Ra 0.8 vs 0.4 vs 0.2: Surface Roughness Explained

Ra requirements on engineering drawings

Engineering drawings for precision stainless steel components often specify surface roughness values such as Ra 0.8, Ra 0.4 or Ra 0.2.

These values concern more than whether a surface looks smooth: they also relate to manufacturing methods, measurement conditions, functional requirements and acceptance criteria.

A lower Ra value does not necessarily mean better corrosion resistance, nor does it establish that a surface is free from scratches or defects.

This article explains what Ra means, how different roughness values compare and what engineers should consider when specifying and accepting stainless steel surface quality.

1. What is surface roughness Ra?

Ra (arithmetic mean height) is a widely used profile parameter for describing surface roughness.

Following appropriate measurement and profile processing, it represents the mean of the absolute height deviations of the surface profile from its mean line.

In precision manufacturing, Ra is commonly expressed in micrometres (μm).

Examples include:

Ra 0.8 μm
Ra 0.4 μm
Ra 0.2 μm

Under the same measurement conditions, a lower Ra generally indicates smaller average height variations in the measured profile.

However, Ra is only one surface texture parameter and cannot describe every surface feature on its own.

2. How do Ra 0.8, Ra 0.4 and Ra 0.2 differ?

Roughness value	Basic meaning	Engineering considerations
Ra 0.8 μm	Relatively greater average profile height variation	Compliance with component function and drawing requirements
Ra 0.4 μm	Relatively smaller average profile height variation	Process stability and measurement consistency
Ra 0.2 μm	Still smaller average profile height variation	Manufacturing feasibility, inspection capability and necessity

This comparison describes roughness specifications. It does not imply that a particular process will necessarily achieve a given value, or that every material or component should have the lowest possible Ra.

For example, Ra 0.2 μm is lower than Ra 0.8 μm, but that difference alone does not establish superior functional performance in every service environment.

Selection must also consider component use, material, manufacturing method and acceptance conditions.

3. Does a lower Ra always mean a better surface?

Not necessarily.

Surface roughness is an important engineering indicator, but surface quality can also involve:

Scratches and local defects
Machining direction and surface lay
Surface contamination
Oxides and residues
The material's own properties
The actual service environment

Components with the same Ra can perform differently because their profile shapes, surface lay or local defects differ.

Ra therefore cannot replace a complete surface quality assessment.

4. What is the difference between Ra and Rz?

Ra and Rz are both common surface profile parameters, but they describe different surface characteristics.

Ra describes average profile height deviations.

Rz relates to extreme height features within a specified profile range. Its precise definition and calculation must be checked against the cited edition of the standard.

Because definitions and evaluation methods for Rz can differ between standards and editions, the applicable standard should be identified before comparing historical drawings with new specifications.

There is no fixed Ra-to-Rz conversion ratio that applies to every surface.

A reliable Rz acceptance result cannot be inferred from Ra alone.

5. Does a smoother stainless steel surface always resist corrosion better?

Surface texture can affect contaminant adhesion, cleaning and local conditions for corrosion.

In some applications, reducing roughness may help limit features that retain contaminants.

However, stainless steel corrosion resistance also depends on alloy grade, contamination, the passive condition, processing history and service environment.

A low Ra must therefore not be equated directly with high corrosion resistance.

Even a component with a low Ra can corrode if free iron contamination, chlorides or other adverse conditions are present.

Corrosion resistance requires an overall assessment of material and service conditions. See why stainless steel rusts and how passivation is assessed.

6. How do SMT, mechanical polishing and electropolishing affect Ra?

Different surface finishing techniques modify stainless steel surface texture, microscopic height variations and surface quality in different ways.

Alongside mechanical polishing and electropolishing, KUN-QI's specialist SMT — Stainless Steel Surface Micro Treatment is relevant when assessing precision stainless steel component finishing.

SMT — KUN-QI's core surface treatment technology

SMT is a core precision surface treatment technology at KUN-QI, focusing on microscopic stainless steel surface condition and processing quality control.

For components with specified roughness, surface uniformity or precision finishing requirements, SMT can be considered as an option for improving surface quality.

Assessment of its effect on Ra should consider:

Initial roughness and machining marks.
Stainless steel grade and material condition.
Component geometry, dimensions and treatment areas.
Required surface quality and roughness.
Measurement conditions and results before and after processing.

KUN-QI assesses SMT against the customer's specified roughness requirements. Where a suitable process and acceptance conditions have been confirmed for the individual component, the target Ra or a lower roughness can be achieved. Materials, geometry, initial surface condition and processing requirements differ, so process conditions and processing time must be assessed and planned for each product. This is not a guarantee that every component can achieve a particular Ra.

Mechanical polishing

Mechanical polishing modifies surface texture mainly through abrasives and mechanical action.

Results can depend on abrasive type, processing conditions, material and initial surface condition.

It may leave directional surface lay, so machining direction and local defects may need consideration alongside Ra.

Electropolishing

Electropolishing improves surface condition through controlled electrochemical material removal.

Under suitable conditions it may reduce some microscopic height variations, but the outcome depends on material, initial surface, component geometry and process conditions.

Electropolishing does not guarantee that every component will meet a specified Ra or that all surface defects will disappear.

How should a surface treatment be selected?

SMT, mechanical polishing and electropolishing have different technical roles and process characteristics.

Selection should consider material, initial surface condition, geometry, functional requirements and acceptance criteria together, rather than comparing treatment names or a single Ra value alone.

KUN-QI can assess the suitability of SMT for precision stainless steel components with specific surface quality requirements and confirm feasibility against the actual processing conditions.

Related services: SMT surface treatment and electropolishing assessment.

7. How is stainless steel surface roughness measured?

Common methods include contact profile measurement and suitable non-contact optical techniques. With optical methods, profile Ra must be distinguished from the areal parameter Sa; they are not interchangeable.

Results can be influenced by:

The instrument and measurement method.
Measurement direction and location.
Profile filtering conditions.
Evaluation length and associated settings.
Component geometry.
Contamination and the measurement environment.
Instrument resolution and measurement uncertainty.

Even when two inspection reports both state Ra, their measurement conditions should be checked for comparability.

For precision components, drawings should clearly identify the applicable standard and necessary measurement conditions to reduce acceptance disputes.

8. ISO 21920 and surface roughness specifications

The ISO 21920 series forms part of geometrical product specifications for profile surface texture.

Its three parts cover:

ISO 21920-1:2021

Indication of surface texture requirements in technical product documentation.

ISO 21920-2:2021 (corrected version 2022-06)

Terms, definitions and surface texture parameters for profile methods.

ISO 21920-3:2021

Specification operators for profile surface texture.

ISO's records list ISO 1302:2002, ISO 4287:1997 and ISO 4288:1996 as withdrawn predecessors of Parts 1, 2 and 3 respectively. The new series updates indication, parameters and specification operators; old and new settings should not be assumed identical.

Where drawings still cite ISO 4287, ISO 4288 or another older standard, establish the specified edition and requirements before applying settings from the new standards.

Inspection and acceptance must follow the drawings, formal standards and customer requirements.

9. What should engineers consider when specifying Ra?

When specifying stainless steel surface roughness, consider:

The component's actual function.
The surface areas where roughness must be controlled.
The specified Ra and acceptance requirements.
The applicable standard edition.
Measurement direction, location and necessary conditions.
Whether additional surface parameters are required.
Any appearance, cleanliness or corrosion resistance requirements.
Manufacturing feasibility and inspection capability.

For precision components with small holes, curved surfaces, thin walls or complex features, measurement feasibility should also be considered early.

10. KUN-QI — assessing precision stainless steel finishing requirements

Surface roughness requirements for precision stainless steel components should be considered together with material properties, manufacturing method, geometry and function.

KUN-QI places importance on processing conditions and surface quality requirements.

If a drawing specifies Ra 0.8, Ra 0.4, Ra 0.2 or another surface requirement, please provide the material grade, engineering drawings, initial surface condition, quantity and acceptance criteria for assessment.

Single-component prototypes, small batches and development trials can be discussed individually, including feasibility, quotation and acceptance conditions. Sample processing is quoted as a paid service.

Contact KUN-QI to discuss precision stainless steel surface finishing requirements.

Technical references

Technical note: This article provides general engineering information. Roughness specifications, measurement conditions and acceptance requirements must be confirmed against the formal standard edition, engineering drawings and customer-approved documents.

Ra 0.8 vs 0.4 vs 0.2: Stainless Steel Surface Roughness Explained