Ra 3.2, Ra 1.6 and Ra 0.8: Choosing Surface Roughness for CNC Parts

Surface roughness can affect friction, sealing, fatigue behaviour, fit stability and appearance. Ra is widely used on mechanical drawings to describe the arithmetic mean roughness of a surface profile. A lower Ra value, however, does not automatically make a component better. Specifying a fine finish on every face can increase machining time, tool-control requirements, grinding operations and inspection cost. A practical drawing identifies the functional surfaces first, then applies a finish that supports contact, movement, sealing or appearance.

This table is an initial guide, not a guarantee that one process always produces a fixed Ra value. Material, hardness, overhang, tool geometry, machine condition, cutting data and measurement position all affect the result.

Ra 3.2 is often suitable for conventional turned or milled surfaces that do not provide a precision sliding or sealing function. It can be economical for external faces, secondary mounting faces, blocks and plates where dimensional accuracy is the primary requirement. Avoid applying Ra 1.6 or Ra 0.8 to an entire component when only selected faces are functional. Marking those faces separately allows the supplier to choose the correct tool, operation and inspection plan.

Ra 1.6 may be considered for locating surfaces, shoulders and fits requiring more consistent contact. Ra 0.8 commonly appears on journals, sliding faces, ground working surfaces or locations where friction and contact stability have a defined role. Before specifying a very low value, confirm that the function requires it. A smooth surface with poor flatness, roundness or coaxiality may still fail in assembly. Roughness and geometric tolerances should be reviewed together.

Turning, milling, grinding and finishing processes create different surface patterns. Turning is affected by feed, tool nose radius, rigidity and vibration. Milling reflects tool runout, step-over and toolpath strategy. Grinding may produce a finer surface but still requires control of wheel condition, dressing, heat and distortion. VietTech reviews CNC milling, CNC turning, surface grinding and cylindrical grinding against each drawing. It should not be assumed that every Ra 0.8 surface requires grinding or that every milled face is limited to Ra 3.2. Geometry, material, tolerance and quantity determine the appropriate route.

A roughness tester moves a stylus across the surface and calculates parameters under selected settings. Results can vary with measurement direction, sampling length, filter, location and surface cleanliness. Inspection should identify the correct face, use a direction appropriate to the machining lay and avoid burrs, scratches or edges unless they are part of the evaluation area. Stylus access must also be considered for small parts, narrow grooves and curved surfaces.

• Ra value and applicable surface.
• Drawing standard or symbol convention.
• Whether material removal is permitted.
• Lay direction where it affects function.
• Inspection condition before or after heat treatment, plating or finishing.
• Report, sampling and measurement-location requirements.
• Material, hardness, quantity and required delivery.

Ra 3.2 is suitable for many general machined surfaces. Ra 1.6 is useful where a finer, more stable contact surface is needed. Ra 0.8 should be applied where the component function justifies a dedicated finishing and inspection route. Appropriate specification balances function, machining time, inspection and cost.

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