JIS Surface Finish Symbols: Reading Triangles, Ra and Rz on a Drawing

Drawings / 9 Sept 2026

Surface finish is the second most common reason a stainless quote comes back wrong, after the general tolerance class. Modern JIS drawings state it clearly. Older ones do not, and older ones are still in wide circulation.

The modern notation

Current drawings specify surface texture to JIS B 0601, using the same tick symbol as ISO 1302 with a number beside it. Ra 1.6 means arithmetic mean roughness of 1.6 micrometres, maximum. This is unambiguous and every shop reads it the same way.

Two parameters do most of the work:

Ra, arithmetic mean deviation. Average the absolute deviations of the profile from the mean line. It is stable, it is easy to measure, and it is what most drawings use. Its weakness is that it is an average, so a surface with one deep scratch and a surface with none can share an Ra.

Rz, maximum height of the profile. Peak to valley, averaged over sampling lengths. It catches exactly what Ra hides. Specified on sealing faces, on surfaces that carry an O-ring, and anywhere an isolated defect matters more than the general texture.

A rough conversion for machined surfaces is Rz around four times Ra. It is a rule of thumb, not a specification. Never convert one to the other on a drawing and treat the result as equivalent.

One note specific to JIS: older editions of B 0601 used Rmax and a ten point height called Rz. The current standard aligns Rz with ISO as maximum profile height, and keeps the ten point parameter as Rzjis. If a drawing says Rz and dates from the 1990s, check which one is meant.

The legacy triangle marks

Drawings produced before the standard was revised carry triangle symbols instead of numbers. They were withdrawn, and they are still everywhere, because drawings outlive standards.

Symbol	Meaning	Typical Ra
Wavy line	No machining required, as supplied	-
One triangle	Rough finish	around Ra 25
Two triangles	Medium finish	around Ra 6.3
Three triangles	Fine finish	around Ra 1.6
Four triangles	Precision finish	around Ra 0.2

Treat this as a reading aid, not a specification. Published conversion tables differ, particularly on the four triangle value, where some sources give Ra 0.4 or Ra 0.8 instead of Ra 0.2. The difference between those is the difference between a fine grind and a lapped surface.

The practical rule: if a drawing carries triangle marks, get the intended Ra confirmed in writing before anyone quotes. It takes one email. The alternative is a shop quoting as machined against a face that needed grinding, and finding out at first article.

N grades

You will also see N numbers, from ISO 1302, which some JIS-era drawings adopted:

Grade	Ra, µm	Grade	Ra, µm
N1	0.025	N7	1.6
N2	0.05	N8	3.2
N3	0.1	N9	6.3
N4	0.2	N10	12.5
N5	0.4	N11	25
N6	0.8	N12	50

These are unambiguous, which makes them preferable to triangles on any drawing you are revising.

What each level costs on stainless

This is the part that decides the quote. Values are typical for austenitic stainless such as SUS304 and SUS316L.

Ra	How it is produced	Cost effect
12.5 to 25	Rough turning or milling	None, this is what roughing leaves
6.3	Medium machining	None
3.2	Normal finish pass	None
1.6	Good finish pass, sharp tooling	None. Specify this as the default
0.8	Careful finish turning or milling, tool changed on schedule	Small. More attention, sometimes an extra pass
0.4	Grinding, or very fine turning on a rigid machine	Real. Often a separate operation and setup
0.2	Grinding	Significant. Separate operation, separate fixturing
0.1 and below	Lapping, honing, polishing, electropolishing	Outside process, its own lead time and minimum lot

Two things follow. Ra 1.6 is free on a normally machined stainless part, so there is rarely a reason to specify looser. And the jump from 0.8 to 0.4 is where the money is, because that is where an additional operation enters the process.

Austenitic stainless makes this harder than carbon steel does. It is gummy, it builds up on the cutting edge, and built up edge tears the surface. A shop that achieves Ra 0.8 on SUS316L is changing inserts on a schedule and running enough coolant pressure to clear the chip, both of which cost something.

Do not blanket specify

The most expensive surface finish mistake is a general note that applies a tight finish to the whole part.

ALL SURFACES Ra 0.8 on a part where two faces need it and eleven do not means every one of the thirteen gets the treatment, or the shop prices it as if they do.

Better:

General surface finish: Ra 3.2
Sealing face A: Ra 0.8 max
Bore B: Ra 0.4 max

Three lines, and the shop can put the effort exactly where it is needed.

Direction, and when it matters

Ra says nothing about lay, the direction of the tool marks. On most parts that is fine. On two it is not:

Dynamic seals. A shaft running in a lip seal wants a plunge ground or lapped surface with no helical lead. A turned surface has a lead angle that pumps fluid along the shaft. The Ra can be perfect and the seal will still leak.
Fatigue loaded parts. Tool marks perpendicular to the stress direction are crack initiation sites.

Both are specified with the lay symbol beside the finish mark. If your part has a rotating seal, use it.

What to write on a drawing you are revising

If you are updating a legacy drawing, replace the triangles. Write Ra values with the JIS B 0601 tick symbol, state the general finish once in the title block area, and call out the exceptions on the features that need them. It removes an entire class of quoting error permanently, and costs one drawing revision.

For the dimensional side of the title block, see JIS B 0405 and B 0419 general tolerances. For what happens to the surface after machining, see passivation of machined stainless parts.

Working from a drawing with triangle marks on it? Get a quote. We flag the finish call-outs and confirm them with you before the drawing is placed with a shop. Priced quote within 24 business hours.

Questions

What do the triangle symbols on an old JIS drawing mean?

They are the withdrawn JIS surface finish marks. The usual reading is one triangle around Ra 25, two around Ra 6.3, three around Ra 1.6 and four around Ra 0.2. Confirm the intended value with the drawing owner before anyone prices a finish from them.

What is the difference between Ra and Rz?

Ra is the arithmetic mean deviation of the profile, an average. Rz is the maximum height of the profile, a peak to valley measure. Rz catches isolated defects that Ra averages away, which is why sealing faces are often specified in Rz.

What Ra can I expect from normal machining?

Ra 1.6 is a standard good machined finish and costs nothing extra. Ra 3.2 is routine. Below Ra 0.8 usually means an additional operation, and below Ra 0.4 normally means grinding, lapping or electropolishing.

Is a mirror finish the same as low Ra?

Not necessarily. Appearance and roughness are related but not identical. If the requirement is cosmetic, say so and give a finish description, not only a number.

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