Surface finish reference

What surface roughness is, what each process delivers, the finish each function needs, and how to call it out on a drawing without over-specifying and paying for it.

Revision1.0

IssuedJune 2026

OwnerIdeambox engineering

CompanionPDF reference

surface finish roughness Ra Rz ISO 1302 machining sealing fatigue DFM mechanical

Abstract

Surface finish is a real cost and performance lever, not a cosmetic afterthought. The same part is cheap at Ra 3.2 µm and expensive at Ra 0.2 µm, and the right value depends entirely on what the surface has to do — seal, slide, resist fatigue, take an interference fit, or just look right.

Section 1 defines roughness and how it's measured. Section 2 gives typical Ra by manufacturing process. Section 3 maps function to a target finish. Section 4 covers drawing call-outs (ISO 1302). Section 5 is cost and DFM. Section 6 is a conversion and N-grade quick reference.

Roughness is the deviation of the real surface from a mean line over a short sampling length. Ra is the average deviation; Rz the mean peak-to-valley. Function sets the target — and finer finishes cost exponentially more.

1.What roughness is and how it's measured

Surface texture has three scales: form (overall shape), waviness (longer-wavelength undulation), and roughness (the fine, closely-spaced peaks and valleys). A profilometer drags a stylus across the surface, filters out form and waviness with a cutoff (λc), and measures the remaining roughness against a mean line over a short sampling length.

1.1Terms

Arithmetic mean of the absolute deviations from the mean line — the default, most-quoted parameter

Mean peak-to-valley height over the sampling lengths — more sensitive to single defects than Ra

Rt / Rmax

Largest single peak-to-valley in the evaluation length — worst-case

Mean line

The reference line that splits the profile into equal areas above and below

Cutoff (λc)

The filter wavelength that separates roughness from waviness (commonly 0.8 mm)

Lay

The dominant direction of the surface pattern (e.g. parallel, perpendicular, circular)

Ra alone can hide problems. Two surfaces with the same Ra can behave very differently — one with a few deep scratches (high Rz, bad for sealing/fatigue) and one uniformly fine. For sealing and fatigue, constrain Rz (or Rmax) as well as Ra.

2.Typical roughness by process

Process	Typical Ra (µm)
Sand casting	6.3 – 25
Die casting	0.8 – 3.2
Hot rolling / forging	3.2 – 12.5
Flame / plasma / laser cut edge	3.2 – 25
Milling	0.8 – 6.3
Turning / boring	0.4 – 6.3
Drilling	1.6 – 6.3
Reaming	0.4 – 1.6
Grinding	0.1 – 1.6
Honing	0.05 – 0.4
Lapping / superfinishing	0.012 – 0.1
Polishing	0.025 – 0.2
EDM	0.4 – 6.3
Injection moulding (SPI A–D)	0.05 – 3.2+
FDM 3D print (layer lines)	6 – 25
SLA / SLS / MJF 3D print	1.6 – 6.3

A surface finer than the base process allows needs a secondary operation (grind, hone, lap, polish) — extra setups and cost.

3.Choosing the finish by function

Specify the loosest finish that works for the job the surface does:

Function	Target Ra (µm)
Dynamic seal / O-ring sliding face	0.1 – 0.4
Static seal / O-ring groove (face & bottom)	≤ 0.8 / ≤ 1.6
Sliding or journal bearing surface	0.2 – 0.8
Fatigue-critical surface	≤ 0.4 (smoother → higher endurance limit)
Press / interference fit	0.4 – 1.6
Mating / clamping / gasket faces	1.6 – 3.2
General machined, non-critical	1.6 – 6.3
Paint / adhesive adhesion	1.6 – 6.3 (some tooth helps)

Notes: rough surfaces lose effective interference in press fits (asperities flatten), so a fit specced from nominal sizes over-predicts holding force if the finish is coarse. Smoother surfaces raise the fatigue endurance limit because scratches are stress raisers — which is why fatigue-critical parts are ground or polished.

4.Drawing call-outs (ISO 1302 / ASME Y14.36)

The surface-texture symbol is a check-mark on the surface or its extension line:

Basic symbol (✓)surface; add a bar for material removal required, or a circle for removal prohibited (leave as-cast/moulded).
Value placementRa value at upper-left (e.g. Ra 1.6). A single value is the maximum (16%-rule); a range uses upper and lower limits.
Other parameterscall Rz/Rmax explicitly when they matter (Rz 6.3), alongside or instead of Ra.
Layadd the lay symbol (= parallel, ⊥ perpendicular, X crossed, M multidirectional, C circular, R radial) when direction matters for sealing or sliding.
All-around / generala circle on the symbol means all surfaces of the outline; a general note (e.g. "Ra 3.2 unless otherwise stated") covers the rest.

State the direction of measurement / lay for sealing and sliding surfaces — a groove machined with the wrong lay leaks even at the right Ra.

5.Cost and DFM

Finer finish costs exponentially. Each step down (3.2 → 1.6 → 0.8 → 0.4 µm) typically adds setups, slower feeds, or a whole secondary process. Don't tighten a finish "to be safe."
Specify per surface, not per part. Put the tight finish only on the faces that need it; leave everything else at the general note.
Match finish to process. Asking for Ra 0.4 on an as-milled face forces grinding; design the feature so the base process delivers it where possible.
Coatings change it. Plating and painting alter the final texture; specify before- or after-coating and account for build-up on fits.

6.Conversion and grade quick reference

Ra (µm)	Ra (µin)	ISO N-grade	~Rz (µm)
0.025	1	N1	~0.15
0.05	2	N2	~0.3
0.1	4	N3	~0.5
0.2	8	N4	~1
0.4	16	N5	~2
0.8	32	N6	~4
1.6	63	N7	~8
3.2	125	N8	~16
6.3	250	N9	~25
12.5	500	N10	~50

Rules of thumb: 1 µm Ra ≈ 39.4 µin; for typical machined surfaces Rz ≈ 4–7 × Ra (use a measured ratio when Rz is critical). N-grades (ISO 1302) are a shorthand some drawings still use — prefer an explicit Ra value.