DESIGN FOR MANUFACTURING (DFM) GUIDE

An Introduction to GD&T

Move beyond simple plus/minus tolerances. This guide introduces the fundamentals of Geometric Dimensioning and Tolerancing (GD&T), a symbolic language for defining how parts function.

Why Go Beyond Basic Tolerances?

While standard tolerances (e.g., ±0.1 mm) control the size of a feature, they don't control its shape or relationship to other features. For example, a hole could be within its diameter tolerance but be drilled at an angle. This is where GD&T comes in.

**GD&T (Geometric Dimensioning and Tolerancing)** is a standardized symbolic language used on 2D engineering drawings to precisely define the allowable variation in a part's geometry. It ensures that every part not only has the right dimensions but also the right form, orientation, and location to function correctly in an assembly.

A CMM machine inspecting a part, a process essential for verifying GD&T callouts

The 14 Symbols of GD&T

GD&T uses 14 standard symbols, grouped into categories, to control different aspects of a part's geometry.

Form Tolerances

These control the shape of a single feature. They include **Straightness, Flatness, Circularity,** and **Cylindricity**.

Orientation Tolerances

These control the angle or orientation of a feature relative to another feature (a datum). They include **Perpendicularity, Parallelism,** and **Angularity**.

Location Tolerances

These control the location of features relative to each other or to a datum framework. They include **Position, Concentricity,** and **Symmetry**.

Profile & Runout

These are complex controls. **Profile** controls the 3D shape of a surface. **Runout** controls the variation of a surface as it rotates around a central axis.

Reading a GD&T Callout: The Feature Control Frame

GD&T requirements are communicated on a drawing using a "Feature Control Frame." This is a rectangular box with compartments that specify the geometric control.

Example: Position of a Hole

Imagine a callout that looks like this: `[⌖|ø0.1(M)|A|B|C]`

`⌖`: This is the symbol for **Position**. It controls the location of the feature (the hole).
`ø0.1`: This is the **tolerance zone**. It means the center axis of the hole must lie within a cylindrical zone that is 0.1 mm in diameter.
`(M)`: This is a **material condition modifier** (Maximum Material Condition). It means the tolerance zone can get bigger if the hole is produced larger than its ideal size, providing a bonus tolerance.
`A|B|C`: These are the **datum references**. They establish the coordinate system. It means the hole's location is measured relative to Datum Plane A, Datum Plane B, and Datum Plane C.

GD&T FAQ

When should I use GD&T?

You should use GD&T when the simple plus/minus tolerances on a part are not enough to guarantee its function. If the assembly requires that two holes align perfectly, that a surface is perfectly flat for sealing, or that a shaft is perfectly perpendicular to a flange, then GD&T is necessary.

Does using GD&T increase the cost of a part?

It can, but not always. Applying very tight GD&T controls will increase cost because it requires more precise machining and inspection. However, using GD&T wisely (especially with material condition modifiers) can sometimes *increase* the allowable tolerance for a machinist, potentially lowering the cost while still guaranteeing the part's function.

Do I need to be a GD&T expert to order parts?

No. For most parts, standard block tolerances are sufficient. However, if you are designing complex assemblies, having a basic understanding of GD&T can greatly improve the quality and functionality of your final product. We have engineers on staff who can review your GD&T callouts and ensure they are manufacturable.