Turning

Usually, parts with an average hardness of HRB 52 have machining properties similar to those of cast iron. At this hardness level, parts should be held rigid, and weak sections should be supported to prevent distortion. Compressed air is used to cool the tool and maintain swarf clearances. Jets are directed onto tool cutting edges and work surfaces. Liquid coolants cannot be used because parts must be kept dry and clean for subsequent sintering. Carbide tips of ISO designation K10 with a hardness of 92.6 HRA give good results and will accept some interruptions on the cutting surface.

Tools must be held rigid, and cutting edges must be sharp with rake angles of up to 3° positive on top and side and frontal clearances of 3 to 5°. Surface speeds of 105 to 120 m/min (350 to 400 sfm) and feeds of 0.050 to 0.10 mm/rev (0.002 to 0.004 in./rev) are satisfactory for form turning, but surface speeds can be increased to 180 to 210 m/min (600 to 700 sfm) in singlepoint turning. Feeds can be increased within the boundaries of economic tool life, the standard of accuracy, and surface finish requirements.

In machining fully sintered parts (average hardness, 90 HRB), K10 carbide tips give a satisfactory life with 0° top rake, 7° frontal clearance, and 5° side clearance. Single-point finish turning is used, with a stock removal of 0.125 to 0.20 mm (0.005 to 0.008 in.) of surface depth and 0.050 mm (0.002 in.) of feed per revolution. Surface speeds are 120 to 135 m/min (400 to 450 sfm), and tips require a radius of 0.20 to 0.25 mm (0.008 to 0.010 in.). Form turning is not advisable because of the work-hardening characteristics of the material in this state.

Abrasive flank wear is the dominating wear mechanism in turning. PVD-TiN coating of the hard metal (HM) inserts reduces the wear rate; CVD coatings (TiN, Al2O3) improves the performance even further. Oil impregnation improves the machinability in general, while cutting fluid is detrimental to the machinability.

The results for the following example show that the axial force is the dominating cutting force component after 1 mm flank wear. The micro surface roughness (within the feed marks) is improved by increased density, and addition of MnS improves the macro surface integrity. Tool life is nearly independent on the feed rate in a range of 0.05 to 0.2 mm/rev. Alloy elements in general decrease the machinability. Correlation to hardness is not enough to explain the different machining performance of P/M material. Micro smearing from "soft" phases due to inhomogeneous microstructure is one explanation for P/M materials performance in relation to conventional steel. MnS addition for intermittent cutting has a strong effect on the machinability.

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