How To Choose Right Blank For Machining Parts?
For processed products, you can usually choose bars, casting blanks, forging blanks, profiles, etc. In the case of small quantities, we give priority to bar processing, because the materials are easy to buy. However, its cost is very high, the processing procedures are many, and the processing is slow. In mass production, we have to consider more cost-effective production methods to save costs and speed up production. Then it is necessary to choose different blanks for machining, but how to choose a blank?
The determination of the blank not only affects the economics of blank for machining, but also affects the economics of machining. Therefore, when determining the blank, it is necessary to consider both the thermal processing factors and the cold working requirements, so as to reduce the manufacturing cost of the parts from the process of determining the blank.
1. Types of commonly used blanks for machining
There are many types of blanks, and there are multiple manufacturing methods for the same blank. The commonly used blanks in mechanical manufacturing are as follows:
The rough parts with complex shapes should be manufactured by casting methods. At present, most castings are made of sand casting, which is divided into wooden mold manual molding and metal mold machine molding. Wooden mold manual moulding castings have low precision, large machining surface allowance, and low productivity. They are 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. It is suitable for mass production of small and medium castings. Secondly, a small number of small castings with high quality requirements can be used in special casting (such as pressure casting, centrifugal manufacturing, investment casting, etc.).
For steel parts with high mechanical strength requirements, forging blanks are generally used. There are two types of forgings: free forging and die forging. Free forging forgings can be obtained by manual forging (small blanks), mechanical hammer forging (medium blanks) or press forging (large blanks). This forging has low precision, low productivity, large machining allowance, and the structure of the parts must be simple; it is suitable for single-piece and small-batch production, as well as the manufacture of large forgings.
The precision and surface quality of die forgings are better than that of free forgings, and the shape of forgings can be more complicated, which can reduce 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 medium and small forgings with larger batches.
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-section profiles. There are two types of profiles: hot-rolled and cold-drawn. The hot-rolled profile has low precision, but the price is cheap, and is used for the blanks of general parts; the cold-drawn profile is small in size, high in accuracy, easy to realize automatic feeding, but the price is higher, and it is mostly used for large batch production, suitable for automatic Machine tool processing.
(4) Welding parts
Welded parts are combined parts obtained by welding methods. The advantages of welded parts are simple manufacture, short cycle time, and material saving. The disadvantages are poor vibration resistance and large deformation. They can only be machined after aging treatment.
In addition, there are other blanks such as stamping parts, cold extruded parts, powder metallurgy and so on.
2. Problems that should be paid attention to in the selection of blank types
(1) Part materials and 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 should be selected when the shape of steel parts is not complicated and the mechanical performance requirements are not too high; for important steel parts, 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. 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 general-purpose stepped shafts, if the diameters of the steps are not much different, round bar materials can be used; if the diameters of the steps are different, in order to reduce material consumption and machining 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) Type of production
For mass-produced parts, a rough manufacturing method with high accuracy and productivity should be selected, such as castings using metal mold machine modeling or precision casting; forgings using die forging and precision forging; profiles using cold-rolled or cold-drawn profiles; when the output of parts is small, it should be Choose a blank manufacturing method with lower accuracy and productivity.
(4) Existing production conditions
To determine the type and manufacturing method of the blank, the specific production conditions must be considered, such as the technological 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 mechanical processing, and sometimes the processing requirements can even be achieved without mechanical processing, and the economic benefits are very significant. We should give full consideration when selecting the blank, and try our best to use it when possible.
3. Determination of the shape and size of the blank
The shape and size of the blank basically depend 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 blank machining allowance. When the blank is manufactured, errors will also occur, and the dimensional tolerance of the blank manufacturing is called the blank tolerance. The size of the blank machining allowance and tolerance directly affects the labor 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 machinery manufacturing is to make the shape and size of the blanks consistent with the parts as much as possible through the refining of the blanks, and strive to achieve minimal and no cutting processing. The size of the blank machining allowance and tolerance is 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 attaching the blank machining allowance to the corresponding processing surface of the part, also consider the influence of various process 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 mechanical processing technology.
(1) Setting of craftsman
Some parts, due to structural reasons, are not easy to be clamped and stabilized during processing. For the convenience and speed of clamping, bosses can be made on the blank, which is the so-called craftsman. The craftsman is only used when clamping the workpiece. After the parts are processed, they are generally cut off, but they can be retained if they do not affect the performance and appearance quality of the parts.
(2) Adoption of the overall blank
In machining, parts such as the three-watt bearing in the main shaft part of the grinder, the connecting rod of the engine and the opening and closing nut of the lathe are sometimes encountered. In order to ensure the processing quality of this kind of parts and the convenience of processing, they are often made into a whole blank and cut after processing to a certain stage.
(3) The use of blanks
In order to facilitate the clamping during the machining process, for some small parts with relatively regular shapes, such as T-shaped keys, flat nuts, small spacers, etc., multiple pieces should be combined into a blank, and after processing to a certain stage or most surface processing After finishing, it is processed into a single piece.
After determining the type, shape and size of the blank, a blank drawing should be drawn as the product drawing of the blank production unit. Drawing a blank drawing is based on the part drawing and adding a blank margin to the corresponding processing surface. However, the specific manufacturing conditions of the blank must be considered when drawing, such as the minimum casting and forging conditions of the hole on the casting, the hole and the gap on the forging, the flange, etc.; the draft angle of the surface of the casting and forging (draft Inclination) and rounded corners; the position of the parting surface and the parting surface, etc. And use the double-dot chain line to show the surface of the part in the rough drawing to distinguish the processed surface from the non-processed surface.
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