How is surface roughness measured and specified?
How Is Surface Roughness Measured and Specified?
What Is Surface Roughness?
Surface roughness is specified on engineering drawings with parameters such as Ra, Rz, or Rt and measured with a contact or optical profilometer over a defined sampling length. The value must name the surface, parameter, unit, measurement direction when needed, and any process or acceptance condition that affects function. A note such as Ra 1.6 µm is useful for a machined sealing land or sliding face, but it is incomplete if the drawing does not say which area is controlled and whether burrs, lay direction, coating, polishing, or post-machining treatment are included. Buyers should specify roughness only where it affects sealing, wear, friction, appearance, cleaning, fatigue, or assembly performance, because unnecessary low Ra values add machining and inspection cost.
Common Measurement Standards and Parameters
Ra, or roughness average, is the most common surface roughness parameter for CNC machined parts. Ra is expressed in micrometers (µm) or microinches (µin) and represents the arithmetic average deviation of the measured profile from the mean line across the evaluation length. Ra is easy to communicate, but Ra alone can hide isolated scratches, torn material, chatter marks, or high peaks that affect seals and sliding parts. For critical surfaces, the drawing should reference the applicable surface texture standard used by the customer or supplier, such as ASME or ISO surface texture practice, and define whether Ra, Rz, Rt, lay, waviness, or a process note controls acceptance.
Other parameters include:
Rz: Average peak-to-valley height over defined sampling lengths. Rz is more sensitive to local peaks and valleys than Ra, so it can be more useful for sealing faces, sliding contact, and surfaces where a single deep valley may hold fluid or start leakage.
Rt: Total height of the roughness profile from the highest peak to the deepest valley within the evaluation length. Rt is stricter than Ra for isolated defects and should be used carefully because one scratch, dent, or handling mark may reject a surface even when average roughness appears acceptable.
Typical surface roughness values should be treated as early RFQ guidance, not as universal guarantees:
Surface Finish Type | Ra (µm) | Application Example |
|---|---|---|
As-machined (standard) | 3.2–6.3, depending on toolpath, feed, tool condition, material, and whether the surface is milled, turned, drilled, or bored. | General CNC parts, brackets, covers, clearance faces, and non-sealing surfaces where visible tool marks are acceptable. |
Semi-finish | 1.6–3.2 when a finish pass, lower feed, and controlled tool wear are planned for the specified surface. | Light-duty mechanical surfaces, locating pads, cosmetic machined faces, and components that need cleaner contact without precision grinding. |
Fine machining | 0.8–1.6 when geometry is accessible, vibration is controlled, and the feature can be inspected in the required direction. | Functional precision components, sliding faces, seal-adjacent surfaces, and visible components where tool marks must be reduced. |
Precision grinding | 0.2–0.8 for suitable flat, cylindrical, or controlled surfaces after machining leaves enough stock for grinding. | Bearing seats, sliding components, precision shafts, and datum surfaces that need tighter texture and dimensional stability together. |
Polishing/electropolishing | Below 0.2 may be possible on suitable materials and geometry, but edge rounding, dimensional change, and process validation must be considered. | Sealing surfaces, cleanability-critical parts, medical-style components, optical-adjacent hardware, and corrosion-sensitive stainless surfaces. |
Surface finish is controlled by process choice as much as by the number on the drawing. A CNC milled surface may meet Ra 3.2 µm directly, a turned shaft may need a dedicated finishing pass, and a hardened bearing surface may need grinding. Stainless parts that need cleaner corrosion behavior may be reviewed for electropolishing, while wear or appearance goals may lead to coatings such as PVD coatings. The buyer should confirm whether the roughness requirement applies before or after finishing, because polishing, coating, anodizing, or electropolishing can change edges, dimensions, and measured texture.
How Surface Roughness Is Measured
Surface roughness is typically measured using:
Contact Profilometers: A stylus traces the selected surface and records profile deviations across a defined sampling length. This method is common for machined metals, but the stylus tip radius, measurement direction, filter setting, access angle, and surface curvature can change the reported value.
Optical Profilometers: Non-contact methods use laser, confocal, or white-light measurement to evaluate texture without dragging a stylus across the part. Optical methods help with delicate, very smooth, coated, or small surfaces, but reflective finish, steep slopes, transparent films, and part cleanliness must be controlled.
CMM with Surface Probes: Surface texture probes can combine dimensional and texture checks in one inspection plan when access and probe type are suitable. A normal CMM dimensional scan should not be assumed to verify Ra unless the machine, probe, software, and method are qualified for surface texture measurement.
Specifying Surface Roughness in Drawings
Roughness should be specified on the exact controlled surface, not only as a general note for the whole part. A drawing note such as Ra 1.6 µm should state the unit, controlled area, standard or company practice, and whether the value applies after deburring, heat treatment, anodizing, coating, polishing, or cleaning. Directionality may matter when the cutting lay crosses a seal path, bearing motion, fluid path, or sliding contact. If the part has multiple functions, assign different roughness levels: for example, keep a visible cover face at an as-machined finish, control a gasket land more tightly, and leave hidden clearance faces at a normal machining finish. That choice gives the supplier room to quote the part without polishing every surface unnecessarily.
Quick Surface Finish Selection Guide
Neway reviews surface finishing services by matching the drawing requirement to the functional surface, process route, and inspection method. Use as-machined finish for non-critical surfaces, polishing when visible tool marks or contact friction must be reduced, anodizing when aluminum corrosion resistance or appearance matters, and powder coating when coating thickness and coverage are acceptable for the part geometry. Before approving a finish, confirm whether the process can affect bore size, thread fit, edge radius, masking needs, or final measurement results.
Explore surface treatment options after the roughness target is tied to a function. Send the surface finish symbols, drawing revision, material grade, post-process sequence, sample approval needs, and inspection report requirements with the RFQ. If a very low Ra value is requested only for appearance, ask whether brushing, bead blasting, polishing, or coating would solve the visual requirement more economically than machining every surface to a fine roughness value.
CNC Grinding Service - consider grinding when the controlled surface needs texture and dimension control that normal milling or turning cannot reliably provide.
Polishing Service - use polishing for visible or sliding surfaces, while checking edge rounding, dimensional change, and inspection after polishing.
Electropolishing - review electropolishing for suitable stainless parts where cleanliness, corrosion behavior, and micro-burr reduction are more important than preserving sharp edges.
PVD Coating - consider PVD when wear, friction, color, or appearance requirements remain after the machined texture and dimensional tolerances are defined.
