Short Answer:

In GD&T (Geometric Dimensioning and Tolerancing), different symbols are used to show the allowable variation in shape, position, and orientation of parts. These symbols are standard and help in understanding the design clearly, without confusion. They are placed inside a feature control frame to describe what type of control is applied to a particular feature.

Some common GD&T symbols include straightness, flatness, circularity, cylindricity, parallelism, perpendicularity, angularity, position, concentricity, and runout. Each symbol has a specific meaning and helps in controlling how the part should be made and inspected.

Detailed Explanation:

Different Symbols Used in GD&T

GD&T uses a set of well-defined geometric symbols to explain the design intention clearly and in a standardized way. These symbols are part of the ASME Y14.5 or ISO 1101 standard, and they help in describing not just the size of a part, but also its shape, form, orientation, and position.

Using these symbols ensures that all manufacturers, inspectors, and designers understand the drawing in the same way, which improves accuracy, reduces mistakes, and supports global production.

Let’s understand the major GD&T symbols in a simple and easy way:

Form Control Symbols

These symbols are used to control the shape or form of a feature.

Straightness (⏤): Controls how straight a line must be.
Flatness (⏥): Controls how flat a surface should be without waves or bumps.
Circularity (Roundness) (○): Ensures the cross-section of a cylinder or hole is perfectly round.
Cylindricity (⌭): Controls the roundness, straightness, and taper of a cylindrical surface.

Orientation Control Symbols

These symbols control the tilt or angle of one feature in relation to another.

Perpendicularity (⟂): Ensures one surface or axis is at 90° to another.
Parallelism (∥): Makes sure surfaces or features are equally spaced and never meet.
Angularity (∠): Controls the angle between two surfaces (not 90°).

Location Control Symbols

These control the exact position of features on a part.

Position (∘ or ⊕): Most commonly used symbol; controls the exact center, axis, or location of features like holes and slots.
Concentricity (◎): Ensures that two or more circles share the same center point.
Symmetry (⌯): Makes sure that a feature is centered between two surfaces or around a central axis.

Runout Control Symbols

These control how much a surface moves or wobbles when the part is rotated.

Circular Runout (↗): Controls the roundness and surface movement in one circular cross-section.
Total Runout (⥀): Controls the entire surface over the full length of rotation.

Other Important GD&T Elements

In addition to symbols, GD&T uses a feature control frame, which is a rectangular box containing:

The geometric symbol
The tolerance value
Datum references, which are fixed reference points used to control how the feature should be measured

Modifiers like MMC (Maximum Material Condition) and LMC (Least Material Condition) are also used with some symbols to give more flexibility during manufacturing.

Purpose of Using GD&T Symbols

To clearly define what is acceptable and what is not in part geometry
To reduce misinterpretation during manufacturing and inspection
To allow flexibility in production without losing part function
To ensure proper fit, form, and function of assembled parts

Using these symbols also supports CAD modeling, CNC machining, and quality control systems, making GD&T a complete solution for modern manufacturing.

Conclusion

GD&T symbols are a key part of engineering drawings that allow for detailed control of how parts should be made and measured. Symbols like flatness, position, straightness, runout, and perpendicularity help in clearly defining the part’s geometry and performance needs. By using these symbols properly, engineers ensure that parts function correctly, fit well with other parts, and meet quality standards. Understanding these symbols is essential for all professionals involved in mechanical design and manufacturing.