Polishing refers to the processing method that makes the surface roughness of the workpiece reduced by mechanical, chemical or electrochemical actions to obtain a bright and smooth surface. In polishing, the surface of the workpiece is machined with polishing tools and abrasive particles or other polishing media.
Polishing does not improve the dimensional accuracy or geometric shape accuracy of the workpiece, but aims to obtain a smooth surface or mirror gloss. Sometimes it can also be used to eliminate gloss (extinction). The polishing wheel is usually used as a polishing tool. The polishing wheel is usually made of multiple layers of canvas, felt or leather. Its two sides are clamped with round metal plates, and its edges are coated with polishing agents made of fine powder, abrasive and oil.
In the polishing process, the high-speed rotating polishing wheel (the circumferential speed is more than 20 m/s) presses the workpiece, making the abrasive on on the workpiece surface rolling and micro cutting. Then the polished surface can be obtained. The surface roughness can generally reach Ra0.63 ~ 0.01 micron. When non-grease polish is used, the gloss on the surface can be eliminated to improve the appearance. The roller polishing method is often used in mass production of bearing steel balls.
In rough casting, a large number of steel balls, lime and abrasive are placed in the inclined canister drum. When the drum rotates, the steel ball and abrasive are randomly rolled and collided in the drum to remove the surface protrusion and reduce the surface roughness, and the remaining amount of 0.01 mm or so can be removed.
During the fine throwing, steel balls and fur fragments are loaded into the barrel, and the shiny surface can be obtained after several hours of continuous rotation. The polishing of the precision linear scale is carried out by soaking the processed surface in the polishing liquid. The polishing solution is a mixture of chromic oxide powder with particle size of W5 ~ W0.5 and emulsion. The polishing wheel is made of fine, degreased wood or special fine felt. Its motion track is uniform and dense mesh. The surface roughness after polishing is not greater than Ra0.01 micron. No surface defects were observed under a 40 - fold magnification microscope. In addition, there are other methods such as electrolytic polishing.
How to polish
1. Mechanical cutting.
Mechanical polishing is achieved by cutting, plastic deformation and removing the convex part after polishing. Mechanical polishing generally USES whetstone bar, wool wheel, sandpaper, etc., mainly manual operation. Special components such as solid of revolution, tools such as turntable can be used. As for the workpieces demanding high surface quality, workers can adopt Ultra precise polishing. Ultra-precision lapping adopts a special grinding tool. In the lapping liquid containing abrasive material, it is pressed on the surface of the workpiece with high speed rotary movement. The surface roughness can reach Ra0.008 displacement m, the highest among all polishing methods. This method is often used in optical lens mould.
2. Chemical polishing.
Chemical polishing gives priority to the dissolution of the microscopic protrusions on the surface of the material in the chemical medium, so as to obtain a smooth surface. The main advantage of this method is that no complex equipment is required, and the workpieces with complex shapes can be polished. What’s more, many workpieces can be polished at the same time, with high efficiency. The key problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is generally 10 (m).
3. Electrolytic polishing.
The basic principle of electrochemical polishing is the same as chemical polishing, that is, smooth surface by selective dissolution of the material surface small protrusions. Compared with chemical polishing, the effect of cathode reaction can be eliminated and the effect is better. The electrochemical polishing process is divided into two steps: (1) macroscopic levelling. The dissolved product diffuses into the electrolyte. The geometrical roughness of the material surface decreases, Ra>, 1 molar. (2)anodic polarization, surface brightness increases, Ra<1 anode m.
4. Ultrasonic polishing.
The workpiece is put into the abrasive suspension, and they are put together in the ultrasonic field. The abrasive polishes the surface of the workpiece by the ultrasonic oscillation. The macroscopic force of ultrasonic machining is small, which will not cause deformation of workpiece. Ultrasonic machining can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution to separate the dissolved product on the surface of the workpiece. Ultrasonic cavitation in liquid can also inhibit the corrosion and facilitate surface lighting.
5. Fluid polishing.
Fluid polishing depends on the high-speed flow of the liquid and its carrying abrasive scour the surface of the workpiece to achieve polishing. Common methods are: abrasive jet machining, liquid jet machining, fluid power grinding, etc. Hydrodynamic grinding is driven by hydraulic pressure, which makes the liquid medium carrying the abrasive to and fro through the workpiece surface in high speed. The medium is mainly made of abrasive and special compounds with good flowability at lower pressure. Abrasive can use silicon carbide powder.
6. Magnetic grinding and polishing.
This method USES magnetic abrasive to form abrasive brushes under the action of magnetic field. The method has high efficiency, good quality, easy control of processing conditions and good working conditions. With suitable abrasive, the surface roughness can reach Ra0.1 pore m.
Three main grinding and polishing techniques
1. The first kind is physical grinding
This kind of abrasive particle is irregular diamond shape, which is available in both coarse and fine grades. The deoxidization and scratch effect is quite good. It is the mainstream grinding and polishing technology in the market. However, rough abrasive with Angle often cause secondary fine scratches, and a finer grade of abrasive is required for more than two times of grinding. The process is complex and inevitably hurts the surfac. If this grinding product and technique is used frequently, the paint will rely on polishing and waxing. The varnish layer will become thinner and the brilliance of the original car paint will fade.
2. The second type is physical + covering grinding
The workpiece that has just been polished, the scratches in the rough disappear, and has a bright effect. But under the sun, it has obvious fine scratches and rotatory light. Reasons: Part of the scratch is filled with waxy or resinous oil, not really removed; The oil property is too large, the polishing ball moves to produce the optical rotation. This kind of polishing method is very easy to reproduce scratches after two or three times of washing, which is highly deceptive. Shop owners and technicians who do not know the reason will be very upset!
3. The third kind is the superior and rare grinding technology
Time-delay crushing and polishing technology, the grinding agent is in the shape of glass beads, which will not cause the second grinding scratch. At the same time, the temperature of the paint surface is low. When the polishing ball runs at high speed and a certain temperature is generated with the grinding agent, the abrasive particles will be immediately broken and more fine particles will be formed. This kind of polishing technology guarantees the effect, also minimizes the damage to the surface (polishing must harm)! At the same time, this kind of abrasive does not contain wax or resin oily ingredients, the gloss is the deep original paint surface gloss, with amazing cold light texture.