Polishing

Polishing improves surface finish and appearance by mechanically or electrochemically smoothing peaks, enabling low Ra values, better cleanability, and reduced friction.

Overview

Polishing is a surface finishing process that smooths a part by removing microscopic high spots using abrasives (manual polishing), controlled material removal under load (lapping), or electrochemical dissolution (electropolishing). The goal is lower roughness (Ra), higher reflectivity, reduced friction, and improved cleanability.

Choose polishing when surface finish drives function or aesthetics: sealing faces, optical/flow surfaces, hygienic equipment, or cosmetic metal parts. It’s often the last step after machining, grinding, or forming.

Tradeoffs are cost and process sensitivity. Polishing adds labor time, can round edges, soften crisp features, and slightly change dimensions—especially on small features. Electropolishing favors conductive metals and can improve corrosion resistance, but it won’t remove deep tool marks without prior prep. Lapping achieves very flat, very smooth surfaces but is slow and typically limited to simple, accessible faces.

Common Materials

  • Stainless Steel 316L
  • Stainless Steel 304
  • Aluminum 6061
  • Titanium Grade 2
  • Brass C360
  • Tool Steel A2

Tolerances

±0.0002" to ±0.001" on critical lapped/polished surfaces (geometry dependent)

Applications

  • Sanitary valve bodies and fittings
  • Hydraulic piston rods
  • Injection mold cavities
  • Optical flats and reference plates
  • Mechanical seal faces
  • Medical device housings

When to Choose Polishing

Polishing fits when a defined Ra, visual finish, or low-friction/contact surface is a primary requirement and the part has accessible surfaces for consistent finishing. It works best when the starting surface is already close (fine machining or grinding) and only small stock removal is needed. Typical volumes range from prototypes to production, with repeatability improving when the finish spec is quantified (Ra, gloss, directionality).

vs Machined Surface Finishing

Choose polishing when the as-machined tool marks are unacceptable for sealing, flow, cleanability, or cosmetic requirements and you need a lower Ra than machining can reliably hold. Polishing can remove lay and micro-peaks, but it may slightly alter edges and dimensions, so define where finish matters.

vs Coatings

Choose polishing when you need the base material itself to be smooth (fit, sealing, cleanability, optical reflectivity) rather than adding a layer. Polishing avoids coating thickness variation and adhesion risks, but it won’t add hardness or barrier performance beyond what the substrate provides.

vs Painting

Choose polishing when you want a bare-metal cosmetic finish, low particle shedding, or a surface that must remain dimensionally controlled without added thickness. Painting hides cosmetic defects and provides color, but it can chip and adds thickness that may interfere with fits or sealing surfaces.

vs Passivation

Choose polishing when roughness, reflectivity, or friction is the driver; passivation primarily improves corrosion resistance by enhancing the passive layer without materially changing surface texture. Polishing is often done before passivation when a smooth stainless surface is required for cleaning or aesthetics.

vs Hard Coatings

Choose polishing when the surface needs to be smooth for contact mechanics or appearance before any wear layer is considered. Hard coatings add wear resistance but typically follow a defined pre-finish; polishing can reduce asperities that would otherwise print through or concentrate stress under load.

Design Considerations

  • Call out finish with a measurable spec (Ra/Rz, lay direction, gloss) and identify the exact surfaces that require it
  • Allow stock or process allowance on critical faces if lapping/polishing must hit a final dimension
  • Avoid sharp edges and delicate ribs on polished areas; add small edge breaks to control rounding and reduce rework
  • Keep polished surfaces accessible; deep pockets, narrow slots, and undercuts drive manual labor and variability
  • Specify acceptable cosmetic limits (scratch direction, max scratch depth, no. of defects per area) for visible parts
  • Provide starting condition expectations (e.g., “pre-grind to Ra X”) when very low Ra or mirror finish is required