Hard Turning Definition
David Richards define “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 generally machined using CBN inserts because other tool materials work as well or better and cost less. Soft materials often stick to PCBN cutting tools causing “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 groove geometry to provide swarf control, not ideal for machining soft steels. However, some steels with a high alloy content and 30+ HRc are successfully finish machined with DR-50 because nothing else will do the job. If there is no adhesion, reliable size control and consistent surface finish can more than justify the cost of the tools.
Aluminum Alloy Machining
Aluminium alloys cannot be machined with CBN inserts. PCBN has a trace content of Aluminium nitride. Aluminium 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 a significant ferrite content are not machined with CBN inserts. The soft gooey ferrite sticks to the CBN insert cutting edge causing rapid wear and poor surface finish.
D2 Machining
Interrupted cutting D2 tool steel is very difficult and unpredictable. D2 contains up to 14% Chromium and was designed to be used at 50-56 HRc. If the material is hardened to +60 HRc and not tempered very carefully, Chromium Carbide formation at the grain boundaries makes the material impossible to machine with interrupted cutting.
HSS Machining
Interrupted cutting of High Speed Steel – HSS is temperature resistant and does not soften in the shear zone. Interrupted cutting Nitrided steel is difficult. When continuous cutting, the super-hard surface is machined away by a part of the cutting edge that is not controlling surface finish and size. When interrupted cutting, the entire cutting edge impacts with a superhard surface resulting in poor tool life.
Hard Facing Alloy Machining
Hard facing alloys – Stellite (Cobalt/Chrome Alloys)and Colmonoy (Nickel/Chrome alloy) with more than 20% Chrome is not practically machined with PCBN – Tool life is too short. Chromium cannot be machined using PCBN. PCBN can be used to remove hard Chromed plated surfaces and expose a hardened steel base material, but it is not possible to machine within the Chrome.
High Temperture Alloy Machining
Machining high temperature alloys – Inconel, Hastalloy, Waspalloy, Titanium, Nimonics etc are not machined with PCBN. Tool life is negligible due to chemical affinity.
The information for the article was given thanks to:
David Richards Engineering Corporation
and
David Richards Engineering Limited
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