How to choose the correct blank

How much do you value raw materials after working in machining for so many years? Do you think you have always chosen the right raw material? There is actually a lot of knowledge about raw materials. Today, the editor will talk to you about the knowledge of rough materials!
The determination of raw materials not only affects the economy of raw material manufacturing, but also affects the economy of mechanical processing. So when determining the blank, it is necessary to consider both the factors of hot processing and the requirements of cold processing, in order to reduce the manufacturing cost of the parts from the process of determining the blank.
1. Types of commonly used blanks in mechanical processing
There are many types of blanks, and there are multiple manufacturing methods for the same type of blank. The commonly used blanks in mechanical manufacturing are as follows:
(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.
In addition, there are other blanks such as stamped parts, cold extruded parts, and powder metallurgy.
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
The shape and size of the blank basically depend on the shape and size of the parts. The main difference between a part and a blank is that a certain amount of machining allowance is added to the surface that the part needs to be machined, that is, the blank machining allowance. When manufacturing raw materials, errors can also occur, and the dimensional tolerance of raw material manufacturing is called raw material tolerance. The size of rough machining allowance and tolerance directly affects the labor and raw material consumption of mechanical processing, thereby affecting the manufacturing cost of products. So one of the development trends of modern mechanical manufacturing is to refine the blank, making the shape and size of the blank as consistent as possible with the parts, and striving to achieve minimal and no cutting processing. The size of the machining allowance and tolerance for the blank is related to the manufacturing method of the blank, and can be determined by referring to relevant process manuals or enterprise and industry standards during production.
After determining the machining allowance of the blank, in addition to attaching the machining allowance to the corresponding machining surface of the part, the shape and size of the blank also need to consider the influence of various process factors such as blank manufacturing, mechanical processing, and heat treatment. The following only analyzes the issues that should be considered when determining the shape and size of the blank from the perspective of mechanical processing technology.
(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.
After determining the type, shape, and size of the blank, a blank drawing should also be drawn as a product drawing for the blank production unit. Drawing a blank drawing is based on the part drawing and adding blank allowance to the corresponding machining surface. But when drawing, it is also necessary to consider the specific manufacturing conditions of the blank, such as the minimum casting and forging conditions for holes on castings, holes and gaps on forgings, flanges, etc; The draft angle and fillet on the surface of castings and forgings; The position of the parting surface and parting surface, etc. And use double dotted lines to indicate the surface of the part in the blank drawing, to distinguish between the machined surface and the non machined surface.