Guide
Surface finish chart: what Ra should you specify for machined parts?
A surface finish chart is a conversion table between roughness grades and Ra values in micrometers and microinches, used to pick the right Ra callout. Ra itself is the arithmetic mean of how far a measured surface profile strays above and below its mean line, and the right value depends on what the surface has to do, not on how smooth it can be made.
By Saola Manufacturing engineering team · Published · Updated
Process charts, which list what each machining process can reach, answer a useful but different question. This guide starts from the other end: what a seal groove, a bearing seat or a sliding bore actually needs, how to write that on the drawing so it cannot be misread, and how the value is measured when the parts arrive. It is written for engineers and sourcing managers sending turned and milled parts to a supplier overseas, including shops in Vietnam. The values on this page come from published standards and from seal and bearing makers’ design data. They describe design practice, not the capability of any particular shop.
On this page
What Ra should you specify for O-ring glands, bearing seats and other functional surfaces?
Specify the roughness the mating part asks for: seal and bearing makers publish Ra limits for the surfaces their products touch, and those limits are the best starting point. The surface finish chart below converts roughness grades to Ra, and the second table collects typical values from that published design data.
| Grade | Ra (µm) | Ra (µin) |
|---|---|---|
| N12 | 50 | 2000 |
| N11 | 25 | 1000 |
| N10 | 12.5 | 500 |
| N9 | 6.3 | 250 |
| N8 | 3.2 | 125 |
| N7 | 1.6 | 63 |
| N6 | 0.8 | 32 |
| N5 | 0.4 | 16 |
| N4 | 0.2 | 8 |
| N3 | 0.1 | 4 |
| N2 | 0.05 | 2 |
| N1 | 0.025 | 1 |
Each grade roughly doubles the Ra of the grade before it. N-grades come from older editions of ISO 1302 and still appear on older prints; on a new drawing, write the Ra value with its unit instead. The ISO 2768 and NADCA tolerance guide explains the grades alongside general tolerances and uses the same chart. The next table turns from conversion to choice: which Ra a seal or bearing surface actually needs.
| Surface | Typical Ra target | Callout form | Published basis |
|---|---|---|---|
| Static O-ring gland, liquids | 32 µin (0.8 µm) | Maximum | Seal maker; rougher surfaces can be tolerated |
| Static O-ring gland, gases or vacuum | 16 µin (0.4 µm) | Maximum | Seal maker |
| Reciprocating seal running surface | 10 to 20 µin (0.25 to 0.5 µm) | Range (upper and lower) | Seal maker; too polished will not hold lubricant |
| Rotary seal shaft / its gland | 16 µin shaft, 32 µin gland | Maximum | Seal maker |
| Roller bearing shaft seat, ground | 1.6 µm (63 µin) | Maximum | Bearing maker |
| Ball bearing shaft seat, shaft under 2 inches | 0.8 µm (32 µin) | Maximum | Bearing maker; 1.6 µm for larger shafts |
| Turned (not ground) shaft seat | 3.2 µm (125 µin) | Maximum | Bearing maker, with a tighter interference fit |
| Bearing housing bore | 3.2 µm (125 µin) | Maximum | Bearing maker, standard-class bearings |
Read the table as a first draft, then check it against the actual seal or bearing you are buying. The Apple Rubber seal guide writes its values as “RMS”, a habit many seal catalogs share; on a new drawing, ask the seal supplier whether the limit is meant as Ra or Rq and write the parameter by name. The Timken Engineering Manual also tightens seat finishes for precision-class tapered roller bearings, down to 0.2 µm Ra on shafts for its highest class, so a spindle drawing needs the bearing class before the finish can be set.
Two groups of surfaces do not need a callout at all. Clearance holes, non-mating faces and outside diameters that touch nothing can take whatever the general finish note says. And faces sealed by a flat gasket should follow that gasket maker’s data, since soft and metallic gaskets want very different textures.
What does Ra actually measure, and what does it miss?
Ra averages the absolute height of the roughness profile over the evaluation length, so it tells you how rough a surface is overall but almost nothing about its shape. Two surfaces with the same Ra can behave very differently against a seal or in a press fit.
The definition comes from the profile standards. ASME B46.1 defines surface texture and splits it into roughness, waviness and lay, then defines the parameters used to specify it. The ISO equivalent was ISO 4287, now replaced by ISO 21920-2:2021. Both compute Ra after a filter has removed the long-wavelength waviness, so Ra describes only the fine texture left by the tool.
Because it is an average, Ra is barely moved by a single deep scratch or a few tall peaks, a point the Mitutoyo quick guide makes directly. A turned surface with sharp peaks and a honed surface with flat plateaus and oil-holding valleys can share an Ra, yet the first will wear an elastomer seal and the second will carry lubricant. When peaks or scratches are the real risk, add a height parameter such as Rz, or a plain-language note banning spiral tool marks or radial scratches across a sealing face.
Lay matters for the same reason. A face seal crossed by radial tool marks can leak along them even at a low Ra, which is why ASME Y14.36 gives a lay symbol: circular lay on a face-seal land is often the real requirement, and the Ra number is secondary.
Should a roughness callout be a maximum or a range?
Use a single maximum for almost every surface, and a range with a lower limit only where too smooth is a functional failure, such as a dynamic seal surface that has to hold oil. A lower limit adds inspection and can add operations, so it should be rare.
ASME Y14.36 sets out the symbols for this, and ISO 21920-1:2021 does the same on ISO drawings, where an upper limit is marked U and a lower limit L. The question “is smoother acceptable?” comes up whenever a part arrives better than the print. With a maximum-only callout the answer is yes, and the part conforms. If smoother would hurt the function, the drawing has to say so with a lower limit, because no inspector can infer it.
A second choice hides inside the word “maximum”. Per ISO 4288 the default comparison is the 16% rule: up to 16% of measured values may exceed the limit and the surface still passes. Writing Ra with the suffix “max” switches to the max rule, where no measurement may exceed it. The ISO 21920 series replaced ISO 4288 in 2021, so name the standard and edition your drawing follows, check which edition’s defaults apply, and write “max” where you mean it.
How do turning and milling set the Ra you get?
On a turned surface, roughness is set mainly by feed per revolution and tool nose radius: finish improves with a larger nose radius and a lower feed. Milling follows the same geometry through the feed per tooth, the cutter corner radius and, on 3D surfaces, the stepover between passes.
The geometry is simple. Each revolution, the tool nose leaves a cusp whose theoretical peak-to-valley height is roughly the feed squared divided by eight times the nose radius, and Ra for that ideal profile is typically about a quarter of the peak-to-valley height. As a typical worked case, a feed of 0.1 mm per revolution with a 0.8 mm nose radius gives a theoretical Ra of about 0.4 µm. Tool makers such as Kennametal publish calculators built on this relation and state that the result is theoretical.
Real parts come out rougher than the formula. Tool wear, built-up edge on soft aluminum, vibration on long slender parts and the material’s own tearing all add texture on top of the feed marks. That is why a finish at the edge of what a process routinely reaches may need a dedicated finishing pass, a fresh insert, or a second operation such as grinding, honing or burnishing, and each of those adds cost.
For CNC turning, bores and shafts with a seal or bearing callout are usually finished in their own pass. For CNC milling, a sealing face is typically cut with a face mill or a wiper geometry, and the drawing should say whether the lay may be crossed or must be circular.
Die-cast surfaces are a separate case. An as-cast face takes its texture from the die, not from a cutting tool, so a machining roughness chart does not apply to it; any face with a seal or bearing callout on a casting should be marked as machined, and the as-cast quality agreed against the NADCA Product Specification Standards.
How is Ra measured, and why does the cutoff length matter?
Ra is measured with a stylus profilometer that traces the surface and filters the trace at a cutoff wavelength, and a different cutoff gives a different number for the same surface. The standards tie the default cutoff to the expected Ra, as the table shows.
| Ra range (µm) | Cutoff λc (mm) | Evaluation length (mm) |
|---|---|---|
| over 0.006 up to 0.02 | 0.08 | 0.4 |
| over 0.02 up to 0.1 | 0.25 | 1.25 |
| over 0.1 up to 2 | 0.8 | 4 |
| over 2 up to 10 | 2.5 | 12.5 |
| over 10 up to 80 | 8 | 40 |
Most machined functional surfaces fall in the middle row, so per ISO 4288 the typical default is a 0.8 mm cutoff over five sampling lengths. For periodic profiles such as turned and milled surfaces, the standard picks the cutoff from the feed-mark spacing instead, which usually lands in the same row. The practical risk is a short feature. A narrow O-ring land or a small bore may not have room for the full stylus travel, and the Mitutoyo guide notes that the number of sampling lengths then has to be reduced and recorded on the drawing.
Measurement scatter is real as well. The same guide says Ra readings typically vary by roughly minus 20% to plus 30% across a surface, which is why the 16% rule exists and why one reading near the limit proves little. For disputes about instrument software, NIST runs a public surface metrology algorithm testing system that checks how a program computes parameters such as Ra against reference results.
How should Ra appear on a drawing sent to an overseas supplier?
Put the parameter, value, unit and limit type on each surface that needs one, one general finish note for everything else, and the governing standard in the title block. A supplier in Vietnam, Mexico or anywhere else can then quote the surfaces as written instead of guessing at intent.
- Name the unit. A bare “32” is microinches on a US print and would be an extremely rough surface if read as micrometers. Write µin or µm every time.
- Name the parameter. Write Ra, Rz or Rq, never “RMS” or “finish” alone. Per ASME B46.1, RMS is Rq, a different measurement from Ra.
- Say max or range. Add a lower limit only where smoother fails the function, as on a dynamic seal surface.
- Say before or after coating. Anodizing and plating change the surface, so state whether the Ra applies to the bare machined part or the finished one; the surface finishing step is part of the same conversation.
- Mark lay where it matters. Circular lay on face-seal lands, no spiral marks on rotary-seal journals.
- Ask for the evidence. Request a profilometer report on first articles that lists the instrument, cutoff and number of readings, so the result can be compared with your own inspection.
A Vietnamese supplier reads ASME and ISO callouts the same way a US or European shop does; what goes wrong across borders is usually an ambiguous unit or an unstated parameter, not the machining. The RFQ package guide covers the rest of what to send, and the tolerance guide converts older N-grade roughness callouts to Ra. When the drawing is ready, send it for a quote; surfaces that need a finishing operation are called out in the reply.
Frequently asked
Is RMS the same as Ra?
No. Per ASME B46.1, RMS is the parameter Rq, which squares the profile deviations before averaging and so reads higher than Ra on most machined surfaces. Older US drawings and many seal catalogs still say RMS; confirm which parameter is meant before quoting to it.
If a part is smoother than the drawing, is it rejected?
Not with a maximum-only callout; smoother conforms. Only a callout with a lower limit makes a too-smooth surface a nonconformance, which is why dynamic seal surfaces are the usual place to use one.
Do I need an Ra callout on every surface?
No. Put individual callouts on the surfaces that seal, locate, slide or carry a bearing, and cover the rest with one general finish note. Every extra callout is another surface the supplier has to control and measure.
Should I write Ra in micrometers or microinches?
Either works if the drawing says which. ASME B46.1 supports both; ISO drawings use micrometers. Pick one per drawing and write the unit with each value.
Does a finish callout change the price?
It can. A callout inside what the normal finishing pass produces adds little, while one that needs grinding, honing or a second setup adds an operation. Send the drawing with the critical surfaces marked and the quote will say which ones need extra work.
Sources
- ASME B46.1-2019, Surface Texture (Surface Roughness, Waviness, and Lay), ASME, 2019 (reaffirmed 2026)
- ASME Y14.36-2018, Surface Texture Symbols, ASME, 2018 (reaffirmed 2024)
- ISO 21920-1:2021, Surface texture: Profile — Part 1: Indication of surface texture, ISO, 2021
- ISO 21920-2:2021 and ISO 21920-3:2021, Surface texture: Profile — terms and parameters; specification operators, ISO, 2021 (replacing ISO 4287:1997 and ISO 4288:1996)
- Quick Guide to Surface Roughness Measurement, Mitutoyo America, 2016
- Timken Engineering Manual, mounting and precision-bearing seat finishes, The Timken Company, 2025
- Seal Design Guide: Surface finishes, Apple Rubber Products, accessed September 2026
- Turning surface finish calculator, Kennametal, accessed September 2026
- Internet-based Surface Metrology Algorithm Testing System, NIST, 2002 (updated 2012)
- Product Specification Standards for Die Castings, NADCA, accessed September 2026