How to Specify Stainless Steel Surface Roughness for OEM Parts

Specify stainless steel surface roughness for OEM parts. Learn how to define Ra limits, measurement methods, finish requirements and inspection records.

2026/10/14

Reading volume： 0

News Details

The surface of stainless steel may look bright and smooth, but may not meet the technical requirements. Similarly, two brushed panels may look similar but produce different roughness readings.

For industrial buyers, appearance is only part of surface quality. The specification should also clearly state measurable requirements for surface functional properties.

This guide helps OEM manufacturers, equipment builders and metal fabricators clearly define surface roughness specifications. It covers finishing selection, measurement, inspection and timing when the final surface must meet the specified requirements.

Understanding Surface Roughness Measurements

Surface roughness describes small variations in surface profile. A common parameter is Ra, expressed in microns (µm). A lower Ra value indicates a smoother measured profile. However, this does not fully describe all characteristics of the surface.

Therefore, buyers should not use terms such as "smooth", "fine polished" or "high quality" as the sole acceptance criteria.

Instead, the engineering team should determine the required parameters, their limits and the applicable areas. This will provide a common basis for the procurement and inspection teams for approval.

Distinguish finish name from roughness limit

Terms such as 2B, BA, No.4, brushed finish and mirror finish describe the category or appearance of a surface. When functionality matters, these terms should not replace specific roughness requirements.

For example, visible panels may require both a consistent brushed pattern and a maximum Ra value. The invisible support parts may only need an acceptable mill finish.

When ordering precision stainless steel sheet, finish names and any numerical roughness limits should be defined separately.

Also identify visible surfaces. If both sides require finishing, state the requirements for each side instead of assuming they will be consistent.

Matching Requirements to the Working Surface

Not every surface of a part requires the same finish.

The machine cover may have a decorative exterior and a concealed interior. Meanwhile, the shaft may have specific contact areas that require closer surface control than the remaining bar stock.

Mark these areas on the drawing. Then explain whether the requirements relate to appearance, cleaning, contact, sealing or other functions.

Surface condition can also affect corrosion performance. Outokumpu notes that rough surfaces are more likely to trap dirt and corrosive substances. However, material grade, environment, design and maintenance also affect performance.

Therefore, the responsible engineer should determine the requirements for the actual application.

Define when the surface must meet the limits

Roughness requirements need a clear delivery stage.

Does this apply to the raw sheet, polished blank or fully fabricated part? Cutting, forming, welding and subsequent finishing can alter surface condition.

For stainless steel bar, distinguish the bar surface as supplied from the final machined surface. If the buyer will remove the outer material during turning, the final shaft requirements should not automatically apply to the raw bar.

Similarly, polished sheet may still require local finishing after welding.

Agree on who performs each operation and who verifies the final surface. This will prevent gaps between material supply and part approval.

Agree on measurement method

Roughness readings are most useful when both parties use compatible measurement methods.

The inspection agreement should specify:

Roughness parameters and units
Applicable measurement standards and versions
Measurement direction
Filter and cut-off settings
Evaluation length
Measurement location
Number of readings
Acceptance Criteria

Mitutoyo's technical guide explains that roughness evaluation uses defined profiles, filters and sampling lengths. Therefore, settings form part of the measurement requirements.

Ask your supplier to confirm the proposed method before production. If a surface is too small or difficult to access, determine how it will be measured during drawing review.

Approve appearance and measured texture separately

Physical samples help determine grain direction, reflectivity and overall appearance. However, visual inspection alone is not sufficient to establish numerical roughness limits.

For trial orders, retain both approved samples and inspection records.

Records should identify material, finish, measurement locations and results. Photos can support visual inspection, while measurement reports support technical acceptance.

For repeat orders, use the same approved reference unless changes are agreed upon. This makes it easier to evaluate batches consistently.

Include surface defects in the inspection plan

Roughness readings do not replace inspection for scratches, dents, stains or uneven polishing.

Therefore, define visual inspection criteria separately. State which surfaces are critical and whether local defects are acceptable.

For stainless steel coil, discuss sampling plans that reflect order size and processing route. A single reading should not be used to prove the condition of the entire coil.

Inspection records should link results to the relevant product or batch. If surface measurements are required, confirm that they will be included in a dedicated report, rather than assuming that the material certificate will include them.

Review Surface Finish Supply with Zhongersheng Metal

Zhongersheng Metal supplies stainless steel sheets, coils and rods through the ZES Metals website. Its product pages list available finishes and processing services, including brushing, polishing and grinding for applicable products.

Customers can submit numerical roughness requirements for review along with dimensions and visual finish requirements. The quotation should confirm agreed processing scope, inspection methods and delivery conditions.

For accurate inquiries, please provide the following information: