Analysis And Optimization Of Cutting Parameters On Machining Of Straddle Screw Holes Between The Locking Blade And Rotor

The manufacture quality of the straddle screw holes, which locate between locking blade and rotor, can directly affect the safety of steam turbine. Due to the special straddle structure and poor cutting properties of the work pieces, tools wear rapidly and quality can not be achieved when machine these screw holes. Base on the mechanical and physical properties of blade (NiCr20TiAl) and rotor (X12CrMoWVNbN10-1-1), twist dills and screw taps made from M42 high speed steel (HSS), S390-PM powder metallurgy high speed steel (HSSPM) and GU20 cemented carbide alloy are selected to do a lot of drilling and tapping tests. The cutting properties of blade and rotor in straddle cutting are analyzed accordingly.According to the tools wear forms, chips forms, topograph of the wear areas observed via scanning electron microscope (SEM), as well as chemical composition of the wear areas observed via energy disperse spectroscopy (EDS), tools failure mechanism in straddle drilling and tapping is studied. Base on test results, as well as properties of tool material and work piece matieral, straddle drilling is simulated with finite element program DEFORM-3D. In the simulation, cutiing force, cutting torque and cutting temperature are studied. The maximum temperature estimated by simulation is 678°C. In order to study the offset of twist drill due to different properties of the work pieces, a dynamical model is set up. According to this model, when the twist drill drills in 30mm, the offset of twist drill is 0.80mm. By changing cutting parameters, the cutting length, cutting force and cutting torque are measured in the cutting tests. According to the test results, the optimized tool material and cutting parameters are as follow: in straddle drilling, use S390PM twist drill with cooling oil, under speed 100r/min and feed 0.10r/min; in straddle tapping, use S390PM screw tap with cooling oil and feed 6r/min.