What Is Polishing？| Basic Knowledge and Processing Methods

This page provides an overview of the basic knowledge and processing methods related to polishing.

For information about TDC’s ultra-precision polishing and machining services, please refer to “Ultra-Precision Polishing and Machining Services“.
To learn more about the specific polishing processes carried out at TDC, see “Precision Polishing Service“.

What Is Polishing?

Polishing is a process that removes surface irregularities from a product by using abrasive grains—tiny, hard particles.

There are various types and materials of abrasive grains used in polishing. By selecting the appropriate abrasives and processing methods to suit the target material, it is possible to achieve surface adjustments at the micrometer (μm) or nanometer (nm) scale. For this reason, polishing is often used as a final finishing step in product manufacturing.

The Difference Between Polishing and Grinding

While both polishing and grinding involve removing unwanted material from a product’s surface, they are technically distinct processes.

Grinding is commonly used to ensure dimensional accuracy and is well-suited for applications such as shape refinement and flatness correction. Polishing, on the other hand, is positioned as an ultra-precision process that aims for even higher levels of accuracy. It is employed to achieve exceptional surface smoothness and to eliminate extremely fine irregularities—often at the nanometer scale. Polishing also plays a vital role in enhancing the overall quality and finish of a product.

Selecting the most appropriate method based on the specifications and required precision is essential for achieving optimal results in any manufacturing process.

At TDC, we combine polishing, grinding, and cutting processes as needed to deliver ultra-precision finishing services, specializing in surface roughness levels as fine as Ra 1 nanometer.

Types of Polishing Processes

Polishing processes encompass a wide range of methods and techniques, selected according to specific objectives such as dimensional accuracy, shape, and surface finish requirements.

Everyday Examples of Polishing

Polishing is not limited to industrial settings or specialized machinery.
There are also many familiar forms of polishing found in everyday life.

Whetstone Polishing (Manual)

Polishing with whetstones is also commonly found in everyday life.
A familiar example is sharpening kitchen knives using a whetstone. While high precision is generally not required, the choice of whetstone and the sharpening technique may vary depending on the item being worked on.

Sandpaper Polishing

Anyone who has done DIY or craft projects has likely used sandpaper at some point.
Though simple, sanding with sandpaper is a legitimate and widely used method of polishing.

Lens Polishing

In the manufacturing of camera and microscope lenses, the final stage often involves manual polishing to complete the surface.
This is another example of polishing that we can relate to in everyday life.

For more detailed information on different polishing methods, please refer to the “Types of Polishing Processes” page.

What Are Abrasive Grains?

Abrasive grains are hard particles used in polishing and grinding processes. They serve as the actual “cutting edges” that remove material from a workpiece. Unlike conventional cutting tools, abrasive grains continuously renew their cutting edges through microscopic fracturing during use.

Abrasives are broadly classified into two categories: conventional abrasives and superabrasives. Conventional abrasives can be further divided into alumina-based and silicon carbide-based types.

Alumina abrasives (also known as alumina or aluminum oxide) are commonly used for materials such as steel and hardened steel, while silicon carbide abrasives (carborundum) are suitable for stone and cast iron.

Below are examples of typical abrasive grains.

• A (Brown Fused Alumina) – Also known as standard A-type alumina abrasive. Commonly used for general steel materials and off-hand grinding. Compared to C-type abrasives, it is softer and sharper, though less friable.
• WA (White Fused Alumina) – A high-purity alumina grain suitable for general-purpose grinding wheels used on ferrous materials. Applied to hardened steel, alloy steel, tool steel, stainless steel, and similar materials.
• HA (Monocrystalline Alumina) – A single-crystal alumina grain with low friability, making it ideal for precision grinding. Typically used for hardened steels, alloy steels, tool steels, and stainless steels requiring fine finishing.
• PA (Pink Fused Alumina) – Offers greater toughness than WA and provides superior shape retention. Suitable for grinding alloy steels, special steels, tool steels, and gears.
• C (Black Silicon Carbide – Known as standard C-type abrasive, used for non-ferrous metals and cast iron. It is harder and more friable than A-type abrasives.
• GC (Green Silicon Carbide) – A high-purity form of silicon carbide. Commonly used for cemented carbide, hard alloys, and special cast irons.

Steps in the Polishing Process

The polishing process is generally divided into three main stages: rough polishing, intermediate polishing, and final finishing.

Rough polishing uses relatively coarse abrasive grains to remove larger surface irregularities.
Intermediate polishing follows, using finer abrasives to further smooth the surface. These two stages are responsible for removing most of the material and shaping the overall form.

In the final finishing stage, even finer abrasives are applied to the already-leveled surface to produce a smooth, lustrous finish and to make fine adjustments to the precision.
In mirror finishing, the material removal is minimal, as the surface is polished gently and gradually. However, this step is critical for achieving high precision and surface quality.

1. Base Preparation

In the base preparation stage, coarse-grit grinding stones are used to remove major surface irregularities and foreign matter. This is a critical step in the polishing process, as it significantly affects the final finish.

Grinding stones used in this stage are selected for their coarseness. Abrasive grains are graded by numbers—the smaller the number, the coarser the grain, and the rougher the stone.

Rather than focusing on detailed surface leveling, this stage involves more aggressive material removal to shape the base.

2. Smoothing

This stage involves using finer-grit stones than those used in the base preparation step to further smooth the surface from which major irregularities and debris have already been removed.

If the surface has been properly leveled during the base preparation, this step is generally not very difficult.

Since higher-numbered, finer-grit stones are used in this stage, the surface is typically close to flat by the time the smoothing process is complete.

3. Polishing for Gloss

This step adds gloss to the already-leveled surface, enhancing the visual appearance of the product.

While the base preparation and smoothing stages focus on flattening the surface, this stage aims to produce a shine while also removing any remaining surface residue. Finer-grit stones are selected here—finer than those used in the smoothing stage.

In some cases, finishing methods such as vibration polishing or hairline finishing are intentionally used to create patterned surface textures rather than a glossy finish.

4. Mirror Finishing

As the name suggests, this is the final polishing stage in which the product surface is brought to a mirror-like gloss.

Not only is a higher-numbered, finer-grit abrasive used for careful polishing, but the finishing quality can also be improved by gradually increasing the grit size in stages.

In some cases, a buffing process may be used, in which a polishing compound is applied to a soft material such as cloth to achieve a smooth, reflective surface.

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