The determination of raw materials not only affects the economy of raw material manufacturing, but also affects the economy of mechanical processing. Therefore, when determining the blank, both the factors of hot working and the requirements of cold processing should be taken into account, so as to reduce the manufacturing cost of parts in 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. There are several commonly used blanks in mechanical manufacturing:

(1) The rough parts with complex casting shapes should be manufactured using casting methods. Currently, most castings are cast using sand molds, which can be divided into manual wood mold molding and machine metal mold molding. Wooden mold handmade castings have low precision, large machining surface allowance, and low productivity, making them suitable for single 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 mass 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 forging blanks. There are two types of forgings: free forging forgings and die forgings. Free forging forgings can be obtained by methods such as manual forging (small billets), mechanical hammer forging (medium billets), or press forging (large billets). 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 allowance. 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) According to the cross-sectional shape, profiles can be divided into round steel, square steel, hexagonal steel, flat steel, angle steel, channel steel, and other special cross-section profiles. There are two types of profiles: hot rolled and cold drawn. Hot-rolled profiles have low accuracy but are cheap, and are used for rough parts of general components; Cold drawn profiles have small dimensions and high accuracy, making them easy to achieve automatic feeding. However, their prices are relatively high, and they are often used for large-scale production, making them suitable for automatic machine tool processing.

(4) Welding parts are a combination of components obtained by welding methods. The advantages of welding parts are simple manufacturing, short cycle, and material saving, while the disadvantages are poor vibration resistance and large deformation, which require aging treatment before mechanical processing.

In addition, there are other blanks such as stamping parts, cold extrusion parts, powder metallurgy, etc.

2. Issues to be noted in the selection of blank 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 the rough parts are generally 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. For general purpose stepped shafts, if the diameters of each step are not significantly different, round bar materials can be used; If the diameters of each step differ significantly, in order to reduce material consumption and mechanical processing labor, it is advisable to choose forging 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 a rough manufacturing method with high precision and productivity, such as using metal mold machine modeling or precision casting for castings; The forging adopts die forging and precision forging; Cold rolled or cold drawn profiles are used for profiles; When the production of parts is small, it is advisable to choose a rough manufacturing method with lower accuracy and productivity.

(4) The existing production conditions determine the type and manufacturing method of the blank, and specific production conditions must be considered, such as the level of blank manufacturing technology, equipment status, 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 the processing requirements without mechanical processing, with significant economic benefits. When selecting a blank, full consideration should be given and it should be used as much as possible under possible conditions.

3. Determination of blank shape and size

The shape and size of the blank largely depend on the shape and size of the part. The main difference between parts and blanks lies in the addition of a certain amount of mechanical processing allowance, i.e. blank processing allowance, on the surface that the parts need to be machined. When manufacturing blanks, errors can also occur, and the dimensional tolerance of blank manufacturing is called blank tolerance. The size of rough machining allowance and tolerance directly affects the labor volume 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 rough machining allowance and tolerance is related to the manufacturing method of the rough, and can be determined by referring to relevant process manuals or relevant enterprise and industry standards during production.

After determining the machining allowance for the blank, the shape and size of the blank should not only be attached to the corresponding machining surface of the part, but also 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 of the process clamp are difficult to clamp and stabilize during processing. In order to facilitate and quickly clamp, bosses can be made on the blank, which is called the process clamp. The process adapter is only used for clamping workpieces. After the parts are processed, they are generally cut off, but can be retained if it does not affect the performance and appearance quality of the parts.

(2) The use of integral blanks in mechanical processing may sometimes encounter parts such as three pad 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 a complete blank and cut open after 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-shaped keys, flat nuts, small spacers, etc., multiple parts should be combined into a blank. After processing to a certain stage or after most of the surface processing is completed, a single piece should be added.

After determining the type, shape, and size of the blank, a blank drawing should also be drawn as the product drawing of 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 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 starting angle (draft angle) and fillet on the surface of castings and forgings; The position of the parting surface and parting surface, etc. And use a double dotted line to represent the surface of the part in the blank drawing to distinguish between the machined surface and the non machined surface.

Article source: CNC tribe, metal processing

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