What is Ra in surface roughness?

Ra (arithmetical mean roughness) is the most commonly used surface roughness parameter. It represents the average absolute deviation of the profile from the mean line, measured in micrometers (μm).

What does the MRR indicator mean on a surface symbol?

MRR stands for Material Removal Required. A circle in the V-symbol means 'any process allowed'. A bar across means 'machining required (material removal required)'. No circle or bar means 'no material removal allowed'.

How are lay direction symbols used?

Lay direction symbols indicate the predominant surface pattern direction: = (parallel to projection plane), ⊥ (perpendicular), X (crosshatch), M (multi-directional), C (circular), R (radial), P (non-directional/particulate).

What is the difference between ISO 1302 and ISO 21920?

ISO 1302 defines how to indicate surface texture on technical drawings (symbols and notation). ISO 21920 defines the actual profile parameters (Ra, Rz, Rq, etc.) and their measurement. They are complementary standards.

How do I choose the right Ra value for my application?

Typical values: Ra 0.1-0.4 μm for bearing surfaces and seals; Ra 0.8-1.6 μm for mating/sliding surfaces; Ra 3.2-6.3 μm for general machined surfaces; Ra 12.5-25 μm for rough machining and castings.

Free Engineering Tool

Surface Roughness Symbols Reference

ISO 1302 drawing symbol reference. Select a machining process or Ra value to see the proper drawing symbol, MRR indicator, and lay direction.

ISO 1302 Ra / Rz Drawing Symbols

Machining Process

Ra Value (μm)

Material Removal

Lay Direction

Quick presets

Symbol & Details

Surface Roughness

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MRR Indicator

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Lay Direction

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Typical Process

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ISO N Grade (approx)

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ISO 1302 Symbol Structure

The ISO 1302 surface texture symbol consists of a basic graphical symbol (check-mark shape) with annotations for Ra/Rz values, machining process, material removal, lay direction, and machining allowance.

Ra vs N Grade Cross-Reference

Ra (μm)	N Grade	Typical Process	Application
0.025	N1	Superfinish / Lapping	Gauge blocks, mirrors
0.05	N2	Lapping	Precision gauges
0.1	N3	Lapping / Honing	Bearing races, seals
0.2	N4	Grinding / Honing	Crankshaft journals
0.4	N5	Grinding	Bearing seats
0.8	N6	Grinding / Fine turning	Mating surfaces
1.6	N7	Turning / Milling	General machined parts
3.2	N8	Turning / Milling	Non-critical surfaces
6.3	N9	Rough machining	Internal surfaces
12.5	N10	Rough turning / Sawing	Rough castings
25	N11	Casting / Forging	Non-functional surfaces
50	N12	Sand casting	As-cast surfaces

Lay Direction Symbols

Symbol	Meaning	Example
=	Parallel to projection plane	Turning, shaping
⊥	Perpendicular to projection plane	Cross-grinding
X	Crossed in two oblique directions	Honing
M	Multi-directional	Lapping, superfinishing
C	Approximately circular	Face turning
R	Approximately radial	Radial grinding
P	Non-directional, particulate	Shot blasting, EDM

Surface Roughness Symbols ISO 1302 | Free Reference Tool

Published by Nikolai Shelkovenko on February 15, 2026 March 5, 2026

Symbol & Details

ISO 1302 Symbol Structure

Ra vs N Grade Cross-Reference

Lay Direction Symbols