Methods For Metal Machined Parts Surface Finishing

Surface finishing of metal processing parts is a process required to change the surface of metal products. It takes longer for workpieces to meet their expectations, thereby providing corrosion resistance, reflectivity, electrical resistance, and conductivity. The scratches on the surface are caused by the sharpness and type of the tool and the material of the product. In some cases, these scratches are negligible, but usually one or more secondary processes are required to produce a truly “finished” part. But usually we will use finishing process or secondary process to make their surface better.

Common surface treatment of metal processing parts can be divided into 4 aspects:

  1. Mechanical surface treatment: sandblasting, shot blasting, polishing, rolling, polishing, brushing, spraying, painting, oiling, etc.
  2. Chemical surface treatment: blue and black, phosphating, pickling, electroless plating of various metals and alloys, TD treatment, QPQ treatment, chemical oxidation, etc.
  3. Electrochemical surface treatment: anodic oxidation, electrochemical polishing, electroplating, etc.
  4. Modern surface treatment: chemical vapor deposition CVD, physical vapor deposition PVD, ion implantation, ion plating, laser surface treatment, etc.

Surface treatment is a process of artificially forming a surface layer on the surface of the substrate material that is different from the mechanical, physical and chemical properties of the substrate. The purpose of surface treatment is to meet the product’s corrosion resistance, wear resistance, decoration or other special functional requirements.

The surface finishing technology of metal processing parts is to repair metal products in regular use or in the production process. The following is a brief description of several main arterial finishing processes that make the surface look perfect.

grinding wheel

Sanding Polishing

This includes manually rubbing sandpaper on metal surfaces. Any abrasive material can be used for this purpose. Sanding is just a method of rubbing abrasive particles with the surface of the workpiece to produce a random, non-linear surface texture. Gravel is the cutting component that removes the roughness, sharp points and unevenness of the metal surface. Use different grinding media and stick it on the backing paper or board. The size of cutting particles is called “particle size”: the higher the particle size, the smaller and finer the particles, so the finer the surface finish they can achieve.

Very coarse abrasives can quickly remove a large amount of material, while finer abrasives can achieve mirror polishing effects. Water or other lubricants are usually used to flush the material and expose the new cut surface. Sanding is particularly useful for contoured or curved surfaces, but it is not suitable for entering very narrow corners or pockets.

Ultrasonic Polishing

When making plastic injection molding or pressure die-casting tools, it is usually necessary to finely polish the inner cavity to produce beautiful finished parts. But tool steels are heat-treated and very hard, so polishing them is difficult. It is not easy to get into holes, pockets and other complicated shapes.

In these cases, ultrasonic polishing is used. A soft, fine-pointed tool is mounted on an ultrasonic spindle vibrating at 30KHz. When used in combination with the grinding slurry media, the tip of the tool does not actually touch the working surface, but it will generate pressure waves and safely act on the surface to form fine polishing. This technique is even suitable for hardened steel, and the possibility of damage to the workpiece is very small.

Rumbling And Tumbling

Similar to sanding, both rumbling and tumbling use abrasive media, but in this case, the sanding particles are “loose” and are not fixed to any backing paper. Depending on the workpiece and the type of surface treatment required, many different types of media can be used, including garnet, walnut shell, stone or grit.

During the tumbling process, the components are placed in a box or bucket together with abrasive particles, and then rotated to randomly mix everything together. This is usually used for “deburring”, or to remove sharp metal points left on the edge of the part after machining.

The rumble is about the same, but the part and the abrasive are in a groove and then vibrated to increase the cutting effect. The rumbling and tumbling method is best for small-sized objects, not large surfaces.

precision grinding

Sandblasting Finishing

Sandblasting uses water and compressed air as the power to form a high-speed jet beam to spray the spray material (copper ore, quartz sand, emery sand, iron sand, Hainan sand) on the surface of the workpiece to be treated at a high speed, so that the outer surface of the workpiece surface The appearance or shape changes. Due to the impact and cutting action of the abrasive on the surface of the workpiece, the surface of the workpiece obtains a certain degree of cleanliness and different roughness, so that the mechanical properties of the workpiece surface are improved, thus improving the fatigue resistance of the workpiece. And enhance its corrosion resistance, increase the adhesion between it and the coating, and extend the durability of the coating film.

Compared with other cleaning processes (such as pickling, tool cleaning), the sandblasting process has the following characteristics:

  1. Sandblasting is the most thorough, most versatile, fastest and most efficient cleaning method.
  2. Sandblasting can be arbitrarily selected between different roughnesses, but other processes cannot achieve this. Manual polishing can make the rough surface but the speed is too slow, and the chemical solvent cleaning will clean the surface too smoothly, which is not conducive to coating bonding.

Polishing/Mirror Polishing

Metal polishing technology uses abrasive materials and polishing tools to modify the surface of metal products. In different types of polishing processes such as glazing, basic, and medium, you can improve the polishing efficiency by using polishing wheels and mirrors, thereby improving the accuracy and size of the finished product. Polishing can not improve the dimensional accuracy or geometric accuracy of the workpiece, but for the purpose of obtaining a smooth surface or mirror gloss, and sometimes it is also used to eliminate gloss (matting). An international standard such as Ra, Rb, Rc grade to judge the roughness grade of the product surface.

The method used depends on the nature, size and position of the metal surface finishing, of course, there are other finishing processes, such as honing finishing, anodizing, electroplating, powder coating, zinc plating, chromium plating, nickel plating, titanium plating, plating silver and other processing technology.

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