If a drawing specifies only Ra ≤ 0.4 μm and the machined surface passes profilometer inspection, no additional polishing is required to meet that value. An Ra 0.4 surface finish and a tool-mark-free appearance are two different acceptance requirements. Treating them as the same can add unnecessary cost and increase the risk of dimensional or geometric nonconformance.
Secondary finishing should be added only when the customer separately requires no visible tool marks, a mirror-like appearance, uniform reflection, or compliance with an approved visual sample.
Ra 0.4 Measures Roughness, Not Appearance
Ra is the arithmetic mean of the absolute profile-height deviations from the mean line over a defined evaluation length. It describes average microscopic roughness, but it does not fully define tool-mark direction, spacing, waviness, or isolated scratches. Ra 0.4 μm is approximately a 16 microinch surface finish in U.S. units.
Two surfaces with the same Ra value can look completely different. Tool-path direction, mark spacing, and reflected light all influence the visible appearance. Fine, uniform patterns left by a diamond cutting tool can still be seen under angled or high-intensity light even when the surface meets its roughness requirement.
ASME B46.1 identifies roughness, waviness, and lay as separate elements of surface texture. Precitech’s guidance on diamond-machined components also recognizes residual tool marks on precision diamond-machined surfaces.
A fine, continuous machining pattern should not be confused with chatter, tearing, scratches, or isolated deep grooves. These are abnormal surface features that can affect function or appearance. Sealing, sliding, optical, and appearance-critical surfaces may also require limits for Rz, Rt, surface lay, waviness, or an approved visual standard.
Diamond Tool Machining Can Achieve Ra 0.4 Without Polishing
On suitable materials and geometries, a final CNC pass with a diamond cutting tool can produce Ra ≤ 0.4 μm directly. Polishing is not required when the machining process has already met the specified roughness.
Finishing the surface on the machine avoids another setup and prevents unnecessary material removal. It also helps preserve the flatness, parallelism, thickness, and step dimensions already established during machining while reducing cost and lead time.
The achievable result depends on:
- Material and part geometry
- Workholding rigidity and part stability
- Tool sharpness, runout, and wear
- Machine and spindle condition
- Feed rate, depth of cut, and tool path
- Chip evacuation, coolant, and vibration control
Thin walls, deep cavities, interrupted cuts, and restricted tool access require separate process evaluation. Results from a stable, accessible flat surface should not be applied automatically to these features.
The final CNC surface finish must be verified with an appropriate roughness tester or profilometer rather than by sight or touch. Inspection location, measurement direction, cutoff wavelength, and evaluation length should remain consistent. For a surface with a clear machining lay, the measurement trace is generally taken across the primary tool marks. Mitutoyo provides the same measurement-direction guidance.
Polishing Requirements Usually Come From Appearance Standards
A part can meet the Ra 0.4 surface finish requirement and still fail a separately defined appearance standard. Common cosmetic or optical requirements include:
- Uniform reflection on a visible face
- No directional machining pattern on a decorative surface
- Compliance with a customer-approved visual sample
- Separate limits for scatter, waviness, or localized defects
These requirements cannot be communicated through Ra alone. “Ra ≤ 0.4 μm” and “no visible tool marks” should be listed as separate acceptance criteria on the drawing or purchase specification.
Roughness is verified by instrument. Appearance should be evaluated under defined lighting, viewing distance, angle, inspection area, or an approved limit sample. A drawing that specifies only Ra 0.4 does not automatically require a mirror finish or the removal of every machining mark.
Hand Polishing Increases the Risk of Tight-Tolerance Failures
Sanding and polishing remove material. During manual work, pressure, dwell time, and tool movement cannot be held perfectly uniform across the entire surface. Large flat faces, softer materials, and edges are particularly sensitive to uneven removal.
Secondary hand polishing creates risk for:
- Flatness, parallelism, and surface profile
- Part thickness, step height, and mating dimensions
- Edges, chamfers, and hole entrances
- Surfaces used as assembly or inspection datums
When CNC machining has already met both the Ra requirement and the dimensional tolerances, unnecessary hand polishing should not be added. Keeping the machined surface preserves the geometric relationships established on the machine and reduces the risk of rework or rejection.
Precision grinding and lapping are different from uncontrolled hand polishing. A validated precision-finishing process can improve both roughness and form accuracy, but it requires a defined stock allowance, suitable workholding, process validation, and final inspection. It should be planned as a separate precision operation rather than added after machining without preparation.
Drawings Should Define Roughness and Appearance Separately
To prevent a part from passing Ra inspection but being rejected for tool marks, the drawing or purchase specification should define:
- The Ra limit, units, and applicable surfaces, such as Ra ≤ 0.4 μm
- Whether a uniform machining pattern is acceptable
- Whether machining marks are prohibited or a mirror finish or specific lay is required
- The lighting, viewing distance, angle, or limit sample used for visual inspection
- Surfaces with critical flatness, parallelism, profile, or datum functions
- Whether sanding or polishing is permitted
- Whether dimensions and geometric tolerances are inspected after finishing
If sealing, friction, lubrication, or optical performance is the design intent, additional parameters such as Rz, Rt, lay, or waviness may also be required. Clear requirements allow the supplier to select the correct process without adding finishing operations that provide no functional value.
Process Recommendation
When the only requirement is Ra ≤ 0.4 μm and the diamond-finished surface passes inspection, keep the part in its machined condition. This protects accuracy while reducing polishing cost and lead time.
When a tool-mark-free appearance is required, quote and plan visual finishing as a separate operation. Define the affected surfaces, allowable material removal, protected areas, and final acceptance method before machining. Reinspect roughness, dimensions, and geometric tolerances after finishing.
When a part combines tight geometric tolerances with a no-tool-mark requirement, use a validated grinding, lapping, or controlled polishing process instead of an unplanned manual operation.
An Ra 0.4 surface finish does not automatically require polishing. Use measured roughness as the acceptance standard when the drawing specifies roughness only. Add secondary finishing only when the drawing also defines the required appearance. Separating these requirements provides the most reliable balance of accuracy, appearance, cost, and lead time.