Complete Guide to Part Polishing: From Process Principles to Application Effects, Comprehensive Analysis of CNC Metal Polishing Technology

What is part polishing?

Part polishing is an indispensable surface treatment process in mechanical manufacturing. Its core objective is to reduce the roughness of the part surface through mechanical, chemical, and electrochemical methods, eliminate tool marks, scratches or oxide layers, and achieve a bright, smooth, and aesthetically pleasing surface effect.
In modern industrial manufacturing, especially in areas such as CNC processing of parts, mold parts, automotive parts, and aerospace components, the polishing process is not only about “beautifying the appearance”, but also directly affects the functional performance of the products, such as wear resistance, sealing performance, friction coefficient, and corrosion resistance.

Common methods of part polishing include:

Mechanical Polishing
Chemical Polishing
Electrochemical Polishing
Ultrasonic polishing, magnetic polishing, vibration polishing, etc.

The core principle of polishing

From a physical principle perspective, polishing is achieved through grinding and plastic deformation, where the surface material is removed or reshaped, thereby reducing the surface roughness.
In mechanical polishing, the microscopic protrusions are removed through repeated friction between abrasive materials and the workpiece surface; while in chemical or electrochemical polishing, the high points are selectively dissolved through chemical reactions to achieve a smooth surface effect.

Surface roughness (Ra) is a key indicator of polishing quality. Generally speaking:

The core objective of part polishing

Its core objectives include:

Enhance appearance: Achieve a shiny and flawless surface to meet consumers’ aesthetic demands.

Improve performance:

Reduce frictional resistance: Smooth surfaces can significantly lower the friction coefficient between moving parts, enhance mechanical efficiency, and reduce energy loss.
Enhance corrosion resistance: Rough surfaces are more prone to accumulate moisture, corrosive media, and contaminants, becoming the starting point of corrosion. Polishing can eliminate these microscopic defects, greatly extending the lifespan of the parts.
Improve cleanliness and hygiene standards: For medical devices, food, and pharmaceutical equipment, highly polished surfaces are less likely to breed bacteria and are easier to clean and disinfect.
Optimize sealing performance: On the mating surfaces of sealing components, high gloss can ensure better sealing contact and prevent leakage.

Eliminating micro-defects: Removing burrs, knife marks, micro-cracks, etc. that occur during the processing, to prevent them from becoming stress concentration points and causing premature failure of the parts.

Prepare for subsequent processing: such as electroplating, spraying, anodizing, etc. A smooth and flat base is the prerequisite for obtaining high-quality coatings.

Key indicators: Understanding of surface roughness (Ra, Rz, Rmax)

To quantify the polishing effect, we must introduce the concept of surface roughness. It describes the microscopic unevenness of the surface through a series of parameters.

Arithmetic Mean Deviation Ra: The most commonly used parameter, representing the arithmetic mean of the absolute values of the contour offset within the sampling length. The smaller the Ra value, the smoother the surface. For example, the Ra value for ordinary milling might be 3.2 μm, while mirror polishing can reach below 0.025 μm.

Microscopic roughness ten-point height Rz: The average value of the sum of the five largest peak heights and the five largest valley depths within the sampling length. It can better reflect the extreme conditions of the surface.

Maximum contour height Rmax: The vertical distance between the highest point and the lowest point of the contour within the sampling length.

Understanding these parameters forms the basis for formulating polishing processes and establishing acceptance criteria.

Common methods and characteristics of part polishing

Mechanical Polishing

Mechanical polishing is the most traditional and widely used process. It achieves surface refinement through the combination of tools such as sandpaper, polishing cloth, and grinding wheels with grinding paste.

Advantages:

The process is mature and the cost is low.
The polishing effect is controllable;
Suitable for various metal and plastic parts.

Disadvantages:

High labor intensity;
Not friendly to complex structure parts;
Prone to over-polishing, overheating and deformation.

Chemical Polishing

Chemical polishing utilizes the action of chemical solutions to cause the microscopic protrusions on the metal surface to dissolve first, thereby achieving a smooth surface.

Advantages:

No need for complex equipment;
Suitable for batch processing;
Can handle parts with complex shapes.

Disadvantages:

The composition of the polishing liquid is complex.
The control is very difficult.
This may lead to slight variations in size.

Electrochemical polishing

This is achieved by applying voltage to the electrolyte, with the workpiece serving as the anode. The purpose of achieving surface finish is accomplished through the difference in current distribution.

Advantages:

The effect is excellent and can reach mirror-like standards.
The corrosion resistance has been significantly enhanced;
Remove residual stress.

Disadvantages:

High investment in equipment;
Not applicable to non-conductive materials;
The operation parameters must be strictly adhered to.

Ultrasonic and magnetic polishing

Modern precision processing often employs ultrasonic or magnetic polishing.
Ultrasonic polishing utilizes high-frequency vibrations to drive abrasive particles, which is suitable for micro-molds;
Magnetic polishing, on the other hand, uses a magnetic field to drive magnetic abrasive particles, enabling efficient polishing of complex surfaces.

Differences in polishing techniques for different materials

Practical Guide: General Parts Polishing Process Flow

Taking a typical metal part (such as a stainless steel part) to achieve a mirror finish as an example, the standard process flow is as follows:

Step 1: Preprocessing – The Foundation of Success/Failure

Cleaning and oil removal: Use alkaline or organic solvents to thoroughly remove oil stains, cutting fluids, and fingerprints from the surface. Any remaining residue will affect the subsequent polishing results.

Initial deburring: Use a file, oilstone or abrasive wheel with a coarse grit to remove the large burrs and flashings.

Coarse grinding / Coarse polishing: Use abrasive belts, wheels or papers ranging from 80 to 400 mesh. The aim is to quickly remove machined grooves and reduce the roughness to Ra 1.6 μm or lower. This stage is crucial in determining the overall flatness.

Step 2: Fine Polishing – Towards Glossiness

Medium-fine grinding: Use 600-mesh to 1000-mesh sandpaper or grinding paste for the transition, further refining the surface texture.

Fine polishing: Use a cloth wheel and a hemp wheel in combination with white (for stainless steel) or green (for chrome steel) polishing paste for polishing. The fine abrasive particles (such as chromium oxide, aluminum oxide) and the grease in the polishing paste work together to produce a shiny effect.

Mirror polishing: Use a softer wool wheel, velvet cloth, and diamond polishing paste (such as W1, W0.5) to perform the final ultra-precision polishing to achieve a mirror-like effect (Ra < 0.05μm).

Step 3: Post-processing – Consolidating the Results

Thorough cleaning: Use an ultrasonic cleaning machine or a dedicated cleaning agent to remove all remaining polishing paste and abrasive particles. This is particularly important for medical devices and food components.

Passivation treatment (for stainless steel): Apply passivating solutions such as nitric acid or citric acid to form a layer of chromium-rich inert oxide film on the surface, significantly enhancing corrosion resistance.

Anti-rust and packaging: Apply anti-rust oil or use anti-rust paper for packaging to prevent rusting or scratches during storage and transportation.

The application of part polishing in industry

Mold industry
Mold polishing can directly affect the demolding performance and appearance quality of the products. Mirror polishing can reduce friction and improve transparency.

Automobile parts
The engine hood, decorative pieces, handles, etc. all need to be polished to enhance the texture and reduce air resistance.

Aerospace
Components with extremely high requirements for surface accuracy and strength often need electrochemical or precision polishing treatment.

Electronics and medical devices
Electrolytic polishing can effectively remove microscopic impurities, prevent the growth of bacteria, and improve hygiene standards.

Future Trends and Summary

With the advancement of Industry 4.0 and intelligent manufacturing, the technology of part polishing is also evolving towards automation, intelligence and environmental friendliness.

Robot polishing: Industrial robots equipped with force control sensors are gradually replacing the high-intensity, low-consistency manual polishing, and are particularly suitable for industries such as automobiles and bathroom products.

Digitalization and AI: Obtain part models through 3D scanning, automatically generate polishing paths; utilize machine learning to optimize polishing parameters and achieve adaptive control.

Green and environmental protection: Developing low-pollution and biodegradable polishing solutions and abrasives, as well as efficient waste liquid recycling and treatment technologies, is an inevitable requirement for the sustainable development of the industry.

Conclusion

Part polishing is a profound discipline that integrates materials science, chemistry, physics, and precise mechanical processing. It is not merely the final “beauty” process; rather, it is a quality assurance link that runs through the entire product design, material selection, and manufacturing process. Understanding the principles of different polishing methods, mastering the process flow, and being able to formulate the optimal polishing plan for specific materials and product requirements are the keys to enhancing product value and strengthening market competitiveness. I hope this detailed guide, which is over 5,000 words long, will serve as a valuable map for you in the world of hand polishing, guiding you to conquer each technical peak in surface treatment and ultimately create flawless industrial artworks.

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