1. Introduction to Polishing
Polishing refers to the machining method that reduces the surface roughness of the workpiece by mechanical, chemical or electrochemical action to obtain bright and flat surface. It is a finishing process of a workpiece surface with a polishing tool and abrasive particles or other polishing medium.
The polishing does not improve the dimensional accuracy or the geometry accuracy of the workpiece, but is sometimes used to achieve a smooth surface or specular gloss and, in some cases, to eliminate the gloss (extinction). A polishing wheel is typically used as a polishing tool. The polishing wheel is generally made of a multi-layer canvas, felt or leather, and the two sides are clamped by a metal circular plate, and the rim of the polishing wheel is coated with a polishing agent which is formed by uniformly mixing the micro-powder abrasive and the grease.
when polishing, the polishing wheel at high-speed rotation (the peripheral speed is over 20 m/ s) is pressed against the workpiece, and the abrasive is pressed against the surface of the workpiece to generate rolling and micro-cutting, so as to obtain a bright working surface, the surface roughness can be generally up to Ra0.63-0.01 micron, and when the non-oil-free extinction polishing agent is adopted, The bright surface may be matt to improve the appearance. In large-scale production of bearing steel ball, the method of roller polishing is often used.
During rough throwing, a large number of steel balls, lime and abrasives are placed in an inclined tank drum. When the drum rotates, the steel ball and abrasive are randomly rolled and collided in the cylinder in order to remove the surface protruding front and reduce the surface roughness, and the margin of about 0.01 mm can be removed.
Steel balls and fur fragments are loaded into the barrel during fine throwing, and dazzling and bright surfaces can be obtained by rotating them for several hours. The polishing of precision thread ruler is carried out by soaking the machined surface in polishing solution, which is made up of chromium oxide powder with particle size of W5~W0.5 and emulsion. The polishing wheel is made of wood with uniform skim treatment or special fine felt. Its motion track is uniform and dense mesh, the surface roughness after polishing is not more than Ra0.01 micron, and no surface defects can be observed under 40 times magnification microscope. In addition, there are electrolytic polishing and other methods.
2. Polishing classification
(I) Mechanical polishing
Mechanical polishing is a polishing method to obtain smooth surface by cutting and plastic deformation of material surface to get smooth surface. Oil stone strip, wool wheel, sandpaper and so on are generally used, mainly manual operation, special parts such as rotating body surface, can use turntable and other auxiliary tools, and ultra-fine polishing method can be used for high surface quality requirements. Ultra-fine polishing is a special grinding tool. In the abrasive solution, it is pressed on the machined surface of the workpiece and rotates at high speed. The surface roughness of Ra0.008 μ m can be achieved by using this technique, which is the highest of all kinds of polishing methods. This method is often used in optical lens mold.
(2) Wheel polishing
The surface of the workpiece is rolled and polished by high speed rotating flexible polishing wheel and very fine abrasive. The polishing wheel is made of multi-layer canvas, felt or leather, and is used for polishing larger parts.
(3) Roller polishing, vibration and polishing
The workpiece, abrasive and polishing liquid are loaded into the rolling or vibration box, the rolling is slow rolling or the vibration of the vibration box, so that the workpiece and the workpiece, the workpiece and the abrasive friction with each other, coupled with the chemical action of the polishing liquid, remove the oil pollution on the surface of the workpiece, rust layer, grind off the convex peak, so as to obtain a smooth surface. Used for polishing small and large parts, the latter has higher productivity and better polishing effect than the former.
(4) Chemical polishing
Chemical polishing is the preferential dissolution of the partially concave portion of the surface of the material on the surface of the chemical medium to obtain a smooth surface. The method has the main advantages of no need of complicated equipment, and can be used for polishing a workpiece with a complex shape, so that a plurality of workpieces can be polished at the same time, and the efficiency is high. The core problem of chemical polishing is the formulation of polishing solution. The surface roughness obtained by chemical polishing is generally 10. m
(5) Electrolytic polishing
The basic principle of electropolishing is the same as that of chemical polishing, that is, the surface of the selective dissolving material is slightly convex, so that the surface is smooth. Compared with the chemical polishing, the effect of the cathode reaction can be eliminated, and the effect is good. The electrochemical polishing process is divided into two steps: (1) the macro-leveling and dissolving product is diffused in the electrolyte, the geometric roughness of the surface of the material is reduced, Ra is more than 1. m
(6) Ultrasonic polishing
The workpiece is put into the abrasive suspension and placed together in the ultrasonic wave field, and the abrasive is ground and polished on the surface of the workpiece by the vibration of ultrasonic wave. Ultrasonic machining has little macro force and will not cause workpiece deformation, but it is difficult to make and install tools. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration stirring solution is applied to separate the dissolved product from the surface of the workpiece, and the corrosion or electrolyte near the surface is uniform. The cavitation of ultrasonic wave in the liquid can also inhibit the corrosion process and facilitate the surface brightening.
(7) Fluid Polishing
Fluid polishing is to wash the surface of workpiece according to the high speed flow of liquid and the abrasive particles it carries to achieve the purpose of polishing. Common methods are: abrasive spray machining, liquid injection processing, hydrodynamic grinding and so on. Hydrodynamic grinding is driven by hydraulic pressure, so that the liquid medium carrying abrasive particles flows through the surface of the workpiece at high speed. The medium is mainly made of special compounds with good flowability at low pressure and mixed with abrasives, which can be made of silicon carbide powder.
(8) Magnetic grinding and polishing
Magnetic grinding and polishing is to use magnetic abrasive to form abrasive brush under the action of magnetic field to grind the workpiece. This method has the advantages of high processing efficiency, good quality, easy control of processing conditions and good working conditions. With suitable abrasive, the surface roughness can reach Ra0.1 μ m.
3. Process anomaly and analysis.
1) phenomenon: sand eye, dark printing on the surface
Reason: insufficient polishing
The method for solving the problem is as follows: a plurality of dotting of the wax is carried out by using a hemp wheel, and then a cloth wheel is used for throwing; the serious passing of the sand, the hemp wheel and the final throwing wheel
(2) Phenomenon: Deformation
Reason: improper operation or offset in the process
Solution: first repair the concave and convex parts, and then carry on the hemp wheel throwing and the cloth wheel throwing
4. Design guidance
Material, maturity, scene, interval cost,
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