Surface roughness chart: Ra values, N grades and finish callouts

Ra is the number most drawings use to call out surface finish, and it is also the number most often over-specified. This page gives the full Ra chart in micrometers and microinches, the range each process can reach, how to write the callout on a drawing, and how to choose the roughest finish that still does the job, which is where the cost savings are.

What surface roughness and Ra mean

Surface roughness is the fine, closely spaced irregularity left on a surface by the process that made it: tool marks from a cutter, grit scratches from grinding, the grain of a sand mold. It is one part of surface texture. The other two are waviness (longer-wavelength undulation from vibration or deflection) and lay (the direction of the dominant pattern).

Ra (arithmetic average roughness, also written AA or CLA on older drawings) is the average absolute height of the profile above and below its mean line over the evaluation length. A profilometer stylus traces the surface, filters out waviness with a cut-off length (λc), and averages the result. For most machined surfaces between Ra 0.1 and 2 µm the cut-off is 0.8 mm and the evaluation length is five cut-offs, 4 mm.

Because Ra is an average, a surface with one deep scratch and a surface with even fine marks can show the same Ra. That is why sealing and bearing surfaces sometimes add Rz (average peak-to-valley height) to the callout.

Surface roughness chart: Ra, microinches and N grades

The N grades come from ISO 1302. Each step roughly doubles Ra. Microinch values are rounded by convention (1 µm ≈ 39.4 µin).

N grade	Ra (µm)	Ra (µin)	What the surface looks like	Typical use
N12	50	2000	Coarse, deep marks	Flame-cut or sand-cast surfaces left as is
N11	25	1000	Heavy saw or rough-cut marks	Clearance surfaces on castings and weldments
N10	12.5	500	Clearly visible, felt with a fingernail	Rough machining, non-functional faces
N9	6.3	250	Visible tool marks	Non-mating machined faces, structural parts
N8	3.2	125	Tool marks visible but smooth to touch	Standard as-machined CNC finish
N7	1.6	63	Faint tool marks	Mating faces, press fits, parts to be anodized for looks
N6	0.8	32	Marks barely visible	Static seal faces, bearing seats, sliding parts
N5	0.4	16	Semi-bright, no visible marks	Dynamic seals, shafts in bushings, high-fatigue areas
N4	0.2	8	Bright	Precision bearing journals, hydraulic spools
N3	0.1	4	Near mirror	Gauges, precision sealing faces
N2	0.05	2	Mirror	Optical mold inserts, lapped faces
N1	0.025	1	Mirror, no visible texture	Gauge blocks, optical components

Ra by manufacturing process

Each process has a range it can hold reliably in production and a finer value it can reach with extra care, slower feeds or a final pass. The table below shows typical industry ranges. The result on a real part also depends on the material, tool condition and machine rigidity.

Process	Typical Ra (µm)	Finer with extra care (µm)
Sand casting	12.5–25	6.3
Die casting	1.6–3.2	0.8
Rough milling	6.3–12.5	3.2
Finish milling (3/4/5-axis)	1.6–3.2	0.8
CNC turning	0.8–3.2	0.4
Drilling	3.2–6.3	1.6 (reamed: 0.8–1.6)
Wire EDM	1.6–3.2	0.4 with skim passes
Surface or cylindrical grinding	0.4–1.6	0.1
Honing	0.1–0.4	0.05
Lapping, superfinishing	0.05–0.2	0.025
Hand polishing	0.1–0.4	0.025 (mirror)

Soft aluminum alloys and free-cutting brass finish more easily than stainless steel or titanium at the same settings. Plastics such as POM machine smoothly; nylon and PEEK tend to leave fuzz on edges. For tighter numbers on a specific part, ask for a DFM review rather than relying on a chart.

Ra vs Rz vs Rq (RMS): can you convert them?

Parameter	What it measures	Relation to Ra
Ra	Average absolute deviation from the mean line	Reference
Rq (RMS)	Root-mean-square deviation	About 1.1 × Ra on turned or milled surfaces; higher on random textures
Rz	Average of peak-to-valley heights over five sampling lengths	Often 4–8 × Ra on machined surfaces, but no fixed ratio
Rt / Rmax	Largest single peak-to-valley height	Catches isolated scratches Ra hides

Rq and Ra are close enough that an old "63 RMS" drawing can be treated as roughly Ra 1.6 µm. Rz is different: it depends on the shape of the profile, so the ratio changes from one process to another. If a European drawing says Rz 6.3, do not convert it to Ra and quote against that. Measure Rz, or ask the designer which parameter matters.

How to read and write a surface finish callout

Surface texture symbols are defined in ISO 21920-1 (which replaced ISO 1302 in 2021) and, on US drawings, ASME Y14.36. The basic symbol is a check mark with the requirement written over the long leg.

• Plain check mark: any process is allowed.
• Check mark with a bar closing the V: material must be removed (machining, grinding).
• Check mark with a circle in the V: material removal is not allowed; the surface stays as cast, molded or forged.

Write the parameter and the value, for example Ra 1.6 on an ISO drawing or 63 (µin) on an ASME drawing. You can add an upper and lower limit (Ra 0.8 max, Ra 0.2 min, which matters for O-ring grooves that must hold lubricant), the process ("ground"), and the lay direction:

Lay symbol	Meaning
=	Lay parallel to the view where the symbol is drawn
⊥	Lay perpendicular to that view
X	Crossed in two directions
M	Multidirectional
C	Circular, about the center
R	Radial
P	Particulate, non-directional (blasted, EDM)

Put a general note in the title block ("All machined surfaces Ra 3.2 unless otherwise specified") and add individual symbols only on the faces that need something finer. One symbol per critical face is easier to quote and inspect than a symbol on every face.

Which Ra to specify: a decision table

The rule engineers repeat is the right one: specify the roughest finish that works. The values below are typical starting points used across industry; seal and bearing suppliers publish their own limits, and theirs win.

Surface function	Typical Ra (µm)	Comment
Non-mating faces, internal pockets	3.2–6.3	Leave as machined
Bolted or mating flat faces	1.6–3.2	Ra 3.2 is enough for most
Cosmetic faces to be anodized	1.6, then bead blast	Anodizing shows tool marks; blasting hides them
Static O-ring or gasket faces	0.8–1.6	Finer (≤0.8) for gas sealing
Press-fit bores and bearing seats	0.8–1.6	Pair with an ISO 286 fit
Dynamic seals, shafts in bushings	0.2–0.4	Usually ground or polished
Optical or mirror surfaces	≤0.05	Lapped or diamond polished

For most housings and brackets, Ra 3.2 on everything and Ra 1.6 or 0.8 on two or three functional faces is the right call. A blanket "Ra 0.8 all over" usually means slower feeds on every face, longer cycle time and more inspection, with no gain in function.

How surface finish affects cost and lead time

Cost does not rise smoothly with smoother Ra. It rises in steps, each time the finish needs a different operation:

1. Ra 3.2: standard finishing pass, no extra cost.
2. Ra 1.6: slower feed or a dedicated finishing tool on the affected faces. Small increase in cycle time.
3. Ra 0.8: careful milling or turning with sharp tools; on hard or gummy materials it may need grinding.
4. Ra 0.4 and below: a separate grinding, honing, lapping or polishing operation, a second setup, and roughness inspection on each part. This is where lead time grows.

Each extra step also adds a measurement. A profilometer reading takes a minute, but on 500 parts with three callouts each it becomes real inspection time.

How finishing processes change Ra

• Bead blasting replaces directional tool marks with a uniform matte, non-directional texture (lay symbol P). It hides marks but does not make a surface smoother in Ra terms; fine media gives a lower Ra than coarse.
• Anodizing (Type II, MIL-A-8625) follows the machined surface closely, so visible tool marks stay visible. Hard anodizing (Type III) builds a thicker layer and usually raises Ra slightly.
• Plating and powder coating add thickness; powder coat fills fine marks but adds 50 µm or more, which affects fits.
• Polishing and electropolishing lower Ra but round sharp edges and can shift tight dimensions.

If a coated face has a functional Ra, state whether the value applies before or after coating. More detail on coatings: CNC aluminum machining and anodizing.

Molded parts: SPI finishes and their Ra

An injection molded part copies the finish of the mold steel, so plastic parts are specified by the mold finish, usually to the SPI (now PLASTICS Industry Association) scale. The Ra values below are the typical ranges published for each grade.

SPI grade	Finishing method	Typical Ra (µm)	Use
A-1 / A-2 / A-3	Diamond buff (#3, #6, #15)	0.012–0.10	Lenses, clear covers, high-gloss parts
B-1 / B-2 / B-3	Grit paper (600, 400, 320)	0.05–0.32	Semi-gloss consumer parts
C-1 / C-2 / C-3	Stone (600, 400, 320)	0.35–0.70	Matte parts, good release
D-1 / D-2 / D-3	Dry blast (glass bead, oxide)	0.8–18	Textured, non-glare, hides sink and flow lines

A-grade polish needs a polishable mold steel. NAK80 is our usual choice for high-gloss surfaces, and E420/S136 stainless is available when the resin or the environment calls for it. Texture and polish are set face by face in the mold quote; see parts of an injection mold and P20 vs H13 mold steel.

What to put on your drawing and RFQ

• A general Ra note for all machined surfaces (Ra 3.2 in most cases).
• Individual symbols with Ra (or Rz) on each functional face, plus lay direction if it matters.
• Whether the value applies before or after anodizing, plating or blasting.
• For molded parts, the SPI grade or texture reference for each visible face.
• Whether you need measured roughness values in the inspection report.

From HC-Mold's side: our CNC machining holds down to Ra 0.4 µm, and every order ships with a 3D measurement report. When we reviewed the 78 RFQs that came through our website, only 4 described a surface finish or texture requirement at all, and just one gave an Ra value (Ra 6.3 µm on all machined faces). Most buyers leave finish to the supplier, which is when cosmetic disputes start. A single general note and two or three face callouts prevent that. The free DFM review flags callouts that are tighter than the function needs. See also CNC machining tolerances and the CNC machining RFQ checklist.

FAQ

What does a 125 or 32 surface finish mean?

They are Ra values in microinches, common on US drawings. 125 µin is Ra 3.2 µm (N8), the standard as-machined finish. 32 µin is Ra 0.8 µm (N6), a fine finish for seal faces and bearing seats.

What Ra can CNC machining achieve?

Standard CNC milling and turning give Ra 1.6–3.2 µm. With sharp tools and a dedicated finishing pass, Ra 0.8 µm is practical, and fine turning can reach Ra 0.4 µm. Smoother values usually need grinding, honing or polishing.

Is Ra the same as RMS?

No. RMS (Rq) is the root-mean-square of the profile, typically about 1.1 times Ra on machined surfaces. For older drawings the two are often treated as equivalent, but on random textures the gap is larger.

Can I convert Ra to Rz?

Only roughly. On machined surfaces Rz is often 4 to 8 times Ra, but the ratio depends on the process and the profile shape. If a drawing specifies Rz, measure Rz rather than converting.

What is the smoothest finish for an injection molded part?

SPI A-1, a diamond-buffed mold finish with Ra of about 0.012–0.025 µm. It needs polishable steel such as NAK80 or S136 and is used for lenses and high-gloss covers.

Get your finish callouts checked

Send your drawing or STEP file with the finish notes. We return a free DFM review that flags over-specified Ra callouts and a quote within 24 hours on a complete package. See our CNC machining services.

Request a quote

Written by HC-Mold Engineering Team · Last updated: 2026-11-01