Surface Roughness Symbols and Ra Values Explained

No machined surface is perfectly flat. Look closely enough and every surface is a landscape of peaks and valleys left by the cutting tool. Surface texture is the drawing’s way of saying how rough that landscape is allowed to be.

It matters because roughness decides how surfaces behave: how they seal, how they wear, how they slide, how well a coating sticks, and how much a fatigue crack likes to start there.

What Ra means

Figure 7.1 — Definition of Ra and the surface profile at different Ra values

Ra is the arithmetical mean of the absolute deviations of the surface profile from its mean line, measured over a defined sampling length. It is given in micrometers (µm), where 1 µm = 0,001 mm.

So Ra 3,2 means the average departure from the mean line is 3,2 thousandths of a millimeter. Ra 0,8 is four times smoother — and considerably more expensive to produce.

Effect of surface roughness on machine performance: engine cylinder and piston, high-pressure hydraulic pump — Why roughness matters: the oil film and sealing of a cylinder–piston pair, and leakage and pressure loss in a hydraulic pump, depend directly on surface quality (Turkish infographic: iyi/kötü yüzey kalitesi = good/poor surface quality)

Ra (µm)	Typical appearance	Typical use
25 – 12,5	visible tool marks, rough	non-functional, as-cast, rough machined
6,3 – 3,2	uniform marks, matt	general machined surfaces
1,6 – 0,8	smooth, slight sheen	fits, bearing seats, sealing faces
0,4 – 0,2	bright, mirror-like	precision fits, hydraulic surfaces
0,1 and below	mirror	gauges, optical and measuring surfaces

Standards note. The old symbol standard ISO 1302 and the roughness standards ISO 4287 / ISO 4288 were replaced in December 2021 by the ISO 21920 series (ISO 21920-1, -2 and -3). Many drawings still in circulation were made to the older standards, so both notations are met in practice. Ra itself is unchanged.

The surface texture symbol

Figure 7.2 — The three basic surface symbols and the a–e information fields added to them

The symbol is a 60° tick whose short arm is half the length of the long one. There are three basic forms:

Basic symbol — the plain tick. It carries no meaning until something is added to it.
Bar added — material removal required: the surface must be machined.
Circle added — material removal not permitted: the surface must stay as cast, forged or rolled.

Information is then written in defined positions around it: the Ra value, a second requirement, the production method or coating, the direction of lay, and the machining allowance.

The symbol touches the surface — or its extension line — with its point, from outside the material.
Each surface is marked once.
If most surfaces share one value, it is given as a general note and only the exceptions are marked individually.
The symbol is always written so it reads from the bottom or the right of the sheet.

The a–e information fields

Field	What is written	Example
a	Ra roughness value or grade number	Ra 6,3
b	Second or further surface requirement	Rz 25
c	Production method, coating, treatment	ground, hardened, plated
d	Direction of lay (tool marks)	=, ⊥, X, M, C, R
e	Machining allowance (mm)	2

Most drawings fill in only field a. When a production method is specified it is written on the horizontal bar added to the long arm of the symbol.

Direction of lay symbols

Symbol	Meaning
=	Lay parallel to the projection plane of the view the symbol is placed on
⊥	Lay perpendicular to that projection plane
X	Lay crossed in two oblique directions
M	Multidirectional, no dominant direction
C	Approximately circular relative to the center of the surface
R	Approximately radial relative to the center of the surface

Placing the symbols on the drawing

Symbols and their text are written so that they read from the bottom or the right of the sheet.
The symbol is placed outside the material with its point towards the surface — directly on the surface, on an extension line, or via a leader.
In holes the symbol is placed where it clearly refers to the internal surface.
On cylindrical parts only one of two symmetrical surfaces is marked.
The symbol goes on the view that carries the size and position dimensions of that surface, and only once.
Equally spaced features (a row of holes, for example) are marked once, following the same rule as their dimension.
On gears the tooth-flank value is placed on the pitch circle line.

Placement of Ra 3,2, Ra 1,6 and Ra 0,8 surface texture symbols on a turned bush — Surface symbols on a bush: the general value Ra 3,2 top right, the exceptions in brackets (Ra 1,6, Ra 0,8); each surface is marked once, from outside the material

When the whole part has one surface requirement

If every surface is to be finished the same way, a symbol 1,5 times the size of the normal one is placed at the top right corner of the drawing, near the part number. Three forms are allowed: the general symbol with the note “except where otherwise stated”; the general symbol followed by the basic symbol in brackets; or the general symbol followed, in brackets, by the individual symbols that appear elsewhere on the drawing.

Which process gives which Ra

Figure 7.3 — Ra ranges achievable by the common production methods

Reading a roughness value is really reading a process. An Ra of 6,3 can be reached by ordinary turning or milling. An Ra of 0,8 normally needs grinding or reaming. An Ra of 0,1 needs honing or lapping. So the surface symbol quietly tells you how many operations the part will take — and therefore what it costs.

Do not over-specify. Asking for Ra 0,4 where Ra 3,2 would do can multiply the cost of a surface several times over for no functional gain. Specify the roughest surface that still works.

N surface grades on older drawings

Older drawings use grade numbers beginning with N instead of an Ra value. The grades were formally withdrawn with ISO 1302, but the drawings are still in circulation, so you need to read both:

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

Test yourself

What does Ra 1,6 mean in millimeters?
0,0016 mm average deviation from the mean line.
Which is smoother, Ra 0,8 or Ra 3,2?
Ra 0,8 — the smaller the number, the smoother the surface.
What does a circle on the surface symbol mean?
Material removal is not permitted; the surface stays as produced.
Which process would you choose for Ra 0,8?
Grinding or reaming; ordinary turning will not reliably reach it.
How many times is one surface marked?
Once.
Why not specify Ra 0,2 on every surface?
Because cost rises steeply with smoothness and most surfaces do not need it.
Which standard replaced ISO 1302?
The ISO 21920 series, from December 2021.

Next lesson. Roughness says how smooth. Lesson 8 says how accurate — dimensional tolerances and the ISO system of limits and fits: what H7 and h6 actually mean and how clearance, transition and interference fits are built.