Surface roughness table with Ra, microinch and N grade conversions

Quick surface roughness table

A surface roughness table is a practical lookup tool for translating common drawing values, usually Ra in micrometers or microinches, into nearby roughness grades and realistic manufacturing routes. Use it as a starting point, not as a substitute for a complete drawing specification. For example, Ra 3.2 µm, often shown as N8 or 125 µin, is a common general machined finish. Ra 0.8 µm, or about 32 µin, is much finer and may require controlled machining, grinding or another finishing operation. The main limitation is that Rz is not a fixed conversion from Ra. It is a separate profile parameter, so the same Ra value can mask very different peak and valley behavior.

The table below gives common reference values used in machining, inspection and surface finishing discussions. Microinch values are rounded from the metric value using 1 µm = 39.37 µin.

Reference grade	Ra µm	Ra µin	Typical interpretation	Common manufacturing route
N12	50	2000	Very rough surface	Rough casting, thermal cutting, heavy stock removal
N11	25	1000	Rough industrial surface	Sawing, rough turning, rough milling, flame-cut edge cleanup
N10	12.5	500	Coarse machined or prepared surface	Rough machining, drilling, coarse milling
N9	6.3	250	Rough to general machining	General turning, milling, boring or drilling
N8	3.2	125	Common general machined finish	Standard CNC milling or turning under controlled conditions
N7	1.6	63	Improved machined finish	Fine turning, fine milling, reaming, light grinding
N6	0.8	32	Fine functional finish	Fine machining, grinding, honing in some applications
N5	0.4	16	Precision finish	Grinding, fine reaming, honing, precision finishing
N4	0.2	8	Very fine finish	Fine grinding, honing, lapping or polishing
N3	0.1	4	High-grade finish	Lapping, polishing, superfinishing
N2	0.05	2	Special fine finish	Fine lapping, superfinishing, controlled polishing
N1	0.025	1	Extremely fine finish	Specialized lapping, polishing or superfinishing

These values are references for communication. They do not guarantee that a process will always produce the listed finish. Tool geometry, feed rate, speed, machine rigidity, workholding, material, coolant, abrasive condition and inspection method can all shift the result.

What the surface roughness numbers mean

Surface roughness is only one part of surface texture. A finished surface can also have waviness, lay, flaws, form error and directional tool marks. Modern profile surface texture practice is described by standards such as ISO 21920 for profile parameters and indication rules. U.S. drawings may also reference ASME B46.1. Older technical documents may still use ISO 4287, ISO 4288 or ISO 1302 terminology, so the controlling drawing note matters.

Ra is an average height parameter

Ra means arithmetical mean roughness. In practical terms, it averages the absolute height deviations of a filtered roughness profile from the mean line over the evaluation length. Because it is an average, Ra is easy to communicate and widely used on machined-part drawings. Its weakness is also its simplicity: isolated peaks, deep valleys or a few scratches may not change the average as much as their functional importance suggests.

Rz describes peak-to-valley behavior

Rz is a height parameter related to peak and valley behavior in the roughness profile. It is often more sensitive than Ra to pronounced peaks and valleys. That makes it useful for surfaces where sealing, wear, lubricant retention or contact stress depends on more than average roughness. However, Rz is not mathematically interchangeable with Ra. A milled surface, ground surface and honed surface can share a similar Ra while producing different Rz values.

N grades are convenient reference labels

N grades, such as N6 or N8, are commonly used as shorthand in roughness charts. They are best treated as reference labels tied to Ra values, not as complete specifications by themselves. If a supplier receives only an N grade without the governing standard, parameter, cutoff and inspection basis, the requirement may be interpreted differently from one shop to another.

Typical surface roughness by manufacturing process

Process capability tables are useful during early design work, especially when comparing machining cost with functional need. The ranges below are broad shop-floor references, not universal limits. A well-controlled machine may beat a range, while an unstable setup, difficult alloy or worn tool may perform worse.

Manufacturing process	Typical Ra range µm	Typical Ra range µin	Design note
Thermal cutting, rough casting, rough forging	12.5–50	500–2000	Suitable for non-functional surfaces or stock allowance before machining
Sawing, rough milling, rough turning	6.3–25	250–1000	Economical for clearance surfaces and heavy material removal
General CNC milling or turning	1.6–6.3	63–250	Common range for many machined faces and housings
Fine turning, fine milling, boring, reaming	0.8–3.2	32–125	Useful for fits, visible faces and moderately demanding functional surfaces
Grinding	0.2–1.6	8–63	Often selected for precision size, form control and finer texture
Honing	0.05–0.8	2–32	Useful for bores, sliding surfaces and controlled crosshatch textures
Lapping, polishing, superfinishing	0.012–0.2	0.5–8	Used where very low roughness or special bearing behavior is required

The lowest number in a process range is usually the most expensive part of the range. It may require slower feeds, sharper tooling, additional passes, controlled abrasives, better fixturing, more inspection time or a separate finishing operation. For cost-sensitive parts, specify fine roughness only on the surfaces that need it.

How to choose the right roughness value

The right roughness value is the one that supports the function of the surface. A number copied from a table may be too rough for a seal, unnecessarily expensive for a clearance face or unsuitable for a coating process. Start with the job the surface must perform, then choose the parameter and value.

• General machined faces: Ra 3.2 µm or Ra 6.3 µm is often sufficient when the surface is not sealing, sliding or carrying a precision fit.
• Precision fits: A finer value such as Ra 0.8–1.6 µm may be appropriate, but size tolerance, roundness and waviness can matter as much as roughness.
• Static sealing surfaces: A low Ra value alone may not prevent leakage. Rz, waviness, lay direction and damage from handling can strongly affect gasket or O-ring performance.
• Sliding and bearing surfaces: Very smooth is not automatically better. Some surfaces need a controlled texture that retains lubricant while avoiding damaging peaks.
• Coated or bonded surfaces: Adhesion may need a prepared texture rather than a polished surface. Follow the coating, plating, adhesive or paint system requirements.
• Fatigue-critical surfaces: Rough valleys and surface damage can act as stress raisers. If fatigue life is critical, roughness should be considered together with material condition, residual stress and process history.

As a drawing practice, avoid applying one strict roughness value to every surface of a part. It increases cost and inspection burden without improving performance. Instead, mark functional surfaces clearly and leave non-critical faces with a practical general finish. See also: CNC Machining.

Common mistakes when using a surface roughness table

A surface roughness table is helpful because it compresses a lot of manufacturing knowledge into a small format. It becomes risky when the table is treated as a full specification. The following mistakes are common in quoting, design review and supplier communication.

• Converting Ra to Rz as a fixed ratio: Many shops use rough estimates during discussion, but inspection requirements should not rely on a universal Ra-to-Rz conversion.
• Ignoring cutoff and evaluation length: Roughness measurement depends on filtering and sampling conditions. Two instruments can disagree if the setup is not aligned with the drawing requirement.
• Confusing roughness with flatness: A surface can be smooth but not flat, or flat but visibly rough. Roughness does not replace GD&T controls for form, orientation or location.
• Forgetting lay direction: Tool marks parallel to a seal path may behave differently from tool marks crossing it. Surface texture symbols can include lay requirements when direction matters.
• Specifying a cosmetic word instead of a measurable value: Terms such as smooth, polished, satin or brushed need measurable acceptance criteria if they affect function or appearance.
• Over-specifying hidden surfaces: Requiring Ra 0.8 µm on every internal pocket or non-contact wall can add machining time without improving the part.

How to specify and inspect surface roughness

A clear surface texture requirement should tell the manufacturer what to make and the inspector what to measure. At minimum, a controlled requirement should identify the parameter, value, unit and surface to which it applies. For more demanding parts, the drawing or purchase specification may also need the standard, cutoff, evaluation length, lay, manufacturing allowance, process restriction or inspection method.

1. Define the function first. Decide whether the surface is for sealing, sliding, coating, fatigue resistance, appearance, fit or simple clearance.
2. Select the parameter. Ra is common and easy to compare, but Rz, Rt or bearing-area parameters may be more relevant for some functional surfaces.
3. Use the correct unit. Do not mix µm and µin without checking the conversion. Ra 0.8 µm is about 32 µin, not 8 µin.
4. Limit the requirement to the right area. Apply the finish to the functional face, bore, groove or land rather than the whole part unless that is truly needed.
5. Agree on inspection conditions. Stylus profilometers are common for profile roughness. Optical areal systems can provide more information, but profile and areal parameters are not automatically interchangeable.
6. Review manufacturability before release. A small change from Ra 1.6 µm to Ra 0.8 µm can be minor on one process and expensive on another.

For supplier communication, include both the engineering requirement and the reason behind it when possible. A note such as “seal land, Ra 0.8 µm max, no radial scratches” gives better guidance than a global roughness callout with no context.

Frequently asked questions

What is the most common surface roughness for machined parts?

Ra 3.2 µm, or about 125 µin, is a common general-purpose machined finish for many non-critical faces. It should not be treated as a default for seals, bearings, precision fits or fatigue-critical surfaces.

Can Ra be converted to Rz?

Only approximately, and only for rough discussion. Ra and Rz measure different characteristics of the profile. If Rz is functionally important, specify and inspect Rz directly instead of deriving it from Ra.

Is a lower Ra value always better?

No. Lower Ra usually means a smoother surface, but it can also increase cost, remove beneficial texture or create a surface that is less suitable for coating or lubricant retention. The correct value depends on the function.

What does N8 mean in a roughness table?

N8 is a reference roughness grade commonly associated with Ra 3.2 µm, or about 125 µin. It is useful shorthand, but a complete drawing should still state the parameter, value, unit and any inspection requirements.

Should drawings use Ra or RMS?

Use the parameter required by the governing drawing standard, customer specification or inspection plan. Ra is widely used for machined parts, while RMS, commonly associated with Rq, is a different parameter and should not be substituted without agreement.