Monday, April 14, 2008

Ceramic Inserts vs. CBN Inserts

CNMA CBN Insert (shown in picture above)


CNGA Ceramic Insert (shown in picture)

An important criterion for the economical application of cutting tools is their life in relation to the metal removal rates, especially with two materials such as ceramic and CBN. The latter is much more expensive. Thus, comparison of tool wear becomes most important.

The two materials show marked performance differences in both size and surface finish in roughing, interrupted, and finishing cuts. In comparison to grinding, however, both materials will achieve much higher metal removal rates.

CBN will out-perform ceramic in interrupted cuts. With depth-of-cut of 0.060 inch to 0.120 inch and a speed of 350 sfpm with 0.010 to 0.020 inch feed per revolution, ceramic often will fail when entering the interruption. With CBN, a tool life up to 20 minutes can be achieved with a half-inch round insert, under the above conditions. Flank wear in interrupted cuts is more irregular than in continuous cuts and can shorten tool life. Solid CBN inserts have greater flank wear resistance and are superior to ceramics for roughing, especially where the turning has interrupted cuts.

For finishing cuts where small depths of cut and low feed rates are required to achieve superior surface finishes, CBN inserts cannot equal the tool life of ceramic inserts. Under these conditions, the ceramic insert has less than half the flank wear of CBN after 30 minutes cutting time.

Considering that CBN is ten times more costly than ceramic for the same size and geometry insert, CBN economically can be justified only for machining heat treated steel in those situations where ceramic fails completely or breaks down before finishing the workpiece.

excerpted from gardner article