Introduction to several commonly used polishing methods (2)
2.1 Basic procedures for mechanical polishing
For high-quality polishing, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond paste. The choice of polishing procedure depends on the surface condition of the previous processing, such as machining, EDM, grinding and so on. The general process of mechanical polishing is as follows:
(1) The surface after rough polishing, electric sparking, grinding, etc. can be polished by a rotary surface polishing machine or an ultrasonic grinder with a rotational speed of 35 000 to 40 000 rpm. A common method is to remove the white spark layer by using a wheel with a diameter of Φ 3 mm and WA # 400. Then there is a manual oil stone grinding, strip of oil stone plus kerosene as a lubricant or coolant. The general order of use is #180 to #240 to #320 to #400 to #600 to #800 to #1000. Many moldmakers choose to start with #400 in order to save time.
(2) Semi-finishing semi-finishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400 ～#600 ～#800 ～#1000 ～#1200 ～#1500 。. In fact, #1500 sandpaper is only used for hardened die steel (above 52HRC), not for pre-hardened steel, as this may cause burns on the surface of pre-hardened steel.
(3) Fine polishing fine polishing mainly uses diamond abrasive paste. If the polishing powder is mixed with a diamond abrasive powder or a polishing paste, the usual polishing order is 9 μm (#1800) to 6 μm (#3000) to 3 μm (#8000). The 9 μm diamond paste and polishing cloth wheel can be used to remove the hair marks from the #1200 and #1500 sandpaper. The polishing is then carried out using a felt and a diamond paste in the order of 1 μm (#14000) to 1/2 μm (#60000) to 1/4 μm (#100000).
Accuracy requires a polishing process of more than 1 μm (including 1 μm) in a clean polishing chamber in the mold shop. For more precise polishing, an absolutely clean space is required. Dust, smoke, dandruff and saliva can all leave high-precision polished surfaces that are obtained after hours of work.
2.2 Problems to be paid attention to in mechanical polishing
Polishing with sandpaper should pay attention to the following points:
(1) Polishing with sandpaper requires the use of soft wood sticks or bamboo sticks. When polishing round or spherical surfaces, use cork sticks to better match the curvature of the round and spherical surfaces. The harder wood strips are like cherry wood, which is more suitable for the polishing of flat surfaces. The end of the trim strip is adjusted to match the shape of the steel surface so that the acute angle of the strip (or bamboo strip) is prevented from contacting the surface of the steel and causing deep scratches.
(2) When changing to different types of sandpaper, the polishing direction should be changed from 45 ° to 90 °, so that the stripe shadow left by the polishing of the former type of sandpaper can be distinguished. Before changing the different types of sandpaper, the polished surface must be carefully wiped with 100% pure cotton and a cleaning solution such as alcohol, because a small gravel left on the surface will destroy the entire polishing work. This cleaning process is equally important when polishing from sandpaper to diamond polishing. All pellets and kerosene must be completely cleaned before polishing continues.
(3) In order to avoid scratching and burning the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is therefore necessary to load a light load and polish the surface using a two-step polishing method. When polishing with each type of sandpaper, it should be polished twice in two different directions, with each rotation between 45 ° and 90 °.
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