HIGH-SPEED MACHINING OF CAST-IRON WITH BZN COMPACTS

Polycrystalline cubic boron nitride (PCBN) cutting tools, as represented by BZN compacts, can be used to mill highly pearlitic cast iron at very high speeds. This is indicated by a uniform flank wear (UFW) of only 0.18 mm (0.007 in) after 36 min at 66.5 m s-1 (13 100 surface feet per minute (sfm)). A single test was carried out at 93.0 m s-1 (18 300 sfm) but the onset of chatter precluded more than two passes across the workpiece. BZN compacts outperformed an SiAlON tool (Kyon 3000) at the three speeds (14.9, 29.9 and 50.0 m s-1; 2932, 5890 and 9817 sfm) for which comparisons were made. The arithmetic average (AA) surface roughness of BZN-compact-machined cast iron was less than half that of SiAlON-machined cast iron. In no case was the roughness of the BZN-compact-machined surface greater than 0.74-mu-m (29-mu-in) and it was only 0.48-mu-m (19-mu-in) after the test at 93.0 m s-1 (18 300 sfm). BZN compact wear in milling ferritic cast iron was much greater than that in milling highly pearlitic cast iron; a flank wear of 0.13 mm (0.005 in) occurred after 0.5 min on the former at 29.9 m s-1 (5890 sfm). For comparison, the BZN compact wear was only 0.071 mm (0.0028 in) after milling the more pearlitic material for 14.5 min at the same speed. The greater BZN compact wear in machining ferritic cast iron may be due to (1) greater workpiece ductility, (2) higher temperature at the cutting interface, (3) greater diffusion of boron from the CBN into the ferrite, or combinations of all three.