Surface finish symbols on engineering drawings communicate the required texture or roughness of a manufactured surface. Correct surface finish selection can affect friction, sealing, wear, appearance and manufacturing cost.
What is Ra?
Ra is the arithmetic average roughness parameter commonly used to describe surface texture. It is normally expressed in micrometres (µm). The required value should be selected according to the function of the surface.
Where surface finish matters
- Bearing and sliding surfaces
- Sealing surfaces
- Precision locating features
- Decorative surfaces
- Fatigue-sensitive components
Do not over-specify surface finish
A very fine finish can require additional machining, polishing or grinding. If a rougher surface performs the function adequately, specifying an unnecessarily low Ra can increase cost without improving the product.
How to choose a practical finish
Start with the function: Does the surface slide, seal, locate, mate, or simply provide appearance? Then select a manufacturing process capable of achieving the requirement consistently.
Drawing checklist
- Use the project drafting standard.
- Place the symbol where the requirement applies.
- Specify Ra or the required parameter clearly.
- Identify special machining or finishing operations when necessary.
- Coordinate finish requirements with material and coating specifications.
Machining and inspection
Surface roughness is measured using appropriate instruments and procedures. The inspection method should match the drawing requirement and applicable standard.
Knowing surface finish symbols on engineering drawings helps mechanical engineers communicate functional requirements without creating unnecessary manufacturing cost.