Definition of hard turning
David Richards defines “hard turning” as machining hardened steels above 40 HRc, not hard in terms of “difficult”. Alloy steels with a hardness below 40 HRc are not typically machined with CBN inserts because other tool materials work just as well or better and cost less. Soft materials often stick to PCBN cutting tools and cause “build up” on the cutting edge. This results in poor surface finish and tool life. The geometry of PCBN tools used for machining hardened steel is very blunt, with no chip flute geometry to provide chip control, which is not ideal for machining mild steels. However, some high alloy steels over 30 HRc will machine successfully with DR-50 because nothing else will do the job. If there is no sticking, reliable size control and consistent surface finish may more than justify the cost of the tools.
Machining of aluminum alloys
Aluminum alloys cannot be machined with CBN inserts. PCBN has a trace content of aluminum nitride. Aluminum builds up on the cutting edge very quickly, causing rapid tool wear and poor surface finish.
cast iron machining
Cast iron and iron-based hard-faced alloys with significant ferrite content are not machined with CBN inserts. Soft, sticky ferrite sticks to the cutting edge of the CBN insert, causing rapid wear and poor surface finish.
D2 machining
Interrupted cutting of D2 tool steel is very difficult and unpredictable. D2 contains up to 14% chromium and was designed for use at 50-56 HRc. If the material is hardened to +60 HRc and not quenched very carefully, the formation of chromium carbide at grain boundaries makes the material impossible to machine with an interrupted cut.
HSS machining
Interrupted cut of high speed steel – HSS is resistant to temperature and does not soften in the cutting zone. Interrupted cutting of nitrided steel is difficult. When performing a continuous cut, the super hard surface is machined by a portion of the cutting edge that does not control the surface finish or size. When cutting is interrupted, the entire cutting edge hits a very hard surface, reducing tool life.
Machined from hardfacing alloy
Hardfacing alloys – Stellite (cobalt/chrome alloys) and colmonoy (nickel/chrome alloy) with more than 20% chromium hardly machines with PCBN – Tool life too short. Chrome cannot be machined with PCBN. PCBN can be used to remove hard chrome surfaces and expose a hardened steel base material, but machining into chrome is not possible.
High temperature machining of alloys
Machining of high temperature alloys: Inconel, Hastalloy, Waspalloy, Titanium, Nimonics, etc., do not machine with PCBN. Tool life is negligible due to chemical affinity.
