Surface roughness affects friction, sealing, wear, coating adhesion, fatigue behavior and appearance. A lower Ra value is not automatically better: it may require slower machining, grinding, lapping or polishing while still failing to describe waviness, lay direction or a local defect. The drawing should specify the characteristic that protects the function and how it will be measured.
Start with the functional surface
Classify the surface as a sliding interface, bearing seat, sealing face, cosmetic area, coating substrate, electrical contact or non-contact clearance surface. Each function responds differently to peaks, valleys and direction. Two surfaces with the same Ra can behave differently when their lay, waviness or process marks differ.
Use Ra as one parameter, not the whole requirement
Ra is an arithmetic average and can hide isolated peaks or valleys. Some applications may need additional profile parameters, a maximum value, a visual standard or a process-specific note. If a customer standard controls the surface, reference its current revision and avoid replacing it with a single generic number.
Typical roughness ranges found in manufacturing guides can help start a discussion, but they are not universal acceptance limits. For example, a machined non-contact surface may permit a broader finish than a bearing or sealing interface. The material, part geometry, process, measurement cutoff and application must be reviewed before a value is released.
Specify lay and measurement conditions
Lay direction can influence sliding, sealing and appearance. Show the required direction when it matters. Define the measurement location, instrument type, cutoff/filter, evaluation length and whether the result is taken before or after coating. Curved, narrow or interrupted surfaces may require a different method from a broad flat face.
Account for post-processing
Blasting, tumbling, anodizing, plating, coating, heat treatment and polishing can change the final texture or obscure the machined surface. State whether the drawing value applies before or after the finish. Coordinate dimensional allowance and masking with the final surface requirement.
Avoid over-finishing
Do not request a mirror appearance or very low roughness without a functional reason and an agreed inspection method. Additional polishing can alter edges and geometry as well as cost. Cosmetic acceptance may require a sample or visual limit rather than only an Ra value.
Include the surface function, location, material, final finish and measurement expectation in the RFQ. If a report is needed, define its format using the quality documents guide.