After so many years of working in machining, how important is roughing to you? Do you think you chose the right blank from the start? There is actually a lot of knowledge about flans. Today, the editor will tell you about the knowledge of blanks!
The determination of the blank not only affects the manufacturing economy of the blank, but also affects the machining economy. Therefore, when determining the roughness, we need to consider both hot processing factors and cold processing requirements, in order to reduce the manufacturing cost of parts from determining the roughness. draft.
1. Types of blanks commonly used in mechanical processing
There are many types of blanks and many manufacturing methods for the same type of blanks. Commonly used blanks in mechanical manufacturing include:
(1) Casting parts Blanks of parts with complex shapes must be manufactured by casting methods. At present, most castings are cast in sand molds, which are divided into hand casting with wooden molds and mechanical casting with metal molds. Hand-shaped wooden castings have low precision, large processing area margin and low productivity. They are suitable for small batch production of unique parts or molding of large parts. The metal casting machine has high casting productivity and high casting precision, but the equipment cost is high and the weight of casting parts is also limited. It is suitable for small and medium-sized castings produced in large quantities. Secondly, special castings (such as die casting, centrifugal manufacturing and investment casting, etc.) can be used for a small number of small castings with higher quality requirements.
(2) Forgings Steel parts with high requirements for mechanical strength generally use forging blanks. There are two types of forgings: free forgings and forgings. Free forgings can be obtained by hand forging (small blanks), mechanical hammer forging (medium blanks) or press forging (large blanks). This type of forging has low precision, low productivity, large machining allowance, and the structure of the part must be simple. It is suitable for single piece and small batch production and large forgings manufacturing.
The precision and surface quality of forgings are better than that of free forgings, and the shape of forgings can also be more complex, thereby reducing the machining allowance. The productivity of 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 larger 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 their cross-section shapes. 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 have smaller sizes, high precision, and are easy to realize automatic feeding, but are mainly used for. large volume production and are suitable for automatic machine tool processing.
(4) Welded parts Welded parts are attachments obtained by welding. The advantages of welded parts are simple manufacturing, short cycle time and material savings. The disadvantages of welded parts are poor vibration resistance and significant deformation. be machined.
In addition, there are stamping parts, cold extrusion parts, powder metallurgy and other blanks.
2. Points to pay attention to when selecting blank types
(1) Workpiece material and its mechanical properties The workpiece material approximately determines the type of blank. For example, casting blanks should be selected for cast iron parts and bronze profiles; mechanical properties, forging blanks should be selected.
(2) Structural shape and exterior dimensions of parts. Blanks of complex shapes are usually manufactured by casting methods. Sand casting is not suitable for thin-walled parts; advanced casting methods can be considered for small and medium sized parts; For general-purpose stepped shafts, if the step diameters are not very different, round bars can be used if the step diameters are very different, in order to reduce material consumption and machining labor, it is better to choose forging; white people. Large parts generally choose free forging; small and medium-sized parts can choose forgings; some small parts can be made into full blanks.
(3) Production type: For mass-produced parts, a blank manufacturing method with relatively high precision and productivity should be selected. For example, castings use metal mold machine modeling or precision forgings use pressure forging or precision forging profiles use cold; cold rolled or drawn profiles When the production of parts is small, blank manufacturing methods with lower precision and productivity should be selected.
(4) Existing production conditions: To determine the type of blank and the manufacturing method, specific production conditions should be considered, such as the technological level of manufacturing the blank, the condition of the equipment and the possibility of external collaboration.
(5) Fully consider the use of new processes, new technologies and new materials With the development of mechanical manufacturing technology, the application of new processes, new technologies and new materials in blank manufacturing are becoming also develops rapidly. For example, investment casting, precision forging, cold extrusion, powder metallurgy and engineering plastics are increasingly used in machinery. Using these methods greatly reduces the amount of mechanical processing, and sometimes even processing requirements can be met without mechanical processing. The economic benefits are very significant. Particular attention should be paid when selecting blanks and, if possible, try to use them.
3. Determination of the shape and size of the blank
The shape and size of the blank essentially depends on the shape and size of the part. The main difference between parts and blanks 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 making blanks, errors will also occur. The dimensional tolerance of blank manufacturing is called blank tolerance. The size of the blank processing tolerance and tolerance directly affects the labor volume of mechanical processing and the consumption of raw materials, thereby affecting the manufacturing cost of the product. Therefore, one of the development trends of modern machine 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 perform the least or no cutting processing. The size of the machining allowance and the tolerance of the blank are related to the manufacturing method of the blank. During production, they can be determined by referring to the relevant process manual or relevant company and industry standards.
After determining the machining allowance of the blank, in addition to adding the machining allowance of the blank to the corresponding processing surface of the workpiece, the shape and size of the blank must also consider from the influence of many process factors such as blank manufacturing, machining. and heat treatment. The following only analyzes the issues that need to be considered when determining the shape and size of the blank from the perspective of machining technology.
(1) Adjustment of process locks: Due to structural reasons, some parts are difficult to clamp stably during processing. To make tightening easier and faster, a boss can be made on the blank, called a process mesh. The craft fastener is only used when tightening parts. After the parts are processed, they are usually cut, but they can be kept if they do not affect the performance and appearance quality of the parts.
(2) Using integrated blanks When machining, parts such as three-tile bearings in grinder spindle components, motor connecting rods, and lathe opening and closing nuts are sometimes encountered. In order to ensure the quality and convenience of processing these parts, they are often made into whole blanks, and then cut after processing to a certain stage.
(3) Use of combined blanks In order to facilitate clamping during processing, for some small parts with relatively regular shapes, such as T-shaped wrenches, flat nuts, small spacers, etc., multiple parts need to be combined into a single draft to be processed. After a certain step or after most of the surface treatment is completed, it is processed as one piece.
After determining the type, shape and size of the blank, a drawing of the blank should also be drawn as the product model of the blank production unit. Drawing the blank drawing is based on drawing the part and adding the gross margin on the corresponding processing area. However, specific manufacturing conditions of the blank must also be considered during drawing, such as minimum casting and forging conditions for holes on castings, holes and gaps on forgings , flanges, etc., the draft angle (draft) of the surfaces of; castings and forgings (slope) and fillet; joint surface and joint surface position, etc. And use a double-dot dotted line to indicate the workpiece surface in the rough drawing to distinguish the machined surface from the unmachined surface.
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