Hard turning with PCBN cutting material is defined as the process of single point cutting of part pieces that have hardness values over 45HRC. CBN is mainly used to turn parts in the region of 58-68HRC.
What is PCBN?
Polycrystalline Cubic Boron Nitride (PCBN) is a man-made product and found nowhere in any form in nature. Its unique properties of high hardness (only diamond is harder), its ability to keep its hardness at elevated temperatures makes PCBN an ideal cutting material for machining hard and abrasive ferrous materials.
PCBN consists of CBN grains which have been bonded together using a ceramic or metallic binder under high pressure and high temperatures. The binder acts as the glue or cement to hold the CBN grains together. In the picture below you can see that its looks like baking flour before it becomes CBN. We can say that PCBN is made from the one of the softest materials we know but it becomes second to diamond the hardest material we know.
Hexagonal Boron Nitride- one of the softest materials known to man. By using high temperatures and high pressure synthesis the soft powder transforms to Cubic Boron Nitride grit- the second hardest material known to us.
Where can we and where can we not apply PCBN as cutting material?
PCBN can be used in many materials but there are materials where we cannot use PCBN.
Where can we use PCBN?
- -Hard steels with hardness 45-62HRC like, Case hardened steels, Bearing steels, Hot work tools steels, High tensile steels, Martensitic stainless steels, Manganese steels.
- -Hard irons like, Chilled irons, White irons, High chrome irons.
- -Soft and abrasive materials like, Grey cast irons, Ductile irons, Compacted graphite irons.
Some other materials we can machine with PCBN are, Valve seat materials, Nickel based super alloys and tungsten carbides.
Where can we NOT use PCBN?
- -Soft steels, Austenitic stainless steels, Ferritic stainless steels with high ferrite content, Chrome plating, High Speed steel HSS, Cemented carbide impossible to mill.
The major growth of PCBN started with hard turning replacing grinding of hardened steel components, the advantages are well documented, and meet all the main trends through higher material rate, a flexible process and dry machining.
An example where PCBN is used. Finish turning in a Cased hardened steel shaft 60HRC.
Type of insert: CNGA120404-01525-L1 CBN060K (Seco Tools)
Cutting speed Vc= 250m/min
Depth of cut D.O.C. = 0.30mm
Feed = 0.15mm/revv
Surface tolerance and roughness superior to grinding and approximatly 6 times faster.
Another application for PCBN is in the machining of Grey Cast Irons , especially in the automotive industry. Two products that are often machined with PCBN are the Cast Iron Brake discs and the Cast Iron Brake drums. These are made of GG25 and are turned with cutting speeds from 600m/min up to sometimes to 2000m/min.
To succesfully machine Grey Cast Iron GG25 is the quality and the material specifications of the castings the crucial factor. The machinability is dependant on the Material Ferrite Content.
Material Ferrite Content
When machining grey cast irons with PCBN, the balance of ferrite and pearlite in the irons’s microstructure is critical to the performance of PCBN. Ideally the stucture should be fully pearlitic with no free ferrite. The amount of free ferrite has a major effect on the life time of PCBN. For the maximal performance of the PCBN the free ferrite levels in the casting should beless than 5% and ideally zero. Free ferrite levels above 8-10% reduces the tool life significantly. The free ferrite in the grey cast iron is known to chemically attack the PCBN, this becomes even more critical with elevated cutting speeds.
In the above is an example of two grey cast iron structures with different amounts of Ferrite percentage’s. A practical example of the effects of the Ferrite percentages can be seen below: When these two materials are machined with PCBN the difference in performance is massive. In the material with 2% Ferrite up to 2000 discs were machined and only 80 discs with the 80% ferrite castings. This is completely due to the free ferrite content of the materials. The image below shows clearly the extensive scaring on the flank face of the cutting tool. This image shows the typical chemical wear on the PCBN insert used when machining the material with 80% ferrite. The distance between the striation peeks is equal to the feed rate being used.
Summary when machining cast iron.
Certain batches of cast iron may exhibit poor machinability.Check that the following are within the recommendations:
- Cutting speed => 400m/min
- Ferrite content =<10%, ideally less than 5%
- Sulphur content =>0.1%
- Ageing => 10 days