPMT can be used for continuous milling operations. This insert is suitable for both semi-finishing and finishing operations. Other milling inserts that can be used for finishing operations include the SPEN, SPGA, SPGW. To mill the slots and square shoulders on any part of the turbine rotor, the ABET insert may be used in combination with a slot or square shoulder milling cutter designed for milling of hard materials like alloy steels.

Conclusion
Choosing the most suitable pcbn inserts for turning and milling turbine rotor, will help to achieve stable conditions and create a balanced rotor that will generate electricity efficiently. These cbn inserts will reduce the heat generated during machining and eliminate chip hammering and reduce premature tool failures.

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