Engineering Drawing Symbols

To keep engineering drawings clean and readable, standard symbols are used instead of writing out long text notes. ASME Y14.5 and other related standards define these symbols so they can be universally interpreted.

Note: This article covers general manufacturing and dimensioning symbols. Geometric Dimensioning and Tolerancing (GD&T) symbols, like Position or Flatness, are covered in separate GD&T fundamental units.

Common Dimensional Symbols

These symbols are placed directly in front of the dimension number to define the shape of the feature being dimensioned.

• Diameter ($\varnothing$): A circle with a diagonal slash. Indicates the dimension applies to a circular feature (e.g., a drilled hole or a cylindrical shaft).
• Radius ($R$): Indicates the radius of an arc. Used for fillets, rounds, and incomplete circles.
• Spherical Diameter ($S\varnothing$) / Spherical Radius ($SR$): Used to dimension spherical features, such as a ball joint.
• Square ($\square$): Indicates the cross-section of the feature is a square.

Machining Feature Symbols

These symbols provide instructions for machining holes in specific configurations.

• Countersink ($\vee$): Indicates a conical recess at the opening of a hole, typically used to allow flat-head screws to sit flush with the surface. The symbol is followed by the diameter and angle of the countersink.
• Counterbore ($\sqcup$): Indicates a cylindrical recess at the opening of a hole, typically used to hide a socket-head screw. The symbol is followed by the diameter and depth of the counterbore.
• Depth ($\downarrow$): Indicates how deep a hole or milled feature should go. Often used in conjunction with counterbores or blind holes.

Surface Texture / Finish Symbols

Surface finish symbols instruct the machinist on how smooth or rough the final surface of the part must be.

The basic symbol looks like a checkmark ($\checkmark$) with the point resting on the surface being defined.

• Basic Symbol: Indicates the surface must be machined.
• Symbol with a horizontal bar: Usually contains a numerical value above the bar, representing the maximum allowable Roughness Average ($Ra$), typically measured in micrometers ($\mu m$) or microinches ($\mu in$).
• Symbol with a circle in the vee: Indicates that material removal (machining) is strictly prohibited on that surface.

Welding Symbols

Welding symbols are highly complex and can convey a massive amount of information in a small space. They are largely governed by AWS (American Welding Society) or ISO standards.

A basic welding symbol consists of:

1. Reference Line: A horizontal line. All other elements are tied to this line.
2. Arrow: Points from the reference line to the joint that needs to be welded.
3. Weld Symbol: Placed on the reference line to indicate the type of weld (e.g., a triangle for a fillet weld, two parallel lines for a square groove weld).
   • If the symbol is below the reference line, the weld is made on the "Arrow side."
   • If the symbol is above the reference line, the weld is made on the "Other side."
4. Tail: Added to the end of the reference line only when it's necessary to reference a specific welding process (e.g., TIG, MIG) or a special note.
5. Dimensions: Numbers dictating the size, length, and pitch of the weld are arranged around the weld symbol.