Polishing (metalworking)

"Buffing" redirects here. For nail polishing, see Nail buffing. For other uses, see Buff.

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An abrasive procedure to achieve smooth surface finishes.

A tarnished and unpolished silver bowl (on the left) showcases a noticeable difference in cleanliness and color compared to its polished counterpart (on the right). The polished piece held above the bowls is also evident. (These items are part of an 18th-century silver epergne from the Indianapolis Museum of Art).

Understanding Buffing and Polishing

Buffing and polishing are essential finishing processes utilized to smoothen a workpiece’s surface. Buffing typically utilizes loose abrasives on a work wheel or a leather strop, whereas polishing refers to the process involving abrasives that are firmly glued to the wheel.

Polishing is often perceived as a more aggressive method compared to buffing, resulting in a smoother and shinier surface. A common misbelief is that a polished surface always achieves a mirror-like finish; in reality, most mirror finishes are actually produced through buffing.

Polishing serves various purposes including enhancing the aesthetic appeal of items, preventing contamination, removing oxidation, generating reflective surfaces, and limiting corrosion, especially in pipes. In fields like metallography and metallurgy, polishing creates a flat and defect-free surface, facilitating examination of a metal's microstructure through microscopy. For this, silicon-based polishing pads or diamond solutions might be utilized. Additionally, polishing stainless steel can improve its sanitary properties.

To eliminate oxidation (tarnish) from metal items, a metal polish or tarnish remover is employed. Post-polishing treatment typically involves applying a coat of wax, oil, or lacquer to deter future oxidation—a major consideration for copper alloy products such as brass and bronze.

Although less frequently used than conventional mechanical polishing, electropolishing presents an innovative alternative that employs electrochemical principles to remove microscopic metal layers from the surface.

The Polishing Process

The type of abrasive used varies based on the material's condition. The process initiates with a coarser abrasive (possibly 60 or 80 grit) for unfinished materials, transitioning to finer abrasives—120, 180, 220/240, 320, and up—until the desired finish is reached. Coarser grits remedy imperfections in the metal surface like nicks and scratches, while finer abrasives impart subtler lines that are not visible to the naked eye. A no. 8 ("mirror") finish necessitates polishing and buffing compounds, employing high-speed polishing machines or electric drills. Utilizing lubricants like wax and kerosene is common, although some materials are tailored for dry use. Buffing can be achieved manually with a stationary polisher or die grinder, or automatically with specialized equipment.

Buffing Techniques

Within buffing, two techniques are prevalent: the cut motion and the color motion. The cut motion aims to deliver a uniform, smooth surface finish by positioning the workpiece against the buffing wheel’s rotation with moderate to firm pressure. In contrast, the color motion seeks to create a clean, shiny surface by aligning the workpiece with the wheel’s rotation, applying light to medium pressure.

When it comes to polishing brass, which is a softer metal, marks caused by impurities can often be observed. To eliminate these finer marks, the surface is polished using a very fine 600-grit to achieve a mirror finish.

Equipment Overview

High tensile strength metals, such as carbon and alloy steels, usually employ white and gray aluminum oxide abrasives. Gray silicon carbide abrasives suit hard and brittle materials like gray iron and cement carbide. Low tensile strength metals tend to benefit from green chromium(III) oxide abrasives, particularly for finished ferrous metals.

Polishing wheels serve diverse needs, made from materials like wood, leather, canvas, cotton cloth, plastic, felt, paper, sheepskin, impregnated rubber, and wool. Commonly, leather and canvas remain prevalent. Wooden wheels are typically enhanced with emery or other abrasives for polishing flat surfaces and maintaining sharp edges. Cloth wheels come in various types, hard cemented wheels intended for rough work, while softer, less aggressive pads cater for fine finishes.

Buffing wheels, often made from cotton or wool, can be bleached or unbleached. Many specific types exist including sisal, spiral sewn, loose cotton, and more, each tailored for particular polishing applications.

Applications of Polishing

Polishing can revitalize and enhance the visual aesthetics of components found in vehicles, handrails, cookware, kitchenware, and architectural metals. Various industry applications, such as pharmaceutical, dairy, and specialized plumbing, utilize buffing to minimize corrosion and eradicate areas that could harbor bacteria or mold.

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