You have a drawing in front of you and there is a symbol on it you do not recognise. This page is for exactly that. 38 sembol, each with three things: what it is, what it means, and what it means for you at the machine. Type the name of the symbol, or any word you remember, into the search box.
Geometrical Tolerances (GD&T)
These state how far the geometry of a part may deviate. To ISO 1101:2017 (TS 1304). There are fourteen basic symbols in all.
How much an edge or an axis may bow. If Ø precedes the tolerance value the zone becomes a cylinder.
The surface must lie between two parallel planes separated by the tolerance value.
The cross-section must be a true circle. Each section is assessed separately.
Roundness + straightness + taper all at once. The whole cylinder must lie between two coaxial cylinders.
Deviation of a curve in one section from the theoretical profile.
Deviation of a whole surface from the theoretical surface. Used on free-form surfaces.
Parallelism of a surface or axis relative to the datum.
A surface or axis must be at 90° to the datum.
Being at a stated angle other than 90° to the datum.
Deviation of a hole or feature from its theoretical position. Usually given as a cylindrical zone with Ø.
Two diameters sharing the same axis.
A feature must be symmetrical about the datum plane.
The total deviation shown by a dial gauge in one section as the part is rotated about the datum axis.
The total deviation seen as the gauge is traversed along the surface. It covers the whole surface.
Tolerance Frame Modifiers
Symbols added next to the value or the datum in a tolerance frame that change its meaning.
The tolerance applies when the part contains the most material (hole at its smallest, shaft at its largest). As the part departs from that condition, bonus tolerance is gained.
The tolerance applies when the part contains the least material.
A boxed dimension carries no tolerance of its own — its deviation is given by the position tolerance.
A Ø in front of the tolerance value states that the zone is a cylinder, not a square.
Surface Texture Symbols
These state how smooth a surface must be and how that is to be achieved. Textbooks cite ISO 1302; that standard was withdrawn in December 2021 and replaced by ISO 21920-1:2021. Most drawings in circulation still use the older notation.
Two arms at 60° to the surface, the short arm half the length of the long one — at least 3 mm and 6 mm. It has no meaning on its own.
A bar closes the top. Machining — turning, milling, grinding, drilling — is mandatory.
A circle closes the top. The surface stays as cast or rolled and must not be machined.
A circle added to the symbol means every surface of the part is to be finished to that value.
The lay is parallel to the projection plane of the view carrying the symbol.
The lay is perpendicular to the projection plane.
The lay crosses in two oblique directions.
The lay has no particular direction.
The lay is approximately circular about the centre of the surface.
The lay radiates outward from the centre of the surface.
Dimensioning Symbols
Placed before or beside a dimension figure to state what the dimension describes.
The diameter of a circular feature. Ø20 means 20 mm diameter.
The radius of an arc or corner fillet. R3 means a 3 mm radius.
The diameter (SØ) or radius (SR) of a spherical surface.
States that the section is square. □20 means a 20×20 square section.
Edge break. At 45° it is written on one line as depth × angle. At other angles the depth and angle are dimensioned separately.
The slope of a surface relative to the horizontal, given as a ratio such as 1:10.
The ratio of a conical surface, calculated as (D−d)/L.
States that the dimension follows the arc, not the chord.
The view from the left is drawn on the right. Standard in Europe and Türkiye.
The view from the left is drawn on the left. Standard in the USA and Japan.
Ra Values and N Surface Grades
The N grade numbers came from ISO 1302:1978 and were formally withdrawn with ISO 21920 — but they still appear on older drawings and in machine catalogues. 1 µm = 0,001 mm.
| Grade | Ra (µm) | Typical production method |
|---|---|---|
| N1 | 0,025 | Superfinishing, lapping |
| N2 | 0,05 | Polishing, honing |
| N3 | 0,1 | Fine honing, lapping |
| N4 | 0,2 | Fine grinding |
| N5 | 0,4 | Grinding, honing |
| N6 | 0,8 | Fine grinding / fine turning |
| N7 | 1,6 | Fine turning, milling, reaming |
| N8 | 3,2 | Medium turning, milling |
| N9 | 6,3 | Rough turning, milling, drilling |
| N10 | 12,5 | Rough milling, sawing |
| N11 | 25 | Rough cutting, parting |
| N12 | 50 | As-cast, forged or rolled surface |
Technical Drawing Training — all lessons
- Lesson 1: Introduction, Paper Sizes, Title Block and Scale
- Lesson 2: Line Types, Line Widths and Lettering
- Lesson 3: Geometric Constructions, Tangency, Polygons and the Ellipse
- Lesson 4: Orthographic Projection, Views and First vs Third Angle
- Lesson 5: Section Views, Hatching Rules and Parts Never Hatched
- Lesson 6: Dimensioning Rules, Systems and Tolerance Accumulation
- Lesson 7: Surface Texture, Ra and Surface Symbols
- Lesson 8: Dimensional Tolerances and the ISO System of Fits
- Lesson 9: Geometrical Tolerances — The 14 Symbols, Frame and Datums
- Lesson 10: Pictorial Projection, Isometric Drawing and Sketching
- Lesson 11: Assembly Drawings, Detail Drawings and the Parts List
- Lesson 12: Reading a Drawing for the CNC Operator
- Technical Drawing Symbols and Abbreviations Glossary ← you are here