Stainless Steel Polishing - Mechanical Polishing

Mechanical polishing covers grinding, polishing and buffing - all processes for improving the surface conditions of a product for decorative or functional purposes. The various techniques are abrading operations although some plastic working of the surfaces may occur, particularly during buffing.

The process is to do grinding first, polishing second, and buffing third. In general, grinding permits far more aggressive abrading action than polishing. Likewise, polishing is a far more aggressive abrading action than buffing.

In grinding, polishing and buffing, labor is a major variable in the process. The requirement if for highly skilled labor with years of experience and a thorough knowledge of the art of their craft.

The basic mill plate and sheet metal finishes for stainless steel include five grades that have finishes that are produced mechanically by using abrasive compositions and buffing wheels. There is also available on the market what is generically known as 'non-directional No. 8."

Special mechanical polishing procedures are required for preparing metal surfaces, such as stainless steel, for electropolishing. Please look at the comparison table and compare the various polished mill finishes against, the abrasive grit number used to produce them, and the surface roughness readings before and after electropolishing.

Comparison Table

Polishing

Polishing is an intermediate step used to improve the surface finish from the grinding step, such as found on a common household stainless steel sink - lustrous, but not mirror-like. Polishing uses abrasives firmly attached to a flexible backing, such as a wheel, belt or orbital motion tool.

Mechanical stainless steel polishing is an abrading operation used to remove or smooth grinding lines, scratches, pits, mold marks, parting lines, tool marks, stretcher strains, and surface defects that adversely affect the appearance or function of the part. The process causes some plastic working of the surface as metal is removed. A mechanically polished surface yields an abundance of scratches, strains, metal debris and embedded abrasives, and always distorts the metal surface. Burnishing metal by lapping or buffing decreases the micro-inch roughness and improves the image-defining quality of a surface, but it never completely removes the debris and damage metal caused by mechanical polishing.

Mechanical Polishing vs. Electropolishing

Mechanical polishing and buffing cannot be viewed as an adequate substitute for electropolishing in most applications due to the embedded abrasives and compounds, exposed grain structure of the metal, and the lack of the non-particulating, non-contaminating, and non-outgassing characteristics of an electropolished surface.

Key point: Any mechanically produced surface (ground, polished, buffed, lapped, honed, etc.) produces a work hardened, disturbed and damaged grain layer approximately 0.001 inch thick. This is accompanied by a scratched, sheared, and torn surface contaminated with embedded abrasives and compounds. Its importance depends on its application. From a cosmetic appearance standpoint, these effects may be unimportant. For high purity applications, e.g., semiconductor or pharmaceutical, particles and contaminants can be of utmost importance affecting purity of product and yield. Often it takes a SEM photomicrograph at very, very high magnification to show these effects - but it is there. A desirable alternative to be considered is electropolishing.

Basic Mill Plate Finishes for Stainless Steel

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