Process defects of large machine tool base plate castings

Machine tools play an important role in the construction of national economic modernization. Machine tool refers to the machine that makes machines, which is conventionally referred to as machine tool. It is generally divided into metal cutting machine tools, forging machine tools and woodworking machine tools. There are many methods to process mechanical parts in modern mechanical manufacturing: in addition to cutting, there are casting, forging, welding, stamping, extrusion, etc. However, parts with high metal precision and fine surface roughness generally need to be processed by cutting on the machine tool. The base plate is often used as the workbench of NC machine tools, so the quality of the base plate of machine tools will directly affect the machining accuracy of large machine tools, especially the base plate of large machine tools. Because of its large volume, shrinkage defects of varying degrees are often found in the base plate hole after drilling, which affects the overall accuracy and stability of the machine tools and can not meet the needs of its work. In addition, the weight of the backing plate of a large machine tool is generally 20 ~ 50 tons. If it becomes a waste due to unqualified quality, it will not only cause great economic losses, but also seriously affect the production progress of the workshop. Therefore, it is of great significance to optimize and improve the casting process of the backing plate of large machine tools to fundamentally solve the problem of unstable casting quality.

The base plate of large press is generally made of high-strength cast iron ht200-300, of which 2500 × four thousand and six hundred × 340 base plate as an example. The original casting process is shown in Figure 1. The casting is divided up and down. The important processing surface is located at the bottom of the lower box. It is molded with resin sand and poured on one side. The pouring temperature is 1470 ~ 1480 ℃, and the carbon equivalent of molten iron is controlled at 3.60 ~ 3.70%. After drilling the casting, it is found that shrinkage defects of different degrees are found in the base plate hole, as shown in Fig. 2.

After rough machining and drilling, hole defects with different areas were found on the inner wall of the hole. Holes generally appear below 70mm on the upper surface of the casting, distributed in the central area of the casting, and the hole wall is dendritic. Through analysis, it can be determined that the casting defect category is shrinkage cavity defect in thick ductile iron.