Engineering Drawing Symbols

To keep engineering drawings clean and easily readable, standard symbols are used to replace lengthy textual notes. The ASME Y14.5 and 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, such as position or flatness, are covered in the foundational GD&T unit.

Common Dimensional Symbols

These symbols are placed directly before the numerical dimension value to indicate the shape of the feature being dimensioned.

• Diameter ($\varnothing$): A circle with a diagonal line through it. Indicates that the dimension applies to a circular feature (like a 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 the ball on a ball joint.
• Square ($\square$): Indicates that the cross-section of the feature is a square.

Machining Feature Symbols

These symbols provide instructions for specific machining operations on holes.

• Countersink ($\vee$): Indicates a conical recess at the top of a hole, typically used to allow a flat-head screw to sit flush with the surface. The symbol is followed by the diameter and the angle of the countersink.
• Counterbore ($\sqcup$): Indicates a cylindrical recess at the top of a hole, typically used to hide the head of a socket-head cap screw. The symbol is followed by the diameter and depth of the counterbore.
• Depth ($\downarrow$): Indicates how deep a hole or feature must be machined. It is often used with counterbores or blind holes (holes that do not go all the way through the part).

Surface Texture (Finish) Symbols

The surface finish symbol instructs 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 requires machining.
• Symbol with a Horizontal Line: Typically contains a numerical value above the line representing the maximum allowable Roughness Average ($Ra$), often measured in micrometers ($\mu m$) or microinches ($\mu in$).
• Symbol with a Circle in the Vee: Indicates that material removal is strictly prohibited on that surface.

Welding Symbols

Welding symbols are highly complex and can convey a vast amount of information in a small space, governed primarily by the AWS (American Welding Society) or ISO standards.

A basic welding symbol consists of:

1. Reference Line: A horizontal line. All other elements are attached 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" (the side the arrow points to).
   • 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 a specific welding process (e.g., TIG, MIG) or specialized note needs to be referenced.
5. Dimensions: Numbers indicating the size, length, and pitch (spacing) of the weld are placed around the weld symbol.