After doing machining for so many years, how much do you pay attention to the blank? Do you feel like you have picked the right blanks all along? There is actually a lot of knowledge about blanks. Today, the editor will talk to you about the knowledge of blanks!
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 not only the factors of thermal processing, but also the requirements of cold processing, so as to reduce the manufacturing cost of parts from the process of determining the blank.
1. Types of blanks commonly used in machining
There are many types of blanks, and there are many manufacturing methods for the same blank. The blanks commonly used in mechanical manufacturing are as follows:
(1) Casting parts blanks with complex shapes should be manufactured by casting. At present, most of the castings are cast with sand molds, which are divided into manual molding of wooden molds and machine molding of metal molds. Wooden mold hand-molded castings have low precision, large machining surface allowance, and low productivity. They are suitable for small batch production of single pieces or casting of large parts. The metal mold machine has high modeling productivity and high casting precision, but the equipment cost is high and the weight of the casting is limited. It is suitable for small and medium-sized castings produced in large quantities. 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, etc.).
(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 forging forgings can be obtained by hand forging (small blanks), mechanical hammer forging (medium blanks) or press forging (large blanks). This kind of forging has low precision, low productivity, large machining allowance, and the structure of the parts must be simple, which is suitable for single piece and small batch production and the manufacture of 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 the 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 small and medium-sized forgings with large batches.
(3) Profiles Profiles can be divided into round steel, square steel, hexagonal steel, flat steel, angle steel, channel steel and other special cross-section profiles according to the cross-sectional shape. There are two types of profiles: hot rolled and cold drawn. Hot-rolled profiles have low precision, but are cheap, and are used as blanks for general parts; cold-drawn profiles are small in size, high in precision, and easy to realize automatic feeding, but the price is high, and are mostly used for large-batch production, suitable for automatic Machine tool processing.
(4) Welded parts Welded parts are combined parts obtained by welding. The advantages of welded parts are simple manufacturing, short cycle time, and material saving. The disadvantages are poor vibration resistance and large deformation. Machining can only be done after aging treatment.
In addition, there are stamping parts, cold extrusion parts, powder metallurgy and other blanks.
2. Problems that should be paid attention to in the selection of blank types
(1) Part material and its mechanical properties The material of the part roughly determines the type of blank. For example, casting blanks should be selected for parts made of cast iron and bronze; steel parts with uncomplicated shapes and low mechanical performance requirements can choose profiles; for important steel parts, in order to ensure their mechanical properties, forging blanks should be selected.
(2) Structural shape and external dimensions of parts The blanks with complex shapes are generally manufactured by casting. Sand casting is not suitable for thin-walled parts; advanced casting methods can be considered for small and medium-sized parts; sand casting can be used for large parts. For stepped shafts for general purposes, if the diameters of the steps are not much different, round bars can be used; if the diameters of the steps are quite different, in order to reduce material consumption and machining labor, forging blanks should be selected. Large-sized parts generally choose free forging; small and medium-sized parts can choose die forging; some small parts can be made into integral blanks.
(3) Production type Mass-produced parts should choose rough manufacturing methods with relatively high precision and productivity, such as castings using metal mold machine modeling or precision casting; forgings using die forging, precision forging; profiles using cold-rolled or cold-drawn profiles; When the output of parts is small, the blank manufacturing method with lower precision and productivity should be selected.
(4) Existing production conditions To determine the type and manufacturing method of blanks, specific production conditions must be considered, such as the level of blank manufacturing technology, equipment status, and the possibility of external cooperation.
(5) Fully consider the use of new technology, new technology and new material With the development of machinery manufacturing technology, the application of new technology, new technology and new material in blank manufacturing is also developing 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 the machining requirements can be met without machining, and its economic benefits are very significant. Full consideration should be given when selecting the blank, 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 basically depends on the shape and size of the part. The main difference between a part and a blank is that a certain machining allowance is added to the surface of the part to be processed, that is, the machining allowance of the blank. When the blank is manufactured, errors will also occur, and the dimensional tolerance of the blank is called the blank tolerance. The size of the blank machining allowance and tolerance directly affects the labor amount of machining and the consumption of raw materials, thus affecting the manufacturing cost of the product. Therefore, one of the development trends of modern machinery manufacturing is to refine the blank so that the shape and size of the blank are as consistent as possible with the parts, and strive to achieve less and no cutting. The machining allowance and tolerance of the blank are related to the manufacturing method of the blank, and can be determined by referring to relevant process manuals or relevant enterprise and industry standards during production.
After determining the blank machining allowance, the shape and size of the blank, in addition to adding the blank machining allowance to the corresponding processing surface of the part, also consider the influence of various technological factors such as blank manufacturing, machining 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 machining technology.
(1) The setting of the process stand Some parts, due to structural reasons, are not easy to clamp and stabilize during processing. For the convenience and speed of clamping, bosses can be made on the blank, which is the so-called process stand. The craft strap is only used when clamping the workpiece. After the part is processed, it is generally cut off, but if it does not affect the performance and appearance quality of the part, it can be kept.
(2) The use of the overall blank In machining, sometimes such parts as the three-tile bearing in the main shaft of the grinding machine, the connecting rod of the engine and the split nut of the lathe are encountered. In order to ensure the processing quality and convenience of this kind of parts, they are often made into a whole blank, and then cut after processing to a certain stage.
(3) The use of composite blanks In order to facilitate the clamping during processing, for some small parts with relatively regular shapes, such as T-shaped keys, flat nuts, small spacers, etc., multiple pieces should be combined into one blank to be processed to After a certain stage or after most of the surface processing is completed, it is processed into a single piece.
After determining the type, shape and size of the rough, a rough drawing should also be drawn as the product pattern of the rough production unit. To draw the blank drawing, add the blank allowance on the corresponding processing 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 hole and the gap on the forging, and the flange; the draft angle (draft) of the surface of the casting and forging Slope) and rounded corners; the position of the parting surface and the parting surface, etc. And use the double dotted line to indicate the surface of the part in the rough drawing to distinguish the machined surface from the non-machined surface
