Third, grinding

Surface grinding, like other surface grinding, has features such as high cutting speed, small amount of feed, easy to control dimensional accuracy, and small surface roughness value, etc. The machining accuracy is generally up to IT7~IT5, surface roughness The value can reach Ra1.6~0.2μm. The grinding quality of surface grinding is higher than that of planing and milling, and it can also process hardened parts, thus it is mostly used for semi-finishing and finishing of parts. When the production volume is large, the plane of the box is often ground to finish the machining.

In the case of grinding of a relatively rigid surface of a process system, strong grinding can be used to not only finish high-hardness materials and hardened surfaces, but also rough-cut rough surfaces with a hard skin and a uniform balance. At the same time, surface grinding can install multiple parts at the same time on the electromagnetic working platform for continuous machining. Therefore, in the finishing process, not only high-quality machining is required for the surfaces of small and medium-sized parts that need to maintain a certain dimensional accuracy and mutual position accuracy. And can get higher productivity.

There are two types of surface grinding: flat grinding and end grinding.

1, flat grinding

As shown in Fig. 8-10a, the working surface of the grinding wheel is a circumferential surface. When the grinding wheel has a small contact area with the workpiece, the heating is small, the heat dissipation is fast, and the chip removal and cooling conditions are good, so that high machining accuracy and surface quality can be obtained. It is usually suitable for parts with high machining accuracy. However, due to the intermittent grinding with flat grinding, productivity is low.

2, end grinding

As shown in Figure 8-10b, the grinding wheel face is the end face. When grinding, the shaft length of the grinding head is short and the rigidity is good. The grinding head is mainly subjected to axial force and the bending deformation is small, so a larger grinding amount can be used. The grinding wheel has a large contact area with the workpiece. At the same time, many abrasive particles participate in grinding, so the productivity is high. However, the cooling and cooling conditions are poor, and the circumferential speed of the end face of the grinding wheel varies from point to point in the radial direction, resulting in uneven wear. Therefore, the grinding accuracy is higher. low. It is generally applicable to the surface processing of parts with low precision in mass production, or directly rough grinding of blanks. In order to reduce the contact area between the grinding wheel and the workpiece, the end face of the grinding wheel is trimmed into an inner cone shape, or the grinding head is tilted at a slight angle. This improves the heat dissipation conditions and improves the machining efficiency. The center of the ground plane is slightly concave, but due to The tilt angle is very small and the amount of sinking is minimal.

When grinding thin-film workpieces, due to poor workpiece stiffness, the workpiece warpage and deformation is more special. There are two main reasons for deformation:
(1) The workpiece has been bent (quenched and deformed) before grinding. When the workpiece is sucked on the electromagnetic table, it is leveled by the magnetic force, but after the grinding is released, it returns to its original shape, as shown in Figure 8-11a. In view of this situation, the suction force of the electromagnetic table can be reduced. The size of the suction force only needs to make the workpiece not slippery during grinding so as to reduce the deformation of the workpiece. A thin piece of paper or rubber (less than 0.5mm) can also be inserted between the work piece and the electromagnetic table, and the deformation of the work piece when the work piece is absorbed on the electromagnetic table can be reduced, so that a flat surface with high flatness can be obtained. As shown in Figure 8-11b

(2) Workpiece grinding generates heat deflection. Grinding heat increases the local temperature of the workpiece. When the upper layer is hot and the lower layer is cold, the workpiece will protrude. If the two ends are caught and cannot be stretched freely, the workpiece must be warped. For this situation, slotted grinding wheels can be used for grinding. Due to the intermittent contact between the workpiece and the grinding wheel, the heat dissipation conditions are improved, and the workpiece heating time is shortened and the temperature rises slowly. The use of sufficient coolant during the grinding process can also achieve better results.

Fourth, the plane of finishing

For parts with high dimensional accuracy and surface roughness, finishing is generally required. There are many methods for flat surface finishing, generally grinding, scraping, superfinishing, and polishing. The following describes grinding and scraping.

(a) Grinding

Grinding is a kind of application of a wide range of finishing. After processing, the precision can reach IT5 level, and the surface roughness can reach Ra0.1~0.006μm. It can process metal materials as well as non-metal materials.

In the grinding process, there are scattered fine-grained sand grains (abrasives and abrasives) between the lap and the surface of the workpiece. A certain amount of pressure is applied between the granules and the surface of the workpiece, and a complex relative movement is generated, so that grinding and grinding of the granules occurs. The chemical and physical action of the agent removes an extremely thin layer on the surface of the workpiece, achieving high precision and a small surface roughness.

The grinding method is divided into the following three categories according to the conditions of use of the abrasive:

1. Dry grinding requires only a small amount of lubricant additives on the surface of the lap. As shown in Figure 8-12a. The sand is basically fixed on the grinding tool during the grinding process, and its grinding action is mainly based on sliding grinding. This method has low productivity, but it can achieve high processing accuracy and a small surface roughness (Ra0.02 to 0.01 μm).

2. Wet Grinding The abrasive is applied to the lap during the grinding process and ground with dispersed sand. Apart from sand, abrasives also include kerosene, engine oil, oleic acid, and stearic acid. In the grinding process, some sand grains exist between the lap and the workpiece, as shown in Figure 8-12b. At this time, the sand is mainly based on rolling grinding, the productivity is high, and the surface roughness is Ra 0.04-0.02 μm, which is generally used for rough processing, but the processing surface is generally dull.

3. Soft abrasive grinding

In the grinding process, an abrasive using chromic oxide as an abrasive is coated on the working surface of the lap. Since the abrasive is softer than the lap and the workpiece, the abrasive is suspended between the workpiece and the lap during the grinding process, mainly using the abrasive and the workpiece. The chemistry of the surface produces a very soft layer of oxide film, and the film at the bump is easily ground away. This method can obtain a very fine surface roughness (Ra0.02 to 0.01 μm).

(b) scraping

The scraping plane is used for unquenched workpieces. It can achieve close contact between the two planes, can obtain higher shape and position accuracy, processing accuracy can reach above IT7, surface roughness Ra0.8 ~ 0.1μm . The scraped plane can form a sliding surface with a lubricating oil film, thereby reducing wear between the opposing moving surfaces and enhancing the contact stiffness between the joining surfaces of the components. The scratched surface quality was evaluated by the number of contact points per unit area, the rough scratch was 1 to 2 points/cm2, the semi-fine scratch was 2 to 3 points/cm2, and the fine scratch was 3 to 4 points/cm2.

Scraping labor intensity, low productivity; but scraping required equipment is simple, production preparation time is short, scratching force is small, heat is small, deformation is small, processing accuracy and surface quality. This method is commonly used in small batch production and maintenance work.

Table 3-16 shows the commonly used planar machining solutions. The plane machining plan should be properly selected according to the shape, size, material, technical requirements and production type of the part.

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