Polishing refers to the processing method used to obtain a smooth surface by using polishing discs made of low-elasticity materials (cotton cloth, felt, artificial leather, etc.) rotating at high speed, or soft elastic or viscoelastic materials (plastic, asphalt, paraffin, tin, etc.) rotating at low speed, with the addition of a polishing agent. In short, there is no essential difference between grinding and polishing, except for the choice of abrasive and the material of the lapping tool.

Polishing generally does not improve the form or dimensional accuracy of a workpiece. Polishing typically uses fine abrasive particles smaller than 1 µm. Polishing wheels are usually made of soft metals or non-metallic materials such as asphalt, paraffin, synthetic resin, artificial leather, tin, etc. The force exerted by the abrasive on the workpiece is small, so the workpiece does not crack. Traditionally, the machining method using hard abrasive tools is called grinding, and the method using soft abrasive tools is called polishing. In reality, in the field of ultra-precision machining, there is no difference between ultra-precision grinding and ultra-precision polishing.

Grinding offers unparalleled advantages over other machining technologies: it is a typical "direct creation" or "evolutionary processing" process. This means that certain measures are taken on the machine tool (micro-feed, low grinding speed, continuous changes in grinding direction, etc.) to directly machine workpieces with a precision exceeding that of the machine tool itself. This is because, when the lapping tool comes into contact with the workpiece, the modeling and simulation of grinding marks on non-ultra-precise surfaces automatically selects the local protrusions for machining, so that material is only removed from the raised areas of both surfaces. Thus, the lapping tool and the workpiece correct each other, and the precision gradually improves. The machining accuracy of ultra-precision lapping is almost independent of the precision of the machine tool's relative movement and is determined primarily by the contact and pressure characteristics between the workpiece and the lapping tool, as well as by the shape of the relative movement path and other factors.

1. Sanding with sandpaper requires the use of soft wooden or bamboo rods. When sanding curved or spherical surfaces, cork rods allow for better adaptation to the surface's curvature. Harder woods, such as cherry, are more suitable for sanding flat surfaces. The end of the wooden rod should be trimmed to match the shape of the steel surface, thus preventing deep scratches caused by sharp edges of the rod (or bamboo) coming into contact with the steel surface.

2. When using different grits of sandpaper, the polishing direction should be changed between 45° and 90° so that the marks left by the previous grit are visible. Before changing to a different grit of sandpaper, the polished surface should be carefully cleaned with 100% pure cotton soaked in a cleaning solution such as alcohol, as even a small abrasive particle left on the surface can ruin the entire polishing process. This cleaning process is also essential when switching from sandpaper to diamond polishing compound. All abrasive particles and kerosene must be completely removed before continuing to polish.

3. To avoid scratches and burns on the surface of the piece, extreme care must be taken when polishing with #1200 and #1500 grit sandpaper. Therefore, it is necessary to apply light pressure and use a two-stage polishing method. With each type of sandpaper, polish twice in two different directions, rotating between 45° and 90° between directions.

1. This polishing should be done with the least possible pressure, especially when polishing pre-hardened steel parts and when using fine-grit pastes. When polishing with #8000 grit abrasive paste, the usual load is 100–200 g/cm², but it is difficult to maintain this pressure precisely. To make it easier, a thin, narrow handle can be made on the wooden rod, for example, by adding a piece of copper, or by trimming part of the bamboo to make it more flexible. This helps control the polishing pressure and ensures that the pressure on the mold surface is not excessive.

2. When using diamond grinding and polishing, not only is it necessary that the work surface be clean, but the operator's hands must also be carefully cleaned.

3. Each polishing operation should not be prolonged. The shorter the time, the better the effect. If polishing is carried out for too long, defects such as "orange peel" and "pitting" may appear.

4. To achieve a high-quality polish, methods and tools that easily generate heat should be avoided. For example, when polishing with a polishing wheel, the heat generated can easily cause an "orange peel" effect.

5. When stopping the polishing process, it is very important to ensure that the surface of the part is clean and that all abrasives and lubricants have been carefully removed. Next, a protective anti-corrosion coating should be applied to the surface of the mold.

Since mechanical polishing is primarily performed manually, the polishing technique remains the main factor affecting the final quality. In addition, the mold material, the surface condition prior to polishing, and the heat treatment process also play a role. High-quality steel is a prerequisite for achieving a good polished finish. If the steel's surface hardness is uneven or its properties are not homogeneous, difficulties often arise during polishing. Similarly, inclusions and porosity present in the steel are detrimental to the polishing process.

Increased hardness increases the difficulty of grinding but reduces surface roughness after polishing. As hardness increases, the polishing time required to achieve a lower surface roughness also increases. At the same time, with increased hardness, the likelihood of overpolishing decreases accordingly.

During cutting and machining processes, the surface layer of steel can be damaged by heat, internal stresses, or other factors. Inappropriate cutting parameters will negatively affect the polishing result. The surface obtained after electrical discharge machining (EDM) is more difficult to polish than the surface resulting from conventional machining or heat treatment. Therefore, before completing the EDM machining, precision gauges and standard EDM parameters must be used for fine-tuning; otherwise, a hardened surface layer will form. If the spark finishing parameters are not selected correctly, the maximum depth of the heat-affected zone can reach 0.4 mm. The hardness of this hardened layer is greater than that of the base material and must be removed. Therefore, it is advisable to add a roughing process to completely remove the damaged surface layer and obtain a metal surface with uniform roughness, which serves as a good base for polishing.

The most important aspect of ultra-mirror polishing is the grit size used. The distance from the sharpening stone to the mirror finish is 10 μm. (Ideally, grinding begins with a #1500 (#1000) stone. The sandpaper grit sequence is: (#320~) #400~#600~#800~#1000~#1200~#1500~#01~#02~#03~#04~#05~#06~#015). The basic principle is that the core of the mold must be thoroughly cleaned before proceeding to the next operation to achieve good results. Repeated modifications should be carefully avoided, as these negatively affect the final polish quality.