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The development of new materials with tough workpieces is driving tool manufacturers to develop new tool geometries, carbide grades and coating technologies. For example, a processing shop serving the aerospace industry must find an effective means of processing 5553 titanium alloys and composites. Similarly, medical parts processing plants need to process PEEK polymers, stainless steel and other special materials. In the automotive industry, a typical difficult-to-machine material is compacted graphite iron (CGI). This material is primarily used in the manufacture of engine blocks, cylinder heads, and bearing cap castings commonly used in large diesel trucks. Because compacted graphite iron is half the weight of conventional gray cast iron, it has higher fuel efficiency for automobiles. In addition, its strength and stiffness are twice as high as that of gray cast iron, allowing the design of engine blocks with thinner wall thicknesses. Therefore, an assembled vermicular cast iron engine weighs about 9% less than a gray cast iron engine.
The application of compacted graphite iron has been in Europe for a long time and has been accepted by more and more people in the United States. Vermicular graphite iron can withstand the highest combustion pressure of diesel engines, but aluminum engine blocks with cast iron cylinder liners do not. Some high-performance V-type racing engines are also made of vermicular cast iron, not only because of the reduced weight, but also the rigidity (especially the rigidity of the recess between the cylinders).
One reason why vermicular graphite iron is more difficult to process is that its tensile strength is 2 to 3 times that of gray cast iron. In milling, higher tensile strength translates into higher cutting forces, and the processing power required to process vermicular cast iron is about 15% to 25% higher than that of gray cast iron. Therefore, some workshops that originally processed gray cast iron need to be converted into vermicular cast iron, which may cause insufficient machine power. In addition, the processing of vermicular cast iron also faces the following challenges:
(1) The thermal conductivity of compacted graphite iron is relatively low, so the cutting heat generated during machining accumulates in the workpiece, which in turn affects the wear of the tool. In contrast, gray cast iron has good thermal conductivity, and the cutting heat is easily taken away by the chips during processing.
(2) The cast hard skin of the vermicular graphite cast iron part has a ferrite structure and is easy to bond with the cutting edge of the tool. The cast hard skin of gray cast iron is a pearlite structure, so no bonding occurs.
(3) Unlike gray cast iron, vermicular graphite cast iron does not contain sulfide. The sulphide in the gray cast iron deposits on the cutting edge of the tool and acts as a lubricant to extend tool life.
(4) In the casting process of vermicular graphite cast iron, titanium is added as an alloying element, thereby producing a high-strength cast outer skin, and at the same time, an abrasive free carbide is formed in the entire casting. The alloying element content in the vermicular graphite cast iron has a great influence on its workability and tool life.
For the above reasons, the tool life for cutting vermicular cast iron is usually only half the life of cutting gray cast iron tools.
Milling and boring Compared to grey cast iron, the surface finish (Rz) of vermicular cast iron can be increased by approximately 50%, which means that the machining process may be reduced, or the finishing tool may not be required to obtain the requirements. Surface finish. During machining, when the tool exits cutting, the edge of the vermicular graphite cast iron workpiece will not be damaged; while the gray cast iron workpiece may be broken, and when the damage is serious, the cylinder may be scrapped. The properties of compacted graphite iron in this respect are similar to those of steel, which produces burrs but does not cause chipping.
Since the use of conventional processes for the production of vermicular cast iron requires a lower cutting speed, it may take up to three times longer processing time than cutting grey cast iron. In order to determine a more efficient method of processing compacted graphite iron, Sandvik has conducted many tests. For milling, the best tool material to be determined is coated cemented carbide with a thick layer of titanium oxynitride (TiCn) and alumina (Al2O3). The thickness of the thick coating layer is 7 to 10 μm, and the thickness of the thin coating layer is generally 2 to 3 μm.
The results of the vermicular cast iron milling test conducted by Sandvik are as follows: processing machine: Heller PFV2; depth of cut: 3mm; length of knife: 80mm; cutting speed: 130m/min; speed: 414r/min; feed rate: 298mm/ Min; feed per tooth: 0.36mm; number of inserts: 2 (for test); total area of ​​milling: 3.08m2; tool life: 130min (full knife); life of each blade: 1.54m2.
The test used a CoroMill 365 milling cutter designed to process cast iron to machine a liquid control part. The thick insert used is a 12° positive angle insert, but it is mounted on a negative angled insert to create a smaller positive cutting angle. It also allows for a higher density of blade arrangements to achieve the highest possible productivity.
For the turning and boring of compacted graphite iron, Sandvik recommends the use of a hard alloy substrate with high abrasion resistance and a wear-resistant thick coating prepared by a medium temperature chemical vapor deposition (CVD) process. Tests have shown that the tool life of boring cast iron with CBN inserts is only 1/10 of that of gray cast iron. At this time, the tool geometry with a small positive angle (5° to 10°) is appropriate, and it is recommended not to use the coolant when machining the vermicular cast iron.
Sandvik has teamed up with Makino to develop a boring process that allows the finishing of rough boring holes in one pass. The multi-blade tool developed for this purpose is called the Long-Edge Tool. The tool feeds down into the cylinder bore in a helical path, and it is said that the time taken to finish a cylinder bore is roughly equivalent to that of a machined gray cast iron cylinder bore. Subsequent boring is the final step before engine assembly.
In developing this new fine-tuning process, Sandvik and Makino determined that roughing is best done with conventional single-end milling cutters, with Si3Ni4 coatings and geometries optimized for boring cast iron. .
The challenge of cutting compacted graphite iron (CGI)
The use of compacted graphite iron (CGI) in diesel and racing engine parts is increasing. In order to effectively cut this challenging material, the choice of tool is critical.