In internal combustion engine, the function of crankshaft is to convert reciprocating motion of piston into rotary motion. This means severe acceleration and deceleration, accompanied by high bending deformation, high torque and vibration impact, results in very high and variable stresses.
Such extreme stress requires careful design and calculation, selection of suitable materials and batch production process. These stresses are mainly located at the fillet radius between the bearing journal and the balance weight plate as well as on the oil hole, so it is necessary to pay special attention to the fine fabrication of these spaces.
In today’s market, automobile manufacturers are required to provide smaller engines while meeting higher power and speed requirements, which further increases the load and stress of the crankshaft. As a result, manufacturers are constantly looking for higher strength materials for these parts, usually including high alloy metals.
Ductile iron is the standard material for lower load engines (mainly gasoline engines), while high-power engines (including most diesel engines) use more expensive alloy steel forgings. The steels used for crankshafts are generally subjected to various heat treatments, such as quenching and tempering. In some special high-end use areas (including racing cars), crankshafts may be made directly from round steel.
At present, the requirement of large-scale precision machining for crankshaft is the most strict. Crankshaft manufacturing line has also become the largest consumer of cemented carbide in the whole process of automobile production.
A simple method of finishing both ends and Journal of crankshaft is turning on a traditional lathe or turning center. This clearly requires sufficient space from the handle to the finished journal appearance. The advantages of this approach include the ability to apply standardized machine tools, tool heads and blades to achieve high flexibility and low cost.
With the ability of turning and turning, a special machine tool can finish the Journal on the crankshaft rotation center. The blades used for rough and fine manufacturing can be installed on the same cutter head at the same time. High speed computer gongs can be used to make a special shape in one complete cutting stroke. Other benefits include repeated positioning accuracy, good finish appearance quality, smaller tolerance, longer tool head life, higher machine tool utilization due to reduced tool change times, and shorter finishing time. The disadvantage of turning and drawing fine production is that the cost of special machine tool and cutter head is high, and a large number of blades are needed. A more professional way of turning is to use multi tooth turning blades to achieve cost reduction efforts conducive to scrap iron control and not to use broaching action.
No matter what method is adopted, the overall goal of crankshaft finishing includes scrap iron control, prolonging tool head life, shortening finishing beat and improving product quality. Crankshafts are usually manufactured in large quantities on a production line consisting of several special machine tools. However, the one-piece refinement method can also be used in some special turnkey projects, such as racing engine crankshaft and marine diesel crankshaft.
External and internal milling represents another approach, especially the finishing of the connecting rod neck. If the balance weight has large allowance to be removed, the milling cutter head with cutter block with blade is an attractive choice. Sandvik has more than 40 years of experience with these uses. Oil hole drilling and milling are the most consumed technologies of cemented carbide in crankshaft manufacturing.
In the crankshaft oil hole, the hole depth is usually 20 times of the diameter, so it is very difficult to finish the task. The most widely used tools include gun drill and carbide twist drill lubricated by oil mist. High quality precision fabrication is also essential because of the possibility of major stresses around the borehole.
Stefan Knecht, global solutions manager of the crankshaft capability center in Dusseldorf, Germany, said: “although there are hundreds of cutting tool head manufacturers in the world, only six have the ability to supply cutting heads for crankshaft precision manufacturing, and even fewer companies really promote the technology development of the industry. Sandvik is one of the few
The center works with OEMs and machine tool manufacturers around the world, with projects in about 20 countries. Emerging markets are expanding in this area, especially in China, where the centre recently opened a new branch.
Crankshaft precision manufacturing is one of the most demanding industries in automobile production. The key requirements of all finishing methods are the same, including scrap iron control, prolonging tool head life, shortening finishing beat and improving product quality.
