Surface finish symbols communicate the required condition of a manufactured surface. For mechanical designers, understanding surface roughness is important because finish can affect friction, sealing, wear, appearance and manufacturing cost.
What Is Surface Roughness?
Surface roughness describes the small-scale irregularities on a manufactured surface. One commonly used parameter is Ra, the arithmetic average roughness.
Why Surface Finish Matters
A bearing seat, sealing surface and decorative cover do not necessarily need the same finish. The correct requirement should be driven by function.
How Finish Is Achieved
Machining processes such as turning, milling, grinding and polishing can produce different surface conditions. Tool geometry, cutting parameters, material and machine condition also influence the final surface.
Do Not Over-Specify
Specifying an unnecessarily fine surface finish can increase manufacturing time and cost. Use a tighter finish only where the application requires it.
Drawing Communication
Surface requirements should be placed clearly on the engineering drawing using the applicable standardized symbol and roughness value. Company standards and the drawing standard in use should be followed.
Surface Finish and Tolerance
Surface finish and dimensional tolerance control different characteristics. A component can have a precise dimension but an unsuitable surface finish, or a good finish but an incorrect dimension.
Design Engineer Checklist
- Is the surface function defined?
- Is the roughness requirement necessary?
- Can the selected manufacturing process achieve it?
- Is the symbol clear on the drawing?
- Have critical surfaces been identified?
Good engineering drawing surface finish requirements are specific enough for manufacturing but not unnecessarily restrictive.