Metal Surface Finishing Tips

Black oxide - a simple low cost finish for steel

Surface finishes are often an important aspect in designing custom parts. Surface finishes can be used to protect your part, make your part more attractive, change the friction of the surface, reflect light, absorb light, etc. Etc.

Also see our surface finish capabilities for more tips and guideliines.

Decorative Finishes 

Custom parts can be categorized as functional or decorative. Functional parts are typically used inside a device or in an application where appearance is less important. Decorative parts are those where appearance is more important. For example a maker of a cell phone would consider the phone case as decorative but an internal bracket for a circuit board as functional. If you need a decorative finish for a part consider these tips:
  • Metal Brushing: Creates a texture that hides imperfections (on sheet metal parts).
  • Buff Polishing: Provides a glossy finish on surfaces accessible by the polishing wheel and slightly rounded edges. For a longer lasting polished finish and/or for the brightest appearance also add chrome or bright nickel plating - otherwise you may need to brighten up the finish periodically with polishing cloths. For 2D parts cuts from sheet material we recommend specifying waterjet. For 3D parts we recommend to specify a surface and edge finish of Ra 32 or 16.
  • Electroplating: Provides various metallic colored finishes - commonly: nickel, zinc, chrome.
  • Anodizing: Provides black, clear and colored finishes on aluminum.
  • Powder Coating: Provides a durable color layer - generally preferred to painting.
  • For Plastics, injection molding generally produces a better appearance than milling.
  • For plastics Acrylic is best for polishing.
  • Milling creates a surface which may have complex patterns from the cutting process. For best appearance apply a mechanical finish. For flat parts brushing can be used. Or finish with powder coat. For non-flat parts this would usually be polishing. Prior to buff polishing we recommend to machine to Ra 32 or 16.

Decorative finishes create an attractive look on parts and can protect the surface at the same time. 

Compensating for finish thickness 

Adding a surface finish can change the size of a part slightly. In some designs it is important to consider this when specifying dimensions. Keep the following in mind: 

  • Mechanical part dimensions apply after non-organic finishes (anodize, electroplate,mechanical etc.) but before organic coatings (powder coat, etc.). For example to make a powder coated one-inch cube your CAD design would need to be a few thousandth's of an inch under one inch in size. But to make an anodized one-inch cube your CAD design should be exactly one-inch.
  • You might also want to specify whether threaded holes should be plugged to prevent finish.
  • In highly critical cases you might want to specify a tolerance on finish thickness, for example .002" +/- .0005".

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If you need custom mechanical parts machined - eMachineShop can help.
Simply download our free CAD software, design your part, get an instant price and click to order!

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