After so many years of machining, how much do you attach importance to the blank? Do you think you've always made the right choice? In fact, there is a lot of knowledge about the blank. Today, I will talk to you about the knowledge of the blank!
The determination of the blank not only affects the economy of blank manufacturing, but also affects the economy of machining. Therefore, when determining the blank, it is necessary to consider both the factors of hot processing and the requirements of cold processing, so as to reduce the manufacturing cost of parts from the link of determining the blank.
1. Types of blanks commonly used in machining
There are many types of blanks, and there are a variety of manufacturing methods for the same kind of blanks, and the blanks commonly used in mechanical manufacturing are as follows:
1) Castings
Parts with complex shapes should be manufactured by casting methods. At present, most of the castings are cast with sand molds, and it is divided into wood mold manual modeling and metal mold machine modeling. The wood mold manual molding casting has low precision, large machining surface allowance and low productivity, which is suitable for single-piece small batch production or large-scale parts casting. The metal mold machine has high molding productivity and high casting precision, but the equipment cost is high, and the weight of the casting is also limited, which is suitable for small and medium-sized castings in mass production. Secondly, a small number of small castings with high quality requirements can be used for special casting (such as pressure casting, centrifugal manufacturing and investment casting).
2) Forgings
Steel parts with high mechanical strength requirements generally use forging blanks. There are two types of forgings: free forging forgings and die forgings. Free forgings can be obtained by hand forging (small blanks), mechanical hammer forging (medium blanks) or press forging (large blanks). The precision of this forging is low, the productivity is not high, the machining allowance is large, and the structure of the part must be simple; It is suitable for single and small batch production, as well as for the manufacture of large forgings.
The accuracy and surface quality of die forgings are better than those of free forgings, and the shape of the forgings is more complex, so the machining allowance can be reduced. The productivity of die forging is much higher than that of open forging, but it requires special equipment and forging dies, so it is suitable for small and medium-sized forgings with large batches.
3) Profiles
According to the cross-sectional shape, the profile can be divided into: round steel, square steel, hexagonal steel, flat steel, angle steel, channel steel and other special cross-sectional profiles. There are two types of profiles: hot-rolled and cold-drawn. The hot-rolled profiles have low precision, but are cheap and are used for blanks of general parts; The cold drawn profile is small in size, high in precision, easy to realize automatic feeding, but the price is high, and it is mostly used for large batch production, which is suitable for automatic machine tool processing.
4) Weldments
The advantages of welded parts are simple to manufacture, short cycle and material saving, and the disadvantages are poor vibration resistance and large deformation, and they can be machined only after aging treatment.
In addition to this, there are other blanks such as stamping parts, cold extrusion parts, powder metallurgy, etc.
2. Problems that should be paid attention to in the selection of blank types
1) Part materials and their mechanical properties
The material of the part roughly determines the type of blank. For example, parts made of cast iron and bronze should be cast blanks; The shape of the steel parts is not complex, and the mechanical properties are not too high. Important steel parts, in order to ensure their mechanical properties, forging blanks should be selected.
2) The structural shape and dimensions of the parts
Blanks with complex shapes are generally manufactured by casting methods. Thin-walled parts should not be cast with sand molds; Small and medium-sized parts can be considered with advanced casting methods; Large parts can be sand cast. For general purpose stepped shafts, if the diameter of each step is not much different, round bar material can be used; If the diameter of each step is quite different, in order to reduce material consumption and the amount of labor in machining, it is advisable to choose the forging blank. Large-size parts are generally free forging; Die forgings can be selected for small and medium-sized parts; Some small parts can be made into a monolithic blank.
3) Production type
Mass production of parts should choose a blank manufacturing method with high precision and productivity, such as metal mold machine modeling or precision casting for castings; Forgings are made of die forging and precision forging; The profiles are cold-rolled or cold-drawn; When the part output is small, the blank manufacturing method with lower accuracy and productivity should be selected.
4) Existing production conditions
To determine the type and manufacturing method of the blank, it is necessary to consider the specific production conditions, such as the process level of the blank manufacturing, the condition of the equipment and the possibility of external cooperation.
5) Fully consider the use of new processes, new technologies and new materials
With the development of machinery manufacturing technology, the application of new processes, new technologies and new materials in blank manufacturing has also developed rapidly. Such as precision casting, precision forging, cold extrusion, powder metallurgy and engineering plastics are increasingly used in machinery. The use of these methods greatly reduces the amount of machining, and sometimes even no machining can be carried out to meet the processing requirements, we should give full consideration to the selection of blanks, and try to use them under possible conditions.
3. Determination of the shape and size of the blank
The shape and size of the blank, basically depends on the shape and size of the part. The main difference between parts and blanks is that on the surface of the parts that need to be processed, a certain machining allowance is added, that is, the blank machining allowance. When the blank is manufactured, errors are also generated, and the dimensional tolerances of the blank manufacturing are called blank tolerances. The size of the blank machining allowance and tolerance directly affects the amount of labor in machining and the consumption of raw materials, thereby affecting the manufacturing cost of the product. Therefore, one of the development trends of modern machinery manufacturing is to make the shape and size of the blank as consistent with the parts as possible through the refinement of the blank, and strive to do less and no cutting processing. The size of the blank machining allowance and tolerance is related to the manufacturing method of the blank, and the production can be determined with reference to the relevant process manual or relevant enterprises and industry standards.
After determining the machining allowance of the blank, the shape and size of the blank, in addition to attaching the machining allowance of the blank to the corresponding processing surface of the part, should also consider the influence of many process factors such as blank manufacturing, machining and heat treatment. The following is only from the perspective of machining technology, the problems that should be considered when determining the shape and size of the blank are analyzed.
1) The setting of the craft pair
Some parts, due to structural reasons, are not easy to clamp and stabilize during processing, and in order to clamp conveniently and quickly, a boss can be made on the blank, that is, the so-called process lap. The process lap is only used when clamping the workpiece, and after the parts are processed, they generally have to be cut off, but if they do not affect the performance and appearance quality of the parts, they can be retained.
2) The use of the whole blank
In machining, parts such as three-watt bearings in grinding machine spindle components, connecting rods for engines, and opening and closing nuts for lathes are sometimes encountered. In order to ensure the processing quality and convenience of this kind of parts, they are often made into an integral blank and then cut after processing to a certain stage.
3) The use of composite blanks
In order to facilitate the clamping in the processing process, for some small parts with regular shapes, such as T-keys, flat nuts, small spacers, etc., multiple pieces should be combined into a blank, and then processed into a single piece after processing to a certain stage or after most of the surface processing is completed.
After determining the type, shape and size of the blank, a blank drawing should also be drawn as a product drawing of the blank production unit. Drawing the blank diagram is to add the blank allowance on the corresponding machined surface on the basis of the part drawing. However, the specific manufacturing conditions of the blank should also be considered when drawing, such as the minimum casting and forging conditions of the holes on the casting, the holes and gaps on the forgings, and the flanges; Die start angle (draft angle) and fillet of the surface of castings and forgings; the position of the parting surface and the parting surface, etc. The surface of the part is represented in the blank diagram with a double-dotted line to distinguish between machined and non-machined surfaces.