Methods For Improving The Effectiveness Of Coated Cutting Tools

- Jan 09, 2019-

Methods for improving the effectiveness of coated cutting tools


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The choice of tool material depends on the cutting conditions and also on the range of surfaces to be re-ground. For example, if the front face of the tool is re-ground, it may be advantageous to use cobalt-containing high-speed steel, which is more resistant to crescent wear after the front face of the tool is uncoated. With the development of tool materials, the use of high speed steel, hard alloy, various toughening ceramics, cobalt base ceramics, polycrystalline diamond and CBN and other materials has greatly improved the efficiency of metal cutting. Each material's knife has its own advantages and disadvantages, so it has a specific purpose.


New requirements for tool geometry are put forward for coated tools. It is generally believed that the improvement of tool geometry, such as rake Angle, chip removal space, etc., should focus on chip removal ability to adapt to the increase in cutting amount at a higher feed rate and a higher speed. Coated knife has higher machining efficiency, it allows a higher feed and cutting speed (can be increased to the original cutting speed of 2 ~ 3 times). For refractory materials, coating can improve tool performance greatly.


The reason why the tool with superhard coating wears less is that the film superhard compound has high hardness, high melting point and excellent thermochemical stability. Superhard compounds are mostly composed of transition metal nitrides, carbides and borides. They bond with strong covalent bonds; It has a very low standard free energy of formation, forming a very stable system, the hardness is not significantly reduced at high temperature. These films show higher resistance to mechanical and heat wear than hard alloys and high speed steels.


Coating conditions, process parameters, pretreatment of substrate before plating are very important for the preparation of high-quality coating. The condition of the tool surface is crucial to the adhesion of the coating. The surface of the plated workpiece must be free from other film layers, burn spots, rust spots, oil stains or other stains. The workpiece should be cleaned by strict sand-blasting and degreasing and ion bombardment before the hard film is grown in vacuum.


Different coating materials of the tool, the use of the effect is not the same. Low speed cutting, TiC coating has the advantage; High speed cutting, TiN is more suitable; Al2O3 has higher thermochemical stability than TiN and is suitable for working at higher cutting speeds. Compared with Al2O3 coating, TiN coating has obvious advantages in high speed cutting, while TiN coating has longer service life in low speed cutting.


The tool life is also related to the film thickness. If the future cutter surface wear is the benchmark, the tool life will increase with the increase of the film thickness, but the film thickness reaches saturation at 5um, that is, the life will not increase significantly; However, if the depth of crescent-depression on the cutter surface is the reference of tool life before, the tool life is proportional to the film thickness, and no saturation observation is found. When the film layer is too thick, it is easy to cause peeling. Now the coating thickness of turning tool is mostly 5 ~ 10um on one side.


The effect of film thickness on the hard coating of milling cutters is different. When the steel workpiece is milling, no matter what kind of film is used, the tool life is the longest when the film thickness is about 2um, but the life decreases when the film thickness is increased. However, for cast iron with less impact, the optimum film thickness changes to a thicker direction. In milling, the TiC coating has the best results, while the Al2O3 coating shows no advantage in turning.


Cemented carbide cutters are usually coated by CVD method, but PVD coating treatment almost does not cause a decrease in blade strength, PVD coating cemented carbide milling cutters than CVD coating more durable. For general high speed steel tools wear resistance, CVD coating is better than PVD coating, but precision, complex shape, expensive high speed steel tools are mostly PVD coating. It is a rather complicated technology to improve the effect of coating tool and give full play to the function of hard coating. In order to achieve optimal combination, the establishment of coating tool database, for different workpieces, through the computer selection of tool coating materials and processing parameters, the situation becomes simple and effective, so as to truly achieve high quality, efficient, low-cost processing goals.