1. Types of commonly used blanks in mechanical processing
(1) The rough parts with complex casting shapes should be manufactured using casting methods. At present, most castings are cast using sand molds, which are divided into manual molding with wooden molds and machine molding with metal molds. Handmade wooden mold castings have low precision, large machining surface allowance, and low productivity, making them suitable for single piece small batch production or casting of large parts. The metal mold machine has high molding productivity and casting accuracy, but the equipment cost is high and the weight of castings is also limited, making it suitable for small and medium-sized castings in large-scale production. Secondly, a small number of small castings with high quality requirements can use special casting (such as pressure casting, centrifugal manufacturing, and investment casting).
(2) Steel parts with high mechanical strength requirements for forgings generally require the use of forged blanks. There are two types of forgings: free forging forgings and die forgings. Free forging forgings can be obtained through methods such as manual forging (small blanks), mechanical hammer forging (medium blanks), or press forging (large blanks). This type of forging has low accuracy, low productivity, large machining allowance, and the structure of the parts must be simple, suitable for single piece and small batch production as well as manufacturing large forgings.
The precision and surface quality of die forgings are better than those of free forgings, and the shape of forgings can also be more complex, thus reducing machining allowances. The productivity of die forging is much higher than that of free forging, but it requires special equipment and forging dies, so it is suitable for large batches of small and medium-sized forgings.
(3) Profile profiles can be divided into round steel, square steel, hexagonal steel, flat steel, angle steel, channel steel, and other special section profiles according to their cross-sectional shapes. There are two types of profiles: hot rolled and cold drawn. Hot rolled profiles have low precision but are inexpensive, and are used for rough blanks of general parts; Cold drawn profiles have smaller dimensions and higher accuracy, making them easy to achieve automatic feeding. However, they are more expensive and are often used for large-scale production, making them suitable for automatic machine tool processing.
(4) Welding components are combination components obtained by welding methods. The advantages of welding components are simple manufacturing, short cycle, and material saving. The disadvantages are poor vibration resistance and large deformation, which require aging treatment before mechanical processing.
2. Issues to be noted in the selection of raw material types
(1) The material and mechanical properties of the parts roughly determine the type of blank. For example, for parts made of cast iron and bronze, casting blanks should be selected; When the shape of steel parts is not complex and the mechanical performance requirements are not too high, profiles can be selected; To ensure the mechanical properties of important steel parts, forging blanks should be selected.
(2) The structural shape and external dimensions of the parts are complex and rough, usually manufactured using casting methods. Thin walled parts should not be cast with sand molds; Advanced casting methods can be considered for small and medium-sized parts; Large parts can be cast in sand molds. A general purpose stepped shaft, if the diameters of each step are not significantly different, can be made of round bar material; If the diameters of each step differ significantly, in order to reduce material consumption and mechanical processing labor, it is advisable to choose forged blanks. Large parts generally choose free forging; Small and medium-sized parts can choose die forgings; Some small parts can be made into integral blanks.
(3) The production type of parts produced in large quantities should choose rough manufacturing methods with high precision and productivity, such as using metal mold machine modeling or precision casting for castings; Forgings adopt die forging and precision forging; Cold rolled or cold drawn profiles are used for profiles; When the production volume of parts is low, a rough manufacturing method with lower accuracy and productivity should be chosen.
(4) The determination of the type and manufacturing method of raw materials under existing production conditions must take into account specific production conditions, such as the level of raw material manufacturing technology, equipment conditions, and the possibility of external cooperation.
(5) Fully considering the utilization of new processes, technologies, and materials. With the development of mechanical manufacturing technology, the application of new processes, technologies, and materials in blank manufacturing is also developing rapidly. The applications of precision casting, precision forging, cold extrusion, powder metallurgy, and engineering plastics in machinery are increasing day by day. The use of these methods greatly reduces the amount of mechanical processing, and sometimes even achieves processing requirements without mechanical processing, with significant economic benefits. When selecting a blank, full consideration should be given and, under possible conditions, it should be used as much as possible.
3. Determination of rough shape and size
(1) Due to structural reasons, some parts in the process of setting up process clamps are difficult to clamp and stabilize during processing. In order to facilitate and quickly clamp, protrusions can be made on the blank, which is called process clamps. The process tool is only used for clamping workpieces. After the parts are processed, they are generally cut off. However, if it does not affect the performance and appearance quality of the parts, they can be retained.
(2) The use of integral blanks in mechanical processing sometimes encounters parts such as three tile bearings in the spindle components of grinding machines, connecting rods of engines, and opening and closing nuts of lathes. In order to ensure the processing quality and convenience of such parts, they are often made into integral blanks and cut open after reaching a certain stage of processing.
(3) The use of composite blanks is for the convenience of clamping during the machining process. For some small shaped parts with relatively regular shapes, such as T-keys, flat nuts, small spacers, etc., multiple parts should be combined into one blank. After processing to a certain stage or most of the surface processing is completed, it should be processed into a single piece.
