(1) Pay attention to the following points when polishing with sandpaper:
① Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing round surface or spherical surface, the use of cork rod can better match the radian of round surface and spherical surface. The harder wood, like cherry wood, is more suitable for polishing flat surfaces. Trim the end of the wooden strip to keep it consistent with the shape of the steel surface, so as to avoid deep scratches caused by the sharp angle of the wooden strip (or bamboo strip) contacting the steel surface.
② When different models of sandpaper are used, the polishing direction should be changed from 45 ° to 90 °, so that the stripe shadow left by the former model of sandpaper after polishing can be distinguished. Before changing the sandpaper of different models, the polished surface must be carefully wiped with 100% pure cotton dipped with a cleaning solution such as alcohol, because a small grit left on the surface will destroy the whole polishing work. This cleaning process is also important when changing from sandpaper polishing to diamond grinding paste polishing. All particles and kerosene must be completely cleaned before polishing continues.
③ In order to avoid scratching and burning the workpiece surface, special care must be taken when polishing with # 1200 and # 1500 sandpaper. Therefore, it is necessary to load a light load and polish the surface with two-step polishing method. When polishing with each type of sandpaper, polish twice in two different directions, and rotate 45 °~90 ° each time between the two directions.
(2) Attention shall be paid to the following points in diamond grinding and polishing:
① This polishing must be carried out under light pressure as far as possible, especially when polishing pre hardened steel parts and polishing with fine grinding paste. When polishing with # 8000 abrasive paste, the common load is 100~200g/cm2, but it is difficult to maintain the accuracy of this load. To make it easier to do this, you can make a thin and narrow handle on the wooden strip, such as adding a copper piece; Or cut a part of the bamboo to make it softer. This helps to control the polishing pressure to ensure that the die surface pressure is not too high.
② When diamond grinding and polishing is used, not only the working surface is required to be clean, but also the workers' hands must be carefully cleaned.
③ Each polishing time should not be too long. The shorter the time, the better the effect. If the polishing process is carried out too long, "orange peel" and "pitting" will be caused.
④ In order to obtain high-quality polishing effect, polishing methods and tools that are easy to generate heat should be avoided. For example, when the polishing wheel is polished, the heat generated by the polishing wheel will easily cause "orange peel".
⑤ When the polishing process stops, it is very important to ensure that the workpiece surface is clean and all abrasives and lubricants are carefully removed, and then a layer of anti rust coating of the mold should be sprayed on the surface.
As mechanical polishing is mainly completed by manual work, polishing technology is still the main reason for affecting the polishing quality. In addition, it is also related to die materials, surface conditions before polishing, heat treatment process, etc. High quality steel is the prerequisite for obtaining good polishing quality. If the surface hardness of steel is uneven or the characteristics are different, polishing difficulties will often occur. All kinds of inclusions and pores in steel are not conducive to polishing.
(3) Influence of Different Hardness on Polishing Process
The hardness increases the difficulty of grinding, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly. At the same time, with the increase of hardness, the possibility of over polishing is reduced accordingly.
(4) Influence of Workpiece Surface Condition on Polishing Process
The surface layer of steel will be damaged due to heat, internal stress or other factors during the crushing process of cutting and machining. Improper cutting parameters will affect the polishing effect. The surface after EDM machining is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, before the completion of EDM machining, precision gauge and standard EDM shall be used for trimming, otherwise the surface will form a hardened thin layer. If the electric spark finishing gauge is not properly selected, the maximum depth of the heat affected layer can reach 0.4 mm. The hardness of the hardened thin layer is higher than that of the matrix and must be removed. Therefore, it is better to add a rough grinding process to completely remove the damaged surface layer and form an average rough metal surface to provide a good basis for polishing.
The most important point in ultra mirror polishing is the size of the polishing. The distance from the whetstone to the mirror polishing is 10µ. (Ideally, # 1500 (# 1000) whetstone starts to grind. The number of sandpaper is (# 320~) # 400~# 600~# 800~# 1000~# 1200~# 1500~# 01~# 02~# 03~# 04~# 05~# 06~# 015). The most basic principle is that the mold core must be thoroughly cleaned before the next operation to achieve good results. Be very careful not to make repeated modifications (repeated modifications will affect the quality).