After several years of machining, some parts are very difficult to process. Now I understand that the raw material is not selected correctly

After so many years of machining, how much attention do you attach to the raw material? Do you think you have chosen the right roughcast all the time? In fact, there is a lot of knowledge about the raw material. Today, I'd like to talk about the knowledge of raw material!

The determination of raw material not only affects the economy of raw material manufacturing, but also affects the economy of machining. Therefore, when determining the raw material, we should not only consider the factors of hot working, but also consider the requirements of cold working, so as to reduce the manufacturing cost of parts from the link of determining the raw material.

1. Types of raw materials commonly used in machining

There are many kinds of raw materials, and there are many manufacturing methods for the same kind of raw material. The raw materials commonly used in mechanical manufacturing are as follows:

1) Casting

Casting method should be used to manufacture the parts with complex shape. At present, most castings are cast in sand mold, which can be divided into wood mold hand molding and metal mold machine molding. It is suitable for small batch production of single piece or casting of large parts. Metal mold machine has high productivity and high precision of casting, but the equipment cost is high and the weight of casting is limited. It is suitable for medium and small castings in mass production. Secondly, special casting (such as pressure casting, centrifugal manufacturing and investment casting) can be used for small castings with high quality requirements.

2) Forging

For steel parts with high mechanical strength requirements, forging raw materials are generally used. There are two kinds of Forgings: free forging and die forging. Free forging forgings can be obtained by hand forging (small raw material), mechanical hammer forging (medium raw material) or press forging (large raw material). This kind of 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 manufacturing large forgings.

The precision and surface quality of die forging are better than free forging, and the shape of forging is more complex, so the machining allowance can be reduced. The productivity of die forging is much higher than that of free forging, but it needs special equipment and forging die, so it is suitable for large batch of medium and small forgings.

3) Profile

Profiles can be divided into round steel, square steel, six angle steel, flat steel, angle steel, channel steel and other profiles with special sections. There are hot rolled and cold drawn profiles. Hot rolled profiles have low precision but low price, which are used for the raw material of general parts; cold drawn profiles have small size and high precision, which are easy to realize automatic feeding, but high price, which are mostly used for large batch production and suitable for automatic machine tool processing.

4) Weldment

Weldment is the combination obtained by welding method. The advantages of weldment are simple manufacturing, short cycle and material saving. The disadvantages are poor vibration resistance and large deformation. It can only be machined after aging treatment.

In addition, there are stamping parts, cold extrusion parts, powder metallurgy and other raw materials.

2. Problems in the selection of raw material types

1) Material and mechanical properties of parts

The material of the part roughly determines the type of raw material. For example, the casting raw material should be selected for the parts made of cast iron and bronze; when the shape of steel parts is not complex and the mechanical property requirements are not too high, the profile can be selected; for important steel parts, the forging raw material should be selected to ensure their mechanical properties.

2) Structure shape and dimension of parts

The raw material with complex shape is usually made by casting. Thin wall parts are not suitable for sand casting; medium and small parts can consider advanced casting methods; large parts can be cast by sand casting. For general purpose stepped shafts, if the diameter difference of each step is small, round bar can be used; if the diameter difference of each step is large, forging raw material should be selected to reduce material consumption and machining labor. Large size parts generally choose free forging; small and medium parts can choose die forging; some small parts can be made into integral raw material.

3) Type of production

For mass-produced parts, the raw material manufacturing method with high precision and productivity should be selected. For example, the casting adopts metal mold machine modeling or precision casting; the forging adopts die forging and precision forging; the profile adopts cold-rolled or cold drawn profile; and the raw material manufacturing method with low precision and productivity should be selected when the output of parts is small.

4) Existing production condition

To determine the type and manufacturing method of the raw material, we must consider the specific production conditions, such as the raw material manufacturing process level, equipment status and the possibility of external cooperation.

5) Fully consider the use of new processes, new technologies and new materials

With the development of mechanical manufacturing technology, the application of new technology, new technology and new materials in raw material manufacturing is also developing rapidly. Such as investment casting, precision forging, cold extrusion, powder metallurgy and engineering plastics are increasingly used in machinery. Using these methods can greatly reduce the amount of machining, sometimes even can not be machined to meet the processing requirements, we should give full consideration to the selection of raw material, under possible conditions, try to use.

3. Determination of raw material shape and size

The shape and size of the raw material basically depend on the shape and size of the part. The main difference between parts and raw materials is that a certain machining allowance is added to the surface to be machined, that is, the raw material machining allowance. When the raw material is manufactured, there will also be errors. The dimensional tolerance of raw material manufacturing is called raw material tolerance. raw material machining allowance and tolerance directly affect the amount of labor and the consumption of raw materials, thus affecting the manufacturing cost of products. Therefore, one of the development trends of modern machinery manufacturing is to make the shape and size of the raw material consistent with the parts as much as possible through the refinement of the raw material, and strive to achieve less and no cutting. The size of raw material machining allowance and tolerance is related to the manufacturing method of raw material. In production, it can be determined by referring to the relevant process manual or relevant enterprise and industry standards.

After determining the raw material machining allowance, the shape and size of the raw material, in addition to the raw material machining allowance attached to the corresponding processing surface of the parts, but also consider the raw material manufacturing, machining and heat treatment and other factors. The following only from the perspective of machining process, analysis to determine the shape and size of the raw material should be considered.

1) The setting of the process lap

Some parts, due to structural reasons, are not easy to be clamped and stable during processing. In order to clamp conveniently and quickly, a boss can be made on the raw material, which is the so-called process lap. The process lap is only used for clamping the workpiece. Generally, the parts should be cut off after processing, but it can be retained if the performance and appearance quality of the parts are not affected.

2) Application of integral raw material

In machining, sometimes we will encounter such parts as three bush bearing of grinder spindle, connecting rod of engine and opening and closing nut of lathe. In order to ensure the processing quality and convenience of this kind of parts, it is often made into a whole raw material, which is cut after processing to a certain stage.

3) Application of the raw material of the combined parts

In order to facilitate the clamping in the process of processing, for some small parts with regular shape, such as t-keys, flat nuts, small spacers, etc., multiple parts should be combined into a raw material, and then processed into a single part after processing to a certain stage or after most of the surface processing.

After determining the type, shape and size of the raw material, a raw material drawing should be drawn as the product drawing of the raw material production unit. Drawing the raw material drawing is to add the raw material allowance on the corresponding processing surface on the basis of the part drawing. However, the specific manufacturing conditions of the raw material should be considered when drawing, such as the minimum casting and forging conditions of the hole on the casting, the hole and neutral position on the forging, the flange, etc.; the mold angle (draft angle) and fillet of the casting and forging surface; the position of parting surface and parting surface, etc. In order to distinguish the machined surface from the non machined surface, the double dotted line is used to show the surface of the part in the raw material drawing.