What is Mechanical Polishing?

This article is to discuss mechanical polishing, one of the physical surface treatments to bring components a mirror finish.

Overview of Mechanical Polishing

Mechanical polishing is generally performed after the grinding or sanding stage. Its purpose is to further remove minute surface irregularities on a metal workpiece to achieve a mirror finish. It can be used for pre-plating preparation of workpieces or as a finishing operation after plating to give the surface a decorative appearance and enhance the part’s corrosion resistance.

Unlike grinding, the polishing process does not result in significant metal loss. The generally accepted mechanism for buffing is as follows: The high-speed rotation of the buffing wheel generates high temperatures upon friction with the metal. This causes the metal surface to undergo plastic deformation, which smooths out the surface’s peaks and valleys. Simultaneously, the extremely thin oxide layer instantaneously formed on the metal surface due to atmospheric oxidation is repeatedly polished away, resulting in a bright finish. Therefore, the buffing process involves a combination of mechanical abrasion as well as physico-chemical effects.

Image Source: ResearchGate

Mechanical polishing relies on the abrasive and burnishing action of extremely fine polishing powder to remove a very thin layer of metal from the prepared surface. If the surface retains deep grinding marks or scratches from the prior grinding step, it is difficult to remove them using mechanical polishing alone. Attempting to buff out deep scratches by increasing the concentration of the polishing liquid, extending the polishing time, or increasing the applied pressure can lead to metal flow and surface disturbance on the buffed area, thickening the deformation layer, and causing phase pullout in some alloys. This is particularly noticeable in softer materials like aluminum alloys, where surface oxidation is intensified, which can negatively affect subsequent microstructural examination.

Polishing Wheels

The polishing wheel is a common abrasive tool in polishing. It is typically constructed by layering multiple discs of canvas, muslin, felt, or leather. The hardness of the buffing wheel is determined by the sewing density or distance between the stitching lines; a smaller distance results in a harder wheel. Ventilated or airway buffing wheels are also used for polishing large workpieces, as they feature special airflow systems for cooling.

Buffing wheels are classified by material and include common types such as:

• Muslin Buffs (Cotton/Canvas)
• Felt Buffs
• Wool Buffing Pads/Wheels (often for glass, stone, or automotive)
• Sisal Buffs (aggressive cutting)
• Airway Buffs
• Flannel Buffs (softest, for final finishes)

Color classifications are also used to denote specific types, though this can vary by manufacturer. For instance:

• Gray buffing wheels are often general-purpose nylon wheels, mainly used for satin finishing or brushed finish on various metals, including stainless steel, copper, and aluminum alloys, as well as wood. The resulting finish is often bright.
• Red buffing wheels are often used for softer metals like copper and aluminum, or for a brushed finish after electroplating.

Polishing Compounds

Based on the required final surface finish (surface quality), mechanical polishing can be divided into three stages: coarse buffing, intermediate buffing, and finished buffing.

Coarse Buffing: Uses a harder wheel (e.g., Sisal or tightly stitched canvas) on surfaces that have already been ground to a lower surface roughness (Ra). This stage focuses on further reducing the roughness.

Intermediate Buffing: Uses a medium-hard buffing wheel, softer than the cut-down wheel, to further process the rough-buffed surface. It removes the visible scratches left by the cut-down process and produces a medium-luster finish.

Finished Buffing: This is the final operation, using a soft buffing wheel (e.g., loose-stitched muslin or flannel) and a very fine compound to achieve a mirror-bright surface or high-luster finish. The material removal is minimal.

Polishing Compounds

Common polishing compounds, typically in bar or stick form, are often referred to by their color, which usually indicates the primary abrasive and intended application.

Type	Key Components	Use/Properties
Green Rouge	Chromium Oxide, Alumina, Binders	Hard and sharp abrasive with good cutting power. Excellent for final polishing of stainless steel and hard metals. Use sparingly and with low pressure on aluminum.
White Rouge	Calcium Oxide, Magnesium Oxide, Binders	Medium performance, versatile. Ideal for polishing copper/brass, nickel plating, and aluminum. Must be kept airtight as it is sensitive to weathering.
Red Rouge	Iron Oxide, Binders	Medium hardness, relatively coarse structure. Mainly used for the oil-finishing step on steel parts after grinding. Effective for lapping aluminum castings.
Brown Tripoli	Stearic Acid, Refined Iron Oxide/Silica, Binders	Brown color, good polishing performance, low cutting force, and minimal substrate loss. Best for soft metals like gold, silver, zinc, and copper, and for aluminum. Often preferred for parts requiring subsequent electropolishing.

Considerations for Mechanical Polishing

The following points should be noted during mechanical polishing:

Selecting wheel peripheral speed based on workpiece hardness. The harder the workpiece, the higher the required peripheral speed of the buffing wheel. Conversely, softer materials require a lower speed. It is recommended to use 30 to 35 m/s for steel with nickel or chrome plating layer and 22~25 m/s for copper and copper alloys; and lower for soft metals like zinc and silver to prevent burn-through or smearing. If the polishing machine’s rotational speed is fixed, the desired peripheral speed can be adjusted by selecting a buffing wheel with an appropriate diameter, since peripheral speed is directly proportional to the wheel’s diameter.

Selecting a buffing compound based on hardness. Different compounds are suited for different substrates. Always select a compound appropriate for the hardness and material of the base metal.

Buffing wheel maintenance. Buffing wheels should be stored separately after use to prevent contamination by coarse abrasives like emery grit, which can cause noticeable gouges or deep scratches on the polished surface.

Proper use of polishing tools. When buffing slender, thin, framed, or small plated parts, use appropriate fixtures, backing plates, jigs, or clamps to prevent the workpiece from deforming, bending, or being dropped, thereby avoiding both damage to the part and workplace injury.

Post-polishing treatment. Finely buffed surfaces may be finished goods or require further processing, but all must be cleaned. Wipe the surface with a degreased cotton cloth or specialized cotton towels. Never use a damp cloth, as this can cause staining or the formation of rust.

About Getzshape

Getzshape is your go-to partner in achieving unparalleled metal component finish. We deliver precision finished metal components using industry-leading processes, including mechanical polishing for high-luster surfaces, electropolishing for passive and hygienic finishes, and controlled chemical polishing for complex geometries. Contact us to start your projects.