Polishing processing methods and applications


This article describes the principles and methods of polishing processing, as well as the operating principles of common polishing processes. Through the correct application of polishing wheel, polishing agent, abrasive particle size and polishing speed, the surface of the parts processed to achieve the requirements of bright as a mirror.

1 Preface
Polishing is the process of finishing parts in machining, can make the surface of the parts bright as a mirror. Various molds, decorative parts and parts with high requirements for appearance must be polished before plating. Polishing process is divided into rough polishing, polishing and fine polishing 3 stages. Coarse polishing, generally used in advance with adhesives sticky abrasive polishing wheel, because the abrasive sticky is very solid, so the polishing process is similar to the use of abrasive wheels, abrasive belts and abrasive cloth impeller, such as the same finishing process; in the polishing and fine polishing, the first polishing agent coated in a softer polishing wheel, and then the workpiece pressed on the high-speed rotating polishing wheel for polishing process.

2 polishing principle
The basic principle of polishing is the elastic polishing wheel by the bonded or coated with abrasive, in high-speed rotating conditions, soft grinding of the workpiece. Polishing can be divided into the following 3 types.

(1) fixed abrasive polishing The use of fixed abrasive polishing with adhesive bonding (see Figure 1). Because the abrasive grains and soft polishing wheel surface bonding firmly, so the cutting force is larger, the principle is similar to grinding. When polishing, the direction of rotation of the polishing wheel is the same as the workpiece feeding direction, and a very glossy surface can be obtained. If the direction is opposite, the polishing wheel in contact with the workpiece, the abrasive grain has a large cutting force, making the workpiece surface scratches, rougher. 

Figure 1 fixed abrasive polishing

(2) Adherent abrasive polishing The use of grease adherent abrasive polishing wheel for polishing (see Figure 2), abrasive in the role of force, can be slowly rolling in the grease, so that all the cutting edge of the abrasive have the opportunity to participate in the work, so that the polishing wheel to maintain long-term working capacity. At the same time, under the action of frictional heat and processing pressure, mediums such as fatty acids in the polishing agent react chemically with the metal surface to produce compounds that can be easily removed, thus speeding up the polishing efficiency.

Figure 2 Adhesive abrasive polishing
(3) polishing in the liquid polishing wheel is generally made of uniform material and degreasing treatment of wood or special fine felt, both of which are highly impregnated materials, in the polishing process can contain a large amount of polishing fluid for processing. The principle is accomplished through four stages of the polishing process, namely the free polishing stage, the inlay polishing stage, the saturation passivation polishing stage and the "shell film" polishing stage. In the use of fine felt polishing wheel, because the felt is soft, uniform, elasticity and immersion is greater, the whole polishing time is shorter, only inlay, saturation passivation and "shell film" 3 polishing stage.

3 polishing wheel material
Commonly used polishing wheel body materials, cotton, hemp, felt, leather, hard shell paper, soft wood and woolen fabrics and other softer materials. Coarse polishing needs to use a large polishing force to improve efficiency, can use canvas, felt, hard shell paper, cork, leather and hemp and other relatively hard polishing wheel materials. In the polishing and fine polishing is the use of good softness, and polishing agent to maintain a better cotton, felt and other polishing wheel materials. Polishing wheel material in the production of the former also need to be processed, the purpose of processing is to increase the rigidity to improve the polishing ability, strengthen the fiber, extend the service life, increase the softness, enhance the "imitation" ability, improve the retention of polishing agent, lubricity and fire resistance. Treatment methods are bleaching, sizing, wax treatment, resin treatment and pharmaceutical treatment. The elasticity and rigidity of the polishing wheel can be adjusted by changing the stitching method and the interval of the mesh of the polishing wheel. Swirling stitching method is easy to manufacture and use, and is widely used. In addition, there are concentric circles, checkerboard and radial stitching. If the polishing wheel stitching the same way, if the stitching interval is large, the elasticity of the polishing wheel is large, and vice versa, the elasticity is small and rigid.

4 polishing agent selection
Polishing agent by the powder polishing material and grease and other appropriate components of the medium evenly mixed. According to its state at room temperature, can be divided into solid polishing agent and liquid polishing agent. Solid polishing agent according to the composition or nature of the medium, can be divided into greasy and non-greasy two. Liquid polishing agent according to the composition or nature of the medium, can be divided into emulsion type, liquid grease type and non-grease type 3 categories. But the most used is the solid polishing agent.

Solid greasy polishing agent including Saizar polishing paste (fused alumina), for carbon steel, stainless steel and non-ferrous metal rough polishing; emery paste (fused alumina, emery), for carbon steel, stainless steel rough polishing and polishing; yellow polishing paste (plate diatomite), for iron, brass, aluminum and zinc, etc. in the polishing; rod-shaped iron oxide, for copper, brass, aluminum and copper-plated surface in the polishing and fine polishing; white polishing paste (roasted dolomite), for fine polishing of copper, brass, aluminum, copper-plated surface and nickel-plated surface; green polishing paste (chromium oxide), for fine polishing of stainless steel, brass and chromium-plated surface; red polishing paste (refined iron oxide), for fine polishing of gold, silver and platinum; polishing agent for plastics (microcrystalline anhydrous carbonic acid), for fine polishing of plastics, leather and ivory.

Liquid polishing agent generally uses chromium oxide and emulsion mixture of liquid.

5 polishing with the choice of abrasive particle size

The particle size of abrasive in polishing agent has a direct impact on the surface roughness value and polishing efficiency of the workpiece after polishing. Abrasive particle size coarse, the workpiece surface roughness value is large, and high processing efficiency; abrasive particle size fine, the workpiece surface roughness value is small, but the processing efficiency is low. The required processing surface roughness value Ra=1.6~3.2μm, the particle size is F46~F60; the required Ra=0.4~0.8μm, the particle size is F100~F180; the required Ra=0.1~0.2μm, the particle size is F240~W28; the required Ra=0.025~0.05μm, the particle size is W20~W5; the required Ra≤ 0.012μm, particle size <W5. In polishing there are coarse polishing, polishing and fine polishing, replacement of different particle size polishing agent, must replace the polishing wheel with different abrasive particle size, so as to avoid the coarse grain size abrasive to bring in and affect the polishing effect.

6 polishing speed and pressure selection
(1) the circumferential speed of the polishing wheel polishing, the higher the circumferential speed under certain pressure conditions, the smaller the amount of abrasive cutting, which is conducive to reducing the workpiece surface roughness value, polishing efficiency is also increased accordingly. Polishing steel, cast iron, nickel and chromium and other harder materials, polishing wheel speed of 30 ~ 35m / s; polishing copper, copper alloy and silver, polishing wheel speed of 20 ~ 30m / s; polishing aluminum, aluminum alloy, zinc and tin and other soft materials, polishing wheel speed of 18 ~ 25m / s. In practice, the choice of polishing wheel speed, according to the specific circumstances of the flexible control, in order to achieve safety, efficiency and quality purposes.

(2) polishing pressure polishing workpiece pressure to the polishing wheel pressure size, and polishing efficiency and workpiece surface quality is closely related. Rough polishing, the pressure is relatively large to improve efficiency; fine polishing, using a smaller pressure to improve the surface quality of the workpiece. General rough polishing pressure of 10 ~ 30MPa, fine polishing for 5 ~ 10MPa.

7 other polishing process
(1) sandpaper (cloth) polishing This method is easy to operate, flexible, is the traditional process method. Can be on the lathe, grinder, in order to further reduce the surface roughness value of the workpiece by hand, no other equipment is required. But must be based on the workpiece surface roughness value requirements, reasonable choice of abrasive cloth grit. For example, the required surface roughness value Ra = 0.1 ~ 0.8μm, abrasive grit F150 ~ F240. For the polishing of the bore and type surface (slot), now widely used abrasive impeller polishing. This kind of impeller is made of resin to bond the abrasive grains on the abrasive cloth, the abrasive cloth on the impeller is involute distribution, very soft, dense and flexible, can be installed on the electric or wind-driven tools when using. According to the surface requirements of the workpiece, the choice of different diameter and size of the abrasive cloth impeller, very convenient.

(2) liquid polishing will hold abrasives and liquid mixed suspension sanding liquid, with compressed air and through the nozzle at high speed spray to the surface of the workpiece, polishing process. This polishing method can generally be based on the surface roughness value Ra = 0.2μm, and soon get Ra = 0.05 ~ 0.1μm, mainly for other methods difficult to finish the surface (such as small holes, complex type surface and small narrow grooves, etc.).

(3) Electrolytic mechanical grinding compound polishing The principle is basically the same as that of electrolytic grinding. When polishing, the polishing head is connected to the negative terminal of DC power supply, the workpiece is connected to the positive terminal, and the electrolyte is injected into the polishing area by the hydraulic pump between them, and the polishing head rotates at a certain speed and pressure. After the DC power supply is connected, the surface of the workpiece is dissolved by the electrolyte and a passivation film is formed. The hardness of this extremely thin passivation film is much lower than the hardness of the workpiece material itself, which is easily removed by the abrasive carried by the polishing head. Because the process is only within 0.1s time cycle, so polishing high efficiency, good quality, low cost.

(4) ultrasonic EDM compound polishing rely on ultrasonic grinding and spark discharge to polish the surface of the workpiece, more than 3 times higher than the efficiency of pure ultrasonic mechanical polishing. Its most important feature is the high efficiency polishing small holes, narrow grooves, gaps and small precision surface, processing surface roughness value Ra up to 0.08 ~ 0.16μm.

(5) magnetic grinding and polishing As shown in Figure 3, the magnetic abrasives into the magnetic field, abrasives along the magnetic line of force direction into a magnetic brush, when the workpiece into the middle of the N-S magnetic poles, for relative movement, the two magnetic poles abrasive grinding and polishing of the workpiece, the workpiece surface roughness value Ra can be 8 ~ 12s to 0.2μm.

Figure 3 Magnetic grinding and polishing

Practice has proved that polishing is a widely used and increasingly mature surface finishing processing method in machining, both the practical experience of traditional processes and the theoretical basis of modern processes, simple operation, economic and practical. For the current stage of research and development of many efficient and excellent new polishing process, should be combined with production conditions, according to local conditions to choose, through the process test to achieve the desired effect before the full promotion.