When Not to Use CBN



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     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 PCBN 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.
     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.
     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 super-hard surface resulting in poor tool life.
     Machining high temperature alloys – Inconel, Hastalloy, Waspalloy, Titanium, Nimonics etc are not machined with PCBN. Tool life is     negligible due to chemical affinity.
     Aluminium alloys cannot be machined with PCBN. 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.
     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 Chrome plated surfaces and expose a      hardened steel base material, but it is not possible to machine within the Chrome.
     Cast iron and Iron based hard facing alloys with a significant ferrite content are not machined with PCBN. The soft gooeyferrite
    sticks to the PCBN cutting edge causing rapid wear and poor surface finish. (See separate Cast Iron information sheet)
     Sintered or Powder Metallurgy Iron materials are effectively machined with PCBN but some heat treated Sintered Irons,
     with significant alloy content, are not. Heat treated materials often measure 40-50 HRc. These can consist of a bulk matrix
     of 40-50 HRc with small hard particles, sometimes 64+ HRc, dispersed within it. These hard particles make the material
     very difficult to machine and almost impossible to interrupted cut.
     Stainless Steels come in a variety of conditions with many chemical compositions.
    Austenitic stainless steels are generally soft and “sticky” causing “build up” on the cutting edge, poor surface finish
     and tool life.     
    Martensitic, and heat treated, stainless steels are successfully machined with PCBN but interrupted cutting can be difficult due to
    formation of unfavourable compounds at the grain boundaries.







© David Richards Engineering  2013