Mechanical Drawing Symbols (Dimension & Geometric Tolerance, Explained)

Mechanical drawings are written in a "language of symbols." Knowing what they mean makes machining and inspection smoother and reduces defects from misreading. This article compiles the symbols most used on the shop floor, with readings and examples.

Dimensional symbols

These qualify a dimension's meaning.

Symbol	Meaning	Example
φ (phi)	Diameter	φ20 = 20mm diameter
R	Radius	R5 = 5mm radius
□	Side of a square	□10 = 10mm square
t	Plate thickness	t3 = 3mm thick
C	45° chamfer	C1 = 1mm chamfer at 45°
SR	Spherical radius	SR10
Sφ	Spherical diameter	Sφ20
⌴	Counterbore	Recess for a bolt head
⌵	Countersink	For flat-head screws
↧	Depth	↧10 = 10mm deep

Geometric tolerance symbols

These control form, orientation, location, and run-out.

Symbol	Name	Group	Controls
⏤	Straightness	Form	Straightness of a line/axis
▱	Flatness	Form	Flatness of a face
○	Roundness	Form	Roundness of a section
⌭	Cylindricity	Form	The whole cylinder
∥	Parallelism	Orientation	Parallel to a datum
⊥	Perpendicularity	Orientation	Perpendicular to a datum
∠	Angularity	Orientation	A specified angle
⊕	Position	Location	Position of holes, etc.
◎	Concentricity	Location	Coincident axes
⌀	Run-out (circular/total)	Run-out	Run-out when rotating

Reading a feature control frame

Geometric tolerance is given in a "frame." Read left to right: symbol | tolerance value | datum.

Example: ⊕ | φ0.1 | A B C = "position, tolerance φ0.1, referenced to datums A, B, C."

Datums

• Datum (A, B, C...): the reference plane/line/point for measurement and geometric tolerance
• A triangle symbol marks which face is the reference
• There's a priority order (primary, secondary, tertiary) — read it in the frame's order

Surface texture symbol

A "√"-shaped symbol specifies the roughness of a machined face, with an Ra value (see the surface roughness article).

Common mistakes

• Confusing φ and R: φ is diameter, R is radius. φ20 and R20 differ by 2×
• Confusing C (chamfer) and R (fillet): C bevels a corner, R rounds it
• Measuring without respecting datum priority

FAQ

Q. How is φ read? A. As "phi." It denotes diameter.

Q. Where do I start reading a geometric tolerance? A. Read the frame left to right: "symbol → value → datum."

Q. Are there geometric tolerances without a datum? A. Yes. Form tolerances (straightness, flatness, roundness) need no datum; orientation/location tolerances do.

Conclusion

The more symbols you know, the faster you read drawings. First grasp φ, R, C, the main geometric tolerance symbols, and how to read the feature control frame.

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