Hard Turning Process
In the grinding process, because the work piece is turning slowly, the wheel speed has to be considered in relation to the cutting speed. In the turning process, on the other hand, the work piece takes care of the cutting speed, whilst the tool performs the contouring. In consequence the high speed of the grinding wheel causes the work piece to produce a large quantity of small chips, whilst the turning process forms a continuous chip, which nearly always disintegrates into smaller pieces. Therefore one can see that grinding requires more time and energy per unit volume than turning. Another difference between turning and grinding is that with the latter it is only possible to remove an extremely small quantity of material, for instance with a thickness of only 1 µm. But turning is characterised by a minimal chip thickness of hundredths of a millimetre. That’s why high-precision hard turning requires a different strategy to reach exactly the dimension required.
The turning process: a brief history
Hard turning really started to develop at the beginning of the nineties. The reason for this was the availability of new tool materials and the capability of designing a turning machine that was rigid, stable and accurate enough to successfully finish hard turn. The result of these developments have made finish hard turning a viable alternative to grinding, as an accurate finishing operation.
For some time, natural and synthetic diamond was being used for precision turning of non-ferrous metals. But the hardest of all materials could not be used for the machining of steel, because diamond
reacts with the carbon in steel. Consequently diamond gets “graphitised”: and therefore another crystalline structure is required. Fortunately boron nitride with cubic crystalline structure solved this problem, because its hardness approximates that of diamond. Only a polycrystalline structure exists, which is called PCBN. High-precision hard turning requires tool bits with a relatively low CBN content of about 50 %; the rest is a ceramic binding agent. (Incidentally one uses so-called pseudo-monocrystalline CBN, but this is a fine-structured polycrystalline material without binding agent. It has extra wear and is only being applied in cases of extreme accuracy.)
In general turning of hardened steel with PCBN requires a negative cutting angle, in most cases without the need to use cutting or cooling fluid. But when using a conventional lathe, turning with a negative cutting angle causes vibrations, which seriously effects workpiece form and roughness. The tool pressure force FP in the X-direction causes those vibrations, because this force is about twice the main cutting force FC. (Conventional turning with a positive cutting angle gives a pressure force that is one third to about one half of the main cutting force.) Turning with high pressure force can be done only on turning machines with extremely high stiffness and stability. Another requirement is rigidity of the work piece, which needs a reasonable length-diameter ratio. But sometimes it is possible to support the work piece near the cutting tool.