Definition of hard turn

David Richards defines “Hard turning” as machining tempered steels above 40 HRc, not hard in “difficult” terms. Alloy steels with a hardness less than 40 HRc are generally not machined with CBN inserts because other tool materials perform the same or better and cost less. Soft materials often stick to PCBN cutting tools causing “buildup” on the cutting edge. This results in poor surface finish and poor wear life. PCBN tool geometry used for machining hardened steel is very dull with no chip groove geometry to provide chip control, not ideal for machining soft steels. However, some high alloy 30+ HRc steels are successfully machined with DR-50 because nothing else will work. If there is no adhesion, reliable size control and a consistent surface finish can more than justify the cost of the tools.

Aluminum Alloy Machining

Aluminum alloys cannot be machined with CBN inserts. PCBN has traces 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 facing alloys with significant ferrite content are not machined with CBN inserts. The soft, sticky ferrite clings to the cutting edge of the CBN insert causing rapid wear and poor surface finish.

Machining D2

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 hardens to +60 HRc and is not tempered very carefully, the formation of chromium carbide at the grain boundaries makes the material impossible to machine with interrupted cuts.

HSS machining

High Speed ​​Steel Interrupted Cutting – HSS is temperature resistant and does not soften in the cutting area. The interrupted cut of nitrided steel is difficult. When making a continuous cut, the super hard surface is machined by a part of the cutting edge that does not control the size or finish of the surface. When cutting is interrupted, the entire cutting edge hits a super hard surface, reducing tool life.

Machining of hard facing alloys

Hardfacing alloys – Stellite (cobalt / chromium alloys) and Colmonoy (nickel / chromium alloy) with more than 20% chromium are practically not machined with PCBN – Tool life too short. Chromium 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

High temperature alloy machining: Inconel, Hastalloy, Waspalloy, Titanium, Nimonics, etc. are not machined with PCBN. Tool life is negligible due to chemical affinity.