Symbols: Read Them Right, Avoid Costly Misunderstandings

Symbols compress a lot of meaning into very little space on a technical drawing. Reading them correctly avoids ambiguity in quoting, machining, inspection, and assembly.

Tip: Units follow the drawing standard in the title block. Unless specified otherwise, ISO drawings are typically in mm. US‑centric drawings often use in.

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Quick reference

Symbol	Meaning	How to read	Example
R	Radius (arc/fillet/round)	Value is the distance from arc center to surface	R6 → radius 6 mm
SR	Spherical radius	Radius of a spherical surface	SR30 → spherical radius 30 mm
Ø	Diameter (circle/cylindrical feature)	Value is full diameter	Ø80 → diameter 80 mm
SØ	Spherical diameter	Diameter of a spherical cap/surface	SØ30 → spherical diameter 30 mm
□	Square section	Cross‑section is square; value is side length	□20 → 20 mm × 20 mm square
t	Thickness (sheet/plate)	Material thickness; often used for sheet metal	t2.5 → thickness 2.5 mm
C	45° chamfer shorthand	Width at 45° (default). Other variants may state angle explicitly	C3 → 3 mm × 45° chamfer

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Core symbols and best practices

Radius R

Radius is used for inside fillets and outside rounds, as well as for general arcs.

• How it’s shown: R<value> before the dimension.
• Good to know: If the entire circle is controlled, use Ø instead of R.
• Example: R6 means the radius is 6 mm.

Common pitfalls

• Calling out a radius where a full diameter is intended → use Ø for circles.
• Omitting the number of instances when multiple identical radii exist → add (xN) if needed, e.g., R3 (x6).

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Spherical radius SR

Used when the feature is part of a sphere described by its radius.

• How it’s shown: SR<value>
• Example: SR30 → spherical radius 30 mm.

Common pitfalls

• Confusing SR (spherical radius) with SØ (spherical diameter). Make sure the requirement matches your tooling/inspection plan.

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Diameter Ø

Used for circles, cylinders, holes, and pins.

• How it’s shown: Ø<value>
• Example: Ø80 → diameter 80 mm.
• Metric vs imperial: You may see Ø0.500 in or Ø12.7 depending on the standard.

Common pitfalls

• Leaving off tolerances for critical fits (e.g., bearings, shafts). Consider adding fit class (e.g., Ø20 H7).

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Spherical diameter SØ

Used for spherical features specified by diameter.

• How it’s shown: SØ<value>
• Example: SØ30 → spherical diameter 30 mm.

Common pitfalls

• Selecting SØ when the mating spec expects SR. Align symbols with the measuring method.

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Square □

Indicates a square cross‑section.

• How it’s shown: □<side>
• Example: □20 → 20 mm square bar.
• Variations: You may also see length appended for stock, e.g., □20 × 100 (square bar 100 mm long).

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Thickness t

Common in sheet‑metal and plate parts.

• How it’s shown: t<value>
• Example: t2.5 → material thickness 2.5 mm.
• Note: Some standards use s for thickness; follow the title‑block convention.

Common pitfalls

• Forgetting that bend deductions/allowances depend on t. Ensure CAM settings match the material and tooling.

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45‑degree chamfer C

Shorthand for a chamfer at 45° unless otherwise stated.

• How it’s shown: C<width> or explicitly with angle: <width> × 45°.
• Example: C3 → 3 mm chamfer at 45°.

Common pitfalls

• Assuming 45° when a different angle is required. If angle matters, write it: 2 × 30°.

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Quality & communication tips

• Declare units once (title block or note) and only annotate when deviating.
• Be explicit about counts for repeated features (e.g., R2 (x8) around a bolt circle).
• Add tolerances for fit‑critical diameters and radii.
• Match symbol to inspection: choose SR vs SØ based on how the feature will be measured.