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Why Grinding, Polishing, and Brushing are Necessary After CNC Machining of Aluminum Alloys? A Comprehensive Analysis of These Three Pre-treatment Processes

I. Surface Treatment Determines the Product's "Appearance" and "Lifespan"

Aluminum alloys are easy to machine and offer a variety of surface treatment options, making them widely used in consumer electronics, automobiles, and other fields.

CNC-machined aluminum alloy workpieces often retain tool marks, burrs, and minor scratches on their surfaces. Grinding, polishing, and brushing are the three key pre-treatment processes that solve these problems.

II. Grinding – The Foundation for Deburring and Smoothing the Surface

Grinding is the first surface treatment process after CNC machining, primarily addressing: Removing residual burrs and flash; Eliminating tool marks and localized unevenness; Providing a smooth base for subsequent polishing or brushing. Grinding is divided into coarse grinding (quickly removing excess material) and fine grinding (eliminating rough marks). Common tools include belt sanders and grinding heads.

III. Polishing – An Advanced Process from Matte to Mirror Finish

Polishing reduces surface roughness through mechanical friction or chemical methods, resulting in a bright and smooth surface. There are three main types of polishing: Mechanical polishing: uses polishing wheels and abrasives, suitable for most aluminum parts; Chemical polishing: dissolves the surface layer with chemical reagents to achieve a uniform gloss; Electrolytic polishing: an electrochemical method, achieving a near-stainless steel mirror finish. The surface roughness of polished aluminum parts can be as low as Ra 0.1μm or less, suitable for medical devices, optical instruments, high-end digital casings, etc.

IV. Brushed Finish – A Decorative Process that Gives Metal a Delicate Texture

Brushed finish is a manufacturing process where lines are repeatedly scratched onto the surface of an aluminum plate with sandpaper. Common types include: Straight-line brushed finish: parallel lines, simple and clean; Random-line brushed finish: irregular texture, strong sense of layering; Swirl-line brushed finish: circular texture, suitable for round workpieces. Brushed finish effectively conceals minor scratches, creating a unique matte metallic texture that combines fashion and technology. Commonly used in laptop casings, mobile phone panels, and appliance panels.

V. How to Choose Between the Three Processes? Achieving a smooth, burr-free finish → Grinding

Achieving a glossy, mirror-like surface → Polishing

Achieving a textured finish → Brushing

Achieving a comprehensive effect → Grinding + Polishing + Brushing + Anodizing combination

VI. Conclusion Grinding, polishing, and brushing each have their own emphasis, and are often used in combination in actual production. The choice of process depends on the product's functional positioning and appearance requirements.