Preferred Ra Surface Roughness Values

MECHANITEC / ENGINEERING REFERENCE

Preferred Ra Surface Roughness Values

Historical preferred Ra values in micrometres with calculated approximate microinch equivalents.

1–13 of 13 matching rows (13 total)

Preferred Ra Surface Roughness Values — page 1 of 1
Preferred Ra (µm)	Approx. Ra (µin)	Series position	Series basis
0.012	0.47	1	Historical preferred series
0.025	0.98	2	Historical preferred series
0.05	1.97	3	Historical preferred series
0.1	3.94	4	Historical preferred series
0.2	7.87	5	Historical preferred series
0.4	15.75	6	Historical preferred series
0.8	31.5	7	Historical preferred series
1.6	62.99	8	Historical preferred series
3.2	125.98	9	Historical preferred series
6.3	248.03	10	Historical preferred series
12.5	492.13	11	Historical preferred series
25	984.25	12	Historical preferred series
50	1968.51	13	Historical preferred series

Search and sort apply to all 13 rows. Print and CSV include only the 13 rows shown on this page.

How to use this reference

The table lists a historical preferred sequence of arithmetic-average roughness values from 0.012 µm through 50 µm. Treat it as a reference scale, not a process-capability chart or a claim that every value suits every part.

The microinch column is calculated using 1 µm = 39.3701 µin and rounded to two decimals. The metric value remains primary where the drawing specifies micrometres.

Understand Ra

Ra is the arithmetic mean of absolute profile deviations over the specified evaluation length. It summarizes the profile, so surfaces with different peaks, valleys, lay directions or local defects can share a similar Ra value. It is not a complete description of sealing, bearing or coating performance.

If individual peaks or valleys affect function, specify additional parameters such as Rz, Rt or a material-ratio parameter. Select parameters according to surface function and the current drawing standard rather than converting a legacy Ra callout into another requirement.

Use current drawing rules

ISO 21920-1:2021 is the current published ISO part for indicating profile surface texture and replaced ISO 1302:2002. The preferred series here is historical, not a complete current standard extract. A controlled drawing should identify its texture specification and non-default evaluation requirements.

For production release, verify which edition and national adoption the contract or customer requires. A legacy drawing may retain an earlier convention; do not silently reinterpret its symbol or acceptance rule using a newer edition.

Choose measurement conditions

A numerical Ra limit only has meaning with a measurement method and suitable evaluation settings. Probe tip, cutoff or filter, sampling length, number of lengths and trace direction affect the result. The metrology procedure should match the drawing and instrument configuration.

Mitutoyo explains primary, roughness and waviness profiles and gives example conditions under an older ISO framework. Those examples are context, not a replacement for the current product specification or applicable measurement standard.

Avoid process-only assumptions

A low Ra value is often associated with more finishing effort, but the value alone does not identify a process or guarantee function. Grinding, honing, lapping, turning and polishing can produce different patterns even where average roughness is similar.

Do not use this series as a universal conversion from machining process to finish. For critical surfaces, define functional requirements, lay direction and acceptance approach with design and metrology teams.

Apply conversions consistently

Because the inch conversion is derived, small rounding differences can appear where another chart rounds to whole microinches. The paired values are approximate comparisons. For calculations, retain unrounded intermediate values and round to the precision required by the drawing or inspection plan.

The table does not convert Ra to Rz, RMS roughness or a grade number. Those parameters have different definitions and cannot be interchanged using one universal factor. Use a separately verified correlation only when the material, process and method support it.

Read the source scope

Each row preserves the source designation, units and conditions available for this reference. A value is useful only when the specification basis matches the part, process, instrument or machine under review. If a source does not state a condition, do not assume one that could change the result.

Keep the table and metadata together when saved or shared. If a current controlled supplier document conflicts with this snapshot, check the revision and use the latest applicable specification. This table is a comparison aid, not a substitute for supplier approval.

Verify the application

The table narrows a lookup but does not decide whether a component or process is acceptable. Confirm the drawing, equipment-maker instructions, safety rules, environment and complete operating duty before making a design, inspection or purchase decision.

For critical applications, have the responsible engineer review the selected value and supporting calculation. Record assumptions, material identity, measurement conditions and approvals that govern the final choice.

Keep units and terminology clear

Read column headings before comparing entries. A number can be a limit, reference value, conversion or one condition from a manufacturer catalogue. Do not mix units, standards, grades or measurement modes simply because the values appear comparable.

When converting units, document the factor and rounding. Retain unrounded values during calculations and round only to the precision required by the governing drawing or specification.

Check current documentation

Technical catalogues and standards can be revised. Check the current source before procurement, production release or inspection. Confirm that the cited edition applies to the customer contract and jurisdiction.

This article explains the table structure and key checks. It does not establish compliance or replace a qualified procedure. When information is incomplete, contact the manufacturer or standards body for the controlling document.