Precision Hardware Finishing Specifications and Their Hidden Cost Implications

When designing a custom precision machined part, the engineer must define all the characteristics of the part and communicate them effectively to the machinist or manufacturer. This is usually done by creating and transferring drawings that contain numerical dimensions that define the size and location of each feature of the part. It also conveys other important part properties, including material type, hardness, allowable dimensional deviations, coatings, manufacturing methods and other industrial manufacturing processes.

A more important requirement that the drawings also define is the surface finish or 'permissible roughness' of the many surfaces of the precision metal-machined part. Surface finish can have a significant impact on a part's ability to perform its intended function. Surfaces that need to be sealed or contain gases or liquids must be defined in a way that allows them to seal to a specified level. Surfaces with relative motion between them may require low or high friction to function as intended. Even if the precision metal surface finish has no effect on the function of the part, it may simply need to be improved during the manufacturing process.

Figure (a) above is an example of the drawing symbols used on the diagram, and Figure (b) shows what each number means and where to place the symbols.

Roughness Average (Ra) is the most common parameter engineers use to define the surface finish of a part. Ra provides the arithmetic mean of surface irregularities measured from the mean line lying between the highest and lowest points.

Precision metal machining part surfaces can be used in many different manufacturing processes to produce a textured surface on the part. They are handy tools that allow engineers to feel the roughness of a surface at the most commonly specified Ra levels. This is a comparator for many common metalworking processes in use today.

If a fabrication shop is forced to use a lot of finishing cuts, surface grinding, polishing and grinding to meet a specified surface finish, it can double or triple the cost of producing a part. Point faces, grooves, bumps, and raised bosses can be added to product designs to minimize the size of surface areas that require extensive surface preparation and inspection.

Producing very smooth and highly polished surfaces over large areas can be an extremely expensive manual process. This is usually done with a fine abrasive compound, paper or tape, where the material removal rate is very slow compared to major machining processes that use cutting tools to remove large amounts of material. If manual grinding and polishing is required, one way to reduce the cost of parts containing highly polished sealing surfaces is to dimensionally define and minimize the surface area that requires additional manual work. When specifying a surface finish, the engineer must analyze and understand the downstream time and cost implications of the roughness average (Ra) being called up on the drawing.

Our well-equipped, state-of-the-art manufacturing facilities allow us to produce to tight dimensional tolerances.

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